6. Geothermal Probes

The basic condition of near-surface geothermal energy utilization is the low temperature of the thermal reservoir. The temperature is typically lower than the working temperature of house heating. The heat transfer fluid in house heating systems requires a minimum temperature of about 20–30 °C, whereas ground temperatures are typically in the range of 5–15 °C. Therefore, in order to use the geothermal energy for the heating of buildings the transfer fluid temperature must be increased by means of a heat pump system. The highest reservoir temperatures are accessible to geothermal probes. Depending on the depth of the probe, drillhole heat exchangers may provide fluid temperatures of 10–12 °C depending on the local conditions (Central Europe). The temperature increase needed by the house heating system is then done by the heat pump. Most heat pumps are driven electrically. Electricity is expensive and produced with large losses from fossil energy resources in some countries.