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Geoexchange |
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Because seasonal temperature variations do not reach very deeply into the earth, engineers can sometimes use the relatively consistent temperatures of deep earth for the heating and cooling of buildings. The most common way to make use of the relatively constant temperatures is through the use of ground-source heat pumps (GSHP), also known as geothermal heat pumps or geoexchange heat pumps.
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Contents
1.Definition
2. Use/Application
a. Established Techniques
b. Emerging Trends
3. Use an Integrated Approach
4. Resources
5. Associated Strategies
6. Case Studies |
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Definition |
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Simple schematic of a vapor compression cycle used by heat
pumps for heating and cooling of (1) Condenser,
buildings. (2) Expansion device, (3) Evaporator, (4) Compressor.
| Because seasonal temperature variations do not reach very deeply into the earth, engineers can sometimes use the relatively consistent temperatures of deep earth for the heating and cooling of buildings. The most common way to make use of the relatively constant temperatures is through the use of ground-source heat pumps (GSHP), also known as geothermal heat pumps or geoexchange heat pumps.
A heat pump is a device that moves thermal energy from one location to another. A refrigerator is a form of heat pump; it removes energy from inside the refrigerator and discharges it into the surrounding area. Air conditioners function in a similar manner. Heat pumps used in buildings, however, generally provide both heating and cooling. During winter, a ground-source heat pump will move heat from the ground into the building; during summer, the GSHP will move heat from the building into the ground.
Almost all heat pumps operate using a vapor compression cycle. This is very similar in concept to what is used in air conditioners or refrigerators. A simple schematic of a vapor compression cycle is shown in the figure above. A compressor (4) compresses refrigerant vapor so it becomes hot, high-pressure gas. The hot gas moves to the condenser (1) where the heat is removed and the refrigerant condenses (becomes liquid). This high-pressure liquid refrigerant is then directed through an expansion device (2) where the refrigerant pressure and temperature are lowered dramatically. This cold liquid then enters the evaporator (3) where it absorbs heat and vaporizes. This vapor then returns to the compressor (4) and begins the cycle again.
In a building cooled by a ground-source heat pump, heat is removed from the building by the evaporator and directed to the ground via the condenser. During heating season, the cycle is reversed; heat is removed from the ground by the evaporator and delivered to the building by the condenser.
Ground-source heat pumps represent opportunities for reducing carbon emissions in two main ways. First, if well-designed applications, ground-source heat pumps can offer energy cost savings relative to more conventional heating and cooling strategies (air-cooled chillers, air conditioners, boilers, etc.). Second, GSHPs offer an effective way to meet both space heating and cooling loads without use of fossil fuels on site. If electricity that powers the heat pumps is obtained sustainably, heat pumps can have very low net carbon emissions.
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Use / Application |
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Established Techniques |
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In most instances, refrigerant is not plumbed directly to the ground. Usually water or an antifreeze solution is pumped from wells or through buried heat exchangers to transfer heat to and from the ground. Because conductivity of ground can vary greatly, knowing ground conditions is important for the sizing of ground heat exchangers.
Because removing or injecting large amounts of heat into the ground over a season can significantly change temperatures of the ground fields, ground-source heat pumps may work best when the heating and cooling loads of a building are fairly balanced.
Ground-source heat pumps are generally divided into two categories with respect to interactions with the ground: open loop and closed loop. In an open loop system, ground water is pumped from a well, heat is removed or added by the heat pump, and the water is run back into the ground. Sometimes water is run back into the same well; other times a drywell nearby is used. Open loop applications require a steady flow of ground water, and they often work best if the wells can tap an aquifer with relatively constant water temperatures.
 
Simple diagrams of a closed-loop vertical geothermal field
(left) and a closed-loop horizontal geothermal field (right).
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Closed-loop systems do not make use of ground water directly; rather a water or antifreeze solution moves between the heat pump and heat exchangers within the ground. Sometimes these heat exchangers are run vertically through deep wells; other times coils of pipe are run in coils horizontally. Closed-loop systems also work more efficiently with high ground conductivity, and they are sometimes preferred over open loop for other environmental reasons (such as depletion of aquifers or dumping process water).
Inside the buildings, there are typically two types of heat pumps: water to air and water to water. In the winter, water-air heat pumps remove heat from ground water and send it to the building air directly – typically in an air handler or fan coil. Water-water heat pumps remove heat from ground water and use it to heat separate process water. This warm water can then be used in several ways: heating a building through a hydronic system (e.g., radiant floors), heating air through a hydro coil, heating service water, etc. During cooling season, the systems run in reverse: water-air systems cool air directly; water-water systems chill water for cooling applications.
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Emerging Trends |
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Some designers have been very creative in combining ground-source technology with other building systems. On the most basic level, ground-source heat pumps are sometimes combined with auxiliary heating systems (e.g., gas or electricity). If ground conditions are very cold, it may be more costly to heat a building with the heat pump than with an efficient gas boiler, for example. This strategy of using other heat sources to meet peak winter loads can sometimes save considerable money in the up-front cost of the heat pump equipment and ground field.
Other engineers have incorporated ways to augment temperatures of ground water. A heat pump will work much more efficiently moving heat from 60°F water than 50°F water, so boosting temperatures of the ground loop can dramatically affect heat pump performance. Some designs use solar energy to heat ground water, where others use waste heat from other building processes.
Some ground-source heat pumps do not use a ground water at all. “Direct exchange” systems actually run refrigerant from the heat pump to the ground heat exchanger. While eliminating the extra heat exchange step (ground to water, water to refrigerant) can improve efficiencies, the added cost for copper piping and refrigerant can be considerable.
If you are considering the use of GSHP, you should consult with an engineer to evaluate the soil and hydrology early in the design phase. |
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Use an Integrated Approach |
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A new way of thinking must be adopted in order to meet the goal of reducing carbon emissions associated with buildings. Your solutions can begin by integrating four possible methods. None works alone, and they are not all relevant in considering every strategy. However, considering the following tactics is necessary:
- Reduce the overall energy use in your building.
- Incremental cost for ground-source heat pumps can be substantial. However, smaller, less-costly systems can be specified to meet smaller building loads.
- As with most heating and cooling equipment, heat pumps can operate more efficiently when well-matched to a small load.
- Specify energy-efficient equipment and technologies.
- GSHP is one option for efficient equipment and technology.
- Use renewable strategies and purchase green power.
- Renewable electricity generated on site or purchased can be used to power heat pumps.
- Educate building owners, operators, and occupants.
- Proper control and maintenance of ground source heat pumps will ensure operation at higher efficiencies
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Last modified at 2/27/2009 11:36 PM by jamie nace
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