True Geothermal

The use of ground source heat pumps is often described as “geothermal heating”. Although this is not incorrect, ‘true’ geothermal energy is derived from greater depths that those used for ground source heating and cooling applications. With typical heat pump applications boreholes are drilled to a maximum depth of about 300m. In true geothermal heating the depths can be several kilometres. There are two principal techniques used to harness the heat contained within the earth at these depths.

Hydrothermal reservoir

At certain places on the planet it’s possible to drill deeply and access and existing reservoir of hot water or steam. If steam is available it can be used directly to drive the turbines of an electricity generator. Where only hot water can be “mined” it’s necessary to turn it into steam at the surface using another method (gas or other fossil fuel) before it can be used to drive the generators.

Hot dry rock and magma

This method utilises the very high temperatures found within the earth at depths of several kilometres. First one or more very deep boreholes are drilled. Then using explosives a “seismic surge” is created to cause fractures in the rock between the boreholes. A fluid (normally water or brine solution) is pumped down the borehole. When this reaches the hot rock below it is turned into steam. The steam then returns to the surface via the cracked rock and additional boreholes.

Currently these techniques are largely experimental and not commercially viable. Some of the reasons for this are:

• The return steam brine/gases are high in methane
• The magma is too hot for conventional materials and processes to be used for commercial or domestic applications.