Hot rocks offer a long-term energy solution

wind doesn't always spin the turbines, the sun cannot shine on solar power stations at night, and even hydroelectricity can run short if the rains don't come.

The inherently erratic behaviour of renewable energy limits how much of these types of renewable power can usefully be fed into electricity grids. The engineering solution is to keep a large amount of reliable base-load power as a component of the generating mix and supplement this with "peaking plants" that can be brought on-line when needs arise. This capacity is built around hydroelectric systems in some countries, but usually it is based on gas, diesel or fuel oils. The base-load power, too, is based on fossil fuels, with around 39 per cent from coal.

The challenge, then, is to reduce our current reliance on fossil and nuclear fuels for base-load power. The answer may be under our feet. Earth is an extraordinarily hot planet. Six thousand kilometres below the surface, the planet's core is as hot as the surface of the sun. Yet, even at shallow depths, useful temperatures for power generation are often available. This "conventional" geothermal energy has been used to generate base-load electricity for more than 100 years, and is now used in many countries including Italy, Iceland, Japan, New Zealand, and the US.

The technology is well established, but conventional geothermal power requires a natural source of large quantities of steam or hot water, and such sources are usually found only in volcanic regions.

More tantalising for humanity's need for available, clean base-load power is the non-conventional geothermal energy called "hot dry rocks," or HDR. With HDR, useful heat is present in rocks only a few kilometres below the surface. But, with no natural steam or hot water to bring the energy to the surface, an engineered solution is needed, and, during the past 35 years, more than $600 million has been spent worldwide devising one.

The concept is simple: drill at least two boreholes five kilometres deep, inject cold water into one, pass it through the hot rocks, and then bring it back to the surface, where the energy is removed in a power station. Then re-inject the now cooled water for another pass through the subsurface. Only the heat is extracted at the surface, and everything else brought up to the surface is re-injected again, eliminating waste.

But the economics of HDR geothermal will eventually determine its long-term role, because deep boreholes are expensive to drill. Deposits of hot dry rocks are common, and large amounts of heat are within reach in many places. But the science and engineering has been challenging, and it is only now that the first power stations are emerging. A small station is operating in Landau, Germany, and others are under construction in France and Australia. These will develop the operational and financial performance histories that will be necessary before HDR geothermal energy can begin making an impact on world energy supplies.

The road to HDR geothermal energy has been long and expensive, but, like all developing technologies, the basic research and development had to be done before commercial development could follow. With power stations now being built, the signs are bright for widespread use of geothermal energy.

Prame Chopra is a founding director of Geodynamics Limited, the world's first hot rock energy company.

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