Are engineered geothermal systems our future energy reserve?

An engineered geothermal system (EGS) harnesses the energy in the earth to produce commercial quantities of electricity. The basic principle of an EGS is to access the high temperature available at depth and manipulate the underground rock mass to enhance permeability so that cooled water can be injected from one well and steam or hot water is returned from other wells. This principle works because water heats up as it circulates and is then brought back to the surface, where the heat is extracted to generate electricity.

The drilling depth varies between 3 and 5 km depending on the quality of the rock mass that has to be penetrated. The key to the technology is the skill in enhancing the fracture permeability in the rock through which the water travels in order to be heated up. On the one hand the width of the aperture needs to be wide enough to let water pass through it with less resistance but on the other hand encompass a large number of fractures to sustain heat extraction for years at a commercially viable rate.

So what are the advantages of the system and what its limits? The good things first: the concept can work anywhere in the world. At the moment there are plans for new EGS projects in Britain, France, Germany and America. Optimists say that there will be 160 GW of geothermal capacity installed worldwide by 2050, about half of which will be EGS. By now conventional geothermal power stations worldwide have a total capacity of 10.7 GW and will generate 67.2 GWh of energy this year. That is enough to supply more than 50 million people in 24 countries! Like other forms of renewable energy, geothermal power produces little or no carbon dioxide and helps to prevent climate change.

The disadvantages and hurdles of the system are high upfront costs including the verification of the potential of the drilling site. This process might take long time which makes it difficult to find investors. The most economically viable projects are those that exploit high temperatures at shallow depth. Unfortunately such sites are rare. To make geothermal power competitive to other technologies financial incentives have to be given such as production tax-credit for renewable energy projects in America or subsidies in the form of feed-in tariffs like in Germany.

If geothermal power systems should be competitive in the future technological improvements have to be made first such as cheaper and better drilling equipment. Another cutting cost factor is the improvement of water-flow rates in the system by creating more fractures per well. By now the rates are around 25 l/s, far short of the 50-100 l/s required to operate profitably. And cracking of the rock is the most vulnerable point in the process because it causes noticeable man-made earthquakes that frighten people. Well, there is no perfect technology!