Geothermal power is becoming more and more significant in the renewable power mix of several countries in the world. The thermal conditions of the geothermal fluids exhausting from geothermal power plants shows additional potential for improved heat utilization through the integration of a low heat recovery system. This paper addresses the optimum integration of an Organic Rankine Cycle (ORC) as bottoming cycle with a geothermal steam power plant as topping cycle over a range of geothermal fluid interface temperatures.
A reference geothermal based steam turbine power plant of 50MW capacity with indirect cycle configuration has been chosen for the study. At design point of the reference plant, optimized Organic Rankine Cycles based on three working fluids n-pentane, R123, and R245fa have been integrated at the exhaust of the geothermal fluid leaving the geothermal plant. An overall optimization of the power plant has been carried out by downsizing and over sizing the topping cycle with the integration of the bottoming cycle. One of the optimization variables for the overall plant is the interface temperature, which is a consequence of the resizing of the topping cycle. The procedure is repeated for the three different organic working fluids. By applying this procedure, it is then possible to know within a given interface temperature range, the organic working fluid that will give optimum plant performance.
The choice of ORC integration option is not only driven by the best techno-economic solution but additionally by environmental, health and safety compliance.