Geothermal energy in a sustainable built environment

Abstract

Geothermal energy is usually perceived to be about gushing geysers and bubbling mud pools and limited to only the small volcanically active parts of the Earth’s surface. Nothing could be further from the truth. Geothermal energy is in fact an incredible store of energy found in all parts of the world which is beginning to be understood and used for our sustainable future. There are two basic forms of this energy. One form (sometimes referred to as hot dry rocks or enhanced geothermal systems), makes use of the heat (>200°C) in the rocks at depths of up to about 5 kms to produce electricity from extracted (but returnable) hot water. There are several locations around the world where “proof of concept” stage has been or about to be reached suggesting that within the next few years, these systems may be providing a significant proportion of our base-load electricity. The other form makes use of the heat (and the cooling potential) of the soils and rocks within the upper few tens of metres from the surface to heat and cool buildings. It involves the circulation of a fluid through pipes built into building foundations or in specifically drilled boreholes, and back to the surface where heat stored in the fluid is extracted by a heat pump, and used to heat a building. The cooled fluid is reinjected into the ground loops to heat up again to complete the cycle. In cooling mode, the system is reversed with heat taken out of the building transferred to the fluid which is injected underground to dump the extra heat to the ground. The cooled fluid then returns to the heat pump to receive more heat. There are many thousands of these systems installed around the world but many counties have been slow to pick up on their enormous potential. The paper explains how these systems work and looks at some of the issues which require attention in the near future for geothermal energy to become a truly sustainable, renewable and most importantly, continuous, energy source.