Replacement of R134a with low-GWP fluids in a kW-size reciprocating piston expander: Performance prediction and design optimization

In this study, low-GWP fluids (R1234yf and R1234ze(E)) have been compared with R134a when used in a kW-size reciprocating piston expander. Semi-empirical models of the pump and the expander are employed to analyze how the different fluids thermodynamic characteristics could influence machines behavior into real operation of a micro-ORC. Parameters related to thermo-fluid-dynamic properties are updated compared to the original values calibrated over R134a. Results show that the use of HFOs alternative fluids leads to a loss of electric power and expander efficiency, whose detriment depends on fluids properties and on operation strategy. At a given pressure ratio the decrease of power output is close to 21% and 42%, while the loss on expander efficiency is more limited, being around 6% and 11%, for R1234yf and for R1234ze(E), respectively. Main factors of influence such as saturation pressure, viscosity, heat transfer coefficients and vapour density are discussed. The expander model has also been used to perform the optimization of the built-in volume ratio for each fluid, revealing that a significant enhancement of the expander overall performance could be obtained modifying the intake valve timing, thus reducing under-expansion losses and improving its volumetric efficiency.