Geothermal heating - efficiency

The idea of geothermal energy sounds great - free heat from the ground! Is this always a good way to heat houses in an environmentally friendly way?

For geothermal heating, heat is usually obtained by pumping water underground where the soil and rocks are warmer, and then back up to the surface. The energy is recovered from the heated water using a heat exchanger. But these processes require energy to run the pumps and other equipment. So the usefulness depends upon how much heat you get out for the energy you put in to run the equipment.

In countries like Iceland where there are hot volcanic rocks just underground, the heat energy that can be transferred is very great compared to the energy required to extract it. In other locations such as the UK, it can be harder to tell whether it is always worth it.

Case Study: domestic ground source heat pumps