Energy underfoot: Bringing up heat from inside Earth
Posted by Unni Skoglund
Ever since Jules Verne wrote in 1864 about a trip to the Earth’s interior, people have dreamed of bringing up heat from the centre of the planet. So far we have only scratched the surface, but researchers are now beginning to work down into the depths.
The fact is that 99 per cent of the planet has a temperature above 1000 degrees Celsius. The heat is what’s left over from when the Earth was first formed, and there is more than enough of it for us to transform it into energy.
“If we can drill and recover just a fraction of the geothermal heat that exists, there will be enough to supply the entire planet with energy – energy that is clean and safe,” says Are Lund, senior researcher at SINTEF Materials and Chemistry.
Geothermal heat offers incredible potential. It is an inexhaustible energy source that is nearly emission-free. Heat energy is found in the different rock types that make up the Earth’s surface, and deeper in the crust. The deeper you get, the hotter it is.
Around one-third of the heat flow comes from the original heat in the Earth’s core and mantle (the layer closest to the Earth’s crust). The remaining two-thirds originate in radioactivity in the Earth’s crust, where radioactive substances continuously decay and generate heat. The heat is transported to rock layers that are nearer the Earth’s surface.
Geothermal energy that comes from 150-200 metres below the surface is called low temperature geothermal energy. At these depths, temperatures hover between 6 and 8 degrees C and can be extracted with heat pumps, combined with an energy well. This type of geothermal energy is exploited at a fairly large scale.
The Norwegian company Rock Energy wants to be an international leader in geothermal heat and energy. A pilot plant has been planned for Oslo that will collect heat from 5500 metres deep. Temperatures from this depth can heat water to 90-95 degrees C and can be used in district heating plants. The pilot plant will be built in cooperation with NTNU, which is studying the thermal aspects of the plant.
The plan is to drill two wells, an injection well where cold water is pumped down, and a production well where hot water flows back up. Between these will be so-called radiator leads that connect the wells. The water is then exchanged with water in Hafslund’s district heating plant.
The normal lifespan for a well like this is approximately 30 years. After that the rock will be so cooled by the cold water that has been injected into the wells that it will no longer produce enough heat. However, after 20-30 years, the heat will have built up again, and the well can be used once more.
The Rock Energy facility will be a major step forward in exploiting Norway’s geothermal heat resources.
However, if we want to reduce CO2 emissions and provide clean energy on a scale that will make a difference, we will need go much further down into the Earth itself.
Researchers at NTNU, the University of Bergen (UiB), the Geological Survey of Norway (NGU) and SINTEF believe this is possible. In 2009, deep geological energy enthusiasts formed the Norwegian Centre for Geothermal Energy Research (CGER), with partners from universities, colleges, research institutions and the industry.
The researchers’ goal is to reach depths of 10,000 metres or more to exploit deep geothermal heat. Drilling that deep will enable wells to reach what is called supercritical water with a temperature of at least 374 degrees C and a pressure of at least 220 bar. That multiplies by a factor of 10 the amount of energy you can extract from such an arrangement, and the amount of geothermal energy produced can match that created in a nuclear power plant.
But there is a very important difference: Geothermal heat does not create radioactive waste. It is clean energy.
Pros at 5000 metres
Today’s oil companies are making a good living by extracting oil that is as deep as 5000 metres, where temperatures are as high as 170 degrees C. Drilling any deeper than this results in a range of engineering problems, both in terms of the drilling itself and materials. Steel becomes brittle, and materials such as plastics and electronics will be weakened or melt. Electronics operate normally only a short time at temperatures hotter than 200 degrees C. These problems will have to be solved for the deep geothermal industry to be profitable.
Nevertheless, SINTEF scientists think that Norway is in a unique position to capture geothermal heat.
“We have a strong and innovative oil industry this country. Because the oil industry has wanted to develop oil and gas deposits from inaccessible areas, drilling technology has evolved tremendously over the past ten years. There are test wells for oil that go 12 000 metres into the Earth. Knowledge from the oil and drilling industry may be used in the future to capture geothermal energy,” say Lund and Lademo.
The Norwegian drilling and oil and gas industries all demand equipment that makes it possible to drill ever deeper at an affordable cost. The oil fields that are being discovered now are generally deeper and more complicated than before. Even though there have been a number of wells in the world that have been drilled to 10-12 000 meters, the technology does not yet exist to allow for precision drilling at these depths.
“We have to have a common commitment. Multidisciplinary expertise is required. Here at Materials and Chemistry, we are working with an internally funded project in which we are assessing SINTEF’s overall ability to contribute. The goal is to work on projects with industry and the Research Council of Norway,” Lund said, adding, “If research and industry succeed in developing the materials and technology needed to bring up the most difficult-to-reach oil, in the long run we will be able to replace oil with geothermal energy for heating and electricity.”
One of the unique aspects of geothermal heat is that it is found everywhere throughout the world. Call it a “democratic” energy source that anyone can take advantage of, regardless of the conditions at the Earth’s surface, such as the weather.
How far down you have to drill into the Earth’s crust to reach the temperature that you’re interested in varies from country to country. This is because the crust varies in thickness, and controls what is called the geothermal gradient. At more northerly latitudes, like Norway, the temperature increases by about 20 degrees per kilometre into the Earth’s crust. In other parts of the world, it is 40 degrees per kilometre. The average is around 25 degrees.
The United States, the Philippines, Mexico, Indonesia and Italy are the international leaders in terms of producing electricity from geothermal energy.
“It will succeed”
“The oil and gas industry is conservative. To begin to develop geothermal energy from ten to twelve thousand metres deep will be expensive. But the benefits will also be enormous. That is why the industry will eventually begin to invest. In the 1960s, we were beginners when it came to pumping oil from the North Sea. Tackling that challenge was a huge boost in many ways. As a nation, we bet and we won,” says Lademo.
“I believe we can develop the knowledge we need about materials to get down to 300 degrees C in ten years time. It might take 25 years or more of research and development to get down to 500 degrees C,” Lund said, with agreement from Lademo.
“We are convinced that this is possible. But it requires us to further develop existing technology. To do that requires money, a lot of money. Public funding is the key that’s needed to get the industry overall to invest. Geothermal energy is a unique opportunity for the oil industry to develop in a new way. They will come to realize this, it’s just a matter of time.”