For very shallow and shallow systems (categories I and II), and with the exception of passive cooling (geocooling), a geothermal heat pump must be used. Powered by electricity, these devices can use the Earth's temperature to regulate the building's temperature. Heat pumps can cool the building and re-inject heat underground, or produce heat by capturing it underground.
Heat pumps consist in a closed and watertight system in which a refrigerant runs at a liquid or gaseous state depending on which component of the system it is in. The four components are: the evaporator, the compressor, the condenser, and the expansion valve.
In the evaporator, the refrigerant captures the thermal energy contained in the geothermal loop field's heat exchange fluid. This increases the refrigerant's temperature, causing it to evaporate.
In the compressor, the vapour is compressed (using an electric motor powered by an external source), which increases its temperature.
In the condenser, the hot refrigerant loses its thermal energy to the fluid circulating through the distribution system. This reduces the refrigerant's temperature, causing it to condense back into liquid form. It now has low temperature and high pressure.
In the expansion valve, the refrigerant's pressure decreases, which lowers its temperature further. The refrigerant is now back to its low-temperature, low-pressure state, and can begin a new cycle.
Heat pump efficiency
A heat pump's efficiency is measured by its coefficient of performance (COP). This measures the relationship between the thermal energy produced by the heat pump (to heat or cool the building) and the energy (generally electric) required to run the heat pump, the circulator pump, etc. For geothermal heat pumps, coefficients of performance above 4 are not uncommon (i.e. to produce 1 kWh of heat, 0.25 kWh is drawn from an electric power source while 0.75 kWh is captured from the Earth).
The coefficient of performance depends on the temperature of the heat transfer fluid inside the main loop (the pipes that run underground) and that of the refrigerant inside de secondary loop (the building's heating). The higher the temperature difference between these two loops, the lower the heat pump's efficiency.
For a low-temperature heating system (e.g. 35 °C) and an appropriately sized geothermal system (i.e. one that does not exhaust the underground heat reservoir), the heat pump's coefficient of performance can be above 5. Conversely, when a very high output temperature is required from the heat pump, its efficiency drops and the risk of exhausting the underground heat reservoir increases. As a result, it is preferable to use a low-temperature heating system, such as underfloor heating, in order to optimise the heat pump's efficiency.
The benefit of heat pumps is therefore that the main loop's temperature remains relatively stable throughout the year (from 10 to 14 °C, depending on the region and on the depth); this means the coefficient of performance never drops dramatically during the winter, which can happen with heat pumps that use outdoor air for cooling (i.e. very low temperatures in the winter). However, the geothermal system must be accurately sized by professionals. This sizing must take into account the thermal characteristics of the ground, as well as the energy requirements of the building itself. Proper sizing prevents the risk of exhausting the geothermal reservoir, and thus a decrease in the heat pump's efficiency.