Ground source heat pumps
Ground source heat pumps (GSHPs) are an innovative renewable energy technology designed for heating and cooling indoor spaces. Unlike conventional geothermal systems that utilize deep Earth heat for electricity or direct heating, GSHPs harness the stable geothermal energy located just below the Earth's surface, typically around 10 degrees Celsius (50 degrees Fahrenheit) year-round. These systems operate efficiently by transferring heat from the ground into buildings during winter and reversing the process to cool them in summer.
GSHPs consist of several key components, including a ground loop, heat exchanger, refrigerant compressor, and an air-handling system. They can be installed in various configurations, such as vertical or horizontal loops, depending on site-specific conditions and land availability. The benefits of GSHPs include high energy efficiency—typically 50 to 70 percent better than traditional fossil fuel systems—low noise, and minimal visual impact since their components are buried underground.
While GSHPs represent a growing sector in renewable energy, challenges such as high initial installation costs and the need for specialized designs remain. Nonetheless, their potential to reduce fossil fuel use and greenhouse gas emissions makes them an appealing option for enhancing energy security and sustainability. The technology is increasingly recognized as vital for smart-grid development, particularly in European markets, and its future growth will be supported by government incentives and public education.
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Ground source heat pumps
Summary: Ground source heat pumps are gaining use as a renewable energy technology that provides heating and cooling for indoor spaces.
Conventional geothermal heat originates deep beneath the Earth’s surface and is often tapped to generate electric power or used for direct heating. In contrast, a ground source heat pump (GSHP) makes productive use of the natural geothermal heat found just below Earth’s surface. Despite seasonal climate variations, the temperature of the Earth several feet belowground remains a relatively constant 10 degrees Celsius (50 degrees Fahrenheit) year-round. Like the inside of a cave, the ground stays warmer in winter and cooler in the summer than the air outside. During the winter, a heat pump operates by using a relatively small amount of electricity to “pump” heat from the ground outside into the building. During the summer, the same heat pump is run in reverse, removing heat from the warm indoor air and transferring it to the ground. Heat pumps are usually sized for either heating or cooling needs, depending on their primary use. While GSHPs are used mainly for homes, commercial buildings, and institutions such as schools, there are numerous other applications—for example, keeping ice rinks cold.
The first GSHPs gained popularity in the late 1940s. Technological advances in the 1980s resulted in better materials for making ground loop “earth connections,” along with improved efficiency. Nowadays, GSHPs represent a small but growing sector of the renewable heating and cooling market. Installations in the United States, for example, have quintupled since 1990. Europe is second to the United States in the deployment of GSHPs, with Sweden, Germany, Switzerland, France, Italy, and Austria out in front. Other countries using GSHPs include Canada, Turkey, Japan, Korea, and China. Interestingly, heat pump technology was used for the athletes’ village during the 2008 Beijing Olympics. The overall energy contribution to global heating and cooling from GSHPs in 2009 was around 15 thermal gigawatts.
A typical GSHP system consists of several components, including an exterior ground loop, a heat exchanger, a refrigerant-based compressor, and an air-handling system. The “closed” ground loop (also called earth-coupled or ground-coupled) is made of sturdy plastic pipe. Selection of a loop type depends on a number of factors: climate, soil composition, the presence of bedrock, and the amount of available land. Vertical loops, which are inserted into boreholes drilled to a depth of 100 to 400 feet, are the best choice when land area is limited. Typically, two vertical pipes are connected at the bottom to form a U shape. Clay or fine soil is used to backfill the loop for good thermal contact.
Horizontal loops are the most cost-effective for residential GSHP installations, especially for new construction. The piping is laid in trenches at a depth of 4 to 6 feet. Sometimes a coiled (or Slinky-shaped) pipe is employed when a more compact loop is desired. Coiled loops can also be located underwater by running a length of pipe underground to a nearby pond or lake. Regardless of their shape or heat source, closed loops are filled with a mixture of water and antifreeze that serves as the heat-exchange medium.
Alternatively, in open, or groundwater, loop heat pump systems, well water circulates through the pipe instead of a water-antifreeze mixture. Once the water has completed its path to the building’s heat-exchanger unit, it is returned to the ground. However, the exit water must meet local discharge codes.
An electrically driven compressor containing a refrigerant in combination with a heat-exchange unit concentrates the heat, which is then delivered to the air-handling system, consisting of a fan, air filter, and condensate removal system. Conventional ductwork directs the conditioned air to the building’s rooms, which may have their own temperature controls. An auxiliary unit, called a desuperheater, is sometimes installed to provide hot water. This unit works by transferring waste heat from the compressor to a hot-water tank.
Heating efficiency is generally 50 to 70 percent higher than with conventional fossil fuel systems. Efficiency is calculated using a coefficient of performance (COP), or the ratio of heat provided to a space compared with the amount of energy required to operate the heat pump. In the cooling mode, the energy efficiency ratio (EER) is calculated as the ratio of heat removed from a space compared with the energy input. The United States also uses a measurement where 1 ton is equal to the amount of British thermal units needed to melt 1 ton of ice in a 24-hour period.
The advantages of GSHPs include low noise and the absence of unsightly exterior units. Since the mechanical components are completely enclosed, potential damage from extreme weather and vandalism is minimized. Use of GSHPs can also help reduce peak demand on the electrical grid. Energy cost savings can be realized over the lifetime of the heat pump. Environmental benefits include reduced use of fossil fuels and a corresponding reduction in greenhouse gas emissions. Disadvantages include the requirement for site-specific loop design and the high initial costs of borehole drilling and trenching. Buildings with GSHPs also require an emergency backup system.
Globally, the heating and cooling of buildings accounted for about 25 percent of all energy demand in the 2020s. Therefore, use of a renewable energy source such as geothermal heat—a virtually inexhaustible resource—is an attractive option for countries that have set goals for energy security and environmental sustainability. Novel hybrid configurations are also emerging—for instance, the integration of heat pumps with solar collectors. In many European countries, heat pumps are considered an enabling technology for the planning of smart-grid infrastructures. Future market growth will depend on the availability of government incentives, education for public awareness, and the training and certification of a range of skilled professionals, such as architects, design engineers, and installers.
Bibliography
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