Hot rocks doon the toon (Geothermal energy)

Geothermal energy refers to heat (thermal) derived from the earth (geo). This article deals with the use of heat from rocks deep within the earth (typically 2 to 5km) and the technology is therefore often referred to as deep geothermal technology. As inferred by the title, there is a local involvement with this technology that is highlighted below. (The heat from a few metres below the surface may be extracted by the use of ground source or air source heat pumps, which are typically installed by individual householders to heat their homes.)

The source of the heat

The earth has a layered structure—a solid crust at the surface, a viscous molten mantle, a runnier liquid outer core and a solid inner core. With increasing depth the temperature rises and whether a given layer is solid or liquid is dependent on its constituents.

The temperature of the inner core may be about 7,000oC though estimates vary.

The earth’s internal heat has two sources:

(i) its formation 4.5 billion years ago through accretion (the coming together) of dust and gas from the Sun’s formation.

(ii) radioactive decay, which is the break-up of large unstable atoms of certain elements in the upper layers (mantle and crust).

Estimates of the contribution of each source vary—from 50% to 90% in the case of radioactive decay.

Harvesting the energy

In deep geothermal technology the heat from the rock is harvested from bore holes drilled to depths of up to 5 km. Water is pumped down one well. Heat is transferred from hot rock to water, which is then pumped to the surface via a second well. The hot water can then be piped to buildings (as in figure 1). A district heating scheme like this currently operates in Southampton.

Figure 1. Geothermal energy harnessed to heat buildings

A similar scheme is being developed at Newcastle Science Central by the Newcastle Institute for Research on Sustainability at Newcastle University. A bore hole has been drilled to a depth of nearly 2 km on the site of the old Newcastle Brewery. The temperature at the bottom of the well is sufficient to heat the water to around 80oC. The possibility that the hot water could be distributed to heat buildings in Science Central and adjoining areas, including the city centre’s Eldon Square shopping mall has been considered.

Electricity generation
Sometimes a rock source is hot enough to boil the water. The resulting steam drives a turbine to generate electricity. If the rock source only provides hot water, the latter can via a heat exchanger transfer heat to another fluid that has a lower boiling point. The resultant vapour can then drive a turbine for electricity generation.

Major developments in geothermal power plants are planned for the south west of the UK starting with a site in Redruth, Cornwall, where drilling is expected to start later this year.

Reduction in CO2 emission and other attractions of geothermal

The potential of geothermal to significantly reduce CO2 emissions is enormous. Although some CO2 results from the combustion of hydrocarbon fuel conventionally used to provide the energy for drilling, the actual heat transfer from rock to water involves no CO2 production (in contrast to fossil fuel-fired power plants). Furthermore, it constitutes a constant source of energy that can be drawn upon according to demand, whatever the weather (unlike wind, solar or wave). 

Alan Myers
2 March 2012 (revised 29 June 2013)