Prame Chopra, former professor of geophysics in the National Australian University, thinks that the energy of “hot dry rocks”, known as geothermal energy, is the answer to humanity's future power requirements.

Put the question of renewable energy sources to any electricity network engineer and he will probably say to you that they do not give constant energy. In other words, it isn't possible to rely on renewable energy sources to feed power networks 24 hours a day, seven days a week: the wind does not always turn wind turbines, the sun does not shine at night on the panels of solar power stations and even hydroelectric energy can be lacking if it does not rain enough.

The inherent unforeseen nature in the main renewable energy sources introduces serious difficulties for the planning of electrical systems.  The technical solution is to make sure that a reliable constant energy supply is the main component of the different sources of production of electricity, supplied by "peak load power stations" integrated into networks when necessary. In some countries, this peak capacity is based on hydroelectric energy, but it is mostly assured by fossil fuels, such as gas, diesel and fuel oil.

Constant energy is also assured by fossil fuels and about 39% of the worldwide production of electricity is based on coal combustion. Some countries have made the bet on nuclear technology, but deposits of good quality nuclear fuels seem to be limited, and the the storage costs of waste and the decommissioning of the power stations are high.

  The challenge is therefore to reduce our current dependence on fossil fuels and nuclear for the basic energy production. The solution could be well under our feet.

The Earth is an incredibly hot planet. At 6000 kilometres under the surface, the Earth's core is as hot as the surface of the Sun. But even at lesser depths, there are sufficient temperatures to produce electricity. This geothermal energy has been used to produce reliable constant energy for more than hundred years, and is commonly used in many countries, including Italy Iceland, Japan, New Zealand and the West coast of the United States.

The technology is well controlled and world production comes to more than 9000 megawatts. But conventional geothermal energy uses natural sources of steam or hot water which are usually found in volcanic regions.  This limits its interest for most of the world.

There is another form of more interesting geothermal energy, which would cover the requirements of humankind for clean and constant energy: the technology of "hot dry  rocks" (HDR). This uses heat present in the rocks at only a few kilometers below under the surface of the Earth. But without natural hot steam or water to transport heat to the surface, a technical solution must be found, and in the course of the last 35 last years, more than 600 million dollars have been spent in the world trying to find one.

The answer is surprisingly simple:  bore two wells at least five kilometres deep, inject some cold water into one, which passes across the faults of the hot rocks and then bring it back to the surface, where energy is recovered in a power plant. Once the energy has been recovered and the water has cooled, simply re-inject the cooled water down for a new journey through the rocks. Only heat is gathered at the surface, everything else is permanently re-injected, eliminating the problem of waste.

Ultimately it will be the cost of Hot Dry Rock geothermal technology which will determine its long-term role, because the drilling is expensive, and because the investment must be found before power stations can produce geothermal electricity. The less deep the sources of heat are, and the less the equipment costs, the more competitive hot rock geothermal energy becomes. The increasingly higher pricesof fossil and nuclear fuels also make this technology more attractive, the long-term economy of geothermal energy being in fact dependant on the movements of oil prices.

The scientific and technological aspects of  HDR geothermal energy have presented many difficulties and it is only today that the first power plants have begun to come into the world. A small power station operates in Landau in Germany, and others are under construction in France and Australia.

The first power stations will have to go through the obligatory operational and financial stages before the hot dry rock geothermal energy can start to truly influence the world energy reserves. The reorganization of the global electricity networks will be an expensive task, whatever the combination of technologies used. The systems selected will also have to be reliable and accessible on a large scale.

The construction of a new alternative energy powerplant is not just the building of a lifeless structure, but it is a sign of hope for a more generalised use of geothermal energy, an inexhaustable source of constant energy without a carbon footprint.

One such source of hope for the future is the Soultz-Sous-Forêts geothermal power project in France, at the western border of the Rhine Graben which began producing electricity in June 2008.  Launched in 1987, this project took 6 years of study. It aims at proving the feasibility of the use of Hot Dry Rock geothermal power.

At the Soultz-Sous-Forêts site, water can be injected to a depth of 5,000 metres.  By thermal exchange the water returns to the surface at a temperature of 200°C and with a flow of 100 liters per second. This heat is then transferred to a separate cyclical fluid system which, creates steam to power an electrical turbine.

If successful, this central pilot project will be followed by an industrial 20 Megawatt prototype power station, providing energy for a city of 20,000 inhabitants in 2015.

The energy challenge demands structural answers. Developed countries like France have little or no control over the price of oil. There is, for them, only one sustainable outcome: to become less dependent on the traditional forms of energy.

Based on an article by JMT