Thermodynamic criteria to select working fluid and configuration of Organic Rankine Cycles for low to medium temperature heat sources

In the thermodynamic optimization of Organic Rankine Cycles both thermal efficiency and heat recovery effectiveness play a major role in the search for the maximum net power output. Optimal thermodynamic parameters are those which simultaneously allow high thermal efficiency and high heat transfer from the heat source to the thermodynamic cycle. In this work, subcritical, regenerative and supercritical ORC systems recovering heat from sensible heat sources in the temperature range from 120 to 180°C are considered. In the search for the best matching between heat source and working fluid a key role is played by the distance between critical temperature and inlet temperature of the heat source. This temperature difference is used in this work to classify the working fluids of subcritical cycles into three categories which help draw a clear picture of the system behavior regardless of the inlet temperature of the heat source. A similar approach is used for supercritical cycles. Fluids having high temperature difference between heat source and critical temperature become attractive in the supercritical cycle configuration because of the increase in thermal efficiency with no significant variation of the heat recovery factor. No significant improvement is instead found for low values of this temperature difference compared to the subcritical configuration. Regenerative cycles are not of interest for low temperature sensible heat sources because of the scarce capability of exploiting the heat available from the heat source in spite of the improved cycle thermal efficiency. Nonetheless, regeneration may become a viable option to increase the power output when the heat source outlet temperature is constrained by a lower limit.