Biogas "refines" geothermal energy

Hybrid system combines both power generation technologies and leads to higher efficiency

Biogas plant © Schmack Biogas
Read out

So that geothermal power plants can work economically, they usually require a temperature of over 120 ° C in the water-bearing layers of the substrate. Thus, they are strongly tied to specific locations. Now, however, a novel concept could greatly expand the uses of geothermal energy. Because the simultaneous supply of heat from a biogas plant already enough water temperatures of just over 100 ° C for an economic power generation.

A community in the Upper Rhine Graben commissioned engineers who are familiar with both the use of biogas and geothermal energy. This provided the starting point for combining the two regenerative energy sources in an innovative hybrid system. In a biogas plant methane is produced in a fermentation process with about seven megawatts (MW) of rated thermal input and then burned in combined heat and power plants. The motors generate electricity that is fed into the grid. The high temperatures from the exhaust gas from the engines and from the cooling water are fed via heat exchangers into the power plant circuit of a geothermal plant.

Increasing efficiency ...

In this way, the power plant cycle can heat up by more than 10 ° C with a waste heat output of the combined heat and power plants of about 2.4 MW. The trick: the temperature of the extracted thermal water now only has to be around 105 ° C. This increases the gross efficiency of power generation by about 0.8 percentage points. In addition, depending on the capacity of the biogas plant, up to 2.4 MW of heat is fed into the geothermal energy process. Thus, the geothermal plant generates up to 500 kW more electricity, which corresponds to a performance increase of more than ten percent through the "hybridization".

... and environmental protection

Biogas plant inside © Schmack Biogas

But in addition to the increase in efficiency, there are further synergy effects. For example, the waste heat from the thermal power generation plants can be made available to consumers in the surrounding area as a combined heat and power system. The potential uses of heat are extremely diverse, ranging from space heating and hot water preparation of swimming pools to commercial use, such as in greenhouses. Normally, these customers must have reliable replacement systems available on a conventional basis, such as oil or gas combustion. However, in the event of a system failure, the hybrid system can continue to supply the heat either from biogas or geothermal energy. In this way, the fossil-fueled additional system is eliminated and thus protects the environment.

If the example project in the Upper Rhine Graben is to be realized, the power generation of up to 44, 000 MWh of electrical power per year could supply up to 28, 000 one-person households. The substitution of natural gas in a gas and steam turbine power plant saves around 18, 000 tonnes of climate-friendly CO2 per year. display

Regional expansion of geothermal energy for power generation

The simple idea of ​​coupling a second regenerative energy source to the geothermal plant allows the use of geothermal energy to generate electricity regionally. For example, regions with reservoir temperatures below 120 C can now be operated more economically. This applies not only to parts of the Upper Rhine Graben known to be particularly hot, but also to other regions in Germany. Especially in the Molasse Basin, where temperatures in the deep reservoir of the Malmkalk are often between nur and 80 -120 C, there is the potential of coupling geothermal energy to another source of regenerative energy, such as biogas extremely high.

(Kreuter Kreuter (GeoThermal Engineering GmbH), B. Kapp (Aufwind Schmack GmbH), 09.02.2007 - AHE)