9.9 Heat Pumps
9.9 Heat PumpsUnder natural circumstances, heat only flows from high temperatures to low temperatures. In order to move heat from a low temperature environment to a high temperature environment, work needs to be done (or rather energy needs to be spent).
A device that moves the heat from a low temperature environment to a high temperature environment is called a heat mover. Recall Lesson 6 when we learned about refrigerators, this is the same principal.
An example of a heat mover is a heat pump. A heat pump is a heating/cooling system and also a forced-air system. Cooled (and sometimes humidified or electronically cleaned) air is usually delivered through the same ductwork and registers used by heated air.
A heat pump uses air-conditioning principles to extract heat from one place and deliver it to another, and vice versa. In addition to expelling heat from indoors, the system can be reversed to heat the home in the winter. Thus, a heat pump is a device that moves heat from a low-temperature to a high-temperature environment with the help of work that is put in.
Heat pumps are classified based on the low-temperature heat source:
- Air-source heat pump or Air-to-air heat pump.
Heat is transferred from the low-temperature air outside to the high-temperature interior. - Ground-source heat pump or Ground-to-air heat pump.
The earth is used as a heat sink in the summer and a heat source in the winter; the pump relies on the relative warmth of the earth for its heating and cooling production. - Water-source heat pump or Water-to-air heat pump.
Heat is transferred from low-temperature water outside (from a pond or a lake) to a high-temperature interior.
Efficiency of a Heat Pump
Efficiency of a heat pump is measured using a term Coefficient of Performance (COP), and it is the ratio of the useful heat that is pumped to a higher temperature, to a unit amount of work that is put in. We will look at COP in terms of air-source heat pumps.
A general expression for the efficiency of a heat engine can be written as:
Using the same logic that was used for heat engines, this expression becomes:
Where, Q Hot = Heat input at high temperature and Q cold= Heat rejected at low temperature. The expression can be rewritten as:
Note: Thot and Tcold must be expressed in the Kelvin Scale.
9.9.1 Air Source Heat Pumps
9.9.1 Air Source Heat PumpsAn air-source or air-to-air heat pump can provide both heating and cooling.
- In the winter, a heat pump extracts heat from outside air and delivers it indoors.
- On hot summer days, it works in reverse, extracting heat from room air and pumping it outdoors to cool the house.
Text description of the Heat Pump image.
The image is divided into two parts. On the left, there is a photograph of an outdoor heat pump unit positioned next to a brick wall, with greenery in the background. The unit is rectangular, featuring a large round fan in a mesh-covered front and is raised slightly above the ground on four black legs.
On the right side of the image, there is a diagram illustrating the refrigeration cycle of a heat pump system. The diagram shows a house with arrows indicating the flow of refrigerant between the outdoor and indoor components, with distinguishing features such as a compressor, fan, outdoor coils, and indoor coils. The components are depicted in a colorful manner, with blue and red lines indicating the flow of gas and liquid along with directional arrows.
Nearly all air-source and air-to-air heat pumps are powered by electricity. They have an outdoor compressor/ condenser unit that is connected with refrigerant-filled tubing to an indoor air handler. As the refrigerant moves through the tubing of the system, it completes a basic refrigeration cycle, warming or cooling the coils inside the air handler. The blower pulls in room air, circulates it across the coils, and pushes the air through ductwork back into rooms.
When extra heat is needed on particularly cold days, supplemental electric-resistance elements kick on inside the air handler to add warmth to the air that is passing through.
The Balance Point
As we have learned, air-source and air-to-air heat pumps work by extracting heat from the outside air. These heat pumps require a backup system to supplement their heating ability when the outdoor temperature gets below a certain temperature.
As the outdoor temperature drops, the heating requirement of the house increases and the output of the heat pump decreases. At some point, the temperature of the home’s heating requirement and the heat pump output match. This temperature is called the balance point and usually falls between 30-45 degrees Fahrenheit. For any temperatures below the balance point, supplemental heat will be required.
To locate the balance point, the heating requirement (BTUs/h) of the house and the heat pump output (BTUs/h) are plotted against the changes in outside temperature. The place where the home heating requirement and heat pump output lines cross is the balance point.
Take a look at the graph of the Balance Point.
Text description of the Balance Point Graph.
The image is a line graph depicting the relationship between temperature and heating requirements, heat pump output, and supplemental heat. The x-axis represents temperature in degrees Fahrenheit, ranging from 0 to 80. The y-axis represents BTUs per hour, ranging from 0 to 70,000. Three lines intersect on the graph:
- A brown dashed line labeled "Heat Pump Output" begins near the bottom right and slopes upwards to the left, indicating increasing BTU output as the temperature decreases.
- A green solid line labeled "Home Heating Requirement" starts at the top left and slopes downwards to the right, showing decreasing BTU requirements as the temperature increases.
- A green shaded area labeled "Supplemental Heat" fills the upper left portion between the green solid line and the y-axis.
The "Balance Point" is marked where the brown and green lines intersect, indicating where the heat pump output meets the home heating requirement.
9.9.2 Ground Source Heat Pumps
9.9.2 Ground Source Heat PumpsGround-source or geothermal heat pumps (GHPs) are similar to the air-source heat pumps, except that the source of heat is the ground instead of outdoor air.
Closed-Looped Systems
Horizontal
The horizontal type of installation is generally most cost-effective for residential installations, particularly for new construction where sufficient land is available. It requires trenches at least four feet deep.
Horizontal systems come in two types of layouts, the two pipes method and the slinky method.
Two Pipes
The most common horizontal layouts include:
Two Pipes Layout (Option 1) - One pipe buried at six feet, and another pipe buried at four feet.
Two Pipes Layout (Option 2) - Both pipes placed side-by-side at five feet in the ground in a two-foot wide trench.
The Slinky™ Method
The pipe is looped to allow more pipes in a shorter trench, which cuts down on installation costs and makes horizontal installation possible in areas not possible with conventional horizontal applications. Large commercial buildings and schools often use vertical systems because the land area required for horizontal loops would be prohibitive
Vertical
This type of system may be used when the soil is too shallow for trenching or when one does not want to disturb the existing landscaping.
For a vertical system, holes (approximately four inches in diameter) are drilled about 20 feet apart and 100 to 400 feet deep. Into these holes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building
Benefits of GSHP
Click on the benefit listed below to find out more information.
- Low Energy Use
The biggest benefit of GHP's is that they use 20 to 50 percent less electricity than conventional heating or cooling systems. This translates into a GHP using one unit of electricity to move three units of heat from the earth.
According to a report by Oak Ridge National Laboratory, statistically valid findings show that the 4,003-unit GHP retrofit project at Fort Polk, LA, will save 25.8 million kilowatt-hours (kWh) in a typical year, or 32.5 percent of pre-retrofit whole-community electrical consumption. This translates to an average annual savings of 6,445 kWh per housing unit. In addition, 100 percent of the whole-community natural gas previously used for space conditioning and water heating (260,000 therms) will be saved.
In housing units that were all-electric in the pre-retrofit period, the GHP's were found to save about 42 percent of the pre-retrofit electrical consumption for heating, cooling, and water heating.
- Free or Reduced-Cost Hot Water
Unlike any other heating and cooling system, a geothermal heat pump can provide free hot water. A device called a super heater transfers excess heat from the heat pump's compressor to the hot water tank. In the summer, hot water is provided free; in the winter, water heating costs are cut roughly in half.
- Year-Round Comfort
While producing lower heating bills, geothermal heat pumps are quieter than conventional systems, and improve humidity control. These features help explain why customer surveys regularly show high levels of user satisfaction, usually well over 90 percent.
- Design Features
Geothermal heat pump systems allow for design flexibility, and can be installed in both new and retrofit situations. Because the hardware requires less space than that needed by conventional HVAC systems, the equipment rooms can be greatly scaled down in size, freeing space for productive use. Also, geothermal heat pump systems usually use the existing ductwork in the building and provide simultaneous heating and cooling without the need for a four-pipe system.
- Improved Aesthetics
Architects and building owners like the design flexibility offered by GHP's. Historic buildings like the Oklahoma State Capital and some Williamsburg, Virginia, structures use GHPs because they are easy to use in retrofit situations and easy to conceal, as they don't require cooling towers.
GHP systems eliminate conventional rooftop equipment, allowing for more aesthetically pleasing architectural designs and roof lines. The lack of rooftop penetrations also means less potential for leaks and ongoing maintenance, and better roof warranties. In addition, the aboveground components of a GHP system are inside the building, sheltering the equipment both from weather-related damage and from potential vandalism.
- Low Environmental Impact
Because a GHP system is so efficient, it uses a lot less energy to maintain comfortable indoor temperatures. This means that less energy is often created from burning fossil fuels than is needed to operate a GHP. According to the EPA, geothermal heat pumps can reduce energy consumption and corresponding emissions up to 44 percent compared to air-source heat pumps, and up to 72 percent compared to electric resistance heating with standard air conditioning equipment.
- Low Maintenance
According to a study completed for the Geothermal Heat Pump Consortium (GHPC), buildings with GHP systems had average total maintenance costs ranging from 6 to 11 cents per square foot, or about one third that of conventional systems. Because the workhorse part of the system, the piping, is underground or underwater, there is little maintenance required. Occasional cleaning of the heat exchanger coils and regularly changing the air filters are about all the work necessary to keep the system in good running order.
- Zone Heating and Cooling
These systems provide excellent zone space conditioning - different areas of the building heated or cooled to different temperatures simultaneously. For example, GHP systems can easily move heat from computer rooms (which need constant cooling) to the perimeter walls for winter heating in commercial buildings. School officials like the flexibility of heating or cooling just auditoriums or gymnasiums for special events rather than the entire school.
- Durability
Because GHP systems have relatively few moving parts, and because those parts are sheltered inside a building, they are durable and highly reliable. The underground piping often carries warranties of 25 to 50 years, and the GHP's often last 20 years or more.
- Reduced Vandalism
GHPs usually have no outdoor compressors or cooling towers, so the potential for vandalism is eliminated.
9.9.3 Water Source Heat Pumps
9.9.3 Water Source Heat PumpsPond
If a home has source surface water, such as a pond or lake, this type of loop design may be the most economical, since there is no need to dig a trench or a well for the pipes in the ground. In this type of system, the fluid circulates through polyethylene piping in a body of water, just as it does in the ground loops. The pipe may be coiled in a slinky shape to fit more of it into a given amount of space. This loop is recommended only if the water level never drops below six to eight feet at its lowest level, to assure sufficient heat-transfer capability. Pond loops used in a closed system result in no adverse impacts on the aquatic system.
Open-Loop Systems
This type of system uses well(s) or surface body water as the heat exchange fluid that circulates directly through the GHP system. Once it has circulated through the system, the water returns to the ground through the well, a recharge well, or a surface discharge. This option is obviously practical only where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met.
