Performance Optimization of ORC Power Plants

Abstract

Electricity production from low enthalpy geothermal sites is typically realized by Organic Rankine Cycles. The selection of the working fluid in the secondary cycle depends on different boundary conditions of the geothermal site. A first decision on the suitable working fluids can be made depending on the brine temperature. At present, it is possible to simulate processes in power plants very precisely, but the vast number of possible fluids and the multiplicity of variables to be taken into account lead to very time-consuming optimizations. The current work therefore addresses the brine temperature range from 100 °C to 300 °C and ambient temperatures from 10 °C to 35 °C. A systematic analysis of typical organic fluids is conducted with the geothermal simulation code GESI. The choice of the fluid and the predominant conditions in the cycle have a great influence on the cycle performance and consequently on the design of the power plant and its net power output. For basic studies on economic and technical issues, developers need models for the expected power output of a geothermal power plant. Site specific conditions have to be taken into account. For this purpose GESI – an in-house thermodynamic code, using fluid properties taken from the NIST database – has been developed. This code uses a bisection method to optimize the heat transfer into the power plant cycle. GESI enables the simulation of optimal live steam parameters under given boundary conditions. To accelerate the pre-calculation process, correlations between net power output and brine respectively ambient temperature were developed.