Increased exergetic efficiency by using a back pressure turbine for high wellhead pressures at Hellisheidi geothermal power plant

Abstract

A study of utilizing geothermal wells with high wellhead pressures by using a backpressure turbine as a topping unit, operating between 17.5 bar-a inlet pressure and 10 bar-a exhaust pressure, was applied for the existing Hellisheidi geothermal power plant. The study was carried out to determine the electrical power output that could be generated and the loss of exergy throughout the process. Using engineering equation solver (EES) and manual calculations, an output of 7.75 MW of electricity was calculated for the topping unit with 98% energy efficiency and 81.76% exergetic efficiency for a back-pressure turbine with an isentropic efficiency of 85%. The total exergy available in the steam is 89 MW while the exergy available in the steam exiting the turbine is 79 MW. Due to irreversibilities, 1.8 MW is destroyed in the turbine. In comparison, 6 MW of exergy is destroyed if a throttle valve is used instead of a back-pressure turbine to reduce the steam pressure from 17.5 bar-a down to 10 bar-a in order to be compatible with the inlet pressure at Hellisheidi geothermal power plant