Energy: Pressure cooker Earth

Geothermal heat from a depth of 4,000 metres to generate power in the future

Why roam far afield, when the good is so close at hand? Humankind burns coal, gas and oil to generate energy. It splits atoms, converts sunlight into electricity and tries to capture the wind. Yet, the earth beneath our feet contains energy enough to satisfy even the greatest demand, because 99% of the Earth is hotter that 1000°C, while 99% of the rest is actually hotter than 100°C. Only the Earth’s surface is comparatively cool. The deeper one goes, the warmer it becomes: on average by 3° Celsius per 100 metres. But how can this geothermal heat be brought to the surface? How can it be used to generate electricity? Scientists at the GeoForschungsZentrum Potsdam are studying questions like these.

Steam and sulphur

In the Larderello Valley, Italy’s region of Tuscany has a completely different face to that seen in travel catalogues. Steam hisses, clouds of fog and mist drift across the ground and there’s a smell of sulphur in the air. Larderello is where the idea of generating electrical energy from geothermal heat was born. The year 1904 saw Count Piero Ginori Conti make five light bulbs glow by means of volcanic steam. Today, the Italians generate 750 megawatts of power from Larderello’s geothermal power stations that meanwhile operate without sulphur precipitation: that is enough energy to light up around 12.5m 60-watt light bulbs. By contrast, one of Germany’s most modern lignite mines, “Schwarze Pumpe” south of Cottbus, has two blocks which each generate 800 megawatts of power.

Larderello has remained an exception. Other than landscapes like those of Iceland, where volcanic geothermal heat is present very close to the surface, this energy is hard to reach. The first German geothermal power station went into operation in autumn 2003 in Neustadt-Glewe in the state of Mecklenburg-West Pomerania. It uses 98° hot water from 2,200 metres below the surface to generate power. However, it only manages around 200 kilowatts, less than 1/20th of the largest wind turbines. In the summer, the publicly-funded plant supplies up to 500 households with power, in the winter it provides the hot water to heat the little town.

The water cycle

At their geothermal plant in Groß Schönebeck, just north of Berlin, the researchers at Potsdam are pursuing the same approach as at the Neustadt-Glewe power station, only this time with a 150°C reservoir temperature. They use two drill holes to create a so-called thermal water cycle. The temperature of the water drawn from these hydrothermal reservoirs is just under 100° at Neustadt-Glewe (from a depth of 2.2 km) and 150° at Groß Schönebeck (from a depth of 4.3 km). Once at the surface, the energy is drawn from the water by means of a heat exchanger. The water is sufficiently hot to generate electrical power. Then, the water is fed back underground through the second drill hole, where this returning water increases the pressure in the reservoir so that the water can be drawn to the surface again through the first drill hole and so on. The advantages of this method are that there is no need to transport it over long distances on the surface where heat could be lost unused. Furthermore, the same water can be used time and time again, without producing any wastewater or residues.

“Stimulating” the rock

This water cycle has not yet been turned into reality. The difficulty lies in the fact that the deep-lying rock is normally not permeable enough to allow the water to circulate. In many experiments, the scientists from Potsdam pressed water into the rock at such high pressures that existing cracks and cavities were artificially enlarged. This gradually makes the rock more permeable. This so-called stimulation technique, known from the oil industry, was used in and developed for hydrothermal geothermics for the first time.

“After a series of stimulation experiments, we have now reached a degree of productivity which makes the generation of power from geothermal heat interesting for the energy industry as well, even under the conditions prevailing here,” explains Dr. Ernst Huenges, Head of the Geothermics Section at the GeoForschungsZentrum Potsdam.

Creating the cycle

This is why the next step involves the drilling of a second deep hole, around one kilometre away from the first one. “Once we’ve done this, we want to spend several months carrying out experimental studies on whether the system of cracks we created in the deep-lying rock is suitable for running the water cycle long-term,” says Huenges. It is important to prove this sustainability. “Because the investment in this form of power generation is only worthwhile if production can be assured long-term,” explains Huenges. Together with an industrial partner, the Potsdam researchers eventually plan to build a geothermal power station at the Groß Schönebeck site that will serve as a demonstration plant and will facilitate process-technology and energy-industry studies.

Incidentally, investors in geothermics receive a government-guaranteed payment for the power they generate in their plants, as governed by the Renewable Energies Act. In 2000, the German government included geothermics in this act and once again improved the conditions in the 2004 amendment to the act. Federal Environment Minister Jürgen Trittin sees geothermal heat as “an exciting chapter in our future energy supply, an energy supply without climate and environmental problems, without marine pollution, without war for the last resources, and without the unforeseeable dangers of radioactivity.”

Providing planning certainty for geothermal power generation

The research work done so far at Groß Schönebeck makes Huenges optimistic for the future of geothermics in Germany. Because in the past it was fraught with major cost risks: kilometre-deep drill holes cost several million euros. The danger of a failure is much smaller, when the rock can be changed by means of stimulation technology to allow the water cycle to work reliably. In addition, the ground beneath the Potsdam researchers’ test site is typical of that of Central Europe; the experimental studies done here can with great probability also be transferred to and applied in other regions. And this means good prospects for the geothermal heat that is so close at hand being able to establish itself in the medium term as a renewable energy source.