Hot Rocks
During the past 50 years, Australia and the continental shelf has been the subject of extensive geological and geophysical surveys, in the search for oil and gas-bearing strata. As a result, there is a large existing database of geological survey information for Australia.
Volcanic geo-thermal energy sources are scarce in Australia. However, a number of large (granite) hot rock bodies have been identified, at depths of 2 - 5 km, which together potentially produce sufficient renewable thermal energy to satisfy the total energy needs of Australia for the next 500 years. The heat is generated by the radioactive decay of the granite, and is not dependent upon the conduction of thermal energy from the earth's core.
Central Australia also has large reserves of water, stored in subterranean acquifers. As these are drawn upon, they must be recycled by the absorption of surface water or by pumped storage.
To extract the energy from a hot rock, it is necessary to fracture the rock body, in order to make it permeable to a liquid, e.g. water, so that the heat may be transferred to heat exchangers for power generation, at the surface. The geological structure of Australia is such that, if water is pumped under pressure into a borehole, it will cause fine horizontal fissures in the granite, producing a large increase in permeability to the steam. In production, the steam is extracted from several other spaced boreholes. Early trials in other countries were only moderately successful, because the granite fissured vertically.
Experimental drilling and testing by a public Company was well-advanced at Innamincka in the Cooper Basin by May 2008. Feasibility testing was completed by the end of April 2008, and production trials were commenced. The Company has fulfilled its undertaking to provide power for Innamincka.
Other hot rocks energy projects were in progress by Geodynamics Ltd (Queensland/SA border), Torrens Energy (Port Augusta) and Petratherm (Beverley uranium mining project) and were expected to come on line within the next 3 to 5 years. Altogether, at May 2008, a total of 8 hot rocks power stations were planned or in progress in Australia.
The use of hot rocks for power generation seems to offer a low-cost, non-polluting solution to Australia's energy needs and the technology has an adequate life-time for the forseeable future. The construction of more hot rock power stations should be progressed as a matter of urgency, if the feasibility and economics of the process can be established. However, it should be noted that the steam temperature of 200C - 250C is much lower than is commonly used in thermal power stations, resulting in reduced efficiency and limiting power station capacity. Rankine Cycle heat engines, using organic fluid, are necessary.
Also, there have been concerns overseas that the drilling of deep bores for hot rocks power generation might trigger severe seismic disturbances, and the drilling of the deep wells has proved to involve technical problems. In Australia, it has been feared that the drilling of deep wells for hot rock power generation, or for the "fracking" of oil-bearing strata, may result in the contamination of water acquifiers. As far as may be determined, there is very little activity internationally in hot rocks power generation.