I'd put my money on the Sun and Solar Energy, what a source of Power! I hope we don't have to wait until oil and coal run out, before we tackle that.

I have no doubt that we will be successful in harnessing the sun's energy. If sunbeams were weapons of war, we would have had solar energy centuries ago.

Figure 1.1: An artist's rendering of Archimedes' Death Ray, (c. 214-212 BC).

Source: Chris Devlin's Blog

People have been leveraging solar thermal energy for millennia. The history of the use of solar energy for heating buildings (c. 4000 BC), creating fire (c. 1000 BC), and driving industrial processes (c. 1800 AD) has been well documented by archeologists and historians. The sun comes up every day (with the exception of locations that are north of the Arctic Circle or south of the Polar Circle where the sun never rises above the horizon in the depths of winter). It is our most reliable source of energy and is the source of most other forms of energy on our planet. While often characterized as intermittent, with intermittence deriving from meteorology, atmospheric physics, and diurnal and seasonal patterns, barring any catastrophic event such as a meteor impact on Earth or the expiration of the sun, the sun is always shining somewhere, will shine again everywhere, and is the primary driver behind the thermal behavior of our environment.

Lesson Objectives

At the successful completion of Lesson 1, students should be able to:

• Identify solar temporal relationships;
• Describe solar geometric relationships;
• Recall key aspects impacting available solar radiation.

What is due for Lesson 1?

This lesson will take us one week to complete. Please refer to the Course Calendar in Canvas for specific time frames and due dates. Brief directions for the lesson assignments are given in the table below, and you can find more details on respective pages of this lesson.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum, located under the Modules tab in Canvas. I will check that discussion forum regularly will do my best to respond. While you are on the forum, please feel free to post your own responses if you are able to help out a classmate.

When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That's relativity."

— Albert Einstein

Einstein, in the special theory of relativity, proved that different observers, in different states of motion, see different realities."

— Leonard Susskind

The location of the sun in the sky through time is relative to the location of the Earth with respect to the sun, the location of the observer on Earth, and the orientation of the collector at the observer’s location on Earth. The geometric relationships are defined by angles separated into three groups; 1) the relationship between the Earth and sun, as spheres, 2) the relationship between the sun and an observer (or point on the Earth), and 3) the relationship between a collector at some orientation and the sun. As the Earth rotates on its axis, the sun appears to traverse across Earth's sky. As the Earth orbits around the sun, the tilt of the axis moves the northern and southern hemispheres closer and further from the sun moving the sun's trajectory across Earth's sky higher and lower in the sky, creating seasons. The temporal and spatial relationships are inextricably linked.

Recall from EME 810 the following four tables showing the Angular Symbols for Standard Solar Relations. These angles are detailed in the D&B textbook in section 1.6. These angles are all critical to the calculation of solar energy on any given surface at some orientation. To calculate the energy balance across any time scale with respect to the incident solar radiation, you must know the geometric relationships that define the spherical realities of planetary motion relative to the sun for specific tilted surfaces on the planet. Each piece is defined relative to another piece. For example, a solar panel may have a tilt, beta, but that tilt is relative to the horizontal surface on the planet, not relative to the sun. Greek symbols, often with a subscript, are used to ensure clarity through unique identifiers for each angle. Additionally, the sign convention for each angle is critical, as an incorrect sign or origin can lead to drastically incorrect results. Be careful when performing calculations to ensure that each angle and sign is correct.

In EME 810, you learned and applied principles regarding the Earth's rotation, the cosine projection effect of light, and some insight into the driving force behind the seasons. These principles are critical for appropriate engineering of solar thermal solutions for utilities and industry. A comprehensive understanding of the solar resource and the physics behind the intermittent and cyclical behavior of solar energy enables the engineering of solar thermal systems that adequately meet a client's needs.

Earth's Rotation

As we have seen in our reading, the Earth rotates with a roughly constant speed, so that every hour the direct beam (a ray pointing from the surface of the sun to a spot on Earth) will traverse across a single standard meridian (standard meridians are spaced ${15}^{\circ }$ apart). The implications are that the unit of one hour is equivalent to the rotation of Earth 15 degrees. When Earth rotates such that the beam of the sun shifts $+ 1^{\circ }$ of longitude from East to West: it takes 4 minutes of time.

• $1 h = + {15}^{\circ } \text{Earth rotation}$
• $4 m i n = + 1^{\circ } \text{Earth rotation}$

Wild fact: a time zone change of one hour is really just 15 degrees of separation between standard meridians.

The axis of rotation of the Earth is tilted at an angle of 23.5 degrees away from vertical, perpendicular to the plane of our planet's orbit around the sun.

The tilt of the Earth's axis is important, in that it governs the warming strength of the sun's energy. The tilt of the surface of the Earth causes light to be spread across a greater area of land, called the cosine projection effect.

Cosine Projection Effect

When you tilt a surface away from a beam of light, you spread the same density of light across a larger area. Recall that irradiance is in units of $W / m^2$, so a larger denominator means a smaller value of irradiance, right?

Explore the concept of the cosine projection effect in the following experiment.

Seasons and the Cosine Projection Effect

The sun is about 93 million miles away from the Earth (equivalent to ~150 million km). That is so far away that the photons from solar irradiation effectively travels in parallel rays. So, unlike the flashlight experiment, the tilt of the sun has no bearing on the intensity of the radiation reaching the Earth's surface. Instead, we find that the Earth's tilt controls the intensity of irradiation and the seasons.

Keep in mind that the Earth's axis points to the same position in space (toward the North Star, Polaris). As the Earth travels in a near spherical (a very small eccentricity into an ellipse) orbit around the sun, the northern hemisphere can be tilted toward or away from the sun, depending on its orbital position.

Forecasters and meteorologists use different criteria to determine the "meteorological seasons." For example, meteorological winter in PA runs from December 1 to Feb 28/29, a period that statistically includes the three coldest months of the year. This is also centered on a time about 25 days after the Winter Solstice. 

Meteorological summer runs from June 1 to August 31, a period that includes the warmest three months of the year. Again, this is a period centered about 25 days from the Summer Solstice.

Please review the following NASA movie from 2000-2001, showing the rhythms of the most intense ultraviolet radiation coinciding with the most direct rays of the sun (around the summer solstices). Again, what may be a surprising observation is that the average air temperatures lag the sun's most direct days.

As one more example, review Pittsburgh's plot of annual average high temperatures. The maximum daily temperature occurs in late July, long after the summer solstice.

Recall from EME 810 that the time that we use in solar energy engineering is the apparent time and path of the sun relative to the aperture or collection device, called Solar Time. Because the Earth is a rotating sphere (360deg) that completes a rotation every 24 hours, 360 divided by 24 gives 15 degrees of rotation per hour, or one degree every 4 minutes. The chronological time that clocks use do not account for your geographic location beyond correcting for a time zone. Within a time zone, if you are geographically close to the next time zone, a larger time correction is necessary to convert to solar time. Additionally, the Earth does not rotate perfectly, rather, it wobbles on its axis creating what is known as an analemma. As such, an "equation of time" is necessary to convert between solar and standard time, as given in D&B equations 1.5.2 and 1.5.3.

Correcting with the Equation of Time: Accounting for Wobbles

Even in Greenwich, where no longitudinal correction is necessary, "noon" UTC will generally not be the time when the sun is directly overhead. We can see in the plot below that watch time and solar time are the same in Greenwich for only 4 days in the year.

• There are deviations of up to 16 minutes (regardless of your location on the planet).

As you will have read, our interpretation of watch time assumes an even progression for Earth's planetary rotation, with no weebles or wobbles or precession of the polar axis. However, you will now know that wobbling occurs, and there is great variability in the rotation of the Earth throughout the months of the year. This is why we add leap years and leap seconds to our calendars. So we create a "mean time" based on the length of an average day to keep things simple. Solar time has to correct for this mean time approximation.

For views of amazing solar analemma photography by Anthony Ayiomamitis, please visit his solar image gallery. The gallery has a series of composite of images, taken at the same watch time every few days for an entire year to record the position of the sun. We call the shape an analemma. An analemma is a beautiful way to capture both the range of declination $\delta$ (along the length of the analemma) and the Equation of Time Et (the expansion or width of the analemma) in a graphical format. Notice how there is a big loop and a little loop, and compare the same big waves and little waves in the first image of the Equation of Time correction above. If you were to draw a line down the center, you would have removed the error from watch time, and you would be one step closer to solar time.

Extraterrestrial Variation

In addition to the geometric (space) and chronologic (time) relationships between collector surfaces on Earth and the sun, the intensity of the sun's irradiation changes based on the distance between the Earth and the sun because of the eliptical orbit of the Earth. While the average solar radiation flux is 1361 W/m², the annual fluctuation due to the Earth's eliptical orbit is greater than 40 W/m², resulting in peak extraterrestrial radiation flux of about 1410 W/m² in January and lows of about 1320 W/m² in June. The following equations can be found in the D&B text section 1.4.

G_on = G_sc (1 + 0.033 cos(360n/365))

OR

G_on = G_sc (1.000110 + 0.034221 cos B + 0.001280 sin B
 +0.000719 cos 2B + 0.000077 sin 2B)

where G_on is the extraterrestrial radiation incident on the plane normal to the radiation on the n^th day of the year and B is given by

B = (n − 1) *360/365

Atmosphere

The Earth's atmosphere is the source of the greatest uncertainty in predicting solar radiation incident on a solar collector. Particulates, aerosols, clouds, moisture, etc. are the drivers behind the intermittency of incident solar energy due to atmospheric attenuation. Some meteorologists and atmospheric scientists have spent their entire careers focused on the problem of modeling the atmosphere to develop predictive tools for increased accuracy in energy models. The application of such atmospheric modeling tools is enabled with an accurate level of solar radiation known outside of the Earth's atmosphere (extraterrestrial).

Facts about Solar Thermal Energy

• Solar thermal energy has been used in various ways for millennia, ranging from simple fire starting with a pocket mirror to solar architecture to capture heat in buildings.
• 48% of the the sun's energy is in the infrared spectrum, invisible to the human eye, as heat.
• Solar thermal collectors can employ (absorb) nearly the entire solar spectrum
• The sun is the most abundant and reliable source of energy
• Financially, solar thermal energy conversion systems have reached grid-parity in many locations

Currently, we (humans) use an abundance of fossil fuels for much of our heat needs. While in the long run our society will switch to the source of all of those fossil fuels (the sun), the reality is that most of you have probably not experienced the direct impact of a solar thermal energy conversion system on your life. The truth is that we can do everything that we currently do in our society with solar energy. Much of the burden can be carried by solar thermal solutions. One terrific modern day example of a solar thermal energy system is the Drake Landing Solar Community in Alberta, Canada, where 95% of the the community's heating needs are supplied by on-site solar thermal collection and a connected seasonal thermal energy storage system. Our society uses a lot of heat. We need to keep working to make solar thermal energy solutions make sense and work well in more places whenever possible.

While solar thermal energy has been studied and leveraged for millennia, the technical difficulty and subsequent costs of appropriately meeting an energy need have hindered the wide adoption of solar thermal energy conversion systems, instead opting for other technologies and fuel sources. In light of your background and professional aspirations, write an essay that identifies your relationship with the course material.  The essays will serve two purposes: (1) sharing your perspective with the class and (2) second, thinking critically about the relevance of the course subject matter to your educational and career goals.

Solar thermal energy conversion systems at the utility scale have been developed for over a century using basic thermal principles, concentration systems, and mechanical engineering practices for heat and power. The article referenced below discusses some aspects for and some against the recent progress at the Ivanpah solar thermal electricity generation plant in the Mojave Desert, California, USA. While we are not seeing a mass conversion of our electricity generation infrastructure to solar thermal generation yet, the industry is experiencing significant growth.

Summary

This week, we’ve looked back at what we learned in EME 810 and refreshed our memories. We have taken this information and laid a foundation for applying our knowledge to the field of solar thermal energy for utilities and industry. Finally, we have introduced ourselves to each other and thought about what impact solar thermal energy has on each of us.

Reminder - Complete all of the Lesson 1 tasks!

You have reached the end of Lesson 1! Please double-check the to-do list on the Lesson 1 Introduction page to make sure you have completed all of the activities listed there before you begin Lesson 2.

In EME 810, you learned about the difference between optoelectronic and optocaloric solar energy conversion. As you may remember, optoelectronic refers to the photovoltaic effect of photons (radiation) being converted to electrons (electricity) while optocaloric refers to photons being converted to heat (thermal energy). In order to maximize solar energy gain and the subsequent conversion to useful thermal energy, it is necessary to understand how different materials interact with the sun’s electromagnetic radiation (photons) at different wavelengths. There are three key pieces to gaining a useful understanding of what is going on during the optocaloric conversion process:

1. A basic understanding of radiation heat transfer (D&B Chapter 3) is useful in light of the fact that solar energy, which passes through the atmosphere and reaches your collector surface, is trasferred via the process of radiation (not conduction or convection, though those heat transfer mechanisms can also play a part depending on the system configuration).

2. To convert photons to heat, the photons must be absorbed by the collector. This process requires opaque materials as opposed to reflective or transparent materials which would not absorb the energy but transfer it elsewhere. As such, an understanding of the radiation characteristics of opaque materials (D&B Chapter 4) is especially useful.

3. Many solar thermal energy conversion systems utilize a cover-absorber system to increase efficiency. Covers are typically glass, but can be made out of any material that transmit radiation while reducing losses from convection to the surroundings. As such, an understanding of the transmission of radiation through glazing (D&B Chapter 5) is useful.

These three main topics together provide important background for selection and use of materials in solar thermal energy conversion systems.

Lesson Objectives

• Identify materials best suited as components of solar thermal collectors.
• Apply selective surface characteristics in calculations.
• Calculate transmittance, reflectance, absorptance, and tau-alpha for various scenarios.

What is due for Lesson 2?

This lesson will take us one week to complete. Please refer to the Course Calendar in Canvas for specific time frames and due dates. A brief list of the lesson assignments is provided in the table below. More specific directions can be found on the pages of this lesson and in Canvas.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum, located under the Modules tab in Canvas. I will check that discussion forum regularly will do my best to respond. While you are on the forum, please feel free to post your own responses if you are able to help out a classmate.

This section focuses on the fundamentals of the radiation heat transfer, the nature of solar energy as electromagnetic radiation, and interactions of solar radiation with various materials. Radiation heat transfer is often addressed only briefly in any heat transfer course. However, In solar energy conversion systems, where the total energy flux is often orders of magnitude smaller than in conventional heat transfer systems, the contribution of the radiation heat transfer mechanism is significant. Conduction and convection also play a significant role in the performance of certain solar energy conversion systems.

Radiation heat transfer is dependent on the specific wavelength of the radiation. The distribution of wavelengths from a blackbody radiation source, such as the sun, is described by Planck’s Law (Equation 3.4.1 Duffie & Beckman, 2013). This equation can be integrated for a wavelength range of interest to find the total energy for different scenarios. The results of this integration are given in various simplified forms, which are convenient for practical use. Two important expressions derived from the Planck's law are Wien's displacement law (Equation 3.4.2 Duffie & Beckman, 2013) and Stefan-Boltzmann equation (Equation 3.5.1 Duffie & Beckman, 2013). Take a closer look at those expressions and understand what they are used for.

Another way Planck's law intergration data are often presented is Radiation Tables. Those become handy when the total emount of energy emitted by a blackbody source needs to be estimated for a specific wavelength interval. The example video (6:45) below specifically illustrates how such data are made useful to answer some practical questions.

Many solar thermal energy conversion systems use flat plate collectors, which are essentially two parallel plates (one transparent and the other absorptive) exchanging radiation. Calculating the radiation heat exchange between the two surfaces is a necessary aspect of understanding the energy balance of a system. Example 3.10.1 (Duffie & Beckman, 2013) is given below in a brief (6:19) video.

When solar radiation hits a surface, the photons can be absorbed, reflected, or transmitted. In the case of opaque (not transparent) materials, none of the photons are transmitted. If the material is dark and dull (not reflective or shiny), very few of the photons are reflected. As such, the majority of photons incident on dark opaque surfaces will be absorbed. As s result of absorption, the photons are converted to thermal energy (or heat). At the same time, because of the temperature of the material, the surface emits radiation back to its surroundings at a rate that is dependent on the emissivity of the material. Heat can also be lost to the surroundings by conduction and convection, but that is not the focus of this lesson.

Some additional note on materials selection:

When designing a solar thermal conversion system, the selection of materials is critical. Choosing collector materials that are good absorbers (i.e. carbon black) will help your system to perform well. Sometimes the best material can be too costly to justify. As such, it becomes a balance of priorities towards an optimal system. If a material of high absorptance (α=0.95) costs $10/lb and a material of higher absorptance (α=0.98) costs $20/lb, the best option to achieve a desired solar gain may be to use the cheaper material and increase the system aperture or total collector area.

In addition to the cost and physical radiation properties of materials, we must be careful to select materials that will hold up under extreme climatic and environmental conditions. For example, in sandy desert environments, the abrasive sand can have a negative impact on the reflective properties of concentrating trough collector systems over time. Thus full understanding the in situ performance of a material over a period of decades is important to the design and optimization of solar thermal energy conversion systems.

On the previous page of this lesson, we looked at how the wavelength of the incident radiation matters because the wavelength determines the amount of energy that is transmitted. Some specially selected or designed materials may absorb radiation in one range of wavelengths very efficiently while may be highly reflective in a longer wave length range. Such materials are referred to as selective surfaces. The concept of selectrive surface is discussed in Section 4.8. of D&B book, and Example 4.8.1. and 4.8.2. show how the radiation properties of such materials can be calculated. Please review those examples in detail. In this lesson assignment, you will be asked to perform a similar calculation.

Many solar thermal energy conversion systems employ glass to reduce convective losses from the absorbing surface, increasing system efficiency. Glass is not perfectly transparent, with some absorption as well as reflective losses that are dependent on the incidence angle of the solar irradiation. The three useful metrics for understanding what the performance of a glass cover will be are transmittance (τ), reflectance (ρ), and absorptance (α).

It is important to realize that when a light beam hits a particular surface or material, radiation can be reflected from the surface of the material, transmitted through the bulk of the material, and absorbed in the material. There are no other options since the energy needs to be conserved. This concept is additionally illustrated below:

Firgure 2.1: Energy conservation principle for the radiation interacting with materials: τ  + ρ + α = 1

Credit: Mark Fedkin

The key radiation properties can be derived for a particular material or surface using Fresnel equations. As we will see further, the transmittance, reflectance, and absorptance are dependent on the thickness, refractive index, and extinction coefficient of the material of interest. Before proceeding to the Fresnel derivation, it would be useful to quickly refresh our mind on the light reflection and refraction laws.

Refresher: Law of reflection and refraction

The following diagram illustrates the main directions of a light ray on a material surface. The key parameters to note:

• The angle of incidence (q₁) - angle between the incoming ray and normal to the interface
• The angle of reflection (q₃) - angle between the reflected ray and normal to the surface
• The angle of refraction (q₂) - angle between the refracted ray and normal to the surface
• Index of Refraction (n_i) - dimensionless number characterizing light propagation in a material. It is constant for every type of medium at a constant temperature. For more information, check the Wikipedia page for this property.

Figure 2.2: Propagation of light rays at the interface between two media.

Credit: Mark Fedkin

As you may recall from physics, by the Law of Reflection, the angle of incidence is equal to the angle of reflection:

θ₁ = θ₃

By Snell's Law, the light refraction process is described as follows:

n₁sinθ₁ = n₂sinθ₂

Now let us see how radiation characteristics can be derived.

Example 5.3.1 is given below in a brief (14.31) video.

Because of the complex intra-system interactions of incident solar energy in a combined cover-absorber system, the transmittance-absorptance product (τα) is defined. The τα parameter should be thought of a property of a cover-absorber combination, rather than the product of the two individual properties, which captures the essence of how clear (transmissive) is the cover as well as how absorbing is the absorber in the same system. Figure 5.5.1 in the D&B book schematically shows the reflection and absorption of light occurring at different material interfaces in a cover-absorber system. Example 5.5.1 showing the calculation of the τα parameter is given below in a brief (5:11) video.

This homework assignment consists of six quantitative problems that are closely tied with the readings. Of course these six problems selected from the D&B textbook do not make a complete assessment of how well versed you are with radiation property calculations, but they are good teasers challenging you to apply some of the key equations you see in the book. In that sense it is more of a learning activity rather than an assessment. The videos posted in the previous sections of this lesson give you an example how to approach this kind of calculations. 

Summary

Materials for optocaloric performance vary significantly in physical properties. Additionally, new materials are being developed worldwide. Table 4.7.1 (in D&B book) gives an overview of some of the most common materials used for solar thermal reflection and absorption. However, in current project proposals in many settings, new materials will be used. It is important to be able to think critically about the physical behavior of materials in a broad sense so that new or unfamiliar materials can be put into perspective against available materials. Reading materials in this lesson provide you with the fundamental theory for material selection, which can be further used for practical purposes.

Reminder - Complete all of the Lesson 2 tasks!

Please double-check the to-do list on the Lesson 2 Introduction page to make sure you have completed all of the activities listed there before you begin Lesson 3.

I take great pleasure in saying that after a thorough trial extending over a year and a half, our solar heater continues to give just as much satisfaction as when first installed. I am ready to admit that [at first] we were unreasonably prejudiced against the heater, and feel that refusing to let you install one in my house for so long a time after you first approached me upon the subject, we lost a great deal of comfort and convenience.

Flat plate collector systems are a very robust technology. With no moving parts, the conversion of solar radiation to heat in a working fluid can be very reliable with simple heat exchanger technology and, essentially, plumbing. Some systems that are configured at higher pressures than what is near ambient pressure can be at risk for leaks at joints in and between the system components due to the pressure difference and, in general, system leaks are the most common mode of failure. Performance and efficiency of various collectors, collector types, and system types can impact capital and operating costs that would be different in different locations based on climate and the subsequent energy losses associated with different technologies in different environments.

Learning Objectives

• Recognize the main components and layers of flat plate collectors.
• Define the main parameters used to characterize the flat plate collectors
• Apply efficiency, performance, and energy balance equations to a flat plate system
• Understand how the flat plate collector performance is tested in practice

What is due for Lesson 3?

This lesson will take us one week to complete. The list of assignments for this lesson is provided in the table below.  More detailed instructions are given on respective pages of this lesson and in Canvas modules.

Please refer to the Course Calendar in Canvas for specific time frames and due dates.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

The flat-plate solar collectors are probably the most fundamental and most studied technology for solar-powered domestic hot water systems. The overall idea behind this technology is pretty simple. The Sun heats a dark flat surface, which collect as much energy as possible, and then the energy is transferred to water, air, or other fluid for further use.

These are the main components of a typical flat-plate solar collector:

• Black surface - absorbent of the incident solar energy
• Glazing cover - a transparent layer that transmits radiation to the absorber, but prevents radiative and convective heat loss from the surface
• Tubes containing heating fluid to transfer the heat from the collector
• Support structure to protect the components and hold them in place
• Insulation covering sides and bottom of the collector to reduce heat losses

Figure 3.1: Schematic of a flat plate solar collector with liquid transport medium. The solar radiation is absorbed by the black plate and transfers heat to the fluid in the tubes. The thermal insulation prevents heat loss during fluid transfer; the screens reduce the heat loss due to convection and radiation to the atmosphere

Credit: Mark Fedkin (modified after Duffie and Beckman, 2013)

The flat-plate systems normally operate and reach the maximum efficiency within the temperature range from 30 to 80 ^oC (Kalogirou, 2009), however, some new types of collectors that employ vacuum insulation can achieve higher temperatures (up to 100 ^oC). Due to the introduction of selective coatings, the stagnant fluid temperature in flat-plate collectors has been shown to reach 200 ^oC.

Some advantages of the flat-plate collectors are that they are:

• Easy to manufacture
• Low cost
• Collect both beam and diffuse radiation
• Permanently fixed (no sophisticated positioning or tracking equipment is required)
• Little maintenance

Flat-plate collectors are installed facing the equator (i.e. South oriented in the Northern hemisphere and North oriented in the Southern hemisphere). The optimal tilt of the collector plate is close to the latitude of the location (+/- 15^o). If the application is solar cooling, the optimum installation angle is Latitude - 10^o, so that the solar beam is perpendicular to the collector during summertime. If the application is solar heating, the optimum installation angle is Latitude + 10^o. It was found however, that for year-round hot water application, the optimum angle is Latitude + 5^o, which provides somewhat better performance during winter, when the hot water is more needed (Kalogirou, 2009)

Transport fluid options

The flat plate collectors can involve liquid or air heat transport.

Water is one of the common options as liquid fluid due to its accessibility and good thermal properties:

• It has a relatively high volumetric heat capacity
• It is incompressible (or almost incompressible)
• It has a high mass density (which allows using small tubes and pipes for transport)

One disadvantage of water is that it freezes during winter, which can damage the collector or piping system. This can be managed by draining down the collector at low solar inputs (below a critical insolation threshold). Drain down sensors are often employed to monitor the system and to ensure complete draining, as pocket water freezing can cause damage. Refilling the system with water on the next morning also is not perfect. Possible air pockets in the collector can be a problem, blocking water flow and decreasing system efficiency (Vanek and Albright, 2008).

Antifreeze mixtures can be used instead of pure water to alleviate the above-said problems. The common antifreeze components are ethylene glycol or propylene glycol. Those chemicals are mixed with water require closed-loop systems and proper disposal due to toxicity. Nominal antifreeze service like is about 5 years, after which it needs to be replaced.

Air can be used as transport fluid in some designs of flat -plate collectors. This option is better suited to space heating applications or crop drying. A fan is usually required to facilitate air flow in the system and efficient heat transport. Certain designs can provide passive (no fan) movement of air due to thermal buoyancy.

Phase-change liquids can also be used with flat-plate collectors. Some refrigerants are included in this group of fluids. They do not freeze, which eliminates troubles explained above for water, and, due to their low boiling point can change from liquid to gas as temperature increases. Those fluids can be practical in settings where quick response to rapid temperature fluctuation is needed.

Collector construction

The key considerations in flat plate collector design are maximizing absorption, minimizing reflection and radiation losses, and effective heat transfer from the collector plate to the fluids. One of the important issues is obtaining a good thermal bond between the absorber plate and changes (tubes or ducts containing the heat-transfer fluids). Different construction designs (shown below) try to address this issue.

Figure 3.2: Various designs of flat-plate collector assembly. Color codes: light blue - glass cover, dark blue - fluid channels, black - absorber material, gray - insulation. Some constructions (b, c) include fluid channels in the absorber plate structure to maximize thermal conductance between the components. Other modifications (a, d) include tubes and channels soldered or cemented to the plate.

Credit: Mark Fedkin (modified after Kalogirou, 2009)

The plate - channel assembly may use a variety of methods of component attachment - thermal cement, solder, clips, clamps, brazing, mechanical pressure applicators. One of the considerations in choosing the assembly method is cost of labor and materials.

Next, we are going to look at the energy transfer and balance within the flat-plate collector.

References:

• Kalogirou, S.A., Solar Energy Engineering, Elsevier, 2009
• Vanek, F.M, and Albright, L.D., Energy Systems Engineering, McGraw Hill, 2008.

A fundamental concept for thermal analysis of any thermal system is the conservation of energy, which can be analyzed through energy balance calculation under steady state conditions. In steady state, the useful energy output of the collector is the difference between the absorbed solar radiation and the total thermal losses from the collector

Useful energy = Absorbed solar energy - Thermal losses

Obviously, the higher the useful energy output from a particular design, the higher the expected efficiency. Thermal efficiency of the collector is an important parameter to consider in this kind of analysis as it creates the basis for comparison of different materials and modifications of collector systems. So many theoretical calculations presented in the books (as well as in this Lesson), are eventually aimed at evaluating efficiency.

Let us define the thermal efficiency (η) first, as it will be the focus and final destination of this chapter. 

$$\eta = \frac{Q_{u }}{A_{c }{G}_{T }}$$

where Q_u is the useful energy output from a collector, G_T is the incident solar radiation flux (irradience), and A­_c is the collector area. So the denominator here is the total energy input for the collector. In this formula the G_T is the parameter characterizing the external conditions, and it is usually known from practical measurements (with a pyranometer) or assumptions for a specific location. The collector area is a set technical characteristic. So the main question here is how to estimate the Q_u  - the useful energy.  

As was mentioned above, to find how much energy remains available for useful thermal work, we need to understand the energy balance within the collector: absorbed energy - losses.

The energy balance can also be expressed via the following key equation: 

$$Q_{u }= A_{c }[ S - U_{L }( T_{\text{plate } }- T_{\text{ambient } }) ]$$

where S is the absorbed solar radiation, U_L is the total losses, T_plate is the temperature of the absorbing plate, and T_ambient is the temperature of the air, and A_c again is the area of the collector surface.

This equation stands as a cornerstone of the energy balance analysis presented in Chapter 6 of Duffie and Beckman's textbook. To implement this question, we need to understand how the quantities S and U_L can be obtained. The most complete explanation can be found in the following reading. 

In a general case, when measurements of incident solar radiation (I_T) are available, the convenient approximation for the absorbed energy is given by: 

$$S = ( \tau \alpha {)}_{a v }{I}_{T }$$

where (τα)_av is the product of transmittance of the collector cover and absorptance of the plate averaged over different types of radiation. In fact, (τα)_av ≈ 0.96(τα)_beam based on practical estimattions.

Now let us see how the radiation losses can be determined. Please refer to the following reading. 

Now as the absorbed radiation and losses are defined, the useful energy gain can be determined via the energy balance equation given above.

The maximum possible useful energy gain can be achieved when the collector is at the same temperature as the inlet fluid. In this case, the heat losses are minimized. However, in an actual operation setting, this is not always the case. To describe the effective (actual) useful energy gain via heat exchange, we should introduce the heat removal factor - F_R

This coefficient shows how much energy remains after heat losses to the surrounding due to collector and inlet temperature difference. Therefore, the energy balance equation for the actual system can be written as follows

$$Q_{u }= A_{c }{F}_{R }[ S - U_{L }( T_{i }- T_{a }) ]$$

This equation reminds us the energy balance equation discussed in the previous page of this lesson, only with the F_R factor. This flow factor depends on the mass flow rate of the fluid and heat capacity, and you can learn more details about the flow factor and practical application of  the above equation from the following reading.

The theoretical models and calculations described in the D&B textbook can be checked in practice by performing collector tests. As new materials and new collector designs appear on market, there is a need for standardized testing procedure, and metrics, which would allow clear comparison and  assessment if a collector performance is good or not so good.

The basic method of assessment of collector performance is to expose the system to solar radiation, run the fluid through it, and measure the inlet and outlet temperature  along with the flow rate. Then the useful energy gain can be calculated from the experimental data as follows  

$$Q_{u }= m C_{p }( T_{o }- T_{i })$$ In addition the incident radiation on the collector (G_T) and ambient temperature (T_a) can be recorded, so we can express the useful gain in terms of incident radiation:

$$Q_{u }= A_{c }{F}_{R }[ G_{T }( \tau \alpha {)}_{\text{average } }- U_{L }( T_{o }- T_{i }) ]$$

and further the experimental efficiency of the system at each instant of operation can be obtained: 

$${\eta }_{i }= \frac{Q_{u }}{A_{c }{G}_{T }}$$

To see how the collector test data and efficiency look like in practice, please refer to the following reading:

Make sure to complete all the assigned reading in this lesson and take the reading quiz.

This homework assignment consists of two quantitative problems that are closely tied with the readings. Please study examples in Chapter 6 (D&B book) as they can be especially helpful in developing the solutions.

Summary

Flat plate collectors come in various shapes, sizes, materials, and configurations. This type of collectors represent a good model for understanding the energy balance and system performance.The basic principle of absorbing as much solar radiation as possible (via black absorptive surfaces) while minimizing losses to the surrounding environment as much as possible (via glazing surfaces, insulation, and vacuum tubes) can be accomplished by various technologies trading off the level of performance and material and manufacturing costs. Being able to discern between flat plate technologies for different applications and locales based on the underlying physics is a valuable skill to have.

Reminder - Complete all of the Lesson 3 tasks!

Please double-check the to-do list on the Lesson 3 Introduction page to make sure you have completed all of the activities listed there before you begin Lesson 4.

We’re trying to figure out how big the problem is and what we can do to minimize bird mortalities.

Solar concentration is the most cost effective way to achieve sufficiently high temperatures for generating steam for useful work. Concentration of solar radiation is acheieved by using an optical device between the source and absorber, and that allows to decrease the effective area of the absorber and associated radiative energy losses.  As such, concentrating collectors are advantageous when high temperatures are needed. The one remaining caveat is that concentrating technologies typically rely on the direct normal irradiance component of the solar resources. Hence, locations with regularly clear skies and high levels of direct radiation (such as the southwest US and southern Spain) are best suited for concentrating solar power.

Learning Objectives

• Recognize various configurations of solar concentrators
• Calculate concentration ratios for concentrating collectors
• Analyze the optical characteristics of non-imaging optics.
• Compare thermal performance of concentrating systems.

What is due for Lesson 4?

This lesson will take us one week to complete. The list of assignments for this lesson is provided in the table below. More detailed instructions are given on respective pages of this lesson and in Canvas modules.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

There are many different types of configurations of solar concentrators, ranging from cylindrical that focus on a line (tube) to circular that focus on a point (power tower). As was already mentioned, the main purpose of these configurations is to increase the radiation flux on the receiver, and that effect is achieved through reflection of light from multiple anlged or curved mirror surfaces. Figure below provides schematics of several common types of concentrating collectors.

Figure 4.1: Types of concentrating collectors: (a) tubular absorbers with a diffuse back reflector, (b) tubular absorbers with specular cusp reflectors, (c) plane receiver with plain reflectors (V-trough), (d) multisectional planar concentrator, (e) compound parabolic concentrator, (f) parabolic trough, (g) fresnel concentrator, (h) array reflectors (heliostats) with a central receiver. Concentration of light on the receiver is acheieved by shaping the reflectors (mirrors) around the receiver (blue circle)

Credit: Mark Fedkin - modified after Duffie and Beckman, 2013.

The light concentration process is typically characterized by the concentration ratio (C). By physical meaning, the concentration ratio is the factor by which the incident energy flux (I_o) is optically enhanced on the receiving surface (I_r) - as shown in the Figure below. So, by confining the available energy coming through a chosen aperture to a smaller area on the receiver, we should be able to increase the flux.

Figure 4.2: Schematic representation of light concentration process.

Credit: Mark Fedkin

The ratio of area of the receiver to the area of the aperture, C_geo = Ar/A_a is called the area concentration ratio (in some sources also called geometric concentration ratio). It is easy to use, as the areas of the devices are known, although it is adequate only when the radiation flux is uniform over the aperture and over the receiver. Also, please note that for some imaging concentrators, the area of the available receiver surface can be different from the area of the image produced by the concentrator on the receiver. So, if the image does not cover the entire surface of the receiver, we need to use the image area to estimate the concentration ratio.

The concentration ratio can be also represented by the energy flux ratio at the aperture and at the receiver. In this case, it is termed flux concentration ratio C_opt (in some sources - optical concentration ratio) and can be directly applied to thermal calculations.

$C_{opt }=\frac{\text{Average flux over the receiver} }{\text{Flux over the aperture (insolation)} }=\frac{\frac{1}{A_r}{\displaystyle \int I_{r}D{A}_r}}{I_O}$

In case the ambient energy flux over the aperture (insolation) and over the receiver (irradiance) is uniform, the geometric and optical concentration ratios are equal (C_geo = C_opt).

The concentration ratios are important metrics used to characterize and rank optical concentrators. There is a theoretical limit to solar concentration. For circular concentrators - 45,000, and for linear concentrators - 212, based on the geometrical considerations; however, these limits may be unreachable by real systems because of non-idealities and losses. In general sunlight, concentration systems are roughly classified into: low concentration range (C<10), medium concentration range (10<C<100), and high concentration range (C>100). However, only some of the systems provide uniform concentrated light flux (e.g., V-troughs or pyramidal plane reflectors) and can be characterized by a single concentration ratio. Many systems with curved reflecting surfaces (e.g., conical, parabolic, spherical) create a distribution of flux density over the receiver and would rather be characterized by a variable C over the receiver width. In that case, a local concentration ratio (C_l) is the main parameter to characterize the performance of the ideal concentrator:<100),>

$C_l=\frac{\text{Local intensity} }{\text{Incident intensity} }=\frac{I\left( y \right) }{I_{ap }}$

where I(y) is determined for any local position y from the center of the produced image, and I_ap is the intensity of the incident radiation at the aperture.

Figure 4.3: Example of distribution of local radiation intensity over the receiver of an imaging collector.

Credit: Mark Fedkin after Duffie and Beckman, 2013

In many typical cases of imaging concentrators, the reflectance of the surface (ρ), i.e., the fraction of light radiation reflected from the surface compared to the total incident radiation, is also taken into account. Then the local intensity of the concentrated light, I(y), can be described as follows:

$I\left( y \right)=I_{ap }\rho C_l$

Next we are going to refer to the following text to learn more about the optical and thermal performance of concentration collectors.

Nonimaging concentrators are called "non-imaging" because they do not produce any optical image of the source, as opposed to imaging concentrators, which produce an image of the sun by reflecting it on the receiver. The non-imaging concentrators are able to reflect to the receiver all of the incident radiation, either beam or diffused, intercepted over a wide range of incidence angles. These systems are not precise, but they are more flexible. For example, if the right hand side of the sun is reflected to the left hand side of the receiver, the image of the sun is not preserved while the total energy incident on the aperture of the concentrator is additive. The most commonly used technology that leverages nonimaging optics is the compound parabolic concentrator (CPC).

The typical concentration ratios of CPCs are in the single digits. Despite the low concentration ratios of nonimaging systems, nonimaging systems can be very useful for increasing the performance of systems are relatively low costs, particularly in regions where the solar resource is less than ideal for concentrating systems to begin with (such as most of Europe and the northeast of the US).

The following reading describes in detail the optical principles and energy paths in the CPC collectors.

Imaging concentrators are used to achieve the highest temperatures that are currently achievable with a solar thermal system. Imaging concentrators enable a very large aperture area with a small absorber area, effectively reducing thermal losses at high temperatures. Ray tracing is used to evaluate such concentrating collectors during the design process. By (often digitally) drawing careful geometric reflections within a concentrating collector system, the distribution and angles of incidence of radiation on the absorber can be determined. This is a useful tool for both imaging and nonimaging concentrators, and can be used to show how active tracking systems on imaging concentrators (where the incident radiation is always perpendicular to the aperture) enable a much wider aperture with reduced reflectors compared with nonimaging systems. The typical concentration ratios of the currently existing large-scale systems that are using imaging concentrator technology (Ivanpah, SEGS, etc.) range from in the tens to as high as the low to mid hundreds.

Parabolic imaging concentrators are probably the most studied, both analytically and experimentally. To understand how these collectors are designed, it is necessary to understand the geometric properties of a parabola.

When light is reflected from the parabolic mirror onto a receiver to produce the optical image, the main parameters considered in the energy analysis are image size (width) and intensity of radiation within that image. Various models and examples of estimating those parameters are provided in the following reading.

This homework assignment consists of two quantitative problems that are closely tied with the readings. Please study examples in Chapter 7 (D&B book) as they can be especially helpful in developing the solutions.

Summary

Concentrating solar power is a very useful tool that enables reaching high temperatures in solar energy conversion systems. Without concentration, the generation of electricity by conventional steam turbines using energy directly from the sun would not be cost effective. Optical concentration proves to be very advantageous since it reduces thermal losses and increases the useful energy gain.

Please double-check the to-do list on the Lesson 4 Introduction page to make sure you have completed all of the activities listed there before you begin Lesson 5.

Imagine that thermal fluid plays the same role in plant processes as blood in the human body. It is thus inextricably linked to the optimum functioning of the entire system; if cholesterol deposits congest arteries, then that directly affects the heart, which unfortunately leads to the organism shutting down.

Thermal fluids are used to move heat through a system. A system could be composed of solar collectors, storage tanks, heat exchangers, valves, pipes, and more. Through all of these components flow a fluid that carries the heat from one point to another within the system. In a heat exchanger, one fluid flows next to another fluid (e.g., air and water) and heat flows from the hotter fluid to the colder fluid (e.g., from a hot air stream to cold water, effectively cooling the air). In a storage tank, the fluid holds the heat and waits to be pumped to where the heat can be used. In a solar collector, the fluid passes through a tube (or pipe) that is either joined to black, radiation absorbing fins (flat plate collector) or contained within a vacuum glass tube (parabolic troughs) or a combination of both (evacuated tube collector). Selecting a thermal fluid requires knowledge of the physical properties of each fluid under different thermal conditions.

Learning Objectives

• Describe the available and developing thermal fluids used in solar thermal energy systems.
• Discuss relevant issues (i.e. cost, corrosivity, etc.) associated with available and developing thermal fluids.
• Apply thermal conductivity calculations.
• Apply heat capacity calculations.
• Select the proper working fluids for various applications.

What is due for Lesson 5?

This lesson will take us one week to complete. The list of assignments for this lesson is provided in the table below. More detailed instructions are given on respective pages of this lesson and in Canvas modules.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

Solar thermal fluids (or heat-transfer fluides - HTF) come in six primary groups:

• Oil-based
• Water-based
• Molten salts
• Air
• Refrigerants
• Silicones

Each type of heat transfer fluid has advantages and disadvantages with respect to different types of solar thermal energy conversion systems. Oil, water, or molten salts can all be used in Parabolic Trough and Linear Fresnel collector systems, while only molten salt and water (oil is excluded here) in addition to the option of air can be used in a power tower system. Parabolic trough systems are the most widely installed type of system worldwide, at 90% of systems installed by the end of the year 2013 (source: iea.org). Refrigerants and silicones are rarely used in flat plate systems and are not used in concentrating systems for various reasons discussed later.

Oil-based fluids come in three categories; synthetic hydrocarbons, paraffin hydrocarbons, and aromatic refined mineral oils. Water based fluids can be either pure water or a water and glycol mixture with either ethylene or propylene glycol which are types of “antifreeze”. Molten salts are nitrate (ionic) salts that are only available in concentrating systems due to the high temperature requirements of the fluid. Air is standard air, comprised mostly of nitrogen and oxygen, useful for specific applications such as a drying industrial process or low grade heating of building spaces. Refrigerants are commonly used in refrigerators, air conditioners, and heat pumps, but are not commonly used in solar thermal systems today. Silicones can be either synthetic or organic, but are also not commonly used in solar thermal systems today.

There are seven key properties of a thermal fluid for solar application that must be understood before engaging in design work or decision-making regarding thermal fluid performance and/or selection. The properties include:

• Maximum temperature is the highest temperature before the fluid begins to break down or decompose. The hottest parts of a system are where this maximum temperature is most probable, and therefore must be carefully designed for those key points in the system. The critical locations include the center of any absorbing tubes in the solar collectors or at the auxiliary heater/boiler.
• Freezing temperature is the coldest temperature before the fluid changes phase into a solid, and cannot be pumped, potentially causing irreversible damage to system components. In the case of each fluid, additional properties such as the expansion of the fluid when it freezes are important when determining the potential impact of freezing.
• Density is the mass per unit volume and is temperature dependent. Density can change drastically for some fluids as they heat up. For example, hydrocarbon oils can have a ~30% decrease in density when increased from 25 degrees C to 390 degrees C.
• Steam pressure is the pressure required to prevent the fluid from changing state to a gas. At higher pressures, the temperature at which the heat transfer fluid changes phase from a liquid to a gas (steam) is also higher.
• Specific heat is measured in units of energy (J) per mass (kg) temperature (K) and represents the amount of energy required to raise the temperature of the heat transfer fluid per unit mass. For example, if a fluid had a specific heat of 2300 kJ/kgK, that fluid would require the supply of 2300 kJ of energy to raise one kg of the fluid by one degree Kelvin (Celsius). This parameter is very useful for energy and mass balance calculations.
• Enthalpy is measured in units of energy (J) per mass (kg), representing the energy contained in the fluid under specific conditions such as temperature.
• Viscosity is a fluid’s resistance to shear stress; essentially a measure of how much pumping force is required to move the fluid in a constrained environment. Viscosity can change greatly with temperature for some fluids. Lower viscosities are preferred to reduce pumping costs.

Learn more about the types and properties of common heat-transfer fluids in the following readings:

Cons and pros of various heat-transfer fluids

Most notably, molten salts bring a high level of corrosion with them. Additionally, molten salts must be kept above their freezing temperature in pipes running to and from collectors or must be drained back from the system components into a holding tank which is configured to deal with the solidification (freezing) of the salt. There has long been a search for molten salts that remain liquid at room temperature, as described briefly in the Advanced Heat Transfer and Thermal Storage Fluids article from the reading. Such salts are anticipated to be low in cost and their high thermal stability (resistance to flammability) makes them very desirable for high temperature applications where alternatives such as oil are highly flammable.

Water-based fluids, such as glycol solutions, degrade over time and must be changed every 3-5 years. If a glycol fluid is subjected to very high temperatures, such as stagnation temperatures when a system is at capacity and the load is not using the heat in the summer, this degradation speeds up, further reducing the life of the fluid. In part, this is one driver for solar air conditioning systems as a way to use excess heat in the summer, increasing the life of the fluid while using free fuel (solar radiation). Pure water is subject to freezing in the winter, but is ideal (very low cost) for locations that do not experience freezing temperatures as well as systems that are equipped (adding additional capital cost) with a drainback tank to hold the water in a thermally controlled space during the times when the collector temperature is below freezing.

Oils are a great fluid for concentrating systems because of their high boiling point (>300 degrees C). With relatively low costs, low freezing points, and high thermal capacity compared to water or air, oils are the best choice for most concentrating systems and are used worldwide.

Some systems use refrigerants, which was discussed briefly above. Chlorofluorocarbon (CFC) refrigerants, such as Freon, have been used historically in some solar thermal systems, but have been phased out due to the negative effect of CFCs on the earth’s ozone layer when CFCs are vented to the environment (either intentionally or accidentally). The benefit of refrigerants is a low boiling point enabling the leveraging of the fluid’s phase change as well as a high heat capacity. CFC refrigerants can be replaced, with some system modifications, by methyl alcohol, ammonia, and more. Research is ongoing in this field.

Silicones is another group of fluids discussed briefly above. Silicones are still rare in solar applications. These fluids require more energy to pump than alternatives due to a high viscosity and they also tend to leak easily through microscopic holes in a solar thermal system. Silicones are interesting and will easily be researched further because of their noncorrosive nature and long lifetime (compared with the 3-5 years of oil).

Applications

Flat plate collectors that are intended for domestic hot water and space heating typically use a water-based heat transfer fluid, either pure water if the location is either not subject to freezing temperatures or if the system contains a drain-back tank, or a water/glycol (antifreeze) mixture.

Parabolic trough collectors typically used an oil based heat transfer fluid, though water and molten salt can both be used as well. When water is used, it is used to directly generate steam for use in a turbine or other steam application. Oil can achieve a higher temperature before boiling, thus increasing the efficiency of the collector.

Power tower systems often use molten salt as a fluid of choice. This is because the molten salt does not need to be pumped through a long series of tubes to pass by each collector. The fluid simply resides in the central boiler tank where all of the power tower mirrors are aimed. When the sun sets and the heat is used up, the molten salt freezes (solidifies) in the boiler tank, ready to be melted again when the sun comes up. This greatly reduces the complexities associated with the fluid solidifying inside a series of tubes across a large area with parts that may or may not be easily subjected to solar radiation or other heat sources to melt the salt the following day. Air is typically only used in flat plate collectors, but can also be used in power towers. Air is a good choice for applications such as food/textile drying or space heating.

Please look through the following presentation at the IEA CSP Workshop - it provides additional summary of application of different heat transfer fluids in various solar thermal systems:

Pumping energy is one piece of the puzzle that is important to consider. Pumping power is calculated as the volume of the fluid per unit time (flow capacity) times the density of the fluid times the gravitational constant times the pumping head (vertical distance to be pumped). Pumping energy is simply power multiplied across time. 100kW of power for one hour is 100kWh of energy. Units must be tracked carefully to ensure the correct answer. Friction within the pipe, particularly for pumping over horizontal distances, can be calculated using the Darcy-Weisbach equation (noted in the second video) to relate friction and fluid speed. The head loss due to friction, and as such the required pumping power, is proportional to the square of the fluid speed. As such, this is an important calculation because if your system is designed in a way that requires high pumping speeds, you will have very high pumping energy costs. Additionally, if you pump too slow, you risk damaging your fluid and system components because of high temperatures gained from the solar radiation and not moving that energy through the system, away from the collectors, fast enough. This optimization problem is key to designing a good system.

The video below explains how we can estimate the pumping power required to move the heat-transfer fluid in the system. This kind of calculations becomes handy when you need to determine the cost of using one or another type of fluid for a specific system design. So, watch and see what parameters of the system and fluid need to be taken into account.

The example above shows how we can estimate the pumping energy for pumping fluid upwards over a certain vertical distance. The video below shows the case when the fluid moves through horizontal pipes, which is quite common of solar applications. How much energy would be needed in this case? Watch as see how this calculation is done using the Darcy-Weisbach equation.

Thermal fluids have several key properties that need to be carefully considered when selecting a fluid for a specific application. These properties include maximum temperature, freezing temperature, density, vapor pressure, specific heat, enthalpy, and viscosity. By performing calculations, one can save a significant amount of money that would otherwise be spent on trial and error experiments and extra equipment, which potentially can be avoided.

This assignment is a series of short analytical problems dealing with thermal fluid assessment. Some problems will require conceptual explanation and some will involved quantitative calculations.

Scenario:

There are two heat transfer fluids - oils that are similar in cost, quality, and expected lifetime. Here are their technical characteristics:

Oil A:

T_max = 300°C
T_freeze = -10°C
_Cp = 1.7 kJ/kgK (specific heat)
ρ = 800 kg/m³ (density)

Oil B:

T_max = 395°C
T_freeze = 13°C
_Cp = 2.3 kJ/kgK (specific heat)
ρ = 700 kg/m³ (density)

Range of reasonable Darcy Friction Factors:

0.01 > f_d > 0.09 (for problem 3, use f_d = 0.04)

T_in = 150°C

T_out = 280 ^oC

Answer the following questions:

1. Which of the two fluids above should be used, and why, if the process requires a temperature of (a) 380°C, (b) 300 ^oC?
2. If the system is required to output 2 MW of thermal energy, what should the mass flow rate (kg/hr) be with Oil A? What about with Oil B? (Hint: you can use equation 10.1.2 from the D&B book to develop this problem),
3. Based on results from Question 2, for each of the two oils, calculate the flow speed (m/s) in a pipe with an inner diameter of 3 cm?
4. What is the resulting pumping head loss for each fluid in a 200 m long collector pipe with Darcy Friction Factor of f_d = 0.04?
5. Based on results from Questions 3 and 4, calculate the pumping power required for each fluid’s head loss and associated annual costs of energy, considering rate $0.10/kWh and system's capacity factor of 20% over the year (i.e., it is pumping 20% of the time). Discuss the difference in pumping power (or cost) between the fluids.

List all your answers in the comparative table Oil A vs Oil B and include it in the end of your report.

Summary

The main function of the thermal fluids is moving heat from the collector to the point of demand in STE systems. Solar thermal heat transfer fluids come in all types and densities (as opposed to shapes and sizes, since that’s not really possible with a fluid). The properties of each fluid make some fluids better for certain applications than others. Sometimes it is a challenge to choose one fluid over another with seemingly equal tradeoffs between the various options. In the end, reliability and historical track records that reduce the risk associated with any single fluid option are typically what steers decisions in the end. As new fluids are researched and developed, we must push forward with systems that use the current state-of-the-art fluids that are proven already.

Please double-check the to-do list on the Lesson 5 Introduction page to make sure you have completed all of the activities listed there before you begin Lesson 6.

Informed decision making via an integrative design process and iterative energy simulations (early and often) is crucial during the development of a building design.

This lesson introduces the concept of load and demonstrates how the fundamental heat transfer computations are applied to simple practical scenarios. In the previous lessons we focused on how the solar raditaion is converted to heat and how much useful gain is acquired from different collectors. Now we are moving into the applications zone. Let us see what kind of work that heat does next. You will have an opportunity to run a SAM model for a small scale solar heating system and see how the performance depends on different initial parameters and how it varies over time. Chapter 10 in D&B contains a number of example problems, which explain the behind-the-scene math for such modeling.

Learning Objectives

• Understand the concept of load.
• Apply themal calculation methods to find energy balance and efficiency of water heating systems
• Apply a simulation tool to a model solar thermal system.
• Analyze solar thermal system performance.

What is due for Lesson 6?

This lesson will take us one week to complete. Specific directions for different assignments are given in the table below and within this lesson pages.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

In each solar energy system, there are supply and demand of energy, which, ideally, should be matched. The supply of solar energy depends on the available solar resource, the technology to convert solar radiation to the usable heat, losses, properties of materials, and system design. The energy demand depends on applications connected to the collectors - let it be water stoarge tank to be heated for domestic or industrial use, a space, a swimming pool, etc. Both supply and demand are time dependent. It is understandable that the solar energy varies on the daily basis, usually peaking during the day and diminishing over the night. The use of available energy also varies over time based on human activities or technical processes involved in the system. Here we use the term "load" to define a time-dependent energy need. Load is the amount of energy obtained from the source to do the work.

In a certain system, we can have a solar collector and another system - the auxiliary - to meet energy demand requirements. The solar system alone is not sometimes sufficient and requires such a backup to make sure the application in use does not run out of energy. The auxiliary can be represented by an on-site natural gas combustion system or grid, for example. Then the system load can be represented as:

L = L_s + L_a

with subscripts s and a standing for solar and auxiliary, respectively.

It is also useful to define the load rates (e.i. demanded power). The load rate is

L' = dL/dt

Note L' (rate) is denoted in the D&B book as L with a dot. Load rates are useful because the load are highly variable, and we may see times when the demands are met by solar energy and times when theya are met by auxiliar energy. The one important purpose of system modeling is to determine the hour-by-hour energy performance of the system, match it with loads, and decide how much auxiliary energy must be secured or purchased.

Here we can also define heating and cooling loads. Those depend on system thermal requirements. For example, if a building is too cold and requires some heating to meet a certain standard, then we deal with a heating load. On the contrary, if the building is too warm, due to internal gains and losses, then we deal with a cooling load, in other words we need to remove energy from the space.

How can loads be estimated?

Please refer to the following reading to understand what heat gains and losses should be taken into account and what equations can be used.

One commonly used method for calculating the heating and cooling requirements of a building is to calculate degree days (DD) as discussed in Section 9.3 of the D&B textbook. The rate of energy transfer from the building to the exterior environment makes up a significant piece of the energy balance calculation. This rate of energy transfer is considered to be directly proportional to degree days. The following video (3:36) provides a brief explanation on the calculation of degree days, showing how the number of degree days is directly calculated by a difference in temperature over a period of time.

The real-life solar energy systems are composed of a number of different components and units. Each of those components has specifics that require certain theoretical background and consideration. Several previous lessons introduced some theory behind those component models - heat transfer from the Sun to the collector; heat transfer from the collector to thermal fluid; concentration of solar radiation on optical devices, etc. While the basic calculations performed for those components can answer questions about what energy parameters can be output and what efficiency can be expected from each part of the system, the question still remains how those component models can be combined into a system model, that would allow optimization of the performance for a target application.

Overview of those component models is given in the first part of Chapter 10 if the D&B book. In these sections we can also read about the role of the heat exchangers, which provide interface between different components and allowheat transfer from one part of the system to another. Figure 10.2.1 from the D&B textbook shows a typical solar water heating system, containing a collector, heat exchanger, storage tank, pipes, and pumps. Throughout the system diagram, temperatures are noted. It is by these temperatures that the system component efficiencies can be calculated and subsequently integrated to find overall efficiency.

"System models are assemblies of appropriate component models." When you put together the equations decribing each of the components into the system model, the simultaneous solving of all those equations may be a serious challenge. Sometimes it is advantageous to treat the systems of equations numerically, especially if some of them are non-linear. A number of computer simulation software have been developed to help with this task. Commonly, models cover annual cycle of system operation based on available meteorological data.

Thermal analysis performed for the whole system over significant period of time provide valuable information for assessing the economics of the project. There are a couple of useful parameters that we need to introduce here. The first one - solar fraction (f) is the ratio of the solar energy obtained by the system to the total load:

f_i = L_S,i /L_i

where L_s is the amount of solar energy used in the load, and L_i is the total load per unit of time.

Or in integrated form (over a year), the same concept will be expressed as annual solar fraction (F):

F = L_S/L

The second parameter useful from economical standpoint is solar savings fraction (F_sav). It accounts for energy expenditures needed to run the solar system equipment (pumps, fans, controllers..) - so call "parasitic energy". 

F_sav = F - (C_efΔE)/L

where C_ef is the ratio of cost of additional electricity for solar system operation to the cost of fuel; ΔE is the amount of required "parasitic" electric energy. Read more about these metrics in the following source:

Equilibrium and steady state are two very different thermal states, but both provide a way to analyze the thermal status of a system. Recall that an object in outer space absorbing solar radiation could be analyzed at thermal equilibrium to calculate the temperature of the object in light of the radiative heat loss and solar gain. A steady state energy balance is a similar method that is used to analyze heat transfer in light of system dynamics. The Alleyne and Jain article from the Mechanical Engineering magazine gives an overview of basic transient system modeling for thermal systems in light of the application of steady state energy balance. This method is how TRNSYS works, under the hood. Note that to simulate a thermal system, at steady state, the energy balance is calculated iteratively across time, and results in a time dependent solution. By calculating and tracking the energy through the system at each interface or sub-system, we can obtain the overall energy balance of the whole system. Careful accounting is required to calculate an accurate energy balance. All energy gain (heat transfer into the system) must equal all energy loss (heat transfer out of the system). When all energy is accounted for, we find a series of energy balance equations and can solve them simultaneously to calculate unknown temperatures, heat flow, and thermal properties.

Computer energy modeling is often used to compare the performance and cost of system alternatives. While many software tools are available for various energy modeling applications, few offer a flexible suite of components for multiple energy system types, such as buildings, solar thermal, photovoltaics, etc. One such tool that is System Advisor Model (SAM), which helps create performance and financial model for a variety of solar systems and projects. This assignment provides you with another opportunity to apply SAM modeling to a solar heating system. 

The Assignment

Use the the example from the Introduction section of D&B online textbook (pages xxi - xxvi) to create a SAM profile of the water heating system and run the simulation for your chosen location.

The goal is to generate an estimate for the annual utilization of a solar water heater. Extract performance information, expected energy gains, and expected costs savings for water heating (here consider only cost of energy, not equipment). The adjustable parameters you can use to optimize the system performance are: type of the collector, tilt of the collector, and size of the collector. 

More detailed instruction for this assignment in posted in Lesson 6 Module in Canvas.

Computer energy modeling is utilized heavily in decision making during the design and retrofit process for solar thermal systems. While rarely “dead on” when it comes to accuracy, the comparative value (as opposed to absolute value) of one benchmarked model against another is tangible. Simulated energetic and financial impact of design alternatives enables the best path forward while developing a solar project.

Please double-check the to-do list on the Lesson 6 Introduction page to make sure you have completed all of the activities listed there before you begin Lesson 7.

For well-being and health, the homestead should be airy in summer and sunny in winter. A homestead possessing these qualities would be longer than it is deep; and its main front would face south.

It is very warm and light, not only from the direct rays of the sun, but by their reflection from the sea.

Passive solar architecture can trace its origins back thousands of years. Buildings have long been used as solar collectors and thermal storage systems (D&B Chapter 14). Today we have some well-defined ways of calculating the time-dependent loads within our buildings, the space heating requirements, and the thermal behavior of buildings (D&B Chapter 9). This enables the active heating of buildings using solar thermal collection systems (D&B Chapter 13). On top of that, there are districts that have centralized or distributed solar heating systems for use at a community wide scale. With the technology available to us today, we could supply the vast majority of all building heating worldwide with solar thermal energy at reasonable cost. The key element is seasonal solar thermal energy storage enabling an adequate solution for regions with long winters due to poor solar access during that time (D&B Chapter 8). Appropriately sizing systems is a critical step towards good and cost effective solutions.

Lesson Objectives

• Describe the differences between active and passive systems/methods.
• Understand the principles of different thermal energy storage types.
• Examine the heating options for a residence or office.
• Perform basic thermal balance calculation for residential storage tank.

What is due for Lesson 7?

This lesson will take us one week to complete. Specific directions for different assignments are given in the table below and within this lesson pages.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

D&B Chapters 13 and 14 cover the two types of district heating; active and passive.

The difference between those types is rather simple. The active heating implies using specially designed solar collectors that are installed on the building and supply energy to partially of wholly satify the building energy loads. The passive heating means that building itself acts as a solar collector, and hence the properties of building materials and building design both play role in the energy balance of the system.

The typical basic components of an active solar heating system include:

• collector,
• storage unit
• load
• auxiliary source

Design approach is similar for both air and water heating systems, but the storage types and equipment are different. When such standard designs are applied to specific building cases, a number of key parameters are manipulated to ensure that the thermal performance of the system meets the space heating requirements. Please refer to the following reading for more details on active solar heating.

Energy storage is a very important element of many sollar heating systems due to inherent intermittency of solar flux. The storage unit is typically represented by medium capable of effectively maintaining its temperature over a certain period of time. When the direct solar gain is unavailable, storage heat can be used for meeting the load requirement. The optimal storage capacity (size of the unit) is always dependent on the expected time variations of solar radiation availability.

The D&B Chapter 8 describes several types of thermal energy storage, which can be used with either liquid-based or air-based heating systems.

The most common thermal energy storage system in many houses is a hot water tank. The insulated tank typically holds 50-80 gallons of water and can be very efficient at storing energy depending on the level of insulation. Solar water heating systems will often use a water tank that is sized larger than would be sized for a conventional domestic hot water system so as to increase the solar fraction of the system.

For solar heating systems that use air, a packed rock bed storage system is a great alternative to requiring water-air heat exchangers with a hot water storage tank. The airflow requirements of a packed bed storage system result in a limitation of rock bed storage, where, unlike in a water tank, heat cannot be input and extracted at the same time.

Both types - water tank and pebble bed - are storage options that were discussed earlier in space heating design.

Seasonal thermal energy storage is a hot topic at present. The type of storage enables the wintertime use of abundant solar thermal energy from the summer months. While as much as half of the collected summer energy that is put into any seasonal thermal energy storage system is lost, the remaining half that is stored and eventually extracted can cover as much as 95% of the total heating requirements. [source: Sibbit et al., The Performance of a High Solar Fraction Seasonal Storage District Heating System – Five Years of Operation Energy Procedia, 2011]. The Drake Landing Solar Community in Alberta, Canada, that you became famliar with in the previous Section, is an example of unprecedented success in the field of seasonal thermal energy storage for the heating of buildings. Seasonal thermal energy storage systems can take many years to achieve their operational goals as there is a lot of thermal lag while the system “charges” over the course of the first few summers. Retrofitting a building or community to use a seasonal thermal energy storage system can be challenging, especially in urban settings, since most such storage systems are large and are installed underground and hence may require digging or drilling, which can be both costly in space-demanding. Some solutions to this cost barrier include compact drilling rigs for urban settings and horizontally oriented storage system layouts.

The method called f-Chart is one of the empirical frameworks that uses standardized metrics to characterize the long-term performance of solar heating systems. It was originally developed by Klein, Beckman, and Duffie in 1976. Although it has some uncertainties and  is mostly applied to standard designs, it provides a quick and robust estimate of system performance using collector parameters and average monthly radiation and temperature data. To this end, understanding the f-Chart data can be useful.

The f value, which is a targeted output in f-Chart computations, characterizes the fraction of a total heating load supplied by solar energy:

$$f = \frac{\mathrm{s} \mathrm{o} \mathrm{l} \mathrm{a} {\mathrm{r}}_{-}\mathrm{e} \mathrm{n} \mathrm{e} \mathrm{r} \mathrm{g} {\mathrm{y}}_{-}\mathrm{i} \mathrm{n} \mathrm{p} \mathrm{u} {\mathrm{t}}_{-}\mathrm{t} {\mathrm{o}}_{-}\mathrm{l} \mathrm{o} \mathrm{a} \mathrm{d} }{\mathrm{t} \mathrm{o} \mathrm{t} \mathrm{a} {\mathrm{l}}_{-}\mathrm{e} \mathrm{n} \mathrm{e} \mathrm{r} \mathrm{g} {\mathrm{y}}_{-}\mathrm{l} \mathrm{o} \mathrm{a} \mathrm{d} }= \frac{L_s}{L_s+ L_a}$$

To set up the calculation you need to choose and specify the following variables: collector area,  collector type,  storage capacity,  fluid flow rates, heat exchanger sizes. Those parameters can be set at the design stage and can be adjusted further to achieve a desirable outcome.

The f-Chart model correlates those parameters with thermal performance. This correlation is empirical and is the result of treatment of hundreds of simulations and case studies of solar heating systems. In the end, f is presented as a function of two dimensionless parameters X and Y:

$$X = \frac{\text{ collector\_energy\_losses }}{\text{ total\_heating\_load }}$$

$$Y = \frac{\text{ total\_energy\_absorbed\_by\_collector }}{\text{ total\_heating\_load }}$$

These metrics are normally defined for a one-month period. In terms of collector properties, these values can also be expressed as follows:

All the parameters and properties used in these metrics are available from collector tests and conditions chosen, so they should be quite straightforward to calculate. Let us define all the properties included (D&B, 2013):

A_c  = collector area (m²)

F'_R = collector heat exchanger efficiency factor

Δt = total number of seconds in month

T_a = monthly average ambient temperature (^oC)

T_ref = empirical reference temperature (100 ^oC)

L = monthly total heating load for space heating and hot water (J)

H_T = monthly average daily radiation incident on collector (J/m²)

N = number days in month

(τα) = monthly average transmittance-absorptance product

The products F_R(τα) and F_RU_L are determined from standard collector tests. The example calculations of X and Y values for a typical solar heating system are given in Example 20.2.1 of the D&B textbook. Further, X and Y are used to calculate f - the monthly fraction of the load supplied by the solar energy. For tha purpose, the following empirical equations are used:

For liquid systems:

$$f = 1.029 Y - 0.065 X - 0.245 Y^{2 }+ 0.0018 X^{2 }+ 0.0215 Y^{3 }$$

For air systems:

$$f = 1.040 Y - 0.065 X - 0.159 Y^{2 }+ 0.00187 X^{2 }- 0.095 Y^{3 }$$

Note: these equations are the result of model data fitting (so do not try to justify it by any theoretical derivations). They are simply used as the approximations of model results fitted to  the pair of X and Y values.

Further, the total solar fraction of annual load (F) can be found by the sum of all monthly fractions:

$$F = \frac{\sum f_{i }{L}_{i }}{\sum L_{i }}$$

For example, for a liquid-based heating system, if we perform this calculation for different collector areas, we observe an increase of solar energy fraction as the system gets larger (Figure below). Based on this output, we can select the collector area for the project to cover the desired amount of energy needs.

The chart above is only one representation of f-Chart performance data. A number of other properties can be presented in similar diagrams.  Please refer to the following reading to learn more details about the f-Chart method and to see more examples and illustrations of how it is applied.

The f-Chart method can be applied to the following systems (Haberl and Cho, 2004):

• Water storage heating
• Pebble bed storage heating
• Building storage heating
• Domestic water heating
• Integral collector-ctorage DHW
• Indoor and outdoor pool heating
• Passive direct gain
• Passive collector-storage wall

The following types of solar collectors can be included in this treatment:

• Flat plate
• Evacuated tube
• Compound parabolic concentrating
• 1-axis and 2-axis tracking

To summarize: the f-Chart method is an express tool for thermal design assessment. It allows quick estimation of solar heating system performance, although errors up to 5-10% may be present due to approximations. That is the price of speed and convenience. The uncertainties may be due to variations in meteorological data, averaging performance data, system use and occupants' behavior, and individual system flaws and imperfections.

This method is especially recommended for exploring relative effects of design changes and also as a benchmark for comparison with real-life performance data from a system.

While the calculations described above can be made by hand, there is licensed software (FCHART) available (but not free) via www.fchart.com (2005).

References:

Duffie, J.A. Beckman, W., Solar Engineering of Thermal Processes, John WIley & Sons, 2013.

Haberl, J.S. and Cho, S., Literature Review of Uncertainty of Analysis Methods (F-Chart Program), Report to the Texas Commission on Environmental Quality, Energy Systems Laboratory, Texas A&M Univeristy, ESL-TR-04/08-04, August 2004.

Klein, S.A., Beckman, W.A.,  and Duffie, J.A.  A Design Procedure For Solar Heating Systems, Solar energy 18, 113- 126 (1976).

Buildings are often not designed to maximize solar utility for the building occupants. As such, there can be some “low hanging fruit” when it comes to increasing solar gain and solar energy utilization in the heating season. Additionally, active solar heating systems can be set up to offset significant portions of annual heating costs.

Consider the heating of your home, apartment, or office during the heating season. If you are in a region where there is little or no heating requirement, please imagine your home, apartment, or office in a location where you would need a heating system. What solar passive solutions would reduce your heating requirements at low capital cost? What solar active systems would help to offset your energy consumption for space heating? Section 13.2 of the D&B text provides some system schematics and specifications for typical, active solar thermal systems to give you some ideas.

Buildings are solar energy conversion systems, absorbing solar radiation and impacting comfort within building zones. Building facades (walls) behave as flat plate absorber systems (with no covers) while windows behave as cavity absorber systems - the radiation is transmitted through the window and part of it is reflected around in the zone until it is absorbed by the floor, walls, and ceiling. The energy balance of a building is comprised of gains, loads, and losses, all of which are time dependent. Careful accounting of each piece of the overall energy balance and choosing the right specifications for collector and energy storage are needed to create an energy-efficient dwelling.

Please double-check the to-do list on the Lesson 7 Introduction page to make sure you have completed all of the activities listed there before you begin Lesson 8.

Last year, I made a refrigerator in my basement. And I needed … to figure how … there is no such thing as 'cold.' There is only less heat.

Cooling needs are an excellent fit for solar thermal energy because of the often-synchronous nature of the energy source and load schedule. What we know of as refrigeration or cooling is simply the moving of heat from our cold zone (building space, refrigerator, etc.) to a heat sink (often the ambient air). The driving of heat from a cold space to a hot space requires the input of energy because that is not the natural direction that heat would flow. An analogy would be pushing a cart uphill, when it wants to roll downhill, requiring the input of energy to get the cart to move uphill. Depending on the type of technology that is used, thermal energy can drive heat in a desired direction, thus creating “cold,” which is actually just less heat. Solar thermal energy is often thought of as some unique source that must have complex technology to convert it to useful work. That is not the case. Solar thermal energy is no different from any other type of thermal energy. Depending on the collector setup and working fluid, different temperatures and operating temperature ranges can be obtained. This heat can then be used for any thermal process that would normally be driven by some other heat source such as natural gas, propane, or electric resistance heat.

Learning Objectives

• Identify different types of solar cooling - e.g. absorption chiller, desiccant, etc.
• Use the psychrometric chart to explain cooling effect
• Select a solar thermal solution for a residence that helps address cooling requirements.

What is due for Lesson 8?

This lesson will take us one week to complete. Specific directions for different assignments are given in the table below and within this lesson pages.

Questions?

If you have any questions, please post them to our Questions and answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

The solar air conditioning immediately seems a very attractive idea when you think of using solar heat to counteract that heat. How does that become possible? The concept however is not new, as it dates back to 1960s and 1970s, when several cooling and refrigeration cycles were investigated and proposed. In general, the use of solar cooling was intended for two main purposes:

1. Refrigeration for food storage and

2. Space cooling (air conditioning)

Furthermore it was realized that such systems as flat-plate solar collectors can be use for heating in the winter and cooling in the summer, thus significantly increasing system usability and efficiency.

The main idea behind this concept is that the physicochemical cycle that is able to transfer thermal energy from one location to another is driven by an external heat source (solar or non-solar). Another important idea here is that removal of heat from any part of the system essentially equals cooling. There are a number of different cooling cycles based on somewhat different principles. The following three are mentioned by Duffie and Beckman (2013) in connection with solar thermal systems: 

• Absorption cycles
• Desiccant cycles
• Solar mechanical cycles

All of these cycles may have some technical variations depending on specific conditions and load.

Absorption cooling technology is considered in more detail here. The cooling effect is based on the evaporative cooling of a refrigerant. Because the vaporization process requires energy input, when it happens it takes heat from the system, thus making the remaining fluid colder.

Desiccant cooling is based on cycling dehumidification-humidification processes. It uses some hygroscopic substances or materials for dehumidification, and those materials are regenerated in the cycle by applying solar heat.

Solar mechanical cycles attempt to combine the solar-powered mechanical work cycles (such as Rankine) with conventional air conditioning systems. The solar-driven part of the system is not actually the chiller, but the engine that produces energy for operating the air conditioner.

At over 50 years of history of commercial systems, why haven't solar absorption chillers simply taken off and taken over the air conditioning market? What about solar desiccant cooling systems? If the theory and proof of concept have been shown to work, the only thing that holds back a technology from taking off is the cost. In this case, for most locations, electricity generated from fossil fuels is currently less costly than the cost of a solar thermal collector, storage tank, and other system components required for cooling with solar thermal energy. Even when combined with solar heating for both domestic hot water as well as space heating (as shown in Figure 15.3.1 of the D&B text), where cost savings through shared system components are expected, the payback period is often too long to justify with current low fossil fuel energy costs as the alternative. In certain applications, however, such systems are indeed the lowest cost alternative, and, as time goes on, these dual-purpose (heating and cooling) systems will become more competitive.

Since cooling is quite expensive, reducing cooling loads via methods of passive building design and insulation are important for overall efficiency. For example, summer solar gains in the building can be minimized by using windows with low transmittance in the infrared part of spectrum. Also various shading designs (internal and external) can be planned to manipulate direct solar gains. Creating passive ventilation loops that take advantage of natural air flows and prevailing winds at a locale can also prevent excessive indoor heating. Finally some building designs may use the ground as heat sink ("earth tempering"). All these methods are discussed in more detail in Passive Cooling Handbook (US DOE, 1980) and should certainly be considered in building designs in warm climates as a way of reducing cooling loads.

Absorption cycle is one of the promising methods to utilize the solar heat for space cooling in domestic and industrial applications. Until recently the absorption cooling technology was not readily available for small capacity applications and was quite expensive compared to the traditional vapor compression cooling technology. However, there is a significant opportunity to combine an absorption system with building envelop design to provide environmentally benign way of controlling internal environment using solar energy.

There are two basic types of absortpion cooling cycles: (1) Lithium Bromide (LiBr)-Water and (2) Ammonia-Water. The LiBr-H₂O appears to be more suitable for small-scale and low-cost solar applications due to lower operating temperature of this cycle (Florides et al., 2001).

Absorption systems are in some way similar to vapor compression systems, but differ in pressurization phase. Maybe it can be useful to briefly review how the conventional vapor compression systems work, such as those found in electric refrigerators.

Now let us look how the absorption cooling cycle works.

It was already mentioned that in comparison with the conventional refrigeration cycle (described above), the absorption cycle has a different kind of pressurization. Instead of using a mechanical compressor (usually power-expensive), it completes pressurization by dissolving the refrigerant in the absorbent. In case of LiBr-H₂O system, LiBr acts as an absorbent, and H₂O acts as refrigerant. LiBr is a salt, so we can guess that if the LiBr-H₂O solution boils, water will go, and salt will stay. The solubility limit of LiBr in water is quite high, so the solution used in the absorption cycle is very concentrated (~60% LiBr by mass).

There are four main components of the absorption cooling cycle: generator, absorber, condenser, and evaporator (where the cooling effect is achieved). The simplified schematic diagram of the absorption cycle is shown below:

Figure 8.1: Schematic representation of the absorption cooling system

Click here for a text description of the figure

Cycle with four main components and the processes that the cycle passes through. The process starts at the generator where heat is input from solar radiation (Q_G), then the solution in the form of vapor passes to the condenser where rejected heat (Q_C) leaves the system. This goes through an expansion valve into the evaporator where heat is input (Q_E) creating a cooling effect. Once again in the form of vapor the solution travels to the absorber where rejected heat leaves the system (Q_A) it is subsequently passed to the heat exchangers where it can return to the generator

Credit: Mark Fedkin - modified after Kalogirou, 2009

The solar (or other external) heat input to the system is denoted Q_G. The heat that is absorbed by the system from the cooled space due to evaporation process is denoted Q_E. The heat rejected from condenser and absorber are shown respectively as Q_C and Q_A. The overall energy balance of the system is therefore:

Q_C + Q_A = Q_G + Q_E

The cycle, step by step goes as follows:

- Absorber contains the absorbent-refrigerant mixture (in this case LiBr-H₂O), which is delivered by a liquid pump to the generator. The solution passes through a heat exchanger where its temperature increases. We can call this pumped mixture "strong solution" because it is rich in refrigerant.

- Generator heats the absorbent-refrigerant mixture using the external heat source - for example, solar heat from a flat-plate collector. The heat causes the solution to boil, and water turns into vapor flows to the condenser. Remaining LiBr with still some water is sent back to absorber via a heat exchanger (this returning fraction can be called "weak solution" since it is depleted of refrigerant).

- Condenser condenses the water vapor coming from the generator. At this point heat is rejected, since condensation process is exothermic. This liquid condensate is now directed to the evaporator through an expansion valve.

- Evaporator absorbs the heat from the load (cooled space) due to evaporation of the refrigerant at low pressure. This creates the cooling effect. Vaporized refrigerant then flows back to the absorber, where it mixes with the "weak solution" of LiBr.

Then the cycle continues. In this scheme, the solar heat provided to the generator is driving this cycle. The cooling effect achieved in the evaporator is applied to either chilling water flow or air flow that is passed through the evaporator. This chilling water flow is not part of the cycle, it simply acts as a vehicle to deliver "cold" to the target space or environment.

The following practical values of the system parameters are typical for LiBr-H₂O system:

- Generator temperature ~75^oC

- Evaporator temperature ~6^oC

- Heat exchanger temperature ~55^oC

- "Strong" solution 55% LiBr(aq) by mass fraction

- "Weak" solution 60% LiBr(aq) by mass fraction

The performance of such system can be characterized by cooling coefficient of performance (COP metric):

COP_cooling = Q_E/Q_G

which is essentially the heat load of the evaporator per unit of heat load of the generator. This metric can become useful when comparing system costs. The COP values for LiBr-H₂O system is typically in the range from 0.6 to 0.8 (Duffie and Beckman, 2013).

The following reading will give you more details and analysis of the adsorption cooling systems.

Solar desiccant cooling systems focus on the dehumidification of air as a means of cooling air for a building space. Psychrometrics is the field of engineering concerned with the amount of moisture (as vapor) in the atmosphere. To understand psychrometrics, we use a psychrometric chart, below (there are many charts available free online from various sources and companies. (The following chart is from engineeringtoolbox.com). Use the following link to access a scalable PDF file of the psychrometric chart.

Figure 8.2: Psychrometric Chart

Source: Engineering ToolBox

The x-axis here is dry bulb temperature (this is the temperature measured by thermometer shielded from radiation or moisture - true thermodynamic temperature). The y-axis is water content, or moisture, measured in mass of water per unit mass of dry air. As you move to the left on the chart, you decrease the sensible heat. As you move to the right, you increase the sensible heat. As you move up you increase the humidity, increasing latent heat. As you move down you decrease the humidity.

You can learn the anatomy of the the psychrometric chart in more detail from the following source: 

The psychrometric charts help to understand how the solar desiccant cooling works. This is the second class of solar cooling systems, which are based on humidification-dehumidification process using a hygroscopic material (desiccant), which is capable of absorbing moisture from air. This  cycle uses water in the direct contact with air as refrigerant. Both liquid or solid desiccant can be used: for example, silica gel, LiCl salt (solid) or glycol, LiCl solution (liquid). An interesting feature of the desiccant cycle is that it is an open system, meaning that water (refrigerant) is discarded from the system via dehumidification process, and then new water is taken in during humidification phase. The desiccant cycles are typically used with flat-plate solar collectors. Solar heat is applied to regenerate the desiccant.

Learn more details about how desiccant cycle works from the following reading:

Figure 15.7.2 in the D&B text shows the psychrometric path through a rotary desiccant solar cooling system, with numbers tagged on the chart that correspond with the numbers tagged on the system diagram. Figures 15.8.1 and 15.8.2 in the D&B text each show an additional system configuration that uses a desiccant and the subsequent psychrometric chart path that results from the use of the system. Revisit each of these three figures and follow the path through the system on the psychrometric chart. 

Solar cooling is a field of great interest for the buildings industry. Solar gains driving cooling loads, and as such, if utilized to drive a cooling system would result in synchronized cooling loads and cooling energy supply. Consider the cooling of your home, apartment, or office during the cooling season. If you are in a region where there is little or no cooling requirement, please imagine your home, apartment, or office in a location where you would need a cooling system. What passive solutions would reduce your cooling requirements? Chapter 15 of the D&B text provides some detailed system schematics and specifications for typical solar cooling systems to give you some ideas.

Course project pre-proposal will help define the scope of your final design proposal and will be used to obtain class feedback before you develop the idea to the final stage. 

Your Project Pre-Proposal should include the following sections:

1. Introduction - briefly explain your project motivation and goals.
2. Project area definition – define the location, buildings, processes, etc.
3. Thermal energy requirements – define peak demand, average demand, scheduling, etc.
4. Alternative technological solutions – describe available alternatives including a no- action alternative if applicable.
5. Potential impact – describe the best case cost and energy savings potential.
6. Overview of methods to be used – list any tools or calculation methods that you will need to employ for the full system design proposal.

Please limit your Pre-Proposal to three pages to facilitate quick review. You can include some originally made graphics if it is important to convey your idea. Consider the content of this pre-proposal as a working outline of your final project.

These pre-proposals will be subject to peer-review, which will happen over the next two weeks. Everyone will be assigned two pre-proposals to critique. Please submit your feedback on each author's submission as comment or attachment to your comment. Again - the deadline for the peer-review will be two weeks from the due date for pre-proposal submission.

Summary

Solar cooling is a field of great interest because of the potential opportunity for decreased costs, synchronized energy supply with energy demand, and the utilization of heat from solar heating processes during times when heat is not needed but rather cooling is needed. The (typical) higher cost of absorption cooling systems over compression cooling systems is what has led to the current widely established norm of compression cooling for most processes. However, in certain applications, absorption cooling wins economically and in the environmental context it is indeed the best option.

Reminder - Complete all of the Lesson 8 tasks!

Double-check the to-do list on the Lesson 8 Introduction page to make sure you have completed all of the assigned tasks before you begin Lesson 9.

In several specific industry sectors, such as food, wine and beverages, transport equipment, machinery, textiles, pulp and paper, the share of heat demand at low and medium temperatures (below 250°C) is around 60%. Tapping into this potential would provide a significant solar contribution to industrial energy requirements.

Global industrial processes have significant thermal energy needs that are addressable by solar thermal energy conversion systems. Any industrial process that requires heat at temperatures below 250°C is readily addressable by a solar thermal energy system. Systems that require higher temperatures are also addressable by solar thermal solutions, but the costs for such systems at small and medium scales are prohibitive. There is more to it than simply the temperature; but, at first glance, using temperature thresholds as a guide enables a rapid assessment of global potential.

Learning Objectives

• Match industrial processes with appropriate solar thermal technologies based on process parameters and inputs.
• Calculate industrial processing system requirements.
• Apply basic cost analysis to the solar thermal system in industry applications.

What is due for Lesson 9?

This lesson will take us one week to complete. Specific directions for different assignments are given in the table below and within this lesson pages.

Questions?

If you have any questions, please post them to our Questions and answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

Industries all over the world use heat. Heat is heat, whether it comes from solar radiation or from fossil fuel combustion. Using solar thermal systems for generating heat for industrial processes is a very attractive avenue since it allows significant reduction in fuel costs and decreases environmental impact due to carbon emissions. Another reason is that a wide range of industrial applications require temperatures from near-ambient to those corresponding to low-pressure steam (<400 ^oC) - the range that fits very well the output temperatures of solar thermal collectors. So why not switch?

The key difference is the dispatchable nature of fossil fuels. Solar heat must be stored to achieve some level of dispatchability, or the industrial process that uses the heat must be flexible to adjust and only use the heat when the solar resource is available. Most often, some combination of a small storage solution along with modifications to the industrial process is the best solution.

At present ~40% of industrial primary energy consumption is provided by natural gas and ~41% by petroleum. So there is a technical potential to increase the share of solar thermal heat and possibly reach the solar thermal deployment in the industrial sector of 33% by 2030 (IRENA, 2015).

Many industries can be potentially benefitted, including but not limited to:

• Chemical processing
• Food production
• Beverage production
• Paper making
• Tanning
• Malt processing
• Desalination
• Textile
• Paint drying
• Masonry curing
• others..

The two key factors to consider in determination of the most suitable solar system for supplying energy to an industrial application are temperature and thermal medium. How much heat does the thermal fluid need to carry? What is the best type of fluid to use in the process?

For example, if hot steam is needed to operate an application, the solar system needs to be designed to deliver temperature significantly over 100 ^oC, and therefore concentrating collectors will be probably required. Or if hot water is needed (e.g. for such applications as desalination or sewage treatment), liquid-based flat plat collectors can be employed.

Learn more details on the status of solar energy implementation in different industries in the following report prepared by International Renewable Energy Agency:

One of the key economic impacts expected from the solar heat applications is fuel deduction costs. While deploying solar collectors involves additional up-front capital costs, the operational costs are much lower compared to fuel-fired units. However, due to reliability requirements, auxiliary energy supply is still typically included in industrial applications. In this case, the useful energy from the solar system is used to reduce the auxiliary fuel consumption.

Solar thermal systems are relatively high in capital costs compared against alternative thermal energy systems. However, the solar fuel is free. As such, the cost structure is inherently different from low capital cost but relatively high and often volatile fuel costs associated with petroleum, coal, natural gas, and other fossil fuel thermal energy systems. So another attractive aspect of solar thermal systems, and one of the primary drivers behind their growing installations worldwide, is the insulation that such systems provide against price shocks of fossil fuels.

For industry, taking into account depreciation rates over the long-term projects is important. In this connection, discounted cash flow analysis is a good tool to estimate the Net Present Value (NPV) of the project and realistic pay-back time. Those metrics are initially used to evaluate the economic viability of the project.

Let us recall what Net Present Value (NPV) is. In brief, NPV is the current value - as of today - of all related cash flows through the time of the project. Because $100 today is worth more than $100 next year (due to inflation rate), expenditures that will occur in the future are not valued as much as the same expenditures occurring today. This is why cheap natural gas boilers often appear as an economic choice in the light of the high capital cost of a solar thermal energy systems. Below we review the basic methods of calculating NPV.

Simple payback approach

This approach is suitable for short-term projects with quick return on investment. In this case, discounting (for money value declining over time) may be unnecessary.

In simple payback evaluation, all cash flows into and out of the project are added up to find Net Present Value (NPV). That includes initial cost (capital investment), annuities (net utility obtained from the operation, i.e. renevue minus maintenance and operation costs), and salvage value (in the end of the project) (Vanek and Albright, 2008):

NPV = Initial Cost + S(Annuities) + Salvage Value

• If NPV is positive, the project is considered financially viable.

The break-even point, i.e., the year when the sum of annuities surpasses the initial cost, and the initial expenditures have been paid back, is characterized by the Simple Payback Period (SPB):

SPB (years) = Initial Cost / Net annuity

• SPB indicates the number of years after which the initial expenditures are paid back.

Capital Recovery Factor (CRF) evaluates the relationship between the cash flow and investment cost. This evaluation is applicable to short-term investments (within N=10 years).

CRF = ACC / NPV

where ACC = Annual Capital Cost

ACC = Annuity – NPV/N

Here, the NPV/N term is the average share of the net present value per each year of the project. So, ACC is the part of the annuity that goes each year to cover the investment; it does not go towards profit.

• CRF factor should not be too high for a project to be considered financially viable.

By recommendation of the Electric Power Research Institute (EPRI), CRF value should not exceed 12%.

Discounted Cash Flow Analysis

This approach is better applied to long-term projects with slow payback. Money value declines over time, so it must be taken into account. This is especially relevant to solar projects, which typically have high capital costs and long pay-pack periods.

In the case of discounted cash flow, we need to evaluate how much any cash flow element would value in the future. That would depend on the interest rate (i) imposed on initial investment and the number of years (N) the project is underway. The following conversion factors are used to adjust the future money values (Vanek and Albright 2008):

$$\frac{\text{ present value }}{\text{ future value }}= \frac{P}{F}= \frac{1}{( 1 + i {)}^{N }}$$

and

$$\frac{\text{ present value }}{\text{ annuity }}= \frac{P}{A}= \frac{( 1 + i {)}^{N }- 1 }{i ( 1 + i {)}^{N }}$$

Then the discounted flow NPV can be derived as follows:

NPV_future = Initial cost + (P/A) × Annuity + (P/F) × Salvage value

In case of solar system providing heating for a facility or a process, the generated thermal energy is not sold to create monetary revenue, but instead is accounted in energy savings. So amount money saved per year can be assigned as monetized benefit.

Example

Let us estimate the NPV values for a hypothetical solar collector system of total area 10 m² designed to offset the thermal energy expenditures of a building. Assume the system cost estimated at $725 per m² of collector surface. This includes cost of collector, associated storage, pumps, and piping. Also assume that the yearly energy savings from the solar system are 7500 kWh, and the monetary value of energy is $0.16 per kWh (this is how much the consumer would need to pay for electric water heating if the solar system is not employed). Would this system be viable over the term of 15 years? or 10 years?

Based on these parameters and assumptions, the total system installation cost will be $725/m² x 10 m² = $7250. We can neglect possible maintenance or repair costs in this example. So this will be the total investment.

Then, the monetary value of the saved energy will be: 0.16 $/kWh x 7500 kWh/year = $1200. This is how much annuity (in terms of energy saving) the system brings.

To assess the economic viability, first we can use the simple payback check:

NPV = - 7250 + (1200 x 15) = $10,750

This is a positive value, therefore the project makes economic sense. However, depreciation rate is not taken into account here. If we assume that depreciation is 5% (and this is how much interest will be put annually on the initial investment), we can estimate the discounted NPV as follows:

NPV_future = Initial cost + (P/A) × Annuity + (P/F) × Salvage value

where the (P/A, 5%, 15) factor is equal to 10.38 (if you apply the above-mentioned formula). The (P/F) factor is not needed in this calculation because we are not considering any salvage value. So finally:

NPV_future = - 7250 + (10.38 x 1200) = $5,206

This is again a positive value, so with the current economic conditions, the project is considered viable.

Another thing to consider in the economics of the solar heat - industrial process combo is investment tax credits. Those incentives effectively reduce the up-front capital costs and may create a more favorable economic scenario and allow solar to compete with relatively cheap fossil fuel heat generation. So if available in specific location, tax credits should be included in the cash flow analysis.

The use of solar heating technologies can be linked to a variety of industrial applications and can replace significant amounts of fossil fuels burned in the process. The potential is high since about 30% of industrial heating demand is withing the range of solar thermal systems. Typically solar heat is supplied via heated water, steam, or air.  

Industrial heating needs can be categorized into three main temperature ranges:

(1) < 80^oC -  low temperature - flat plate solar collectors are capable of meeting these temperatures;

(2) 80 - 250 ^oC - medium temperatures  - concentrating collectors are needed;

(3) >250 ^oC - high temperatures - this range requires imaging concentrated systems to achieve such high temperatures.

 All of these categories can be matched with solar systems.

For learning about some specific applications and system designs, you are now re-directed to the following textbook chapters:

The Solar Heating and Cooling Program of the International Energy Agency is documenting the majority of solar thermal projects in a database. Understanding system components and how external forces, such as policy and alternative fuel costs, impact decision is critical to being effective in the sector of industrial processing.

This assignment is a series of short answer problems and calculations. These problems will require both conceptual thinking and some quantitative operations.

Lesson 9 Problem Set:

1. Take a look at Figure 16.6.1 in the D&B text. Why would a system be designed with a flat plate collector in series with a concentrating collector? Please discuss at least two benefits of this configuration.
2. Equation 16.4.1 in the D&B text says that the useful energy gain (Q_u) of an open-circuit air heating system is proportional to the area of the collector (A_c), how much heat is removed from the system (F_R) based on fan speed, and how much solar energy is incident on the collector minus optical losses (S). Check out a project in Germany of an air heating system for a vehicle paint shop. Which of the three variables (A_c, F_R, S) that impact Q_u must be increased if the system owner wanted to supply air to a second paint room? Why?
3. There are several ways that a solar thermal energy system’s capital cost can be paid back within a reasonable time frame. Tax credits, other tax benefits such as depreciation, high alternative fuel costs (including a carbon tax), and more. Download the Lesson7-STE-Payback.xlsx file from Canvas in the Lesson 7 Module. If you do not have access to Microsoft Excel or an equivalent spreadsheet editor, use Google Drive (Upload the file to drive.google.com, check the box next to the file once uploaded to drive, then click More>Open with>Google Sheets). In that spreadsheet, there are several cells highlighted in green that are the input cells to the payback calculator. Adjust any/all of those values appropriately to achieve a system payback period less than 10 years. Your adjustments should match a specific project that actually exists (e.g., from the solar thermal projects database), including location specific details such as annual solar resource. Policy related adjustments (tax credits, carbon tax, etc.) should be based on real policies for your location. If you are not able to find actual policies for your location, policies from other locations are okay to use if necessary. All policy related adjustments must be supported by references. Deliverable: a write-up describing the project you are assessing including a table of your adjustment values, an exported payback graph from the spreadsheet, and written justification (including references) for each of the values that you selected.

Summary

Solar heating for industrial processes is one of the largest potential markets for solar energy. However, unlike the photovoltaic industry, the solar heating for industrial processes has yet to really take off. In addition to being one of the largest potential markets for solar energy, solar thermal systems are really fundamental to the field of solar energy conversion systems. Not only are flat plate collectors comprised simply of pipes, black, and glass (technologically simple), but also the concept of absorbing solar radiation as heat has been around for thousands of years. It is about time that we start to make solar heating for industrial processes really work. The opportunity is very large.

Conversion of solar to mechanical and electrical energy has been the objective of experiments for over a century.

Solar radiation is a great source of heat for generating steam. Steam is a highly useful working fluid that has been used to drive mechanical systems for centuries. Heat is heat, no matter what its origin. As such, as solar thermal technologies continue to be developed and decrease in cost, more and more thermal systems that are currently driven by steam that is generated from the burning of fossil fuels can have their fuel source replaced by the sun.

Learning Objectives

• Identify the steps of the processes required to convert solar energy to mechanical and electrical energy.
• Compare the thermal behavior of solar thermal power systems.
• Summarize the technical details of the solar thermal power systems existing and emerging over the globe.

What is due for Lesson 10?

This lesson will take us one week to complete. Specific directions for different assignments are given in the table below and within this lesson pages.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

Steam has been used for centuries to perform mechanical work. Steam locomotive engines are probably one of the most popular machines known for converting steam to mechanical work. Any modern steam turbine does a similar conversion at higher energy conversion efficiency. Many steam turbines are used because of their high efficiency at turning steam energy into kinetic rotational energy. This rotational energy can be further used to drive an electricity generator or any other process that requires mechanical energy to operate. Historically, the steam required for such processes was derived from burning fossil fuels such as coal or natural gas, while solar thermal energy was used experimentally for over a century. Steam that is generated by renewable methods (such as solar radiation) is identical to steam generated by burning a fuel to heat water, and the principles of conversion of solar heat to mechanical and electrical energy are fundamentally similar to those used in combustion systems. Concentrating solar thermal technologies are best suited to achieve high temperatures under higher pressures, simultaneously meeting the demands of large-scale turbines that require a significant amount of high-quality steam. The general strategy of energy conversion using solar thermal energy is presented on the diagram below.

Figure 10.1: Schematic of a generic solar thermal power system

Credit: Mark Fedkin

The solar energy obtained and converted to heat by the collector system is transferred by the thermal fluid to the storage and further to a boiler, where steam is generated. Further steam is supplied to a turbine in the heat engine, where it is converted to mechanical energy, while some heat is rejected. If the electric power is desirable output, the mechanical energy is supplied to a generator, where it is converted to electricity. At each conversion step, we can expect some losses due to non-100% efficiency. One of the challenges here is that the efficiency of the solar collectors decreases with increasing operating temperature, while the efficiency of the heat engine increases at a higher temperature (Duffie and Beckman, 2013). Therefore optimization is needed to select system operation conditions. Typically the temperatures delivered by the flat-plate collectors are too low for heat engines to be efficient; thus concentrating collectors (e.g. parabolic systems) or evacuated tubular collectors are more preferable choices.

The main configurations of solar thermal power systems include: 

• Parabolic troughs
• Parabolic dish
• Central receiver systems (power tower)
• Solar updraft tower

You can re-visit those technologies on the Energy Information Association website

The overall efficiency of the power conversion system is composed of the efficiency of the solar collectors (with parabolic troughs, max ~75%), the efficiency of the heat engine (~35%). Minus field losses, the typical average overall efficiency of solar trough thermal plants is around 15-20%.

The following pages of this lesson refer you to various types and system designs.

Parabolic trough technology is the most widespread among utility-scale solar thermal plants. The potential of this type of concentrating collectors is very high and can provide output fluid temperatures in the range up to 500°C. Parabolic trough is the linear-focus collector, which consists of a cylindrically curved parabolic mirror, which reflects the sunlight onto a tubular receiver positioned in the focus line of the parabola. The tubular receiver contains the fluid that absorbs heat and transfers it via circulation to the boiler or another device to produce steam.

Figure 10.2: Kuraymat parabolic trough solar plant, Egypt. The plant has the total solar aperture area of 130,800 m² and expected electricity generation of 34,000 MWh/year. It has been operating since 2011.

Credit: Green Prophet via Flickr

Rows of parabolic mirrors are mounted in parallel on either a north-south axis (typical) or an east-west axis (there are pros and cons to each orientation based on location and energy production requirements) and move to track the sun across the sky. The tubes are very carefully designed to absorb solar radiation and transfer the heat to the heat exchange fluid passing through the tube. Fluid is pumped through the absorber tubes that are connected in series and parallel. Some systems employ an insulated storage tank to enable power generation when the solar resource is either intermittent (due to something like cloud cover) or unavailable (typically during the early evening hours). The heat transfer fluid is then passed through the storage tank, if it exists, and then pumped to heat exchangers to transfer the heat to water (except in the case of direct steam generation where water is already the heat transfer fluid and a heat exchanger is not needed) to generate steam for expansion in a steam turbine to generate electricity.

Solar Energy Generating Systems (SEGS) is the name of the world’s largest parabolic trough solar thermal electricity generation system, developed by Luz in southern California, USA. SEGS is the second largest solar thermal power plant in the world at 354 MW (surpassed by the 377MW Ivanpah Solar Power Tower system discussed in the next section). The three largest plants in the world currently range in size from 250 MW to 354 MW and are all located in the US. The next twelve largest plants in the world range in size from 100 MW to 200 MW and are all located in Spain.

Unlike linear concentrating systems (troughs), which reflect light onto a focal line, the central receiver systems send concentrated light onto a remote central receiver. A typical example of such a system is a solar power tower system, which consists of multiple tracking mirrors (heliostats) positioned in the field around a main external receiver installed on a tower. Such systems are capable of reaching of much higher levels of concentration than linear systems. Concentrated radiation is further used as heat to produce steam and convert it to electricity (like in a regular power plant), or the generated thermal energy can be stored in a molten salt storage.

Figure 10.3: SOLUCAR PS10 (Planta Solar 10) solar power plant, Spain. Operational since 2007, PS10 produces 23,400 MWh/year. Aperture size is estimated at 74,880 m². The light is concentrated on the top of the 115 m high tower.

Credit: Afloresm via Wikimedia Commons

Central receiver systems are typically large-scale plants that are usually built to power a steam cycle. The central position of the receiver offers a universal advantage to collect all energy at one location and save on transport networks. At the same time, the fixed position of such a central receiver results in a limitation of light collection: heliostats are always oriented at an angle to the direct beam, so the amount of energy collected is less compared to a parabolic concentrator. Therefore, to reach necessary efficiencies of light concentration, the size of the collecting field is increased, which brings into considerations such issues as land use, higher environmental impacts, and higher capital costs. Significant potential for developing large-scale central receiver solar plants is hence attributed to deserts and flat arid areas which have plenty of sunshine and lower land value with respect to other applications and industries.

The world’s current largest solar thermal power system is a power tower system named Ivanpah. Located in Southern California on the border with Nevada, Ivanpah has three main towers, nearly 2.5 million square meters of heliostats (mirrors), and can generate as much as 377MW of power under the right conditions. Worldwide, solar power tower systems have been used for decades to generate steam for both electricity generation and various industrial processes, with large-scale implementations in the tens of MW often for research purposes. Ivanpah is the first and only solar power tower plant in operation in the world that is larger than 20MW. While there are plans and ongoing construction in many countries around the world to build plants that are similar in size and even five times as large as the Ivanpah system, our global experience with such large solar power tower systems is very limited.

In the Ivanpah plant, there are several buildings near the base of the tower that contain the components of a typical steam electricity generation station. Once high-quality steam is generated in the tower and pumped down to the generation station at the base of the tower, the remaining components of the electricity generation system are no different than conventional electricity generation components.

Globally, some countries have much more history and subsequent experience with solar power towers. Spain and the USA are the two leading nations, with many other countries operating small power tower systems or currently developing plans to construct large (>10 MW) solar power tower electricity generation stations. The USA currently houses the largest solar power tower plant in the world and has the history of Solar One and Solar Two, which are currently decommissioned, but were 10 MW in size. Spain houses three active solar power tower systems larger than 10 MW with plans to build three more of that magnitude.

Further please proceed further to the following sources to learn about basic configurations and design of the central receiver solar power technology and some specifications of the heliostats and receivers utilized in known facilities of this type.

Solar updraft towers for generating electric power were first conceived over a hundred years ago. Several prototypes have been developed over the decades, and some have been implemented and operated over the course of several years. These prototypes vary in size and scale, with the largest ones capable of producing tens of kilowatt of power with towers that are a couple of hundred meters tall (most notably, the solar updraft tower in Manzanares, Spain with a tower of 194 m tall and capable of producing 50 kW of electrical power).

Figure 10.4: Solar Chimney prototype at Manzanares, Spain. The tower is seen through the polyester roof.

Widakora via Wikimedia Commons

The principle of operation of solar updraft towers is based on the stack effect: difference in the density of air due to temperature and humidity differences can drive air movement. Solar energy is used to increase the air temperature at the bottom of the tall chimney (tower), creating a gradient in density, which creates upward air movement. This solar-induced "wind" is used to rotate wind turbines installed in the confined space in the chimney. The power output of a solar updraft tower depends on two design variables - solar collector area and height of the tower. The collector is represented by a greenhouse-like structure at the surrounding ground at the base of the tower, and the commercial size collectors are designed up to 4-5 square miles in area. A greater height of the tower creates greater pressure gradient due to stack effect and results in more usable wind power. The known system heights are in the range 100-200 m, while systems as tall as 1500 m are being proposed (Wikipedia- Solar Updraft Tower).

There have been and continue to be many proposals for projects in countries around the world for solar updraft tower systems that are incredibly large compared to the scale that has been proven to date. The high capital cost remains the main barrier to commercialization and widespread use of this technology.

A lot of solar technologies appear to be very similar at face value. However, the history of various technologies as well as knowledge of how each system works help reveal why certain technologies are selected over other solutions. Parabolic trough was mentioned as most widespread solar thermal power technology, and there should be reasons.

For this lesson assignment:

1. Summarize in a table the main cons and pros of the four types of the solar power systems listed in this lesson - parabolic trough, central receiver, parabolic dish, and solar updraft tower (Take into account both technical and economic factors).
2. Provide a discussion as to why certain technologies in this list out-pace others. What are the primary barriers preventing any of these technology options from becoming more widely accepted? Do you have any reason to think that one of the current technologies will become the sole winner in solar thermal power market?

Some supplemental reading resources that can be helpful in this assignment are provided in Module 10 in Canvas.

Summary

Solar thermal power systems are a great way to convert solar radiation to electricity. Historically, there have been many complex issues that have been addressed one by one, making utility scale solar thermal power systems a reality today. Concentrating solar power systems are the main enabler of this reality. By concentrating solar radiation with parabolic troughs or heliostats, higher temperatures and, thus, higher quality steam can be achieved at competitive cost.

Reminder - Complete all of the Lesson 10 tasks!

Solar thermal energy can be applied to any process that would otherwise require the use of heat or electromagnetic radiation (within the solar spectrum). Three main areas that will be covered in this lesson are solar drying, desalination, and chemistry applications. All three areas are closely connected to the topic of energy sustainability and related question - how can we use renewable energy for any process or application if in some foreseeable future there is no more access to fossil fuels? Furthermore, desalination technologies may become a major part of the sustainable water management, especially in the arid areas with acute water shortages. 

Learning Objectives

• Explain the active and passive designed for solar dryers.
• Estimate the parameters of the solar desalination system for an application.
• Articulate the features of solar system designs for drying and desalination processes. 

What is due for Lesson 11?

This lesson will take us one week to complete. Specific directions for different assignments are given in the table below and within this lesson pages.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

Solar dryers are predominantly used in agricultural industry to dry foods for storage purposes. There are direct and indirect types of solar dryers.

The direct drying systems expose the product to the direct sunlight. The key processes here are heat transfer from the heating source to the target product and mass transfer of moisture from the product to the air. In these systems, the drying process is sometimes assisted by air flow, which helps to transport moisture convectively.

Figure 11.1

Source:A.K.Karthik; via Wikimedia Commons.

Indirect dryers use absorbing surfaces, which heat the air, which is further passed over the target product. So there is no direct contact of the solar radiation with the product in this design. In indirect systems, it is easier to protect the agricultural product from external damage (weather, birds, insects, direct radiation).   

The main idea of dryer system is to supply more heat to the process than would be available under ambient exposure and also to improve moisture migration via active or passive design.

Active and passive solar energy dryer designs are overviewed in the following reading source:

Solar desalination is an elegant combo since the solar heat, which in arid areas creates problems with fresh water shortages, is used to counter-act the problem. This idea has been under development and implementation in zones which will probably have to rely on desalination of seawater to meet their potable water needs, such as California, Israel, Arabian Peninsula, and some others.

One of the processes that can be used for desalination is water distillation. It is one of the oldest approaches when thermal energy is used to evaporate water from a saline solution and condense it in a separate collector. What is left behind is more concentrated brine solution or in case of complete evaporation? Solid salt. This process has been used for millennia for production of salt fromseawaterr. There are salt production facilities around the world that still use the same salt production techniques that were developed hundreds of years ago. The earliest use of solar distillation for obtaining fresh water is attributed to Aristotle (4th Century BC).

Example of a small scale self-made system is given in the figure below. In this design, black-backed water basin is absorbing solar radiation, and the evaporating water is condensed on the tilted dome or roof, further dripping down to the channel and directed  for collection. This is the same process that is used by nature to power the evaporation/condensation/rain/stream cycle, but in a contained and controlledenvironment.

Figure 11.2: Solar distillation still used to purify saline water

Source: Wikimedia Commons

While on the topic of the history of solar desalination, during World War II, the use of a portable solar still that was included in U.S. Air Force and Navy emergency kits saved many lives in the South Pacific. When airmen and seamen were stranded at sea for extended periods of time, survival was possible through the production of clean drinking water by a solar driven desalination process.

Today, there are many solar thermal desalination water production facilities around the world of much larger scale, but based on the same physical principles. Those may use direct sunlight or standard collector technologies to provide heat for the extracting of clean water from seawater or brackish water.

Learn more about solar distillation and evaporation processes in the following reading:

Distillation vs. Reverse Osmosis

Distillation is much cleaner process compared to the reverse osmosis technique used by many contemporary desalination plants. In this technology, water is pushed through the semipermeable membrane, which separates out the ions, macromolecules, particles, and other contaminants contained in water. One of the cons of this method is the need for pressurization, which is commonly achieved by applying conventional energy sources (electric high-pressure pumps, etc). Hence this technology is a potential contributor to carbon emissions if fossil fuels are used in powering the process. Its energy efficiency has been improving, so it is still the most widespread method of commercial desalination. Recent reports boast 3 kWh/m³ of processed water as energy rate. Another disadvantage of the reverse osmosis is waste water production. Not all water entering the system passes through the membrane - 15 to 85% (depending on the operating pressure) of it is rejected and sent to the wastewater stream. The necessity to dispose off the wastewater containing increased amounts of salts and contaminants is an additional problem, which adds to the cost and complexity of operation.

Passive vs. Active Solar Desalination

The passive methods of solar desalination do not use any heat collectors, but rather use the direct sunlight for evaporating the input water. Although this creates a limitation on the system efficiency and rate of fresh water production, the simplicity of design makes passive devices applicable at the field conditions at different scales (e.g Watercone^TM or Carocell^TM products etc.).

The active methods employ flat-plate, CPC, or parabolic trough collectors to provide higher temperature heat and enable faster distillation process. One of the examples is Aqua4 technology designed by WaterFX with the purpose of reclaiming process water and make businesses and communities less dependent on imported fresh water. The technology is modular, and each module combines a parabolic trough collector and steam-heated evaporator, which is able to produce fresh water with 90% efficiency as well as recover solid salt components.

You can learn more about this project from the video and WaterFX website:

The systems such as the one demonstrated in the video are classified among the indirect systems because there is not direct exposure of the input water to the sunlight. Instead the solar heat is absorbed by a solar collector and further used to run a disalination process. Learn more about the indirect desalination processes in the following reading: 

Solar Moisture Extraction 

There are relatively new methods being developed at the research level that use liquid desiccants to extract water from air. This does not exactly fall within the desalination field, but pursues the same purpose - providing potable water. Liquid desiccants, such as brine solutions (e.g. LiCl) are used to absorb moisture from air; then solar heat is used to regenerate the desiccant by evaporating water, which is collected as condensate. This technology is even applicable in very hot and dry climates (deserts). Source: Science Daily

Solar thermal energy can be used to drive chemical processes in such areas as fuel reforming, materials processing, detoxification, catalysis, and photolysis to name a few. This is not a complete list, and new ideas and technologies are constantly research and analyzed to take advantage of the available solar resource.

Here are a couple of examples:

Photolysis is the process in which solar radiation is directly absorbed by reagents to create a certain reaction product. For instance, photolysis can be used for the disinfection of clear water for drinking by placing contaminated water in clear plastic (PET) bottles in the sun for several hours of direct radiation. Through the combined use of increased temperature and ultraviolet radiation, water that contains pathogens, such as microorganisms or bacteria, will become potable as the pathogens are killed by the UV light and increased temperature over the course of several hours in full sun. This process is typically used at small scales for individual household use in the developing world, and is recommended by the World Health Organization for purifying and storing household water.

Photocatalysis uses solar radiation as a catalyst of a chemical reaction. In this case not necessarily solar thermal energy is the main input. Either visible light or ultraviolet components of the solar spectrum can be used to trigger electrochemical reactions or to increase the rate of reactions in the prosence of a catalyst. An example of photocatalysis is photo-catalytic oxidation (PCO). In this process visible and UV photos are combined with the nano-catalytic material to destroy volatile organic compounds, fungi, and bacteria. This approach is applied in sanitation and air purification.  

The following reading provides you with an overview if solar chemistry applications: 

Robinson Crusoe Problem

Scenario: pretend that as a result of ship wreckage you find yourself on an isolated island with plenty of sunshine, but with no fresh water supply. Luckily some materials and some tools are washed out to the shore that can be used for building a solar desalination still.  

Assignment: create a design for the solar desalination still, identify the materials, and make a corresponding schematic of the system.

Key Question: How large should be the water basin under the still to safely provide for your daily potable water needs? Support your answer with necessary calculations.

Tip: The property to determine is m_w - the hourly distillate output per square meter. Please use the chapter book resource referenced below to treat this problem analytically. You will need to find data or make educated assumptions on the system parameters and properties to include in this treatment. Please provide references to your sources.

Summary

This lesson reviwed some of the smaller size applications of solar thermal energy, which may create significant value in rural areas as sustainable technologies that do not require extensive infrastructure, fuels, or sophisticated materials for operation. Our consideration was mainly confined to solar drying and solar desalination as most widespread and most assessed processes. However, there are certainly more applications, such as chemical synthesis, washing and sterilization, timber treatment, brick curing, and others which comprise a long list of possible uses of solar heat. 

Double-check the to-do list on the Lesson 11 Introduction page to make sure you have completed all of the assigned tasks before you begin Lesson 12.

"Compared with other forms of renewable energy, solar heating’s contribution in meeting global energy demand is, besides the traditional renewable energies like biomass and hydropower, second only to wind power… In terms of cumulated installed capacity in operation solar thermal is almost equal with wind power by end of 2015"

Solar Thermal Energy (STE) is available in many different market sectors. The rate of growth is variable, and recent report show stiff competition of STE with emerging PV, wind, and other renewables. The purpose of this lesson is to give an overview of the current status of STE markets as well as trends for future development. In the dynamic energy market picture, in the recent years, we can see significant re-orientation of solar thermal towards smaller scale applications.

"In 2014, 94% of the energy provided by solar thermal systems worldwide was used for heating domestic hot water..." [Mauthner et al., 2016] 

Learning Objectives

• Recognize markets available for utility scale solar thermal energy.
• Recognize markets available for small-medium scale solar thermal.
• Differentiate markets by system attributes (i.e. scale).

This lesson will take us one week to complete. Specific directions for different assignments are given in the table below and within this lesson pages.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

What is the main rationale for detailed market analysis in solar energy field today?

Understanding the current state of the market and future trends will help with the following:

• Identifying key growth and investment opportunities in the global solar thermal market;
• Identifying key players and stake holders on market and their strategic moves;
• Making decisions with respect to technology development based on history and forecast data
• Develop business strategies based on the latest economy, customer base, and policy related to the solar thermal field
• Dealing with potential issues and restraints
• Minimizing economic risks

Solar thermal energy markets are diverse and range widely in their fields of application. We can identify six main market areas:

• Solar water heating
• Building heating
• Solar cooling
• Industrial process heat
• Thermal power systems
• Solar ponds

Each of these fields is briefly summarized below. Note that each field may have their own trends in market, which depend on local conditions, economy, and policies.

Solar water heating is performed all over the world and is one of the most widely used forms of solar thermal energy. Systems can be active with a mechanical pump, or passive with either a thermosyphon or geyser pump driving the fluid flow through the collector(s). Additionally, collectors can be constructed from a range of materials as simple as plastic sheets, laminated together with no glazing, for pool heating and as complex as multi-layered glass over black copper fins in a black aluminum enclosure.

Solar building heating can also be an active or passive activity. While active solar building heating is very similar in nature to solar water heating, passive solar building heating design results in some creative and interesting architectural requirements that are derived from the use of strategically placed thermal mass and the subsequent aperture requirements to effectively use a building as a solar thermal collector with high heat capacitance. Passive solar building design is a large field that combines meteorology, thermodynamics, fluid mechanics, and human thermal comfort. The Passive House (Passivhaus) Standard is the highest bar that is widely used for designing buildings that use significantly less energy than is required by most building codes. Solar heat gains are one of the required energy inputs to accomplish this level of energy reduction in a building.

Solar cooling has been a field of great interest for many people for many years. The concept of using heat from the sun to drive a cooling process that exists primarily because of that same source, the sun, is very desirable. This is because if solar radiation is not available over some time frame, presumably the loss of that source of heat reduces the cooling loads over that same time frame. Coupling the cooling source’s energy supply to a same driver of the cooling loads enables a balanced system design. Methods for solar cooling include desiccants, absorption chillers, mechanical systems such as Rankine engines, and passive systems.

Solar industrial process heat can range in use case from large textile factories to small rural agricultural processes and anything in between. Any industrial process that requires heat (particularly as steam) can be integrated with a solar thermal energy conversion system to increase the solar utility of that company or individual. This is particularly applicable in cases where a solar thermal system can supplement either an existing heat system such as steam or a process that does not need to run continuously and can instead run in batches when the sun is shining with sufficient intensity to generate the heat required.

Solar thermal power systems are one of the most publicized forms of solar thermal heat. Using solar thermal energy to generate high quality steam for use in a standard electricity generating steam turbine is happening world wide on the scale of hundreds of MWs of power, the largest of which, at 392 MW, is currently the Ivanpah Solar Power Facility in California that we read about in an earlier lesson. These types of systems have been under development for several decades now, and have now started crossing over the levelized cost of energy barrier through cost reductions and economies of scale.

Solar ponds can be used for multiple purposes ranging from active electrical power generation to distilling water and producing salt. Most of the solar pond research and testing for novel applications has occurred in Israel. One of the oldest solar driven evaporative processes is the production of salt, which occurs worldwide and accounts for about one third of the world’s annual salt production.

All of these STE markets have been capitalized on to some extent, but all of them have a lot more room to grow in both research as well as implementation. As energy costs continue to rise, solar energy of all forms may become more attractive in various locations. When working in any field, it is good to know what alternatives and options are available as both competitors in your sector (one solar thermal water heating system versus another) as well as alternative application of your core technology (solar thermal concentrators for power generation versus industrial processes).

When determining which types of STE systems are best for a certain area or application, several key pieces of information need to be identified:

• The temperature of targeted application
• The solar resource at the given locale
• Schedule for the required energy use.

With these three pieces of information, the system selection can be made relatively straightforward. Once the system types are known, the capital and operating costs can be evaluated and compared with alternative (conventional) fuel system costs.

To determine a required temperature, you may need to do some research on similar systems or to contact the facility management directly.

To assess the solar resource, there are a handful of tools available for free that can provide solar radiation information at varying levels of detail depending on your location. The most widely used meteorological information is a Typical Meteorological Year (TMY) file, which contains solar radiation data. While the TMY data does not contain inter-annual extremes, it does capture typical seasonal variation, which should be sufficient for any first level market assessment. TMY data is available for the whole world from the EnergyPlus website but is formatted for EnergyPlus. The data can be extracted manually using a spreadsheet or script but is not readily available on the web. Historical actual meteorological data, containing seasonal variations from year to year, can be obtained for the U.S. from solaranywhere.com. Basic solar resource information for Africa, Asia, and Europe can be obtained from PVGIS.

To assess the load schedule for a given solar thermal application, the existing schedule, requirements, seasonal and daily variations, and overall flexibility of the process need to be explored. This requires good understanding and some research on the targeted process.

The following table matches some applications with solar thermals technologies implemented. The data contained in the table are from Chapter 16 of Power From the Sun.

Solar heating and cooling, especially distributed energy systems, are considered especially promising in the context of strong emergence of PV solar technologies to the solar power markets. Interestingly, in 2014, 94% of the energy supplied by solar thermal systems worldwide was used for heating domestic hot water, mainly by small-scale systems, mainly including in single-family houses (68% of load) . Other larger applications such as multi-family houses, hotels, schools, accounted for ~27%. [Solar Heat Worldwide, 2016]. This niche is expected to expand.

This assignment is linked directly to your course project, which you are working on right now. Markets for solar thermal systems are very diverse, and current trends differ at different locations. In this lesson we reviewed some energy agency reports that list promising applications and analyze how the industry will develop in the near future. Now you need to narrow this information down to your locality. 

The Assignment

Prepare a Market Overview section for your Course Project Propsal. Include both global and local data, and discuss how the observed trends may affect the success of your project. The recommended length of the market review is <2 pages. The following sub-sections may be included: 

1. Brief history and current status of the STE industry and applications in your area and how it is related to the global  and country-side trends
2. Future promising areas of STE applications taking into account local businesses, policy, or politics
3. Stake holders that will play role in project, competitors, and other players
4. How your project fits in the targeted market situation

Summary

Markets for solar heat applications are very diverse, and experts predict significant promise for thermal systems in various areas of industry, utility, and residential energy sectors. Society and industry use a lot of heat, both low and high temperatures. As such, there is a lot of room for converting conventional heat systems over the NEW (yet surprisingly old and timeless) convention of solar thermal systems in hopes of increasing energy independence and overall sustainability of our society. 

You have reached the end of Lesson 12, which concludes the course content! Double-check the to-do list on the Lesson 12 Introduction page to make sure you have completed all of the activities listed there.

The remaining time of the semester will be spent on completing your course project proposal. Good luck!