Image credit: Mike Newman / Alamy Stock Photo
Three types of geothermal technology are available now to meet heating loads in both new builds and retrofits: closed loop Ground Source Heat Pump (GSHP) systems, Augmented GSHP systems and open loop systems (see figure below).
Closed loop Ground Source Heat Pump (GSHP) systems are an established and well understood technology that utilises low to moderate temperatures in the ground near the surface. Plastic pipe loops are installed in trenches or shallow boreholes, and water is circulated to bring low grade heat to the surface, where a heat pump boosts the temperature to, typically, 35-60ºC. Properly designed systems operate with a Coefficient of Performance of between 3 and 4 which means that they provide 3 to 4 times as much heat energy as they consume in electrical energy.
These systems can be installed anywhere provided there is space for the necessary trenches or boreholes, and they can supply heat loads from a few kilowatts to several megawatts. They can also be operated to provide cooling by removing heat from buildings and discharging it to the ground.
Closed loop systems can also be deployed in rivers, lakes, or the sea, eliminating the need to drill boreholes or dig trenches.
Augmented GSHP (AGSHP) systems take advantage of favourable geological conditions or the availability of warm water resources, for example mine water, to supply water to the heat pumps at a higher temperature; typically, up to about 25ºC. This improves the thermal efficiency and increases the Coefficient of Performance to above 5, resulting in both greater carbon savings and reduced running costs.
Open loop systems can be applied in two ways. For shallow resources they can replace the closed ground loops in GSHP systems, supplying water to the heat pumps directly from an underground aquifer accessed by drilling and then reinjecting it into a second borehole. If there is sufficient permeability in the rocks at depth to provide warm enough water directly for the end use, without the need for heat pumps. Typically, two boreholes (or wells) are required; a production well and an injection well. To reach directly usable temperatures (say, 40ºC) would require drilling to depths of 1,000m or more, depending on location. This type of system is in common use in Europe. In some applications, shallower, warm water aquifers are accessed in this way, but heat pumps are required to boost the temperature.
A fourth technology, single borehole heat exchange systems, is more novel and has yet to be widely applied commercially. A single borehole is drilled and a coaxial heat exchanger is installed within it to extract the energy. Such systems eliminate the geological risk because they do not rely on the natural permeability of the rocks and the heat is extracted purely by conduction. However, the drilling depth is greater than for the other systems, which carries its own risk and is more expensive. This type of system may be suitable for a new build development where grant funding is available for low-carbon heating.