Working Fluid Analysis for Supercritical Organic Rankine Cycles for Medium Geothermal Reservoir Temperatures

A steady state model of a supercritical organic Rankine cycle (SORC) was created in MATLAB and validated. Fluid properties were obtained using NIST REFPROP. Various working fluids were tested, including pentane (R601), isopentane (R601a), butane (R600), isobutane (R600a), butene, and cis-butene. Pentane and isopentane have not been of focus for SORCS at these temperatures. Varying turbine inlet temperatures ranging from 170 to 240°C were tested with the heat source provided by a medium temperature geothermal reservoir. A parametric analysis was performed on varying inlet pressure and turbine inlet temperature in comparison to first law efficiency, second law efficiency, effectiveness, and net work produced to analyze the overall and exergetic performance of each fluid. Optimum first law efficiency ranged from 17 to 22%. Cis-butene and pentane performed the best in all performance factors analyzed. Pentane and isopentane performed the best at pressures near or below their critical point. It was also found that near the critical temperature, a subcritical ORC has better performance than an SORC. This study is beneficial for not only geothermal energy but for applications that can provide operating temperatures between 170 to 240°C.