Geothermal power generation in Germany – challenges for plant and materials engineering

Depending on the region and geothermal reservoir, electric power generation may pose specific requirements which need to be managed. This applies in particular to electricity generated by hydro-geothermal systems which is possible in three regions in Germany: the Molasse basin in Southern Germany, the North German basin and the Upper Rhine rift valley. At these three locations, there are basic geological differences in aquifer properties and the geochemical composition of the geothermal fluids.

Depending on drilling depths, the geothermal waters are more or less saline (and may contain up to ten times more salt than sea water). In light of the above, the entire infrastructure of a geothermal plant – piping, pumping, water treatment and filtration systems and all plant modules of extraction and injection boreholes – must also be protected against corrosion, for example.

In the majority of cases, cost-efficient electricity generation requires production horizons at depths of 3000 m to 4000 m or deeper. Depending on the location, these low-enthalpy geothermal systems permit the extraction of geothermal fluids at temperatures of 100°C to over 150°C by means of deep drilling. To generate electric power, the geothermal water heats a second cycle which contains a fluid that boils at low temperatures. The vapour thus generated drives a special turbine. The substances separated in the geothermal plant depend on the minerals and gases contained in the geothermal water and result in various forms of corrosion.

Typical forms of corrosion

In the Upper Jurassic limestone of the Molasse basin in Southern Germany, the mineralisation of geothermal waters extracted at depths far in excess of 3000 m may still only be <1 g/l, while mineralisation in the North German basin and in the Upper Rhine rift valley reaches values between 100 and 250 g/l. In addition, geothermal waters differ in their content of gases such as methane or carbon dioxide.

The geothermal waters in the Upper Rhine rift valley, which are frequently characterised by high chloride concentrations, place high demands on material strength and resistance. Chlorine compounds, for example, attack the steel’s passive layer depending on the alloy components. Pitting corrosion may also occur in the heat-affected zone (heat tint) of welds.

Problems in connection with deposits caused by precipitation are relatively rare in the Upper Jurassic limestone. Elevated hydrocarbon concentrations are more likely – a methane content of up to 30 ml/l is quite common. The fluids frequently contain hydrogen sulphide. Consequently, material corrosion caused by chemical reactions between metal surfaces and elevated hydrogen sulphide concentrations in geothermal waters also poses a risk in this case.

Dimensioning of pressure level

In fluid extraction in the Upper Rhine rift valley, the pressure levels of the more confined aquifers may extend to levels just below the ground surface. In the Upper Jurassic limestone, by contrast, pressure levels are often several hundreds of meters below the ground. This calls for extraction pumps with an especially high pump capacity.

In light of the above, the primary cycle of a geothermal power station requires a high extraction capacity in the Upper Jurassic limestone but a high re-injection capacity in the Upper Rhine rift valley. This sometimes imposes extreme strain on the strength, as well as fatigue the life and corrosion resistance of pumps, pipe systems and plant components. Both materials and components should be specifically qualified or appropriately selected, dimensioned and/or adjusted. Apart from the above, chemical precipitation reactions must be checked from the perspectives of process and plant engineering.

In real-life projects, the hydro-chemical properties of the fluids and the differences in pressure levels which need to be overcome can only be determined in detail after evaluation of the pump testing of the extraction borehole. The chemical composition of the fluids must be comprehensively analysed to determine the corrosion effects in plant engineering and the required power station chemistry. Key success factors for ensuring a high level of plant availability are the integrated evaluation of the pumping results from the perspectives of process and plant engineering and an engineering approach aligned therewith.

So far, experience with the operational management of power station locations in Germany has shown that geothermal plants must be individually designed down to the smallest detail in materials and plant engineering and constructed with appropriately dimensioned parts and components to ensure reliable operation for years. Improved quality checks already help to prevent weaknesses during construction which may otherwise result in increased maintenance efforts or plant downtime. Integrated weakness analysis is a basic prerequisite for the construction of a suitable and reliable geothermal power station following successful borehole drilling.