Effect of Working Fluid Type on Low Temperature Rankine Cycle Optimization

orc2019 Tracking Number 197

The characteristics of the working fluid play an important role on the performance of low temperature Rankine cycles. One important criterion categorizes fluids based on the slope sign of the saturated vapor curve in T-s diagram to distinguish between so-called wet and dry expansion. Wet expansion is preferably avoided due to the damage inflicted on turbine blades by liquid droplets. On the other hand, superheat in expander inlet and outlet increases the loads on heat exchangers and increases cost of the system. Therefore, in this paper a methodology is presented to minimize expander superheat, while maximizing net power and taking the isentropic efficiency of the expander into account. Results indicate that so-called wet fluids do not necessarily need to be excluded from the fluid candidate selection. Depending on the evaporating and condensing temperature levels, by using an expander with a specific isentropic efficiency, it is possible to decrease the superheat in the expander and in some cases close to zero. However, comparison should be between the performances of various fluids in a cycle with all external parameters and fluids should not be excluded from the screening list only based on their types.