Small device, large contribution: in the Unterhaching geothermal power station, a Sitrans flowmeter monitors the correct ammonia-water ratio in the steam cycle and helps in the cost-effective exploitation of geothermal energy in an innovative power generation process.

The Unterhaching geothermal power station produces electricity and heat from the 120°C water in a limestone layer lying at a depth of approximately 3,500 m directly below the power station. Because the thermal water is not hot enough to be used in a conventional steam process, the Unterhaching plant uses the Kalina process (see box). In this process, the thermal water heats up an ammonia-water mixture that drives the turbine for producing electricity. After heat exchange, the thermal water is pumped back into the ground through a second borehole.

The Unterhaching geothermal plant produces electrical power from a low-temperature source through the Kalina process (Photo: K. Bauer)

The perfect mix

The heart of the system is the steam turbine that drives the generator for power generation. Deviations in the mixing ratio of the ammonia-water mixture directly affect the efficiency of the plant. Process parameters have to be monitored and processed in real time to ensure a swift and targeted parameter correction, so the process has to be equipped with a suitable measuring device that is integrated into the plant’s process control system, in this case Simatic PCS 7.

The Sitrans F C flowmeter monitors the ammonia-water ratio in the steam cycle (Photo: K. Bauer)

The mixing ratio of the two liquids in the pressure cycle should ideally be measured with a single measuring instrument that is inexpensive and accurate, and preferably requires little maintenance. For this application, Siemens proposed the Sitrans F C Coriolis mass flowmeter, which can also be used for measuring density. The instrument operates without wear and can be easily installed and engineered, as only one instrument must be integrated into the pipe. The reliability and accuracy of the measured values ensure efficient operation of the turbine and high economy of the total plant.The Sitrans instrument determines the ammonia-water ratio through fraction measurement calculating the respective densities and masses from the known specific densities of the two components in the temperature range of the process. The Pt1000 temperature sensors built into the transmitter measure the exact temperature of the mixture for the temperature compensation. This measuring method achieves an accuracy of 0.15 percent for the determination of mass and density in an ammonia/deionized water mixture.

Sustainable and efficient

Thanks to the precisely measured values supplied by the Sitrans instrument, the turbine in Unterhaching can be operated reliably and efficiently, resulting in more economical power generation. The geothermal plant reliably supplies electrical power of 3.4 MW and is therefore a real basic load power station. At the same time, the power station reduces carbon dioxide emissions and makes an important contribution to climate protection and a sustainable energy mix in Germany.

The Kalina process in Unterhaching (Photo: Siemens AG)

The Kalina process in Unterhaching

The Kalina process is a heat-transfer process for producing steam at a low temperature level for power generation. A mixture of ammonia and distilled water, with a boiling point of 50°C, is used instead of thermal water. The ammonia evaporates at much lower temperatures, increasing the efficiency of the heat exchange. Especially with low-temperature sources like that in Unterhaching, a much higher conversion efficiency can be achieved in small steam-turbine power stations with the ammonia-water mixture.In Unterhaching, the 122°C thermal water heats up the mixture in the evaporator from 55°C to 115°C. A pump increases the pressure of the working medium to approximately 20 bar. Residual liquid is separated in a high-pressure separator. The compressed and heated steam mixture is then fed through the turbine. The working medium leaves the turbine at approximately 7 bar, still as steam, and is cooled at a smaller heat exchanger. A low-pressure separator collects the remaining liquid, and a cooling-water system reduces the temperature of the steam until it condenses into liquid. After passing through a condensation tank, the mixture is compressed again to 20 bar and preheated at the smaller heat exchanger to 55°C before being fed back into the evaporator.

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