geothermal heat pump

A geothermal heat pump (GHP) is a heating and cooling system that takes advantage of the relatively stable moderate temperature conditions within the first 300 meters (1,000 feet) below Earth’s surface to heat a building in the winter and cool it in the summer. Unlike boilers or furnaces, GHPs do not rely on the combustion of fossil fuels to produce heat, and thus they produce no direct emissions of greenhouse gases. Moreover, because they are unaffected by outside air temperatures, they are significantly more efficient than traditional air conditioners for cooling and can work well in almost all climates. Although GHPs use passive geothermal energy—drawing on a renewable energy source—the systems also require electricity, which may come from renewable or nonrenewable sources, depending on a system’s location. GHP systems that are powered by renewable energy are increasingly seen as an important tool in the fight against anthropogenic climate change, and even those that use electricity generated from fossil fuels are a greener alternative to many other heating and cooling systems.

Mechanism and design

Most GHPs are installed within 6 meters (about 20 feet) of Earth’s surface, where the ground maintains a near-constant temperature of 10 to 16 °C (50 to 60 °F). Consequently, that heat can be used to help warm buildings during the colder months of the year, when the temperature of the air falls below that of the ground. Similarly, during the warmer months of the year, warm air can be drawn from a building and circulated underground, where it loses much of its heat, and returned. Thus, heat—either from the ground or from the building—can be pumped in either direction for heating or cooling, as desired. GHPs can be added to new construction or installed retroactively in existing buildings in rural to urban environments. Large GHP systems can even be implemented as a network to serve an entire new community or larger construction, such as a college campus or an industrial park.

A GHP system is made up of a heat exchanger (a vertical or horizontal loop of pipes buried in the ground), a pump, and a distribution system for the heated or cooled air (typically conventional ductwork). The heat exchanger transfers heat energy between the ground and the air at the surface by means of a fluid that circulates through the pipes. The fluid used is often water or a combination of water and antifreeze. Some so-called open systems circulate water from a nearby water source, such as a pond, and then return it. In closed systems the water or antifreeze mixture stays in constant circulation. During warmer months heat from the building’s warm air is transferred to the heat exchanger and into the fluid. As it moves through the pipes, the heat is dispersed to the rocks, soil, and groundwater. The pump is reversed during the colder months. Heat energy stored in the relatively warm ground raises the temperature of the fluid. The fluid then transfers this energy to the heat pump, which warms the air inside the building. Some GHP systems can also be used as a water heater and provide hot water to the building.

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geothermal energy: Geothermal heat pumps

Advantages and disadvantages

GHPs have several advantages over more-conventional heating and air-conditioning systems. They are very efficient, using 25–50 percent less electricity than comparable conventional heating and cooling systems, and they produce less pollution. The reduction in energy use associated with GHPs can translate into as much as a 44 percent decrease in greenhouse gas emissions compared with air-source heat pumps (which transfer heat between indoor and outdoor air). Compared with air-source heat pumps, GHP systems are generally quieter, require less maintenance, last longer, and function independently of the temperature of the outside air. In addition, compared with electric resistance heating systems (which convert electricity to heat) coupled with standard air-conditioning systems, GHPs can produce up to 72 percent less greenhouse gas emissions.

In addition, GHPs have a very minimal effect on the environment because they make use of shallow geothermal resources. GHPs cause only small temperature changes to the groundwater or rocks and soil in the ground. In closed-loop systems the ground temperature around the vertical boreholes is slightly increased or decreased; the direction of the temperature change is governed by whether the system is dominated by heating (which would be the case in colder regions) or cooling (which would be the case in warmer regions). With balanced heating and cooling loads, the ground temperatures remain stable. Likewise, open-loop systems using groundwater or lake water have very little effect on ground temperature, especially in regions characterized by high groundwater flows.

Given that most homes and businesses already have heating and cooling systems, a major drawback to GHPs is the installation costs, which can be several times those of air-source systems of the same capacity. However, depending on the cost of energy in an area, the upfront costs may be recuperated as energy savings over time. In addition, incentives and rebate programs for GHPs are available in some places. Another potential disadvantage of GHPs is that open-loop systems, which are less common than closed-loop systems, may contaminate groundwater and are not permitted in all areas.

A dual-source heat pump is generally less expensive than a comparable GHP unit. It combines an air-source heat pump with a geothermal heat pump. While dual-source heat pumps have higher efficiency ratings than air-source units, they are less efficient than GHPs.

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