Steam Turbines for Geothermal Plants

Geothermal energy is the term given to heat stored beneath the upper part of the earth’s crust. This heat is permanently available regardless of weather conditions and the time of day. It is an attractive and economical source of energy since it is classified as regenerative and affords emission-free generation of power. Compared to a modern combined cycle power plant of the same capacity, a geothermal power station can reduce CO₂ emissions by more than 10,000 tonnes. This is why some countries offer tax incentives for their construction.

There are many technologies used in geothermal plants. Siemens focuses on the Kalina cycle, which enables the use of low potential heat. In this process, a mixture of 15 % water and 85 % ammonia is used to produce steam. The ammonia-water mixture vaporizes at a much lower temperature than in traditional steam-water systems and is thus extremely suitable for low-temperature applications. The geothermal water is used to heat the ammonia-water mixture, which vaporizes at a pressure of 25 bar and a temperature of 150 ºC.

Such parameters, combined with the unique features of the Kalina cycle, permit the adoption of small and cost-efficient back pressure steam turbines instead of condensing steam turbines. The Kalina cycle also enables considerably higher efficiency rates.

A single-casing axial turbine with a mid-inlet presents the customer with the lowest life-cycle costs. In order to achieve high performance, a high volumetric flow of steam is necessary. Consequently, Siemens recommends different turbines depending on the volumetric flow, for example SST-300 and SST-400 for small power outputs, up to the SST-800, for the larger outputs.