Hydrogeochemical Restrictions for Flexible Hydrogeothermal Power Generation

Abstract

Renewable energy sources are contributing significantly to the national energy market. Although weather conditions and energy became much more predictable of the years, there is still a need for short term control reserves to cover a sudden energy demand or overproduction. This project focussed on the potential of geothermal power plants to provide a control reserve. The gradients recorded in operation data sets of power plants in the Bavarian Molasse Basin are much higher than the initial guesses, and geothermal plants providing district heating have always been run controlled by sometimes rapidly changing weather conditions. However, the inertia of the thermal water system limit the flexible operation to tertiary control reserve. The long-term operational data suggest a generally benign behaviour with just one exception: a complete shutdown and restart of the pumps has proven to increase the risk of failure and enhanced precipitations in and above the pumps. A simulation comparing the current operation and a hypothetical operation providing tertiary reserve by a variation of the flow rate and temperature spread showed no significant changes in terms of precipitates and dissolution rates in the reservoir. These positive results cannot be transferred to the Upper Rhine Valley and the North German Basin. In both regions the hydrochemical conditions favor precipitates at lower temperatures, therefore the injection wells are less fault tolerant and will react sensitively to temperature changes. A flexible operation of the geothermal system makes sense with regard to a sustainable exploration of the reservoir. However, under the current subsidies for geothermal power, it makes more sense to run the power plant at full throttle; at least from a financial point of view.