View source for Ground source heat pump

{{Short description|System to transfer heat to/from the ground}}
{{About|one type of heat pump|more general information|heat pump|direct heating from hot rocks|geothermal heating|electricity generation from hot rocks|geothermal power|}}
[[Image:HeatAndColdStorageWithHeatPump.svg|thumb|right|A heat pump in combination with heat and cold storage]]
A '''ground source heat pump''' (also '''geothermal heat pump''') is a heating/cooling system for buildings that use a type of [[heat pump]] to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through the seasons. Ground-source heat pumps (GSHPs) – or geothermal heat pumps (GHP), as they are commonly termed in North America – are among the most energy-efficient technologies for providing [[HVAC]] and [[water heating]], using far less energy than can be achieved by burning a fuel in a [[Furnace (house heating)|boiler/furnace]] or by use of resistive [[electric heater]]s.

Efficiency is given as a [[coefficient of performance]] (CoP) which is typically in the range 3 – 6, meaning that the devices provide 3 – 6 units of heat for each unit of electricity used. Setup costs are higher than for other heating systems, due to the requirement to install ground loops over large areas or to drill bore holes, and for this reason, ground source is often suitable when new blocks of flats are built.<ref>{{Cite web |title=Heat pumps are hot items. But for people living in condos, getting one presents some challenges |url=https://www.cbc.ca/news/climate/heat-pumps-are-hot-items-but-for-people-living-in-condos-getting-one-presents-some-challenges-1.7088494}}</ref> Otherwise [[air-source heat pump]]s are often used instead.

==Thermal properties of the ground==

Ground-source heat pumps take advantage of the difference between the ambient temperature and the temperature at various depths in the ground.

The thermal properties of the ground near the surface<ref name=kalogirouflorides>Kalogirou, Soteris & Florides, Georgios. (2004). Measurements of Ground Temperature at Various Depths, conference paper 3rd International Conference on Sustainable Energy Technologies, Nottingham, UK, https://www.researchgate.net/publication/30500372_Measurements_of_Ground_Temperature_at_Various_Depths
https://ktisis.cut.ac.cy/bitstream/10488/870/3/C55-PRT020-SET3.pdf</ref><ref name=williamsgold>Williams G. and Gold L. Canadian Building
Digest 180m 1976. National Research Council of Canada, Institute for Research in Construction. https://nrc-publications.canada.ca/eng/view/ft/?id=386ddf88-fe8d-45dd-aabb-0a55be826f3f,</ref>
can be described as follows:

* In the '''surface layer''' to a depth of about 1 metre, the temperature is very sensitive to sunlight and weather,
* In the '''shallow layer''' to a depth of about 8–20 metres depending on soil type, the thermal mass of the ground causes temperature variation to decrease exponentially with depth until it is close to the local annual average air temperature; it also lags behind the surface temperature, so that the peak temperature is about 6 months after the surface peak temperature
* Below that, in the '''deeper layer''', the temperature is effectively constant, rising about 0.025&nbsp;°C per metre according to the [[geothermal gradient]].

The "penetration depth"<ref name=williamsgold /> is defined as the depth at which the temperature variable is less than 0.01 of the variation at the surface, and this depends on the type of soil:

{| class="wikitable" style="text-align:right;"
|+ Penetration depth in metres of diurnal and annual temperature cycles
|-
! style="text-align:left;" | Soil Type || Day (m) || Year (m)
|-
| style="text-align:left;" | Rock || 1.10 || 20.5
|-
| style="text-align:left;" | Wet clay || 0.95 || 18.0
|-
| style="text-align:left;" | Wet sand || 0.80 || 14.5
|-
| style="text-align:left;" | Dry clay || 0.40 || 6.5
|-
| style="text-align:left;" | Dry sand || 0.30 || 4.5
|}

==History==
The [[heat pump]] was described by [[Lord Kelvin]] in 1853 and developed by [[Peter Ritter von Rittinger]] in 1855. [[Heinrich Zoelly]] had patented the idea of using it to draw heat from the ground in 1912.<ref name="zogg">
{{Citation
 | first = M.
 | last = Zogg
 | title = History of Heat Pumps Swiss Contributions and International Milestones
 | url = http://www.zogg-engineering.ch/Publi/IEA_HPC08_Zogg.pdf
 | series = 9th International IEA Heat Pump Conference
 | date = 20–22 May 2008
 | place = Zürich, Switzerland
}}</ref>

After experimenting with a freezer, Robert C. Webber built the first [[direct exchange ground source heat pump]] in the late 1940s; sources disagree, however, as to the exact timeline of his invention<ref name="zogg"/><ref>{{cite web |url=http://www.igshpa.okstate.edu/about/about_us.htm |title=History |access-date=2009-03-24 |work=About Us |publisher=International Ground Source Heat Pump Association |archive-url=https://web.archive.org/web/20090404051034/http://www.igshpa.okstate.edu/about/about_us.htm |archive-date=2009-04-04 |url-status=dead }}</ref> The first successful commercial project was installed in the [[Commonwealth Building (Portland, Oregon)]] in 1948, and has been designated a [[National Historic Mechanical Engineering Landmark]] by [[ASME]].<ref name="bloomquist" /> Professor Carl Nielsen of [[Ohio State University]] built the first residential open loop version in his home in 1948.<ref>{{Citation
 | last = Gannon | first = Robert
 | title = Ground-Water Heat Pumps – Home Heating and Cooling from Your Own Well
 | newspaper = Popular Science | pages = 78–82
 | date = February 1978
 | volume = 212 | issue = 2
 | publisher = Bonnier Corporation
 | url = https://books.google.com/books?id=qQAAAAAAMBAJ
 | access-date = 2009-11-01
}}</ref>

As a result of the [[1973 oil crisis]], ground source heat pumps became popular in Sweden and have been growing slowly in worldwide acceptance since then. Open loop systems dominated the market until the development of [[polybutylene]] pipe in 1979 made closed loop systems economically viable.<ref name="bloomquist" />

As of 2004, there are over a million units installed worldwide, providing 12&nbsp;GW of thermal capacity with a growth rate of 10% per year.<ref name="world">{{Cite news | last1 =Lund | first1 =J. | last2 =Sanner | first2 =B. | last3 =Rybach | first3 =L. | last4 =Curtis | first4 =R. | last5 =Hellström | first5 =G. |date = September 2004| title =Geothermal (Ground Source) Heat Pumps, A World Overview | periodical =Geo-Heat Centre Quarterly Bulletin | location =Klmath Falls, Oregon | publisher =Oregon Institute of Technology | volume =25 | issue =3 | pages =1–10 | url =http://geoheat.oit.edu/bulletin/bull25-3/art1.pdf | issn =0276-1084 | access-date =2009-03-21 }}
</ref> Each year (as of 2011/2004, respectively), about 80,000 units are installed in the US<ref>{{cite web|url=http://www1.eere.energy.gov/geothermal/pdfs/40665.pdf |title=Geothermal – The Energy Under Our Feet: Geothermal Resources Estimates for the United States |access-date=2011-03-30}}</ref> and 27,000 in Sweden.<ref name="world" /> In Finland, a geothermal heat pump was the most common heating system choice for new detached houses between 2006 and 2011 with market share exceeding 40%.<ref>{{cite web|url=http://www.motiva.fi/rakentaminen/lammitysjarjestelman_valinta|title=Choosing a heating system}}</ref>

==Arrangement==

===Internal arrangement===
[[Image:Water to Water Heat Pump.jpg|thumb|right|Liquid-to-water heat pump]]
A [[heat pump]] is the central unit for the building's heating and cooling. It usually comes in two main variants:

''Liquid-to-water'' heat pumps (also called ''water-to-water'') are [[hydronic]] systems that carry heating or cooling through the building through pipes to [[wall radiator|conventional radiator]]s, [[underfloor heating]], [[baseboard radiator]]s and [[hot water tank]]s. These heat pumps are also preferred for pool heating. Heat pumps typically only heat water to about {{convert|55|°C|°F|abbr=on}} efficiently, whereas boilers typically operate at {{convert|65|–|95|C|F}} {{Citation needed|date=December 2023}}. The size of radiators designed for the higher temperatures achieved by boilers may be too small for use with heat pumps, requiring replacement with larger radiators when retrofitting a home from boiler to heat pump. When used for cooling, the temperature of the circulating water must normally be kept above the [[dew point]] to ensure that atmospheric humidity does not condense on the radiator.

''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, high-velocity systems, and ductless systems. Heat pumps cannot achieve as high 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 ductwork may have to be enlarged to reduce the noise from the higher air flow.

===Ground heat exchanger===
[[Image:3-ton Slinky Loop.jpg|thumb|right|A horizontal slinky loop prior to being covered with soil.]]
Ground source heat pumps employ a ground heat exchanger in contact with the ground or groundwater to extract or dissipate heat. Incorrect design can result in the system freezing after a number of years or very inefficient system performance; thus accurate system design is critical to a successful system <ref>{{cite web|url=http://www.carbonzeroco.com/ground-source-heat-pumps/ground-source-heating-cooling/|title=GSHC Viability and Design – Carbon Zero Consulting|website=carbonzeroco.com|access-date=19 March 2018}}</ref>

Pipework for the ground loop is typically made of [[high-density polyethylene]] pipe and contains a mixture of water and anti-freeze ([[propylene glycol]], [[denatured alcohol]] or [[methanol]]). Monopropylene glycol has the least damaging potential when it might leak into the ground, and is, therefore, the only allowed anti-freeze in ground sources in an increasing number of European countries.

;Horizontal
A horizontal closed loop field is composed of pipes that are arrayed in a plane in the ground. A long [[trench]], deeper than the [[frost line]], is dug and U-shaped or slinky coils are spread out inside the same trench. Shallow {{Convert|3-8|foot|m|adj = on}} 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 will harvest heat deposited by the sun several months earlier, while being weighed down in late winter and spring, due to accumulated winter cold. Systems in wet ground or in water are generally more efficient than drier ground loops since water conducts and stores heat better than solids in sand or soil. If the ground is naturally dry, soaker hoses may be buried with the ground loop to keep it wet.

;Vertical
[[File:Geothermal drilling for residential heating.jpg|thumb|Drilling of a borehole for residential heating]]
A vertical system consists of a number of boreholes some {{convert|50|to(-)|400|ft}} deep fitted with U-shaped pipes through which a heat-carrying fluid that absorbs (or discharges) heat from (or to) the ground is circulated.<ref name="Review">Li M, Lai ACK. Review of analytical models for heat transfer by vertical ground heat exchangers (GHEs): A perspective of time and space scales, Applied Energy 2015; 151: 178-191.</ref><ref name="H">Hellstrom G. Ground heat storage – thermal analysis of duct storage systems I. Theory. Lund: University of Lund; 1991.</ref> Bore holes are spaced at least 5–6 m apart and the depth depends on ground and building characteristics. Alternatively, pipes may be integrated with the [[foundation pile]]s used to support the building. Vertical systems rely on migration of heat from surrounding geology, unless recharged during the summer and at other times when surplus heat is available. Vertical systems are typically used where there is insufficient available land for a horizontal system.

Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole or comprises two small-diameter high-density polyethylene (HDPE) tubes thermally fused to form a U-shaped bend at the bottom.<ref>ASHRAE. ASHRAE handbook: HVAC applications. Atlanta: ASHRAE, Inc; 2011.</ref> The space between the wall of the borehole and the U-shaped tubes is usually grouted completely with grouting material or, in some cases, partially filled with groundwater.<ref>Kavanaugh SK, Rafferty K. Ground-source heat pumps: Design of geothermal systems for commercial and institutional buildings. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.; 1997.</ref>  For illustration, a detached house needing 10&nbsp;kW (3 [[ton#Refrigeration|ton]]) of heating capacity might need three boreholes {{convert|80|to|110|m|ft|abbr=on}} deep.<ref name="NRCnumbers" />

;Radial or directional drilling
As an alternative to trenching, loops may be laid by mini [[horizontal directional drilling]] (mini-HDD). This technique can lay piping under yards, driveways, gardens or other structures without disturbing them, with a cost between those of trenching and vertical drilling. This system also differs from horizontal & vertical drilling as the loops are installed from one central chamber, further reducing the ground space needed. Radial drilling is often installed retroactively (after the property has been built) due to the small nature of the equipment used and the ability to bore beneath existing constructions.

;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. 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 [[fouling|foul]] the system over time and require periodic acid cleaning. This is much more of a problem with cooling systems than heating systems.<ref>[[Hard water#Indices]]</ref> A standing column well system is a specialized type of open-loop system where water is drawn from the bottom of a deep rock well, passed through a heat pump, and returned to the top of the well.<ref>{{Cite news|last1=Orio |first1=Carl D. |last2=Johnson |first2=Carl N. |last3=Rees |first3=Simon J. |last4=Chiasson |first4=A. |last5=Deng |first5=Zheng |last6=Spitler |first6=Jeffrey D. |year=2004 |title=A Survey of Standing Column Well Installations in North America |periodical=ASHRAE Transactions |publisher=ASHRAE |volume=11 |issue=4 |pages=637–655 |url=http://www.hvac.okstate.edu/research/Documents/Orio_Johnson_Rees_Chiasson_Deng_Spitler_05.pdf |access-date=2009-03-25 |url-status=dead |archive-url=https://web.archive.org/web/20100626095818/http://www.hvac.okstate.edu/research/Documents/Orio_Johnson_Rees_Chiasson_Deng_Spitler_05.pdf |archive-date=2010-06-26 }}</ref> A growing number of jurisdictions have outlawed open-loop systems that drain to the surface because these may drain [[aquifer]]s or [[water contamination|contaminate]] wells. This forces the use of more environmentally sound injection wells or a closed-loop system.

;Pond
[[Image:Pond Loop Being Sunk.jpg|thumb|right|12-ton pond loop system being sunk to the bottom of a pond]]
A closed 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. Artificial ponds are used as heat storage (up to 90% efficient) in some [[central solar heating]] plants, which later extract the heat (similar to ground storage) via a large heat pump to supply [[district heating]].<ref name=epp2019-05>{{cite web |last1=Epp |first1=Baerbel |title=Seasonal pit heat storage: Cost benchmark of 30 EUR/m<sup>3</sup> |url=https://www.solarthermalworld.org/news/seasonal-pit-heat-storage-cost-benchmark-30-eurm3 |website=Solarthermalworld |archive-url= https://web.archive.org/web/20200202103359/https://www.solarthermalworld.org/news/seasonal-pit-heat-storage-cost-benchmark-30-eurm3 |archive-date=2 February 2020 |language=en |date=17 May 2019 |url-status=live}}</ref><ref name=utes2019>{{cite web |editor1-last=Kallesøe |editor1-first=A.J. |editor2-last=Vangkilde-Pedersen |editor2-first=T. |title=Underground Thermal Energy Storage (UTES) |at=sec. 4 PTES (Pit Thermal Energy Storage), p. 99 |url=https://www.heatstore.eu/documents/HEATSTORE_UTES%20State%20of%20the%20Art_WP1_D1.1_Final_2019.04.26.pdf |website=heatstore.eu}}<!--also aquifer, borehole and mines--></ref>

;Direct exchange (DX)
The [[direct exchange geothermal heat pump]] (DX) is the oldest type of geothermal heat pump technology where the refrigerant itself is passed through the ground loop. Developed during the 1980s, this approach faced issues with the [[refrigerant]] and oil management system, especially after the [[Montreal Protocol|ban of CFC refrigerants]] in 1989 and DX systems now are infrequently used.{{citation needed|date=June 2021}}

==Installation==
Because of the technical knowledge and equipment needed to design and size the system properly (and install the piping if heat fusion is required), a GSHP system installation requires a professional's services. Several installers have published real-time views of system performance in an online community of recent residential installations.  The [[International Ground Source Heat Pump Association]] ([[IGSHPA]]),<ref>{{cite web|title=IGSHPA|url=http://www.igshpa.okstate.edu/|publisher=www.igshpa.okstate.edu|access-date=17 May 2015|archive-url=https://web.archive.org/web/20150503162422/http://www.igshpa.okstate.edu/|archive-date=3 May 2015|url-status=dead}}</ref> Geothermal Exchange Organization (GEO),<ref>{{cite web|title=White House Executive Order on Sustainability Includes Geothermal Heat Pumps|url=https://www.geoexchange.org/|publisher=www.geoexchange.org|access-date=17 May 2015}}</ref> Canadian GeoExchange Coalition and Ground Source Heat Pump Association maintain listings of qualified installers in the US, Canada and the UK.<ref>{{cite web|url=http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12670 |title=Energy Savers: Selecting and Installing a Geothermal Heat Pump System |publisher=Apps1.eere.energy.gov |date=2008-12-30 |access-date=2009-06-08}}</ref> Furthermore, detailed analysis of soil thermal conductivity for horizontal systems and formation thermal conductivity for vertical systems will generally result in more accurately designed systems with a higher efficiency.<ref>{{cite web|url=http://www.carbonzeroco.com/field-services/soil-thermal-conductivity-testing/|title=Horizontal & Vertical Thermal Conductivity |publisher=Carbonzeroco.com |date=2016-03-23 |access-date=2016-03-23}}</ref>

==Thermal performance==
{{main|Coefficient of performance|Energy Efficiency Ratio}}
Cooling performance is typically expressed in units of BTU/hr/watt as the [[Energy Efficiency Ratio|energy efficiency ratio]] (EER), while heating performance is typically reduced to dimensionless units as the [[Coefficient of Performance|coefficient of performance]] (COP). The conversion factor is 3.41 BTU/hr/watt. Since a heat pump moves three to five times more heat energy than the electric energy it consumes, the total energy output is much greater than the electrical input. This results in net thermal efficiencies greater than 300% as compared to radiant electric heat being 100% efficient. Traditional combustion furnaces and [[electric heater]]s can never exceed 100% efficiency. Ground source heat pumps can reduce energy consumption – and corresponding air pollution emissions – up to 72% compared to electric resistance heating with standard air-conditioning equipment.<ref>[http://www.nrel.gov/docs/legosti/fy98/24782.pdf Geothermal Heat Pumps]. National Renewable Energy Laboratory.</ref>

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.<ref name="survival" /><ref>{{cite web |url=http://www.ahridirectory.org/ahriDirectory/pages/wbahp/defaultSearch.aspx |title=AHRI Directory of water-to-air geothermal heat pumps}}</ref> 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.<ref>{{cite web |url=http://www.energystar.gov/ia/partners/product_specs/eligibility/geo_heat_pump_elig.pdf |title=Energy Star Program Requirements for Geothermal Heat Pumps |access-date=2009-03-24 |work=Partner Commitments |publisher=Energy Star}}</ref>

Standards ARI 210 and 240 define [[Seasonal Energy Efficiency Ratio]] (SEER) and [[Heating Seasonal Performance Factor]]s (HSPF) to account for the impact of seasonal variations on air source heat pumps. 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.<ref name="NRCnumbers">{{cite web |url=http://oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/gsheatpumps.cfm |title=Ground Source Heat Pumps (Earth Energy Systems) |access-date=2009-03-24 |work=Heating and Cooling with a Heat Pump |publisher=Natural Resources Canada, Office of Energy Efficiency |url-status=dead |archive-url=https://web.archive.org/web/20090403212025/http://oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/gsheatpumps.cfm |archive-date=2009-04-03 }} Note: contrary to air-source conventions, the NRC's HSPF numbers are in units of BTU/hr/watt. Divide these numbers by 3.41 BTU/hr/watt to arrive at non-dimensional units comparable to ground-source COPs and air-source HSPF.</ref> 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.

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]]. Standard ARI 330 ratings were intended for closed-loop ground-source heat pumps, and assume secondary loop water temperatures of {{convert|25|C|F}} for air conditioning and {{convert|0|C|F}} for heating. These temperatures are typical of installations in the northern US. Standard ARI 325 ratings were intended for open-loop ground-source heat pumps, and include two sets of ratings for groundwater temperatures of {{convert|10|C|F}} and {{convert|21|C|F}}. ARI 325 budgets more electricity for water pumping than ARI 330. Neither of these standards attempts 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. The new ISO standard produces slightly higher ratings because it no longer budgets any electricity for water pumps.<ref name="survival" />

Soil without artificial heat addition or subtraction and at depths of several metres or more remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air temperature of the chosen location, usually {{convert|7|–|12|C|F}} at a depth of {{convert|6|m}} in the northern US. Because this temperature remains more constant than the air temperature throughout the seasons, ground source heat pumps perform with far greater efficiency during extreme air temperatures than air conditioners and air-source heat pumps.

=== Analysis of heat transfer ===
A challenge in predicting the thermal response of a ground heat exchanger (GHE)<ref>[https://www.sciencedirect.com/topics/engineering/ground-source-heat-pump-system definition of GHE]</ref> is the diversity of the time and space scales involved. Four space scales and eight time scales are involved in the heat transfer of GHEs. The first space scale having practical importance is the diameter of the borehole (~ 0.1 m) and the associated time is on the order of 1 hr, during which the effect of the heat capacity of the backfilling material is significant. The second important space dimension is the half distance between two adjacent boreholes, which is on the order of several meters. The corresponding time is on the order of a month, during which the thermal interaction between adjacent boreholes is important. The largest space scale can be tens of meters or more, such as the half-length of a borehole and the horizontal scale of a GHE cluster. The time scale involved is as long as the lifetime of a GHE (decades).<ref>Li M, Li P, Chan V, Lai ACK. Full-scale temperature response function (G-function) for heat transfer by borehole ground heat exchangers (GHEs) from sub-hour to decades. Appl Energy 2014; 136: 197-205.</ref>

The short-term hourly temperature response of the ground is vital for analyzing the energy of ground-source heat pump systems and for their optimum control and operation. By contrast, the long-term response determines the overall feasibility of a system from the standpoint of the life cycle. Addressing the complete spectrum of time scales require vast computational resources.

The main questions that engineers may ask in the early stages of designing a GHE are (a) what the heat transfer rate of a GHE as a function of time is, given a particular temperature difference between the circulating fluid and the ground, and (b) what the temperature difference as a function of time is, given a required heat exchange rate. In the language of heat transfer, the two questions can probably be expressed as
<math>q_l = [T_f(t) - T_0]/R(t)</math>

where ''T''<sub>f</sub> is the average temperature of the circulating fluid, ''T''<sub>0</sub> is the effective, undisturbed temperature of the ground, ''q<sub>l</sub>'' is the heat transfer rate of the GHE per unit time per unit length (W/m), and ''R'' is the total thermal resistance (m'''<sup>.</sup>'''K/W).''R''(''t'') is often an unknown variable that needs to be determined by heat transfer analysis. Despite ''R''(''t'') being a function of time, analytical models exclusively decompose it into a time-independent part and a time-dependent part to simplify the analysis.

Various models for the time-independent and time-dependent R can be found in the references.<ref name="Review"/><ref name="H"/>  Further, a [[Thermal response test]] is often performed to make a deterministic analysis of ground thermal conductivity to optimize the loopfield size, especially for larger commercial sites (e.g., over 10 wells).

===Seasonal thermal storage===
[[Image:HeatAndColdStorageWithHeatPump.svg|thumb|right|A heat pump in combination with heat and cold storage]]
{{Main|Seasonal thermal energy storage}}
The efficiency of ground source heat pumps can be greatly improved by using [[seasonal thermal energy storage]] and interseasonal heat transfer.<ref>{{cite web|url=http://www.icax.co.uk/interseasonal_heat_transfer.html |title=Interseasonal Heat Transfer |publisher=Icax.co.uk |access-date=2011-09-16}}</ref> Heat captured and stored in thermal banks in the summer can be retrieved efficiently in the winter. Heat storage efficiency increases with scale, so this advantage is most significant in commercial or [[district heating]] systems.

[[Geosolar combisystem]]s have been used to heat and cool a greenhouse using an [[aquifer]] for thermal storage.<ref name=utes2019/><ref name=vlaanderen>{{cite book|last1=Van Passel|first1=Willy |last2=Sourbron|first2=Maarten |last3=Verplaetsen|first3=Filip |last4=Leroy|first4=Luc |last5=Somers|first5=Yvan |last6=Verheyden|first6=Johan |last7=Coupé|first7=Koen |editor=Organisatie voor Duurzame Energie Vlaanderen|title=Warmtepompen voor woningverwarming|url=http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf |access-date=2009-03-23|page=28|archive-url= https://web.archive.org/web/20090318230625/http://www2.vlaanderen.be/economie/energiesparen/doc/brochure_warmtepomp.pdf |archive-date=2009-03-18|url-status=dead}}</ref> 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.<ref name=vlaanderen/><ref>{{cite web|url=http://www.zonneterp.nl/english/index_uk.html |title=Schematic of similar system of aquifers with fans-regulation |publisher=Zonneterp.nl |date=2005-11-11 |access-date=2011-03-30}}</ref> 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<ref>{{cite web|url=http://www.icax.co.uk/renewable_cooling.html |title=Capture, storage and release of Renewable Cooling |publisher=Icax.co.uk |access-date=2011-03-30}}</ref> to all kinds of buildings.

Also the efficiency of existing small heat pump installations can be improved 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 by installing one-inch [[Polyethylene|PE]] pipes into the outer layer.

==Environmental impact==
The US Environmental Protection Agency (EPA) has called ground source heat pumps the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available.<ref>{{cite journal|last1= Environmental Protection Agency|title= Space Conditioning: The Next Frontier – Report 430-R-93-004|publisher= EPA|year= 1993}}</ref> 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.

GSHPs 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. Their environmental impact, therefore, depends on the characteristics of the electricity supply and the available alternatives.

{| class="wikitable" style="float:center; margin:0.5em 0 0.5em 1em;"
|+ Annual greenhouse gas (GHG) savings from using a ground source heat pump instead of a high-efficiency furnace in a detached residence (assuming no specific supply of renewable energy)
!rowspan="2"| Country !!rowspan="2"| Electricity CO<sub>2</sub><br />[[Emissions Intensity]] !!colspan="3"| GHG savings relative to
|-
! [[natural gas]] !! [[heating oil]] !! [[electric heating]]
|-
| Canada || 223 ton/GWh<ref name="EEA">{{Cite book|last=European Environment Agency|title=Energy and environment report 2008|url=http://www.eea.europa.eu/publications/eea_report_2008_6|access-date=2009-03-22|series=EEA Report |volume=No 6/2008|year=2008|publisher=Office for Official Publications of the European Communities|location=Luxemburg|isbn=978-92-9167-980-5|issn=1725-9177|doi=10.2800/10548|page=83}}</ref><ref name="EIA" /><ref>{{cite book  | title = National Inventory Report 1990–2006:Greenhouse Gas Sources and Sinks in Canada  | journal = Canada's Greenhouse Gas Inventory | publisher = Government of Canada  |date = May 2008| chapter = annex 9  | chapter-url = http://www.ec.gc.ca/pdb/ghg/inventory_report/2004_report/ta9_7_e.cfm  | issn = 1706-3353  | isbn = 978-1-100-11176-6 }}</ref> || 2.7 ton/yr || 5.3 ton/yr || 3.4 ton/yr
|-
| Russia || 351 ton/GWh<ref name="EEA" /><ref name="EIA" /> || 1.8 ton/yr || 4.4 ton/yr || 5.4 ton/yr
|-
| US || 676 ton/GWh<ref name="EIA">{{cite web  | last = Energy Information Administration, US Department of Energy  | title = Voluntary Reporting of Greenhouse Gases, Electricity Emission Factors  | year = 2007  | url = http://www.eia.doe.gov/oiaf/1605/pdf/Appendix%20F_r071023.pdf | access-date = 2009-03-22  }}</ref> || style="color:red" | −0.5 ton/yr || 2.2 ton/yr || 10.3 ton/yr
|-
| China || 839 ton/GWh<ref name="EEA" /><ref name="EIA" /> || style="color:red" | −1.6 ton/yr || 1.0 ton/yr || 12.8 ton/yr
|}

The GHG emissions savings from a heat pump over a conventional furnace can be calculated based on the following formula:<ref name="strategic">{{Cite news | last1 =Hanova | first1 =J | last2 =Dowlatabadi | first2 =H | date =9 November 2007 | title =Strategic GHG reduction through the use of ground source heat pump technology | periodical =Environmental Research Letters | place =UK | publisher =IOP Publishing | volume =2 | issue =4 | pages =044001 8pp | url =http://www.iop.org/EJ/article/1748-9326/2/4/044001/erl7_4_044001.pdf?request-id=dd247af1-1eb0-4c8d-a20b-426d37d8ee5f | issn =1748-9326 | doi =10.1088/1748-9326/2/4/044001 | access-date =2009-03-22 | bibcode =2007ERL.....2d4001H | archive-date =2016-04-06 | archive-url =https://web.archive.org/web/20160406132555/http://iopscience.iop.org/article/10.1088/1748-9326/2/4/044001/pdf;jsessionid=F54959800F580EA1CC5C518716D0CDC6.c3 | url-status =dead }}</ref>

<math>\text{GHG Savings}=\mathrm{HL} \left( \frac\mathrm{FI}{\mathrm{AFUE} \times 1000\frac\mathrm{kg}\mathrm{ton}}-\frac\mathrm{EI}{\mathrm{COP} \times 3600\frac\mathrm{sec}\mathrm{hr}}\right)</math>
*HL = seasonal heat load ≈ 80 GJ/yr for a modern detached house in the northern US
*FI = emissions intensity of fuel = 50&nbsp;kg(CO<sub>2</sub>)/GJ for natural gas, 73 for heating oil, 0 for 100% [[renewable energy]] such as wind, hydro, photovoltaic or solar thermal
*[[AFUE]] = furnace efficiency ≈ 95% for a modern [[condensing furnace]]
*COP = heat pump coefficient of performance ≈ 3.2 seasonally adjusted for northern US heat pump
*EI = emissions intensity of electricity ≈ 200–800 ton(CO<sub>2</sub>)/GWh, depending on the region's mix of electric power plants (Coal vs Natural Gas vs Nuclear, Hydro, Wind & Solar)

Ground-source heat pumps always produce fewer greenhouse gases than air conditioners, 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, 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 cities like Beijing or Pittsburgh 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.  For areas not served by utility natural gas infrastructure, however, no better alternative exists.

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]], which is an ozone-depleting substance.<ref name="survival">{{Cite news | last = Rafferty | first = Kevin |date = April 1997| title =An Information Survival Kit for the Prospective Residential Geothermal Heat Pump Owner | periodical =Geo-Heat Centre Quarterly Bulletin | location =Klmath Falls, Oregon | publisher =Oregon Institute of Technology | volume =18 | issue =2 | pages = 1–11 | url =http://geoheat.oit.edu/bulletin/bull18-2/art1.pdf | issn =0276-1084 | access-date =2009-03-21 |archive-url=https://web.archive.org/web/20120217184539/http://geoheat.oit.edu/bulletin/bull18-2/art1.pdf |archive-date=17 February 2012}} The author issued an [http://geoheat.oit.edu/ghp/survival.pdf updated version] {{Webarchive|url=https://web.archive.org/web/20130217043915/http://geoheat.oit.edu/ghp/survival.pdf |date=2013-02-17 }} of this article in February 2001.</ref> Although harmless while contained, leaks and improper end-of-life disposal contribute to enlarging the [[ozone hole]].  For new construction, this refrigerant is being phased out in favor of the ozone-friendly but potent greenhouse gas [[R410A]]. Open-loop systems (i.e. those that draw ground water as opposed to closed-loop systems using a borehole heat exchanger) need to be balanced by reinjecting the spent water. This prevents [[aquifer depletion]] and the [[soil contamination|contamination of soil]] or surface water with [[brine]] or other compounds from underground.{{citation needed|date=January 2013}}

Before drilling, the underground geology needs to be understood, and drillers need to be prepared to seal the borehole, including preventing penetration of water between strata. The unfortunate example is a geothermal heating project in [[Staufen im Breisgau]], Germany which seems the cause of considerable damage to historical buildings there. In 2008, the city centre was reported to have risen 12&nbsp;cm,<ref>[http://www.spiegel.de/wissenschaft/natur/0,1518,589944,00.html Spiegel.de report on recent geological changes] (in German, partial [http://backreaction.blogspot.com/2008/11/town-rips-up.html translation])</ref> after initially sinking a few millimeters.<ref>{{cite news|url=https://www.telegraph.co.uk/news/worldnews/1583323/Geothermal-probe-sinks-German-city.html|title=Geothermal probe sinks German city|first=Bojan|last=Pancevski|date=30 March 2008|access-date=19 March 2018|work=[[Telegraph.co.uk]]}}</ref>  The boring tapped a naturally pressurized aquifer, and via the borehole this water entered a layer of anhydrite, which expands when wet as it forms gypsum. The swelling will stop when the anhydrite is fully reacted, and reconstruction of the city center "is not expedient until the uplift ceases". By 2010 sealing of the borehole had not been accomplished.<ref>{{cite journal |url=http://carsologica.zrc-sazu.si/downloads/392/Sass.pdf |title=DAMAGE TO THE HISTORIC TOWN OF STAUFEN (GERMANY) CAUSED By GEOTHERMAL DRILLING THROUGH ANHYDRITE-BEARING FORMATIONS |first=A |last=FORMACIJE |journal=Acta Carsologica |year=2010 |page=233 |volume=39 |issue=2 |url-status=dead |archive-url=https://web.archive.org/web/20120813041725/http://carsologica.zrc-sazu.si/downloads/392/Sass.pdf |archive-date=2012-08-13 }}</ref><ref>{{cite journal | doi = 10.1007/s00767-010-0154-5 | last1 = Butscher | first1 = Christoph | last2 = Huggenberger | first2 = Peter | last3 = Auckenthaler | first3 = Adrian | last4 = Bänninger | first4 = Dominik | title = Risikoorientierte Bewilligung von Erdwärmesonden  | journal = Grundwasser | year = 2010 | volume = 16 | issue = 1 | pages = 13–24|bibcode = 2011Grund..16...13B | s2cid = 129598890 | url = http://doc.rero.ch/record/321504/files/767_2010_Article_154.pdf }}</ref><ref>{{cite journal | doi =10.1007/s10040-009-0458-7 | last1 =Goldscheider | first1 =Nico | last2 =Bechtel | first2 =Timothy D. |title =Editors' message: The housing crisis from underground—damage to a historic town by geothermal drillings through anhydrite, Staufen, Germany |journal =Hydrogeology Journal | volume =17 | pages =491–493 | year =2009 | issue =3|bibcode = 2009HydJ...17..491G | doi-access =free }}</ref> By 2010, some sections of town had risen by 30&nbsp;cm.<ref name="BZ_15.10.010">[http://www.badische-zeitung.de/staufen/risse-in-staufen-pumpen-reparieren-und-hoffen--36628144.html badische-zeitung.de, ''Lokales, Breisgau'', 15. Oktober 2010, hcw: ''Keine Entwarnung in der Fauststadt – Risse in Staufen: Pumpen, reparieren und hoffen''] (17. Oktober 2010)</ref>

==Economics==
{{Update section|date=September 2023|reason=probably needs to say more about larger systems such as district heating}}
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, 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, and only in a handful of countries where it is exceptionally cheap, or where electricity is exceptionally expensive.<ref name="strategic" /> 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.<ref name="ghpcCons">{{cite web |title=Geothermal Heat Pump Consortium, Inc. |url=http://geoexchange.us/ |access-date=2007-10-19}}</ref><ref name="economics">{{Cite journal | last1 =Lienau | first1 =Paul J. | last2 =Boyd | first2 =Tonya L. | last3 =Rogers | first3 =Robert L. | date =April 1995 | title =Ground-Source Heat Pump Case Studies and Utility Programs | place =Klamath Falls, OR | publisher =Geo-Heat Center, Oregon Institute of Technology | url =http://geoheat.oit.edu/pdf/hp1.pdf | access-date =2009-03-26 | archive-date =2009-10-07 | archive-url =https://web.archive.org/web/20091007151857/http://geoheat.oit.edu/pdf/hp1.pdf | url-status =dead }}</ref>

Capital costs and system lifespan have received much less study until recently, and the [[return on investment]] is highly variable. 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. 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 the living area, the home's age, insulation characteristics, the geology of the area, and the location of the property. Proper duct system design and mechanical air exchange should be considered in the initial system cost.

{| class="wikitable" style="float:right; margin:0.5em 0 0.5em 1em;"
|+ Payback period for installing a ground source heat pump in a detached residence
!rowspan="2"| Country !!colspan="3"| Payback period for replacing
|-
! [[natural gas]] !! [[heating oil]] !! [[electric heating]]
|-
| Canada || 13 years || 3 years || 6 years
|-
| US || 12 years || 5 years || 4 years
|-
| Germany || style="color:red" | net loss || 8 years || 2 years
|-
| colspan="4" |<small>Notes:</small>
* <small>Highly variable with energy prices.</small>
* <small>Government subsidies not included.</small>
* <small>Climate differences not evaluated.</small>
|}

Capital costs may be offset by government subsidies; for example,  Ontario offered $7000 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 or cooling their building.<ref name="geothermal_incentive">{{cite web |url=http://www.capitalelec.com/Energy_Efficiency/ground_source/index.html |title=Geothermal Heat Pumps |publisher=[[Capital Electric Cooperative]] |access-date=2008-10-05 |url-status=dead |archive-url=https://web.archive.org/web/20081206122801/http://www.capitalelec.com/Energy_Efficiency/ground_source/index.html |archive-date=2008-12-06 }}</ref> Where electrical plants have larger loads during summer months and idle capacity in the winter, this increases electrical sales during the winter months. Heat pumps also lower the load peak during the summer due to the increased efficiency of heat pumps, thereby avoiding the 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. They lease the systems to their customers for a monthly fee, at a net overall saving 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. Most loop fields have warranties for 25 to 50 years and are expected to last at least 50 to 200 years.<ref name="ghpcCons"/><ref name="ghpcFAQ"/> 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 US.<ref name="Ref-1">{{cite web |url=http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640 |title=Energy Savers: Geothermal Heat Pumps |publisher=Apps1.eere.energy.gov |date=2009-02-24 |access-date=2009-06-08 |archive-date=2009-04-01 |archive-url=https://web.archive.org/web/20090401183350/http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640 |url-status=dead }}</ref><ref name="economics" /><ref name="ghpcFAQ">{{cite web |title=Geothermal heat pumps: alternative energy heating and cooling FAQs |url=http://www.econar.com/faq.htm |access-date=2007-10-19 |url-status=dead |archive-url=https://web.archive.org/web/20070903210053/http://www.econar.com/faq.htm |archive-date=2007-09-03 }}</ref> The payback period for larger commercial systems in the US is 1–5 years, even when compared to natural gas.<ref name="economics" /> Additionally, because geothermal heat pumps usually have no outdoor compressors or cooling towers, the risk of vandalism is reduced or eliminated, potentially extending a system's lifespan.<ref>{{cite web |title=Benefits of a Geothermal Heat Pump System |url=http://energy.ltgovernors.com/benefits-of-a-geothermal-heat-pump-system.html |access-date=2011-11-21}}</ref>

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. (The ground source is warmer in climates that need strong air conditioning, and cooler in climates that need strong heating.) The financial viability of these systems depends on the adequate sizing of ground heat exchangers (GHEs), which generally contribute the most to the overall capital costs of GSHP systems.<ref>{{Cite book|url=https://www.icebookshop.com/Products/Geothermal-Energy,-Heat-Exchange-Systems-and-Energ.aspx|title=Geothermal Energy, Heat Exchange Systems and Energy Piles|last1=Craig|first1=William|last2=Gavin|first2=Kenneth|publisher=ICE Publishing|year=2018|isbn=9780727763983|location=London|pages=79}}</ref>

Commercial systems maintenance costs in the US have historically been between $0.11 to $0.22 per m<sup>2</sup> per year in 1996 dollars, much less than the average $0.54 per m<sup>2</sup> per year for conventional HVAC systems.<ref name="bloomquist">{{Cite news | last =Bloomquist | first =R. Gordon | date =December 1999 | title =Geothermal Heat Pumps, Four Plus Decades of Experience | periodical =Geo-Heat Centre Quarterly Bulletin | location =Klmath Falls, Oregon | publisher =Oregon Institute of Technology | volume =20 | issue =4 | pages =13–18 | url =http://geoheat.oit.edu/bulletin/bull20-4/art3.pdf | issn =0276-1084 | access-date =2009-03-21 | archive-date =2012-10-31 | archive-url =https://web.archive.org/web/20121031190555/http://geoheat.oit.edu/bulletin/bull20-4/art3.pdf | url-status =dead }}</ref>

Governments that promote renewable energy will likely offer incentives for the consumer (residential), or industrial markets. For example, in the United States, incentives are offered both on the state and federal levels of government.<ref name="mmar2009">[http://www.dsireusa.org/searchby/searchtechnology.cfm?&CurrentPageID=2&EE=1&RE=1 Database of State Incentives for Renewables & Efficiency] {{webarchive|url=https://web.archive.org/web/20080222100703/http://www.dsireusa.org/searchby/searchtechnology.cfm?&CurrentPageID=2&EE=1&RE=1 |date=2008-02-22 }}. US Department of Energy.</ref>

==See also==
{{Portal|Renewable energy|Energy|Technology}}
* [[Ground-coupled heat exchanger]]
* [[Deep water source cooling]]
* [[Solar thermal cooling]]
* [[Renewable heat]]
* [[International Ground Source Heat Pump Association]]
* [[Glossary of geothermal heating and cooling]]
* [[Uniform Mechanical Code]]

==References==
{{Reflist|30em}}

==External links==
* [http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640 Geothermal Heat Pumps] {{Webarchive|url=https://web.archive.org/web/20090401183350/http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640 |date=2009-04-01 }} ([[EERE]]/[[USDOE]]).
* [https://web.archive.org/web/20150224174436/http://toolbase.org/Technology-Inventory/HVAC/geothermal-heat-pumps#initialcost Cost calculation]
* [http://geoexchange.org Geothermal Heat Pump Consortium]
* [https://web.archive.org/web/20150503162422/http://www.igshpa.okstate.edu/ International Ground Source Heat Pump Association]
* [http://www.gshp.org.uk/ Ground Source Heat Pump Association] (GSHPA)

{{Renewable energy}}
{{HVAC}}
{{Geothermal power}}
{{Authority control}}

[[Category:Energy conversion]]
[[Category:Building engineering]]
[[Category:Heat pumps]]
[[Category:Sustainable technologies]]