Geothermal heat pump

A geothermal heat pump, ground source heat pump (GSHP), or ground heat pump[1] is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer) (IBM ThinkPad X41 Tablet battery). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems, and may be combined with solar heating to form a geosolar system with even greater efficiency(IBM ThinkPad R51 battery). Geothermal heat pumps are also known by a variety of other names, including geoexchange, earth-coupled, earth energy or water-source heat pumps. The engineering and scientific communities prefer the terms "geoexchange" or "ground source heat pumps" to avoid confusion with traditional geothermal power(IBM ThinkPad R60 battery), which uses a high temperature heat source to generate electricity. Ground source heat pumps harvest a combination of geothermal energy (from the Earth's core) and solar energy (heat absorbed at the Earth's surface) when heating, but work against these heat sources when used for air conditioning(IBM ThinkPad R50 battery).

Depending on latitude, the upper 3 metres (9.8 ft) of Earth's surface maintains a nearly constant temperature between 10 and 16 °C (50 and 60 °F). Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat from there. Heat pumps can transfer heat from a cool space to a warm space(FUJITSU LIFEBOOK P2120 Battery), against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cool one. The core of the heat pump is a loop of refrigerant pumped through a vapor-compression refrigeration cycle that moves heat(Fujitsu Lifebook S6120 battery). Heat pumps are always more efficient at heating than pure electric heaters, even when extracting heat from cold winter air. But unlike an air-source heat pump, which transfers heat to or from the outside air, a ground source heat pump exchanges heat with the ground(COMPAQ Presario R3000 Battery). This is much more energy-efficient because underground temperatures are more stable than air temperatures through the year. Seasonal variations drop off with depth and disappear below seven meters due to thermal inertia(COMPAQ Presario 2100 Battery). Like a cave, the shallow ground temperature is warmer than the air above during the winter and cooler than the air in the summer. A ground source heat pump extracts ground heat in the winter (for heating) and transfers heat back into the ground in the summer (for cooling) (COMPAQ Presario M2000 Battery). Some systems are designed to operate in one mode only, heating or cooling, depending on climate.

The geothermal pump systems reach fairly high Coefficient of performance (CoP), 3-6, on the coldest of winter nights(ASUS Eee PC 1000HE Battery), compared to 1.75-2.5 for air-source heat pumps on cool days. Ground source heat pumps (GSHPs) are among the most energy efficient technologies for providing HVAC and water heating. Actual CoP of a geothermal system which includes the power required to circulate the fluid through the underground tubes can be lower than 2.5(ASUS Eee PC 900 Battery). The setup costs are higher than for conventional systems, but the difference is usually returned in energy savings in 3 to 10 years. System life is estimated at 25 years for inside components and 50+ years for the ground loop(ASUS A3000 Battery). As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity, with an annual growth rate of 10%.

Differing terms and definitions

There is a great deal of controversy and confusion with regard to exactly what geothermal heat pumps do(ACER Aspire 5020 Battery). There are several concepts commonly attached to the idea of geothermal:

Using geologically hot rocks, which have little relationship to the surface climate and derive their heat from deep in the earth, to run a heat engine which produces electricity. Such a system can be operated only until the rock around the bore cools(ACER Aspire 3000 Battery), then it gradually loses its generating ability. All of these systems are in tectonically or volcanically active areas. Most people are pretty clear that this should be called "geothermal power".

Using geologically hot rocks to heat some type of liquid or gas which is pumped up to be used to heat a building is often called "geothermal heating"(ACER Aspire 3020 Battery).

Using a heat exchanger with a finite amount of external material to incorporate additional thermal mass to a building. This makes the building change temperature slowly, and allows the inhabitants to go through a time period with less overall temperature variation(ACER Travelmate 2300 Battery). This is the main focus of this article, and many terms have been applied. The most common ones appear to be "geothermal heat pump" by laymen and "ground-source heat pump" by experts, but even these are broad, barely understood terms about which there is no consensus(Toshiba NB200 battery).

Builders may try to smooth out the indoor climate over surface temperature variations resulting from the day-night cycle, variations due to short-term weather patterns, or variations due to entire seasons(Toshiba NB100 battery). The amount of thermal mass incorporated is on a spectrum, so one cannot say their system addresses any of these cycles specifically – a system sized for day-night cycling will still help somewhat in a week-long blizzard. Such a system requires power to pump the coolant, but can be operated indefinitely(Toshiba PA3641U-1BAS battery).

To further complicate things, even though most home-sized systems termed "geothermal" operate primarily on the former principle, the thermal mass in such systems is rarely perfectly finite and closed(Toshiba PA3640U-1BRS battery). Groundwater flows through the area, and heat leaks out and warms/cools the surrounding area. True geothermal heat may play a small or large role in such systems.

When trying to explain this subject, experts may go through a series of explanations and divisions.

First, people separate out terms for geothermal electricity generation(Toshiba PA3640U-1BAS battery):

geothermal power

Then, they split out geothermal heating, which is commonly used in tectonically or volcanically active regions:

geothermal heating

Then, they explain the traditional concept of a heat pump which uses only inside and outside air(Toshiba PA3591U-1BAS battery):

heat pump

After that, they try to identify simple systems in which the coolant is air which is pumped directly out of and back into the building, going through a simple hole in the ground:

earth tube or earth air heat exchanger(Toshiba PA3591U-1BRS battery)

ground-coupled heat exchanger

After that, they remove systems which depend on large quantities of water or wet ground, primarily for cooling:

lake water cooling

deep water source cooling

At this point they may explain the concept of a seasonal thermal store or a thermal mass climate control strategy(Toshiba PA3535U-1BAS battery):

trombe wall

seasonal thermal store

thermal mass

Then, they may try to figure out the size of the system. Is it targeted at a home? A building? Is it a full-scale district heating system?

Then they go into the specifics of the system. First, is the coolant water, and if so is it "open loop" – exposed to groundwater – or "closed loop" – not exposed(Toshiba PA3506U-1BRS battery).

Are other energy sources helping? Is solar absorbed from the house or from a dedicated thermal collector?

annualized geothermal solar or annualized geo solar

geosolar or solar combisystem

After this they concentrate on the specific form factor of the system. Is it a grid of pipes buried 3 feet (0.91 m) underneath the owner's garden? Does it consist of a hundred-foot borehole? A thousand-foot borehole? Dozens of 8-foot (2.4 m) boreholes(Toshiba PA3480U-1BRS battery)?

downhole heat exchanger or borehole heat exchanger

Finally they may try to decide what the locals call the system, as identical systems are often called different things in different countries, and in some countries generic terms may be trademarked in others(Toshiba PA3478U-1BRS battery):

Geoexchange is a trademarked product in the US, but is a standards coalition in Canada.

Earth tubes, Air-earth heat exchangers and "heat exchanger" in general, appear to be primarily used in the UK(Toshiba PA3476U-1BRS battery).

History

The heat pump was described by Lord Kelvin in 1853 and developed by Peter Ritter von Rittinger in 1855. After experimenting with a freezer, Robert C. Webber built the first direct exchange ground-source heat pump in the late 1940s(Toshiba PA3475U-1BRS battery). The first successful commercial project was installed in the Commonwealth Building (Portland, Oregon) in 1946, and has been designated a National Historic Mechanical Engineering Landmark by ASME. The technology became popular in Sweden in the 1970s, and has been growing slowly in worldwide acceptance since then(Toshiba PA3383U-1BRS battery). Open loop systems dominated the market until the development of polybutylene pipe in 1979 made closed loop systems economically viable. As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity(Toshiba PA3356U-1BRS battery). Each year, about 80,000 units are installed in the USA (geothermal energy is used in all 50 U.S. states today, with great potential for near-term market growth and savings) and 27,000 in Sweden(Toshiba PA3395U-1BRS battery ).

Ground heat exchanger

Heat pumps provide wintertime heating by extracting heat from a source and transferring it to the building. In theory, heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency. A ground source heat pump uses the shallow ground as a source of heat, thus taking advantage of its seasonally moderate temperatures(Toshiba PA3285U-1BRS battery ).

In the summer, the process can be reversed so the heat pump extracts heat from the building and transfers it to the ground. Transferring heat to a cooler space takes less energy, so the cooling efficiency of the heat pump gain benefits from the lower ground temperatures(Toshiba PA3450U-1BRS battery).

Shallow horizontal heat exchangers experience seasonal temperature cycles due to solar gains and transmission losses to ambient air at ground level. These temperature cycles lag behind the seasons because of thermal inertia, so the heat exchanger can harvest heat deposited by the sun several months earlier(Toshiba PA3465U-1BRS battery). Deep vertical systems rely heavily on migration of heat from surrounding geology, unless they are recharged annually by exhaust heat from air conditioning.

Ground source heat pumps must have a heat exchanger in contact with the ground or groundwater to extract or dissipate heat. This component accounts for a third to a half of the total system cost(Toshiba PA3285U-1BRS battery). Several major design options are available for these, which are classified by fluid and layout. Direct exchange systems circulate refrigerant underground, closed loop systems use a mixture of anti-freeze and water, and open loop systems use natural groundwater(Toshiba PA3399U-1BRS battery).

Direct exchange

The Direct exchange geothermal heat pump is the oldest type of geothermal heat pump technology. It is also the simplest and easiest to understand. The ground-coupling is achieved through a single loop circulating refrigerant in direct thermal contact with the ground (Toshiba PA3399U-2BRS battery) (as opposed to a combination of a refrigerant loop and a water loop). The refrigerant leaves the heat pump appliance cabinet, circulates through a loop of copper tube buried underground, and exchanges heat with the ground before returning to the pump(Toshiba PA3534U-1BRS battery). The name "direct exchange" refers to heat transfer between the refrigerant and the ground without the use of an intermediate fluid. There is no direct interaction between the fluid and the earth; only heat transfer through the pipe wall(Toshiba PA3535U-1BRS battery). Direct exchange heat pumps are not to be confused with "water-source heat pumps" or "water loop heat pumps" since there is no water in the ground loop. ASHRAE defines the term ground-coupled heat pump to encompass closed loop and direct exchange systems, while excluding open loops(Dell Studio 1457 battery).

Direct exchange systems are significantly more efficient and have potentially lower installation costs than closed loop water systems. Copper's high thermal conductivity contributes to the higher efficiency of the system(Dell Inspiron 1764 battery), but heat flow is predominantly limited by the thermal conductivity of the ground, not the pipe. The main reasons for the higher efficiency are the elimination of the water pump (which uses electricity) (Dell Inspiron 1564 battery), the elimination of the water heat exchanger (which is a source of heat losses), and most importantly, the latent heat phase change of the refrigerant in the ground itself.

While they require much more refrigerant and their tubing is more expensive per foot, a direct exchange loop is shorter than a closed water loop for a given capacity(Dell Inspiron 1464 battery). A direct exchange system requires only 15 to 30% of the length of tubing and half the diameter of drilled holes, and the drilling or excavation costs are therefore lower. Refrigerant loops are less tolerant of leaks than water loops because gas can leak out through smaller imperfections(Dell Inspiron 1320N battery). This dictates the use of brazed copper tubing, even though the pressures are similar to water loops. The copper loop must be protected from corrosion in acidic soil through the use of a sacrificial anode or cathodic protection(Dell Studio 1450 battery).

Closed loop

Most installed systems have two loops on the ground side: the primary refrigerant loop is contained in the appliance cabinet where it exchanges heat with a secondary water loop that is buried underground(Dell Inspiron 1320 battery). The secondary loop is typically made of High-density polyethylene pipe and contains a mixture of water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the internal heat exchanger, the water flows through the secondary loop outside the building to exchange heat with the ground before returning(Dell Latitude E6500 battery). The secondary loop is placed below the frost line where the temperature is more stable, or preferably submerged in a body of water if available. Systems in wet ground or in water are generally more efficient than drier ground loops since it is less work to move heat in and out of water than solids in sand or soil(Dell Latitude E6400 battery). If the ground is naturally dry, soaker hoses may be buried with the ground loop to keep it wet.

Closed loop systems need a heat exchanger between the refrigerant loop and the water loop, and pumps in both loops. Some manufacturers have a separate ground loop fluid pump pack(Dell Studio 1737 battery), while some integrate the pumping and valving within the heat pump. Expansion tanks and pressure relief valves may be installed on the heated fluid side. Closed loop systems have lower efficiency than direct exchange systems, so they require longer and larger pipe to be placed in the ground, increasing excavation costs(Dell Studio 1735 battery).

Closed loop tubing can be installed horizontally as a loop field in trenches or vertically as a series of long U-shapes in wells(see below). The size of the loop field depends on the soil type and moisture content(Dell Vostro 1710 battery), the average ground temperature and the heat loss and or gain characteristics of the building being conditioned. A rough approximation of the initial soil temperature is the average daily temperature for the region.

Vertical

A vertical closed loop field is composed of pipes that run vertically in the ground. A hole is bored in the ground(Dell Inspiron 1501 battery), typically 75 to 500 feet (23–150 m) deep. Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole. The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a thermal connection to the surrounding soil or rock to improve the heat transfer(Dell Inspiron 6000 battery). Thermally enhanced grouts are available to improve this heat transfer. Grout also protects the ground water from contamination, and preventsartesian wells from flooding the property. Vertical loop fields are typically used when there is a limited area of land available(Dell INSPIRON 1525 Battery). Bore holes are spaced at least 5–6 m apart and the depth depends on ground and building characteristics. For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need three boreholes 80 to 110 m (260 to 360 ft) deep(Dell N3010 battery). (A ton of heat is 12,000British thermal units per hour (BTU/h) or 3.5 kilowatts.) During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil. Reliable heat transfer models have been developed through sample bore holes as well as other tests(Dell RM791 battery).

Horizontal

A horizontal closed loop field is composed of pipes that run horizontally in the ground. A long horizontal trench, deeper than the frost line, is dug and U-shaped or slinky coils are placed horizontally inside the same trench(Dell Latitude D620 Battery). Excavation for horizontal loop fields is about half the cost of vertical drilling, so this is the most common layout used wherever there is adequate land available. For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need 3 loops 120 to 180 m (390 to 590 ft) long of NPS 3/4 (DN 20) or NPS 1.25 (DN 32) polyethylene tubing at a depth of 1 to 2 m (3.3 to 6.6 ft) (Dell Inspiron E1505 Battery).

As an alternative to trenching, the horizontal loop field may be laid by mini horizontal directional drilling. (mini-HDD) This technique can lay piping under yards, driveways or other structures without disturbing them, with a cost between those of trenching and vertical drilling(Dell INSPIRON 1420 Battery).

A slinky (also called coiled) closed loop field is a type of horizontal closed loop where the pipes overlay each other (not a recommended method). The easiest way of picturing a slinky field is to imagine holding a slinky on the top and bottom with your hands and then move your hands in opposite directions(HP PAVILION DV3000 Battery). A slinky loop field is used if there is not adequate room for a true horizontal system, but it still allows for an easy installation. Rather than using straight pipe, slinky coils, use overlapped loops of piping laid out horizontally along the bottom of a wide trench. Depending on soil, climate and your heat pump's run fraction( HP PAVILION DV3 Battery), slinky coil trenches can be anywhere from one third to two thirds shorter than traditional horizontal loop trenches. Slinky coil ground loops are essentially a more economic and space efficient version of a horizontal ground loop(HP PAVILION DV2000 Battery).

Pond

A closed pond loop is not common because it depends on proximity to a body of water, where an open loop system is usually preferable. A pond loop may be advantageous where poor water quality precludes an open loop(HP PAVILION DV2 Battery), or where the system heat load is small. A pond loop consists of coils of pipe similar to a slinky loop attached to a frame and located at the bottom of an appropriately sized pond or water source(Sony VAIO PCG-5K1L battery).

Open loop

In an open loop system (also called a groundwater heat pump), the secondary loop pumps natural water from a well or body of water into a heat exchanger inside the heat pump. ASHRAE calls open loop systems groundwater heat pumps or surface water heat pumps, depending on the source(Sony VAIO PCG-6W2L battery). Heat is either extracted or added by the primary refrigerant loop, and the water is returned to a separate injection well, irrigation trench, tile field or body of water. The supply and return lines must be placed far enough apart to ensure thermal recharge of the source(Sony VAIO PCG-7112L battery). Since the water chemistry is not controlled, the appliance may need to be protected from corrosion by using different metals in the heat exchanger and pump. Limescale may foul the system over time and require periodic acid cleaning(Sony VAIO PCG-8Z1L battery). Also, as fouling decreases the flow of natural water, it becomes difficult for the heat pump to exchange building heat with the groundwater. If the water contains high levels of salt, minerals, iron bacteria or hydrogen sulfide, a closed loop system is usually preferable(Sony VAIO PCG-8Z2L battery).

Deep lake water cooling uses a similar process with an open loop for air conditioning and cooling. Open loop systems using ground water are usually more efficient than closed systems because they are better coupled with ground temperatures. Closed loop systems, in comparison, have to transfer heat across extra layers of pipe wall and dirt(Sony VAIO PCG-8Y2L battery).

A growing number of jurisdictions have outlawed open-loop systems that drain to the surface because these may drain aquifers or contaminate wells. This forces the use of more environmentally sound injection wells(Sony VAIO PCG-8Y1L battery).

Standing column well

A standing column well system is a specialized type of open loop system. Water is drawn from the bottom of a deep rock well, passed through a heat pump, and returned to the top of the well, where traveling downwards it exchanges heat with the surrounding bedrock(Sony VAIO PCG-7Z2L battery). The choice of a standing column well system is often dictated where there is near-surface bedrock and limited surface area is available. A standing column is typically not suitable in locations where the geology is mostly clay, silt, or sand. If bedrock is deeper than 200 feet (61 m) from the surface, the cost of casing to seal off the overburden may become prohibitive(Sony VAIO PCG-7Z1L battery).

A multiple standing column well system can support a large structure in an urban or rural application. The standing column well method is also popular in residential and small commercial applications(Sony VAIO PCG-7133L battery). There are many successful applications of varying sizes and well quantities in the many boroughs of New York City, and is also the most common application in the New England states. This type of ground source system has some heat storage benefits(Sony VAIO PCG-7113L battery), where heat is rejected from the building and the temperature of the well is raised, within reason, during the Summer cooling months which can then be harvested for heating in the Winter months, thereby increasing the efficiency of the heat pump system(Sony VAIO PCG-6W3L battery). As with closed loop systems, sizing of the standing column system is critical in reference to the heat loss and gain of the existing building. As the heat exchange is actually with the bedrock, using water as the transfer medium(Sony VAIO PCG-7111L battery), a large amount of production capacity (water flow from the well) is not required for a standing column system to work. However, if there is adequate water production, then the thermal capacity of the well system can be enhanced by discharging a small percentage of system flow during the peak Summer and Winter months(Sony VAIO PCG-6W1L battery).

Since this is essentially a water pumping system, standing column well design requires critical considerations to obtain peak operating efficiency. Should a standing column well design be misapplied, leaving out critical shut-off valves for example(Sony VAIO PCG-6V1L battery), the result could be an extreme loss in efficiency and thereby cause operational cost to be higher than anticipated.

Building distribution

The heat pump is the central unit that becomes the heating and cooling plant for the building. Some models may cover space heating, space cooling, (space heating via conditioned air, hydronic systems and / or radiant heating systems) (Sony VAIO PCG-6S3L battery), domestic or pool water preheat (via the desuperheater function, demand hot water, and driveway ice melting all within one appliance with a variety of options with respect to controls, staging and zone control. The heat may be carried to its end use by circulating water or forced air(Sony VAIO PCG-6S2L battery). Almost all types of heat pumps are produced for commercial and residential applications.

Liquid-to-air heat pumps (also called water-to-air) output forced air, and are most commonly used to replace legacy forced air furnaces and central air conditioning systems. There are variations that allow for split systems(Sony VAIO PCG-5L1L battery), high-velocity systems, and ductless systems. Heat pumps cannot achieve as high of a fluid temperature as a conventional furnace, so they require a higher volume flow rate of air to compensate. When retrofitting a residence, the existing duct work may have to be enlarged to reduce the noise from the higher air flow(Sony VAIO PCG-5K2L battery).

Liquid-to-water heat pumps (also called water-to-water) are hydronic systems that use water to carry heating or cooling through the building. Systems such as radiant underfloor heating(Sony VAIO PCG-5J2L battery), baseboard radiators, conventional cast iron radiators would use a liquid-to-water heat pump. These heat pumps are preferred for pool heating or domestic hot water pre-heat. Heat pumps can only heat water to about 50 °C (122 °F) efficiently, whereas a boiler normally reaches 65–95 °C (149–203 °F) (Sony VAIO PCG-5J1L battery). Legacy radiators designed for these higher temperatures may have to be doubled in numbers when retrofitting a home. A hot water tank will still be needed to raise water temperatures above the heat pump's maximum, but pre-heating will save 25-50% of hot water costs(Sony VAIO PCG-5G3L battery).

Ground source heat pumps are especially well matched to underfloor heating and baseboard radiator systems which only require warm temperatures (40 °C) to work well. Thus they are ideal for open plan offices(Sony VAIO PCG-5G2L battery). Using large surfaces such as floors, as opposed to radiators, distributes the heat more uniformly and allows for a lower water temperature. Wood or carpet floor coverings dampen this effect because the thermal transfer efficiency of these materials is lower than that of masonry floors (tile, concrete) (Sony VGP-BPS21A/B battery). Underfloor piping, ceiling or wall radiators can also be used for cooling in dry climates, although the temperature of the circulating water must be above the dew point to ensure that atmospheric humidity does not condense on the radiator(Sony VGP-BPS21/S battery).

Combination heat pumps are available that can produce forced air and circulating water simultaneously and individually. These systems are largely being used for houses that have a combination of air and liquid conditioning needs, for example central air conditioning and pool heating(Sony VGP-BPS21B battery).

Seasonal thermal storage

The efficiency of ground source heat pumps can be improved by using seasonal thermal storage. If heat loss from the ground source is sufficiently low, the heat pumped out of the building in the summer can be retrieved in the winter(Sony VGP-BPS21 battery). Heat storage efficiency increases with scale, so this advantage is most significant in commercial or district heating systems. Geosolar combisystems further augment this efficiency by collecting extra solar energy during the summer (more than is needed for air conditioning) and concentrating it in the store(Sony VGN-FW31J battery).

Such a system has been used to heat and cool a greenhouse using an aquifer for thermal storage. In summer, the greenhouse is cooled with cold ground water. This heats the water in the aquifer which can become a warm source for heating in winter(Sony VGP-BPS13A/S battery). The combination of cold and heat storage with heat pumps can be combined with water/humidity regulation. These principles are used to provide renewable heat and renewable cooling to all kinds of buildings(Sony VGP-BPS13B/S battery).

Also the efficiency of existing small heat pump installations can sometimes be improved a lot by adding large, cheap, water filled solar collectors. These may be integrated into a to be overhauled parking lot, or in walls or roof constructions simply by putting lots of one inch PE pipes into the outer layer(Sony VGP-BPS13S battery). A very simple option is to add a large mechanically ventilated out door water-air heat exchanger (like the one that is in front of your car engine, but larger). In the summer they allow to pump lots of heat, almost free of running cost, into the ground(Sony VGP-BPS13AS battery). This only works well when ground water mobility is not too high, and it works better when more houses install this system next to each other. (In the winter such outdoor components have to be drained of water.)

Thermal efficiency

The net thermal efficiency of a heat pump should take into account the efficiency of electricity generation and transmission, typically about 40%(Sony PCGA-BP2EA battery). Since a heat pump moves 3 to 5 times more heat energy than the electric energy it consumes, the total energy output is much greater than the input. This results in net thermal efficiencies greater than 100% for most electricity sources(Sony VGP-BPS13/B battery). Traditional combustion furnaces andelectric heaters can never exceed 100% efficiency, but heat pumps provide extra energy by extracting it from the ground.

Geothermal heat pumps can reduce energy consumption— and corresponding air pollution emissions—up to 44% compared to air source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment(Sony VGP-BPS13/S battery).

The dependence of net thermal efficiency on the electricity infrastructure tends to be an unnecessary complication for consumers and is not applicable to hydroelectric power, so performance of heat pumps is usually expressed as the ratio of heating output or heat removal to electricity input(Sony VGP-BPS13A/B battery). Cooling performance is typically expressed in units of BTU/hr/watt as the Energy Efficiency Ratio, (EER) while heating performance is typically reduced to dimensionless units as the Coefficient of Performance. (COP) The conversion factor is 3.41 BTU/hr/watt(Sony VGP-BPS13B/B battery). Performance is influenced by all components of the installed system, including the soil conditions, the ground-coupled heat exchanger, the heat pump appliance, and the building distribution, but is largely determined by the "lift" between the input temperature and the output temperature(Sony VGN-FZ11E battery).

For the sake of comparing heat pump appliances to each other, independently from other system components, a few standard test conditions have been established by the American Refrigerant Institute (ARI) and more recently by the International Organization for Standardization(Sony VGN-FZ430E battery). Standard ARI 330 ratings were intended for closed loop ground-source heat pumps, and assumes secondary loop water temperatures of 77 °F (25 °C) for air conditioning and 32 °F (0 °C) for heating. These temperatures are typical of installations in the northern USA(Sony VGN-FZ32 battery). Standard ARI 325 ratings were intended for open loop ground-source heat pumps, and include two sets of ratings for groundwater temperatures of 50 °F (10 °C) and 70 °F (21 °C). ARI 325 budgets more electricity for water pumping than ARI 330(Sony VGN-FZ440N battery). Neither of these standards attempt to account for seasonal variations. Standard ARI 870 ratings are intended for direct exchange ground-source heat pumps. ASHRAE transitioned to ISO 13256-1 in 2001, which replaces ARI 320, 325 and 330(Sony VGN-FZ340E/B battery). The new ISO standard produces slightly higher ratings because it no longer budgets any electricity for water pumps.

Efficient compressors, variable speed compressors and larger heat exchangers all contribute to heat pump efficiency. Residential ground source heat pumps on the market today have standard COPs ranging from 2.4 to 5.0 and EERs ranging from 10.6 to 30(Sony VGN-FZ38 battery). To qualify for an Energy Star label, heat pumps must meet certain minimum COP and EER ratings which depend on the ground heat exchanger type. For closed loop systems, the ISO 13256-1 heating COP must be 3.3 or greater and the cooling EER must be 14.1 or greater(Sony Vaio VGN-FZ21J battery).

Actual installation conditions may produce better or worse efficiency than the standard test conditions. COP improves with a lower temperature difference between the input and output of the heat pump, so the stability of ground temperatures is important(Sony VGN-FZ140E battery). If the loop field or water pump is undersized, the addition or removal of heat may push the ground temperature beyond standard test conditions, and performance will be degraded. Similarly, an undersized blower may allow the plenum coil to overheat and degrade performance(Sony Vaio VGN-FZ31J battery ).

Soil without artificial heat addition or subtraction and at depths of several meters or more remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location(Sony Vaio VGN-FZ31B battery), usually 7–12 °C (45–54 °F) at a depth of six meters in the northern USA. Because this temperature remains more constant than the air temperature throughout the seasons, geothermal heat pumps perform with far greater efficiency during extreme air temperatures than air conditioners and air-source heat pumps(Sony VGN-FZ18L battery).

Standards ARI 210 and 240 define Seasonal Energy Efficiency Ratio (SEER) and Heating Seasonal Performance Factors (HSPF) to account for the impact of seasonal variations on air source heat pumps(Sony VGN-FW11M battery). These numbers are normally not applicable and should not be compared to ground source heat pump ratings. However, Natural Resources Canada has adapted this approach to calculate typical seasonally adjusted HSPFs for ground-source heat pumps in Canada(Sony VGN-FW11 battery). The NRC HSPFs ranged from 8.7 to 12.8 BTU/hr/watt (2.6 to 3.8 in nondimensional factors, or 255% to 375% seasonal average electricity utilization efficiency) for the most populated regions of Canada. When combined with the thermal efficiency of electricity, this corresponds to net average thermal efficiencies of 100% to 150%(Sony Vaio VGN-FZ18S battery).

Environmental impact

The U.S. Environmental Protection Agency (EPA) has called ground source heat pumps the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available. Heat pumps offer significant emission reductions potential, particularly where they are used for both heating and cooling and where the electricity is produced from renewable resources(Sony Vaio VGN-FZ210CE battery).

Ground-source heat pumps have unsurpassed thermal efficiencies and produce zero emissions locally, but their electricity supply includes components with high greenhouse gas emissions, unless the owner has opted for a 100% renewable energy supply(Sony VGN-FZ230E battery). Their environmental impact therefore depends on the characteristics of the electricity supply.

The GHG emissions savings from a heat pump over a conventional furnace can be calculated based on the following formula:

HL = seasonal heat load ≈ 80 GJ/yr for a modern detached house in the northern USA

FI = emissions intensity of fuel = 50 kg(CO2)/GJ for natural gas, 73 for heating oil, 0 for 100% renewable energy such as wind, hydro, photovoltaic or solar thermal(Sony Vaio VGN-FZ21S battery)

AFUE = furnace efficiency ≈ 95% for a modern condensing furnace

COP = heat pump coefficient of performance ≈ 3.2 seasonally adjusted for northern USA heat pump

EI = emissions intensity of electricity ≈ 200-800 ton(CO2)/GWh, depending on region

Ground-source heat pumps always produce less greenhouse gases than air conditioners(Sony Vaio VGN-FZ21E battery ), oil furnaces, and electric heating, but natural gas furnaces may be competitive depending on the greenhouse gas intensity of the local electricity supply. In countries like Canada and Russia with low emitting electricity infrastructure(Sony VGN-FZ18 battery), a residential heat pump may save 5 tons of carbon dioxide per year relative to an oil furnace, or about as much as taking an average passenger car off the road. But in countries like China or USA that are highly reliant on coal for electricity production, a heat pump may result in 1 or 2 tons more carbon dioxide emissions than a natural gas furnace(Sony VGN-FZ190E battery).

The fluids used in closed loops may be designed to be biodegradable and non-toxic, but the refrigerant used in the heat pump cabinet and in direct exchange loops was, until recently, chlorodifluoromethane(Sony VGN-FZ190 battery), which is an ozone depleting substance. Although harmless while contained, leaks and improper end-of-life disposal contribute to enlarging the ozone hole. This refrigerant is being phased out in favor of ozone-friendlyR410A for new construction(Sony VGP-BPS9/B battery). The EcoCute water heater is an air-source heat pump that uses Carbon Dioxide as its working fluid instead of Chlorofluorocarbons.

Open loop systems that draw water from a well and drain to the surface may contribute to aquifer depletion, water shortages, groundwater contamination, and subsidence of the soil. A geothermal heating project inStaufen im Breisgau(Sony VGN-FZ15G battery), Germany, is suspected to have caused considerable damage to buildings in the city center. The ground has subsided by up to eight millimeters under the city hall while other areas have been uplifted by a few millimeters.

Ground-source heat pump technology, like building orientation, is a natural building technique (bioclimatic building) (Sony VGN-FZ11L battery).

Economics

Ground source heat pumps are characterized by high capital costs and low operational costs compared to other HVAC systems. Their overall economic benefit depends primarily on the relative costs of electricity and fuels, which are highly variable over time and across the world. Based on recent prices(Sony Vaio VGN-FZ31S battery), ground-source heat pumps currently have lower operational costs than any other conventional heating source almost everywhere in the world. Natural gas is the only fuel with competitive operational costs(Sony Vaio VGN-FZ38M battery), and only in a handful of countries where it is exceptionally cheap, or where electricity is exceptionally expensive. In general, a homeowner may save anywhere from 20% to 60% annually on utilities by switching from an ordinary system to a ground-source system(Sony VGN-FZ19VN battery). However, many family size installations are reported to use much more electricity then their owners had expected from advertisements. This is often partly due to bad design or installation: Heat exchange capacity with groundwater is often too small, heating pipes in house floors are often too thin and too few, or heated floors are covered with wooden panels or carpets(Sony Vaio VGN-FZ31Z battery).

Capital costs and system lifespan have received much less study, and the return on investment is highly variable. One study found the total installed cost for a system with 10 kW (3 ton) thermal capacity for a detached rural residence in the USA averaged $8000–$9000 in 1995 US dollars(Sony Vaio VGN-FZ31M battery). More recent studies found an average cost of $14,000 in 2008 US dollars for the same size system. The US Department of Energy estimates a price of $7500 on its website, last updated in 2008. Prices over $20,000 are quoted in Canada, with one source placing them in the range of $30,000-$34,000 Canadian dollars(Sony VGN-FZ11M battery). The rapid escalation in system price has been accompanied by rapid improvements in efficiency and reliability. Capital costs are known to benefit from economies of scale, particularly for open loop systems, so they are more cost-effective for larger commercial buildings and harsher climates(Sony VGN-FZ11Z battery). The initial cost can be two to five times that of a conventional heating system in most residential applications, new construction or existing. In retrofits, the cost of installation is affected by the size of living area, the home's age, insulation characteristics, the geology of the area, and location of the home/property(Sony VGN-FZ220E battery). Proper duct system design and mechanical air exchange should be considered in the initial system cost.

Capital costs may be offset by substantial subsidies from many governments, for example totaling over $7000 in Ontario for residential systems installed in the 2009 fiscal year. Some electric companies offer special rates to customers who install a ground-source heat pump for heating/cooling their building(Sony VGN-FZ290 battery). This is due to the fact that electrical plants have the largest loads during summer months and much of their capacity sits idle during winter months. This allows the electric company to use more of their facility during the winter months and sell more electricity(Sony VGP-BPL12 battery). It also allows them to reduce peak usage during the summer (due to the increased efficiency of heat pumps), thereby avoiding costly construction of new power plants. For the same reasons, other utility companies have started to pay for the installation of ground-source heat pumps at customer residences(Sony VGP-BPS12 battery). They lease the systems to their customers for a monthly fee, at a net overall savings to the customer.

The lifespan of the system is longer than conventional heating and cooling systems. Good data on system lifespan is not yet available because the technology is too recent, but many early systems are still operational today after 25–30 years with routine maintenance(Sony VGP-BPS15 battery). Most loop fields have warranties for 25 to 50 years and are expected to last at least 50 to 200 years. Ground-source heat pumps use electricity for heating the house. The higher investment above conventional oil, propane or electric systems may be returned in energy savings in 2–10 years for residential systems in the USA(Sony VGP-BPS18 battery). If compared to natural gas systems, the payback period can be much longer or non-existent. The payback period for larger commercial systems in the USA is 1–5 years, even when compared to natural gas(Sony VGN-FZ17 battery).

Ground source heat pumps are recognized as one of the most efficient heating and cooling systems on the market. They are often the second-most cost effective solution in extreme climates, (after co-generation), despite reductions in thermal efficiency due to ground temperature(Sony VGN-FZ11S battery). (The ground source is warmer in climates that need strong air conditioning, and cooler in climates that need strong heating.)

Commercial systems maintenance costs in the USA have historically been between $0.11 to $0.22 per m2 per year in 1996 dollars, much less than the average $0.54 per m2 per year for conventional HVAC systems(Sony VGP-BPS14 battery).

Governments that promote renewable energy will likely