Deep Geothermal Energy

Geothermal energy – heat from the Earth – is one of the less well recognized forms of renewable energy. This chapter explains the nature of deep geothermal energy, why it is treated as renewable energy, its usage over the past hundred years and its probable future. Several energy technologies are currently labelled ‘geothermal’ and it is important to distinguish between them: (1) Deep geothermal energy usually draws heat from hot rocks or aquifers that can be a kilometre or more below the Earth’s surface. (2) Ground source heat pump technology, described in Chapter 2, particularly that used for domestic heating, is perhaps best described as ‘shallow geothermal energy’ and only uses flows of heat between the air and the top layer of the ground down to a depth of about 40 metres. (3) Aquifer Thermal Energy Storage, also described in Chapter 2, is another ‘shallow geothermal’ heat pump technology and uses heat flows to and from underground aquifers, typically 100 metres below the surface. (4) Heat pumps may also be used in conjunction with deep geothermal wells to raise the temperature of water up to a usable level for heating buildings. As such they do have a place in this chapter. Deep geothermal energy is one of the few renewable energy sources that is constantly available. Although it used to be thought the preserve of regions prone to volcanic activity, that has now changed; new technologies have made this energy source more widely available and it is realistic to think that geothermal energy could meet a significant fraction of the world’s energy demand in the near and longer term future. This chapter documents its current usage and outlines the steps that are being taken to make this forecast a reality