Improved technology can lead to better use of the energy stored in the Earth. Geothermal energy can be used for the continuous production of heat and electricity and thus can serve as a reliable source of power. Having previously been reserved for certain geologically favourable areas geothermal energy is on the way to becoming an alternative source of energy in the world.
Major energy assets in the interior of the Earth
Geothermal energy, the heat stored in the Earth, is literally a resource that is many times greater than the total world resources of oil and natural gas.
It all started about 4.5 billion years ago when the planet Earth and the other planets were formed from the solar nebula. This was a disk-shaped mass of dust and gas left over from when the sun was formed. In the beginning, the Earth was a ball of fire, but gradually cooled the outer parts so much that water could accumulate in the atmosphere. The rain that fell was gradually cooled by the surface and the solid crust was formed. Deeper down in the mantle and the core, there is still a very large stock of thermal energy. New heat is also being developed continuously by the decay of radioactive material in the mantle. The deeper down you go, the more heat there is to extract. The temperature increases by about 25-30 degrees Celsius per kilometre of depth. In the Earth’s core, the temperature is above 5000 ° C.
Crust thickness varies between 10 and 100 km, and in places where it is thinner, it can warm streams of magma or heated water to make heat energy accessible. This is especially true in areas where the tectonic plates meet, which often involves areas with volcanic activity. This includes Iceland, which gets most of its energy for heating and hot water, and a quarter of its electricity from geothermal sources.
Energy from the depths of the Earth
Energy can be extracted by drilling geothermal wells down to the hot areas deep into the Earth’s crust. The heat can be used directly in heating systems but can also be used for power generation. In the latter case, water vapour can drive turbines, which requires a high temperature. This means that energy must be retrieved from the depths or have other technical solutions applied. Today’s wells can be up to three kilometres deep, but in the future there is hope to drill down to 10 kilometres. Such wells are technically complicated and costly, but have been conducted in a research context and in oil extraction.
There are some places in Sweden that are very suitable for geothermal energy, and one of these is Skåne. Drill a well deep enough – about 2 km – in some places in Skåne you’ll get between 75 and 85 ° C of heat. This works great as a source of heat. When the heat is used, the water can be pumped back into another borehole and heated up again. Hot steam requires further drilling.
Currently, geothermal energy only accounts for a fraction of the world’s energy needs, but the percentage is expected to increase in the future. How to use geothermal energy is well known and the technology to drill deep enough has subsequently been refined and become cheaper. This has raised hopes that geothermal energy will become an alternative to the world at large, even outside the volcanic zone. New technology also makes it possible to produce renewable electricity even if the water that is pumped up from the Earth’s interior does not reach the boiling point.
An important factor to efficiently utilize geothermal energy for electricity generation is the use of heat exchangers. The technology has been tested in pilot plants in countries such as Germany, France and the United States. The secret is a combination of heat and fluids that boil at a temperature below 100 ° C. The system converts water into steam that makes the turbines spin and that ultimately generate renewable electricity.
The Swedish company Energeotek is planning a small geothermal power plant in Hungary with the new technology and hopes for similar facilities in Sweden.
Energy from the dry rock
Geothermal energy also includes enhanced geothermal systems (EGS) or HDR (Hot Dry Rock). This involves techniques to lead water or other media into the dry rock where natural hot springs are missing.
It pumps down the water at high pressure to create or develop a crack in the rock, so the water then in a closed system can be circulated from one borehole to another. On the way it absorbs heat that can be utilized in systems with heat exchangers. A number of such demonstration projects are under construction around the world.
A source of power
One advantage over other renewable alternatives such as solar and wind power is a continuous production. The heat flows without interruption so that it can be used as a main source of power.
The argument against geothermal energy is the high investment costs and possible environmental impacts. Land and environmental impacts are important and fluids can bring dissolved gases and heavy metals up from the earth. In such cases, treatment technology must be used to take care of the pollution. A potential risk with EGS / HDR technology can be a negative impact on soil stability and the triggering of seismic activity.
Drilling for heat at great depths in the crust of the Earth is still something of a gamble. Here, for example, EGS / HDR help make drilling less dependent to hit exactly the right source, reducing the financial risks. The technical development in geothermal electricity is also very interesting, and overall there is much to suggest that the heat from the Earth’s interior is an increasingly important component of our future energy supply.
The article was published in June 2013