Geothermal air conditioning (heating or cooling) is a system that uses the high thermal inertia of the subsoil, which at a depth of about three metres has a constant temperature of between 10 and 16 °C, depending on the latitude (north or south) of the site.
Geothermal air conditioning (heating or cooling should not be confused with geothermal energy, which requires high subsoil temperatures, usually associated with volcanic activity.
There are two different systems: high-enthalpy (with an air-conditioning pump) and low-enthalpy (without an air-conditioning pump).
Without a heat pump
The low-enthalpy system is simpler and uses less energy. Subsoil temperatures vary much less than above-ground temperatures (and may never vary throughout the year beyond certain depths). This stable temperature (hot or cold) can be extracted simply by means of a pumped fluid. The liquid remains at cave temperature, seeming warm in the winter and cool in the summer, although in reality it is the ambient temperature that changes, not the temperature of the subsoil.
This system avoids the complexity and cost involved with a heat pump, while the cost of a water pump is minimal. However, it is less powerful and may not be sufficient, or may need to be supported by another auxiliary renewable energy system, unless it is used for underfloor heating.
With heat pump
The high-enthalpy system requires the use of a heat pump, and is very similar in principal to air conditioning that works for cooling and heating. The difference is that instead of expelling the heat outside of the house, the subsoil is used as either a sink or a source of heat.
To understand the concept of geothermal heating and cooling, it is necessary to first understand how a heat pump works: pumps capture heat on one side of the circuit and release it on the other. It other words, they cool down one side at the expense of having to heat up the other. One typical example are air conditioning machines that use the principle of compression cooling: when the machine is set on cool, the fluid circulating around the internal circuit of the house is absorbing heat from the environment. This fluid then goes into a compressor, where as the pressure goes up, so does the temperature, then passes to the exterior circuit outside. As the fluid is now very hot (more than the air outside) it releases heat into the air. It then circulates towards a valve where the fluid cools down again as it expands, and the cycle begins again.
The higher the outside temperature, the less heat the fluid will be able to release into the exterior circuit, and therefore the machine will perform less efficiently. This is where the advantage of geothermal solutions lie.
Geothermal heating is also a heat pump, but instead of exchanging heat with the atmosphere, it does so with the ground. In the winter, the heat pump absorbs heat from the ground and releases it inside the building. In the summer, it absorbs heat from the building and releases it into the ground.
The advantage is that the earth maintains a more constant temperature (between 7°C and 14°C all year round), from just a few metres below the surface. This enables a more efficient heat exchange, and therefore less energy use. In other words: By exchanging more heat in the same cycle, the compressor has to carry out less cycles (compressing the fluid less often), and therefore, energy consumption is lower. The rate of return is then also said to be higher.