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.