Geothermal energy, extracted by pumping water through hot rocks underground and using the resulting steam to generate electricity, can replace more greenhouse-intensive energy such as electricity from coal.
Geothermal Power. Wikipedia
Geothermal power is electricity generated by utilizing naturally occurring geological heat sources. It is a form of renewable energy.
Large scale electrical generation is possible in areas near geysers or hot springs by utilizing naturally occurring steam, superheated ground water or using geothermal heat to heat a heat-transfer fluid.
Experiments are testing whether a fourth method, deep wells into "hot dry rocks" can be economically used to heat water pumped down from the surface.
Hot Rock Energy
Hot Rock Energy. ANU
Hot Rock Energy is a vast, environmentally friendly, economically attractive energy source.
Water is injected into a borehole and circulated through a "heat exchanger" of hot cracked rock several kilometres below the surface. The water is heated through contact with the rock and is then returned to the surface through another borehole where it is used to generate electricity. The water is then re-injected into the first borehole to be reheated and used again.
Hot Rocks. Quantum. ABC
Cold water from the surface is pumped at high pressure down a hole the diameter of a large jam jar. Three-to-five kilometres underground, the cold water is forced sideways under pressure through tiny fractures in the hot dry granite.
Underground granites are not very permeable. Before enough water can be forced through them, natural fractures in the rock have to been widened. The way it's done is to pump water down the borehole at immense pressures - pressures high enough to overcome the weight of four or five kilometres of earth above.
Australia's geological stability works in its favour. It's so far from active volcanic zones, our underground granite bodies are held in the vice-like grip of an unyielding continent. When water is pumped down to widen the cracks here, the result is horizontal fracturing, making extraction of heat much more efficient.
In geologically unstable locations. When water is pumped down to enlarge the cracks in hot rocks, the result is vertical fracturing. That's bad because, when water is eventually pumped in to extract heat, it's forced downwards (making it expensive to recover) or upwards into cooler rock. Either way, it's inefficient.
Surprisingly, underground rocks wouldn't stay hot if they relied on heat bubbling up from the earth's core. Most of the earth's crust is in fact kept hot by the decay of radioactive elements within granites.
If you had one cubic kilometre of rock at a temperature of say 250° and you drop the temperature of that cubic kilometre by 100° then the energy equivalent that you would obtain is the energy equivalent to about 40 million barrels of oil which is quite a large oil field by Australian standards.
Hot Dry Rocks
Hot Dry Rock Geothermal Reservoir Development. Australian Greenhouse Office
The hot dry rock program aims to generate carbon-free electricity by circulating water through hot subsurface granites.
Under certain conditions, subsurface granites can reach 250C and higher at depths of 3 to 5 kilometers. These granites are hot for a number of reasons. They are relatively high in decaying radioactive elements, and heat is conducted from very hot sources below. In most cases (and preferably), the granites are buried beneath thick insulating sedimentary rocks.
The aim of a hot dry rock (HDR) program is to harness the energy in these granites by injecting water into a borehole and circulating it through a permeable reservoir created by hydraulically fracturing pre-existing, minute cracks in the rock. Success primarily depends on the presence of these natural fractures. The injected water is superheated as it passes through the hot rocks and returned to the surface via adjacent boreholes, where it is converted to carbon-free generated electricity using conventional steam turbine technology.
Geothermal Energy. Energy Australia
Geothermal energy is energy from the heat of the earth. It has been used for thousands of years in some countries for hot water, cooking and heating. It can also generate electricity using steam produced from heat found beneath the surface of the earth.
One form of geothermal energy is called "hot rock". This is where water is pumped below the surface to areas of hot rock. The water then turns to steam, and is pumped back to the surface to drive a turbo-generator.
Hot Rock Energy. Earthbeat. ABC. 2004.6.19 - Scientists say that pumping water underground to heat it and drive clean electricity production is just around the corner.
Hot Dry Rock Geothermal Energy. Earthbeat. ABC. 1999.1.30