Zeotropic mixtures as working fluids in Organic Rankine Cycles for low-enthalpy geothermal resources

This work presents detailed simulations of Organic Rankine Cycle processes for energy conversion of low-enthalpy geothermal resources. The working fluids considered in this analysis are zeotropic mixtures. Second law efficiency of subcritical cycles is calculated for isobutane/isopentane and R227ea/R245fa depending on mixture composition, heat source temperature and temperature difference of cooling water. The use of mixtures as working fluids leads to an efficiency increase compared to pure fluids, due to a glide match of temperature profiles in the condenser and evaporator. For heat source temperatures below 120 °C the raise is up to 15%. In case of pure fluids a significant efficiency increase occurs, when the maximum pressure is reached and the pinch point shifts to the inlet of the preheater. This effect appears for R227ea at 120 °C and for isobutane at 170 °C. Using mixtures this behavior can be adjusted to higher temperatures by adding a less volatile component.

► Investigation of zeotropic mixtures as ORC working fluids for geothermal applications. ► Non-isothermal phase change of mixtures leads to an efficiency increase. ► Pure fluids are suitable, if the pinch point shifts. ► Using a mixture the shift can be adjusted to higher temperatures. ► Mixtures permit a higher variety concerning the choice of working fluid for ORC power plants