91av

THE people of Aachen in western Germany are no strangers to hot water bubbling up from deep inside the Earth. The city was built on a hot spring and is still renowned for its spa. But Aachen may soon have another source of hot water to be proud of. A team of geologists and engineers hopes to sink a huge borehole in the city centre and show the world that geothermal heating is there for the taking, anywhere on the planet.

The idea of using the Earth’s natural warmth to heat homes and businesses is an old one. As early as 1910, steam from hot aquifers in and around Larderello in Tuscany was used to heat buildings and greenhouses. And in 1928 Iceland began using its geothermal hot water for central heating. Today, geothermal heating is used in 58 countries worldwide, according to the International Geothermal Association.

Heat from the Earth has obvious attractions. It is free, clean and abundant. Yet despite a century of development, its contribution to the world’s heating bill remains negligible. The problem is that extracting heat has only been possible where unusual geological features push hot water or steam close to the Earth’s surface.

Hot aquifers, for example, can be tapped using something called dual-well technology: you drill two boreholes, one to extract the hot water and the other to return it once it has cooled down. But such geological features are few and far between, and it is very rare for large populations to live on top of one – an absolute must for geothermal heating because you cannot transport heat long distances.

Engineers have attempted to overcome this limitation using the “hot dry rock” method. The idea is to make an artificial hot aquifer by drilling two boreholes into deep rocks then fracturing the rock between them so water can percolate from one to the other. Then you can pump cold water down one borehole and extract it hot from the other.

Hot dry rock can work, but it is expensive and failure is common. Most countries are abandoning it, or looking to use unusual geological features, which rather defeats the object. In Sweden, for example, geologists are investigating the possibility of extracting heat from pre-fissured rock under the Björkö impact crater close to Stockholm (91av, 28 October 2000, p 10).

Shallow geothermal systems have also been developed to extract heat from around 100 metres down. They are more common under new houses, but the heat they produce is low grade and requires an electric heat pump to concentrate it. Although they do cut heating bills, their environmental impact is minimal.

Economics has also played a part in holding back geothermal heat. Following the oil shocks of 1973 there was a surge of interest and the European Union backed a series of projects but support fizzled out in the 1980s as oil became cheap again.

Still the idea of tapping the ground for its abundant heat hasn’t gone away. All over the globe, the temperature of the crust increases the deeper you go, typically by 30 to 33 °C per kilometre. Surely there must be a reliable and economical way of extracting this heat wherever it is needed? That’s what the Aachen project hopes to prove.

The technology at its heart is called a deep geothermal heat-exchanger, which consists of a single borehole sunk 2.5 kilometres into the Earth’s crust. The hole will act as both source and sink – cold water in, hot water out. That makes it simpler and more reliable than a hot dry rock system. And according to the consortium of architects and engineering firms behind the project, you could install one under any building anywhere in the world.

Deep geothermal heat-exchangers already exist, but haven’t quite taken the world by storm. The first was built – somewhat by accident – in Prenzlau in north-eastern Germany several years ago. It was intended to be a hot dry rock system but the second borehole failed, and the designers had to dream up a way of exploiting a single borehole. The exchanger they put in produces heat, although nowhere near as much as intended. Since then three more deep geothermal heat-exchangers have been built, all in Switzerland. All three appear to be a success but none has released any public information so they have yet to inspire copycats.

Aachen will be different. It is intended to demonstrate the value of the technology to a sceptical world. “We want to show that the project is applicable anywhere,” says Roland Gaschnitz of the Institute of Mine Surveying at the Technical University of Aachen, who is overseeing the technical issues surrounding the drilling and heat-extraction.

But surely Aachen’s hot spring is exactly the kind of geological oddity the proponents of geothermal heating need to get away from? Not so, according to Gaschnitz. “The hot springs are 500 metres away, but there is a big geological fault between us and the spring,” he says. As a result, the ground they will drill into probably has a normal thermal gradient, give or take a few degrees.

“There are two possibilities,” Gaschnitz explains. “The water may help us by heating up the rocks a bit, and we might get more heat than we expect. But the crust is also thickened here. It is part of the Rheinisch Massif, a hilly region where two continents collided 200 million years ago. This might mean that the thermal gradient is slightly lower than normal. We will see.”

Drilling is scheduled to begin in July. Within three months the hole should be 2.5 kilometres deep, reaching rock where the temperature is at least 80 °C. Once the borehole is finished, a large building called SuperC will be erected on top of it. This will be the university’s central hub with offices, conference rooms and a cafe. The borehole will provide all the heat the building needs in winter, as well as enough energy to cool it in summer. By replacing fossil-fuel energy, the Aachen team reckon the borehole can reduce the University’s CO₂ emissions by 380 tonnes a year.

The heat-exchanger itself consists of an outer steel pipe 19 centimetres in diameter and an inner pipe made of fibreglass. Cold water will flow down the outer pipe and heat up as it descends. It will then be pumped up through the inner pipe and into the building, insulated on its way by the fibreglass.

Outside the steel casing is a layer of concrete that connects the pipe to the rock, vital for allowing heat exchange. “This is an important problem,” says Gaschnitz. “A small gap between the concrete and the rock will reduce performance dramatically. Also, concrete is not a good thermal conductor. We are working to develop a concrete with a high thermal conductivity which also does not shrink when it solidifies.” One of the companies in the consortium is Anneliese Zementwerke of Ennigerloh, which produces concrete for the oil industry, and is working on a new type of concrete for heat exchange. It will contain fine metal grains or graphite in order to raise its thermal conductivity.

In the winter, hot water from the heat-exchanger will be piped through the building’s walls, floors and ceilings to keep it at a toasty 22 °C. The spent water will then flow back into the heat-exchanger. During the summer, the hot water will be fed into an air-conditioning system called an adsorption cooler. This works by evaporating water under a near-vacuum, cooling the air just as an alcohol swab cools the skin. And when the weather is mild the building can be kept comfortable by reducing the volume of water flowing through the heat-exchanger.

The biggest unknown is exactly how much heat can be extracted from the rocks. SuperC’s architects and engineers are assuming the heat-exchanger will provide water at 70 °C, but Gaschnitz admits that’s an estimate. “At the moment we lack data to calculate the amount of heat we can extract. All we can do is use models, but that is just a best guess.” The scarcity of data from the wells in Prenzlau and Switzerland hasn’t helped, but Gaschnitz says he is confident his estimate is no more than 10 per cent out.

Outside observers are not so sure. Alain Gringarten, director of petroleum studies at Imperial College, London, was involved in geothermal projects in France during the 1970s. He is very sceptical that SuperC will deliver the amount of heat its supporters claim. “To exchange heat you have to think about conductivity and surface area,” he says. “If there is only one well there is not much surface area. And then you have to bring the water to the surface – how much heat will you lose on the way up? It strikes me as a scheme to get funding, but I think it is far less likely to get significant energy from it.”

The economic benefits are also debatable. Even if oil and gas prices rise significantly, it will take at least the first 16 years of the heat-exchanger’s 30 to 40-year lifespan to recoup what it cost to build. In a world that typically demands returns within two years, such a long payback time is a serious problem. Projects like SuperC are not going to replace gas or oil heating systems overnight.

But in Germany at least they may find a receptive audience. Two other commercial geothermal heat-exchangers are already on the drawing board for the Ruhr region, to the north of Aachen, and if the demonstration project is a success there will probably be more. According to Brian Cody, an associate director of Berlin-based architectural engineers Ove Arup, German builders are very aware of green issues, and environmental concerns are high on the agenda for almost every new building in the country. Even Berlin’s revamped Reichstag building is heated using geothermal energy, thanks to its location above a natural aquifer.

The Aachen team want to go further, however. The main benefit of geothermal heat is that it reduces greenhouse-gas emissions – something the EU is politically committed to but apparently lacks the vision to achieve. The project’s backers see deep geothermal heat-exchangers as the solution, and they’re determined to win the politicians round.

That’s quite an ambition for a hole in the ground.