What is shallow geothermal energy?
Shallow geothermal energy (also: near surface geothermal energy) is the heat available or rather stored in the ground. It is available everywhere and anytime, regardless of daytime or season. In central Europe, the temperature in a depth of 20 metres amounts to a constant temperature of roundabout 10 °C. Every 100 metres deeper the temperature increases by 3 K. It can be used for cooling and heating purposes (deep geothermal energy also for electricity production). The heat of the ground is usually extracted in closed loop systems, rarer in open loop systems. Geothermal energy is renewable, ecologically friendly and space-saving at the surface.
Closed loop systems
Closed loop systems use pipes made of polyethylene for heating and cooling. They can be installed vertically down to several hundred meters (tube systems) or horizontally meandering in depths of 1,0 to 1,5 meters (collectors). There are also more compact collectors combining vertical and horizontal energy extraction. Furthermore, foundation piles of buildings are also used for geothermal installations. Several tubes, piles or collectors can be combined to install higher capacity systems.
All closed systems use brine (a mixture of water and a refrigerant like gylcol or ethanol) which continuously circulates in the pipes. Below the surface this fluid absorbs heat from the ground and flows back to the top. A heat exchanger transfers the fluid’s heat to the heat pump and its refrigerant fluid. Compression raises the temperature of the refrigerant fluid in the heat pump from around 10 up to 60 °C. After passing the heat exchanger the brine returns to the ground and a new cycle begins. For cooling in summer, the process is reversed: the heat is extracted from the building and carried back to the ground. This can be done in a very economical way as a free cooling process.
Open loop systems
The process of open loop systems is very similar to closed loop systems, but it uses groundwater directly as heat source. No additional water or fluids are needed. In an extraction well ground water is pumped to the surface, where it transfers its energy via heat exchangers to the heat pump. Afterwards the water is reinjected to the groundwater horizons using an injection well.