At a few meters depth the surface has a constant temperature of about 10 degrees. The deeper you drill, the higher the temperature becomes. Every kilometre the temperature increases by about 30 degrees, this is the temperature gradient. This temperature gradient is not the same everywhere in the Netherlands. The gradient is relatively low in the Netherlands, in countries like Iceland and Italy the temperature gradient is much higher due to volcanic activity. Geothermal energy is therefore used much more frequently in these countries because you do not have to drill as deep.
Geothermal systems are open systems at great depths (kilometres deep) that use a permeable rock reservoir. They are usually doublets that consist of an extraction well and an injection well that are about 1 to 2 kilometres apart at depth. The heat from the hot pumped water is extracted via a heat exchanger and the cooled formation water is then injected back into the same deep layer. The lifespan of such a doublet is about 30 years. Then the layer has cooled too much to be able to supply enough heat.
In addition to the financial risks for the geothermal operator, there are also other risks. One of the biggest risks is contamination of groundwater due to leakage of the formation water. This water is very salty and sometimes also contains higher concentrations of radioactive minerals. Gas can also be drilled during drilling or unexpected high pressure may be found in the layers. In addition, there is a chance that earthquakes will be induced, especially if drilling near faults or in earthquake areas such as Limburg or Groningen. All kinds of measures can be taken to limit the impact of these risks. Seismic monitoring of geothermal wells is often used abroad to intervene if earthquakes start to occur.
Instead of using the subsurface for extracting heat, the surface can also be used for the temporary storage of hot water. Temporary storage can be a solution for the strongly seasonal demand for heat. The greatest heat demand is in winter, while in the summer the heat demand is limited. In periods of surplus energy or heat, this surplus can be stored in aquifers in the form of hot water.