Geothermal energy is both an environmentally friendly and renewable form of energy. The underground is kept warm by the continuous flow of heat from the center of the earth, where the temperature is around 5,000 °C, and from the natural decay of isotopes in the earth’s crust. If the energy is not used in a geothermal plant, it will simply vanish out into icy cold space. Geothermal energy is therefore renewable in the overall perspective.
Locally, though, an over-exploitation is happening if compared to the inflow of energy from below. The cooled water from the injection well will after a period of 25-30 year (depending on the spacing between the wells) begin to influence the temperature in the production well. The plant can continue to produce heat for years, but the production temperature and through that the plant output will over time decline slowly. If it is at some point estimated, that continued production on this location is no longer economically viable, a new plant can be established a few kilometers away.
When a location is abandoned, the reservoir will slowly be reheated. It will though take a very long time – up to 5,000-6,000 years – before the reservoir again comes close to its original temperature.
A geothermal plant does normally not cause any direct emissions to the air – neither of CO2, NOx, SOx, particles, heavy metals or similar unwanted components. By substitution of district heating production based on coal or natural gas, substantial emission reductions can be obtained.
There can though be indirect emissions of CO2 connected to the extration of geothermal energy. Electricity is needed to operate the geothermal plant, especially for the production and injection pumps. Furthermore, the steam that is in some cases used for powering the absorption heat pumps and ends up as district heating could – depending on the plant layout – have been used for production of electricity through a low pressure steam turbine. In this case there is a loss of electricity production related to the steam usage.
All in all, the combined electricity consumption and possible loss of electricity production is in the order of 5-20 % of the produced amount of heat. As part of the electricity that is used in Denmark is still based on fossil fuels, there will be an indirect emission connected to the electricity consumption of the geothermal energy plant. Likewise, diesel is used in the logging and transportation of wood chips or pellets, electricity is used for powering the circulation pumps in a solar thermal plant, etc. As the fossil fuels are phased out in the electricity production, the small CO2 emissions connected to the extraction of geothermal energy will be gradually lowered.
During operation of the geothermal plan it will furthermore be necessary to dispose of some geothermal brine to a recipient, typically the sea. The need typically arises in connection to startup of the plant after prolonged periods of plant shutdown and after repair works, where oxidized and highly saline geothermal brine should not be injected into the reservoir. Through a sensible plant design and operation, the disposal of warm, saline water to e.g. the sea will not pose an environmental problem.