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The new geothermal power plant in Soultz-sous-Forêts generates an electricity volume that is mathematically sufficient to supply 3,000 four-person households.
© EnBW, Hanna Mergner
Geothermal power generation
Projektinfo 10/2017

Power plant schematic showing the ORC plant in Soultz-sous-Forêts
© EnBW

The three wells for the deep reservoir
© BINE Information Service based on BESTEC 2008
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German-French geothermal power plant completed

In the Upper Rhine Graben, natural geological thermal deposits can be tapped at comparatively low depths with bore wells. In a Franco-German research project in Alsace, scientists have been researching the process of utilising heat from crystalline rock for years, whereby they have optimised the concept and technology. At the site of the research facility, two power companies opened a new commercial power plant in 2016. The electricity is fed into the French grid. In Germany, around 95% of geothermal resources are also found in deep rock and 5% in natural thermal water layers.

A place in the Upper Rhine Graben with particularly good geological conditions for producing geothermal energy is Soultz-sous-Forêts in Alsace. The temperature prevailing at a depth of 1,000 metres is two and a half times the average for central European countries. This geothermal heat anomaly, which is the largest in central Europe, is caused by high fractured rocks in great depth from which hot thermal water rise. In the mid-1980s, these conditions provided the impetus for setting up a European research facility for utilising the heat contained in the deep rock, i.e. petrothermal geothermal energy.

The German-French project, in which the European Union was also involved for a period, investigated the scientific and technical bases for petrothermal geothermal energy. In 2016, the two energy suppliers Electricité de Strasbourg (ES) and EnBW Energie Baden-Württemberg AG replaced an old research power plant in Soultz dating from 2008 with a new power plant. It is intended to feed approximately 12 million kWh into the French electricity grid every year. This is sufficient to supply, for example, 3,000 four-person households (à 4,000 kWh/p.a.). In France, the feed-in tariff for geothermal electricity amounts to 22.19 ct/kWh and is based on the gross output of the power plant generator minus the electricity requirement of the ORC power plant. The power requirement of the production pumps is not taken into account.


The new power plant

In 2015, the two energy suppliers ES and EnBW decided to transform the Soultz geothermal power plant from a research facility into a commercial operation. They renewed the entire power generation plant to enable stable, economical operation. The results obtained during the course of the research phase also considerably help in dealing with corrosive salt water and gaseous thermal waters and in sustainably managing a geothermal reservoir.

The geothermal heat is converted using a special power plant process, the Organic Rankine Cycle (ORC). This method can utilise heat at a comparatively low temperature level to generate power. In a geothermal plant, in the heat exchanger the heat from the thermal water is transferred to the isobutane working fluid circulating in the secondary circuit, which thereby evaporates. At this point of the circuit the working fluid is under high pressure. The hot steam is then expanded via a turbine. The working fluid is liquefied again by the air-cooled condenser, and is then once again pressurised by a pump.

The new power plant replaces the former research power plant (2008–2015). Its main focus was to test and optimise components in conjunction with manufacturers. The pilot plant was designed for a wide range of operating conditions to enable it to be used in as many different ways as possible for research purposes.

With the step taken towards commercial use, the operating parameters and power plant design were then precisely defined. The new ORC system operates more efficiently than the pilot facility and the power plant entered into regular operation in 2016.

Projektinfo 10/2017:
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