Geothermal energy

What is and how does a geothermal heat pump work?

What is and how does a geothermal heat pump work?

The geothermal heat pump is an air conditioning and heating system for buildings that exploits the heat exchange with the surface subsoil. In this case, the exchange is carried out by means of a heat pump.

Since the heat from the ground largely from within the Earth, low-temperature geothermal energy is classified as a renewable energy source. 

The heat pump allows transferring heat from a cold source to a hot source. In a heating system, the building represents the hot spot. In an air conditioning system, the building is the cold source from which the heat is drawn.

According to the second law of thermodynamics, heat is only transferred from a hot bulb to a cold bulb. For this reason, it is required to supply work, energy. That is, heat pumps require an external source of electricity such as fossil fuel. 

The ground represents for the heat pump a "source" (when it works in heating) or a "well" (in cooling mode) of heat. The heat may be carried to its end use by circulating water or forced air.

Compared to atmospheric air (which is the source of air source heat pumps), the temperature of the soil at a certain depth undergoes much smaller annual variations. At a depth of 5-10 m the temperature is stable, almost all year round it remains constant.

Therefore, geothermal heating and cooling allow having high efficiency in comparison with air-source heat pumps, which means energy savings.

Geothermal heat pumps also perform better during extreme air temperatures than air conditioners and air-source heat pumps.

Ground source heat pumps are especially well matched to underfloor heating and baseboard radiator systems. There are geothermal heat pumps that incorporate an accumulator for the production of domestic hot water.

The ground temperature at this depth is roughly equivalent to the mean annual outside air temperature.

How is the heat exchange of a geothermal pump carried out?

The different heating and cooling systems can work in three ways:

  • Direct exchange, where the heat pump's evaporator/condenser circuit is in direct contact with the ground.

  • Closed-loop systems, where the heat pump carries out the heat exchange with the ground indirectly, through a hydraulic circuit in which a cooling fluid flows.

  • Open-loop systems, in which underground water is taken in which heat exchange takes place.

In cold climates, the building where the heat load is unbalanced in favor of heating, the floor could cool due to heat removal. However, it is possible to couple the geothermal heat pump for solar thermal panel installation and store heat accumulated in the summer in the ground.

Direct exchange

In the direct exchange geothermal heat pump, the heat exchange takes place with the ground. The refrigerant that leaves the heat pump, circulates in a pipe inserted in direct contact with the ground, exchanges heat with it, and returns to the heat pump.

These systems are much more efficient than closed-loop systems. This is due to the absence of an intermediate circuit and high thermal conductivity of the copper tubes used.

Closed-loop systems

Most low enthalpy geothermal systems are made up of three circuits:

  • air conditioning circuit;

  • the primary circuit of the heat pump;

  • secondary heat exchange circuit with the ground.

The circulation pump can be external or be included within the heat pump. In the secondary circuit, there are also expansion tanks and safety valves for pressure control

There are geothermal heat pumps that incorporate the refrigeration circuit in the indoor unit. 

The closed-loop can be installed horizontally to a depth of 1-3 m, or vertically in a specially designed hole (geothermal probes) or on a foundation post (geothermal piles).

Vertical closed-loop

A vertical closed-loop consists of two or more pipes installed vertically in the ground. These tubes form a closed-loop in which the heat energy transfer fluid flows.

Geothermal probes are widely-used when there is not enough space for a closed-loop horizontal plant.

In the fields of the probe, the distance between the perforations is between 5 and 10 m. Geothermal probes can provide power between 40 and 70 W per meter of drilling.

Horizontal closed-loop

The closed-loop can be placed horizontally in a trench, placed at depths greater than those at which freezing of the ground can occur.

The tube can be linear or spiral (ground coils). The exchangeable power depends on the length of the pipe and the area occupied: approximately, the power exchanged with the ground is 15-40 watts / m².

The pipes are installed at a depth of 1-3 m: the greater the installation depth, the greater the thermal inertia, and the better the efficiency of the heat pump.

Compared with vertical geothermal probes, the efficiency of the heat pump is lower, but the lower installation costs make this solution competitive.

Open-loop geothermal pump

In this type of installation, the heat exchange takes place with groundwater or, more rarely, from rivers and lakes. The extracted water can be reintroduced into a surface water body or the same aquifer from which it was extracted.

The advantage, compared to closed-loop systems, is:

  • Offers a greater energy efficiency of the geothermal heat pump.

  • Lower installation costs

  • Fewer occupied spaces compared to geothermal probe systems and even more than horizontal closed-loop systems.

The main disadvantage of these plants is the risk of cracks and encrustations, which shorten the useful life of the plant.

What is the energy efficiency of a geothermal heat pump?

To study the energy efficiency of a geothermal heat pump, the COP concept is used. The term COP is referenced to the coefficient of performance of a refrigerator. This is defined as the heat removed from the cold reservoir Q divided by the work W done to remove the heat.

The COP of a geothermal heat pump varies between 3 and 6: this means that for every kWh of electricity consumed, 3-6 thermal kWh are produced. 

The primary energy efficiency of a geothermal heat pump is, therefore, variable between 120% and 240%. It might be translated as a cost-effective reduction. On the other hand, the best condensing boilers obtain efficiencies of 90%. 

The COP of the heat pump is highly dependent on the temperatures of the two thermostats: the smaller the difference, the higher the COP.

Publication Date: September 27, 2018
Last Revision: September 4, 2020