Ground source heat pump

A ground source 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 or geothermal heat pumps, as they are commonly termed in North Americaare among the most energy-efficient technologies for providing HVAC and water heating, using less energy than that consumed by resistive electric heaters.
Efficiency is given as a coefficient of performance 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 of installing ground loops over large areas or of drilling bore holes, hence ground source is often installed when new blocks of flats are built. Air-source heat pumps have lower set-up costs but have a lower CoP in very cold or hot weather.

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 can be described as follows:

In the surface layer to a depth of about 1 meter, the temperature is very sensitive to sunlight and weather.
In the shallow layer to a depth of about 8–20 meters, 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 °C per metre according to the geothermal gradient.

The "penetration depth" is defined as the depth at which the temperature variable is less than 0.01 of the variation at the surface. This also depends on the type of soil:

Soil Type	Day	Year
Rock	1.10	20.5
Wet clay	0.95	18.0
Wet sand	0.80	14.5
Dry clay	0.40	6.5
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.
After experimentation 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. The first successful commercial project was installed in the Commonwealth Building in 1948, and has been designated a National Historic Mechanical Engineering Landmark by ASME. Professor Carl Nielsen of Ohio State University built the first residential open loop version in his home in 1948.
As a result of the 1973 oil crisis, ground source heat pumps became popular in Sweden and have since grown slowly in worldwide popularity as the technology has improved. 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 with a growth rate of 10% per year. Each year, about 80,000 units are installed in the US and 27,000 in Sweden. 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%. In 2021, heat pumps accounted for 10% of global heating equipment sales.
In the United Kingdom, the 2022 Boiler Upgrade Scheme has driven demand for ground source heat pumps. In 2023, 2,469 ground source heat pumps were installed in the UK. The scheme closes in 2027.

Arrangement

Internal arrangement

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 are hydronic systems that carry heating or cooling through the building through pipes to conventional radiators, underfloor heating, baseboard radiators and hot water tanks. These heat pumps are also preferred for pool heating. Heat pumps typically only heat water to about efficiently, whereas boilers typically operate at . 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 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

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
Pipework for the ground loop is typically made of high-density polyethylene pipe and contains a mixture of water and anti-freeze. 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 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
A vertical system consists of a number of boreholes some deep fitted with U-shaped pipes through which a heat-carrying fluid that absorbs heat from the ground is circulated. 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 piles 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 tubes thermally fused to form a U-shaped bend at the bottom. 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. For illustration, a detached house needing 10 kW of heating capacity might need three boreholes deep.
;Radial or directional drilling
As an alternative to trenching, loops may be laid by mini horizontal directional drilling. 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 due to the small nature of the equipment used and the ability to bore beneath existing constructions.
;Open loop
In an open-loop system, 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 foul the system over time and require periodic acid cleaning. This is much more of a problem with cooling systems than heating systems. 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. 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 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 in some central solar heating plants, which later extract the heat via a large heat pump to supply district heating.
;Direct exchange
The direct exchange geothermal heat pump 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 ban of CFC refrigerants in 1989 and DX systems now are infrequently used.