Hot Rocks

The main problem with renewable energy sources has always been that they can’t be relied upon to provide power 24 hours a day, seven days a week. But new geothermal technology promises to reduce our dependence on fossil and fissile fuels.

CANBERRA – Ask any power system engineer about renewable energy and you are likely to be told that it doesn’t deliver “base-load” power. In other words, renewable energy can’t be relied upon to provide power 24 hours a day, seven days a week: wind doesn’t always spin the turbines on the hill, 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 behavior of the major renewable energy technologies presents serious problems for power system planners. It limits how much of these types of renewable power can usefully be fed into the world’s electricity grids. After all, consumers expect power always to be available.

The engineering solution is to keep a large amount of reliable base-load power as a major component of the generating mix and supplement this with “peaking plants” that can be brought on-line when needs arise. This peaking capacity is built around hydroelectric systems in some countries, but usually it is based on burning fossil fuels such as gas, diesel, or fuel oils.

The base-load power, too, is predominantly based on fossil fuels, with around 39% of global electricity generation sourced from burning coal. In some countries, nuclear power has been seen as an answer, but deposits of high-grade nuclear fuel worldwide appear to be limited, and the long-term costs of waste storage and plant decommissioning are high.

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 kilometers 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 reliable base-load electricity for more than 100 years, and is now used in many countries including Italy, Iceland, Japan, New Zealand, and the western United States.

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The technology is well established, and the track record of reliable power generation includes more than 9000 megawatts of generating capacity. 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, which rules out its use in large parts of the world.

More tantalizing for humanity’s need for widely 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 kilometers below the Earth’s 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 beguilingly simple: drill at least two boreholes five kilometers 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 that is brought up to the surface is re-injected again, eliminating waste.

But it is the economics of HDR geothermal that will eventually determine its long-term role, because deep boreholes are expensive to drill, and their costs must be met before power stations can begin to generate electricity. The shallower the heat resources and the cheaper the capital, the more competitive an HDR project will be. The rising costs of fossil and fissile fuels will also make HDR more compelling, since the long-term economics of geothermal power is effectively quarantined from fuel price movements.

Deposits of hot dry rocks are common, and large amounts of heat are within reach in many places. But the science and engineering of HDR has been challenging, and it is only now that the first power stations are emerging. A small power station is operating in Landau, Germany, and others are under construction in France and Australia.

These first power stations 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. Re-engineering humanity’s power systems is going to be an expensive undertaking, regardless of what mix of technologies are used, and the chosen systems will have to be reliable and widely available.

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 to generate clean, emissions-free base-load power.

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