Conference article

The increasing concern on environment problems has led to the development of renewable energy sources, being the geothermal energy one of the most promising ones in terms of power generation. Due to the low heat source temperatures this energy provides, the use of Organic Rankine Cycles is necessary to guarantee a good performance of the system. In this paper, the optimization of an Organic Rankine Cycle has been carried out to determine the most suitable working fluid. Different cycle layouts and configurations for 39 different working fluids were simulated by means of a Gradient Based Optimization Algorithm implemented in MATLAB and linked to REFPROP property library. The heat source was hot water from a geothermal reservoir with an inlet temperature of 120ºC and an outlet temperature limit of 75ºC. For each working fluid, an optimal configuration was obtained, based on the optimization of the second law efficiency. In addition, a sensitivity analysis for the polytropic efficiencies of the pump and turbine was carried out. Results show that those working fluids with a critical temperature close to the maximum temperature of the cycle give the highest plant efficiencies (being propylene and R1234yf the best ones). Using a recuperator increases the plant efficiency in all cases with exception of wet working fluids. The cycles experiencing the highest sensitivity on the pump performance are those using working fluids with low critical temperatures. Increasing the number of stages of the turbine increases the overall plant efficiency for all working fluids, but some fluids are more sensitive to the turbine efficiency than others.

Process modelling, process simulation, working fluid selection, parametric optimization, second law efficiency.