Hot-dry-rock process

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The hot-dry-rock method (HDR for short) is a method for using the geothermal energy contained in the earth's body from a depth of between 3,000 and 6,000 meters. The same principle is described with the terms HFR ( Hot Fractured Rock ) and Enhanced Geothermal System (EGS).

technology

The principle is based on the production and operation of an oversized heat exchanger underground between at least two boreholes. By injecting water with a pressure of up to 150 bar, the existing cracks in the rock widen despite the rock pressure , and new ones are formed. With an average width of less than one millimeter, these remain permanently open. This creates a heat exchanger with a huge surface in the rock between the boreholes. During operation, cold water is supplied to the system through one borehole and heated again at another borehole, enriched by any naturally existing deep water. The natural thermal buoyancy of the hot water facilitates circulation.

The technology is used, for example, in Deep Heat Mining Basel and in the European Deep Geothermal Power Program in Soultz-sous-Forêts.

history

The first major HDR project in Europe was created in Soultz-sous-Forêts ( Alsace ). In a four-month test in 1997, the heat exchanger created there produced at least three square kilometers of hot water at a temperature of 142 ° C. In the meantime, the wells have been deepened to over 5,000 m in order to reach a temperature level of 200 ° C. A first power plant with a steam temperature of approx. 180 ° C could then be operated as a scientific pilot plant for generating electricity.

The Google Foundation sees the HDR process as a technology that could be used on a large scale as an energy source in the future, and has therefore funded the development of the HDR process with over 10 million US dollars.

criticism

By injecting water, the stress state of the rock changes when using the HDR process. Tiny earthquakes occur, which lead to the expansion of existing cracks, so that the surface area on which the injected water can heat up increases. According to many geothermal experts, however, the strength of these earthquakes is so small that they are barely noticeable and do not cause any damage. Critics of the process, however, fear that the HDR process can also trigger larger tremors. In December 2006, the deep heat mining project in Basel not only resulted in the expected smaller earthquakes, but also in noticeable tremors with a strength of up to 3.5 points on the Richter scale . There was no major damage, but the uncertainty of the population led to the project being stopped. Geothermal experts said in the newspaper Die Welt that these artificially triggered earthquakes could reduce the risk of another Basel earthquake . However, the Swiss Seismological Service considers this effect to be negligible.

Individual evidence

  1. T. Hettkamp, ​​J. Baumgärtner, D. Teza: Status of the European hot dry rock research project Soultz-sous-Forêts , 2002.
  2. google.org: A Googol of Heat Beneath Our Feet ( Memento from December 29, 2012 in the Internet Archive )
  3. Basler Zeitung: Basler geothermal project triggers earthquake again ( memento from September 30, 2007 in the Internet Archive ), February 2, 2007.
  4. Die Welt: Artificial earthquakes in Basel justified , January 9, 2007.
  5. Swiss Seismological Service: The Deep Heat Mining Project in Basel , accessed on October 12, 2014.

Web links