Comparative Efficiency of Geothermal Vapor-Turbine Cycles

Abstract

Optimal design of the geothermal power generation units is considered as a part of development of the Verhne-Mutnovsky Geothermal Power Station. A well drill provides a hot water with temperature of 420 K (162 C) at 0.65 MPa and steam fraction of 0.3. In addition, the IV unit of the station consumes a hot water streams separated from the others three units built earlier. Thermodynamic exergy analysis displays losses due to constraints associated to heat exchangers scaling. The analysis also helps to optimize a bottom cycle configuration, designed to transform geothermal heat to electricity for both summer and winter seasons. The analysis takes into account both the bottom cycle configuration and working fluid properties. Selecting of a working fluid is based on the engineering aspects and environmental characteristics and safety considerations.
Thermodynamic analysis shows influence of both intrinsic and extrinsic losses on the power unit efficiency, which can be varied with cost considerations. Properties of the working fluid in the bottom cycle influence on optimal equipment design: turbine efficiency and size, volumetric flow rate at the turbine outlet, and overall heat transfer coefficient in the heat exchangers are important. It is shown that application of the mixed working fluids provides more possibilities for improving the efficiency. This is due to better temperature profiles in the heat exchangers and pressure ratio in the bottom cycle. In addition, mixed working fluids allow more flexibility for optimal regulation at variable loads and ambient temperature in the field.