HFOs as substitute for R-134a as working fluids in ORC power plants: A thermodynamic assessment and thermal stability analysis

In this paper we investigate the potential replacement of HFC-134a in ORC applications by two low-GWP refrigerant fluids, namely HFO-1234yf and HFO-1234ze(E). After revising and discussing their main thermo-physical properties, we adopted in our calculations the Peng Robinson EOS available in Aspen Plus v7.3, integrated with literature data. By assuming as reference a geothermal plant operated with HFC-134a, we first consider the direct replacement of the original fluid by the two refrigerants. Results of such off-design simulations show a decrease of the net power of about 13% in case of HFO-1234yf and 1% in case of HFO-1234ze(E). Then, in case of heat recovery from a hot water source, from a comparison among the three refrigerants, with the hypothesis of a completely new design simulation, it turns out that the turbine power results lower than HFC-134a of about 20% and 28% for the cycles using HFO-1234yf and HFO-1234ze(E) respectively. We also show that in case of HFO-1234ze(E) the recuperative heat exchanger could be removed without tangible effects on the useful power and on the cycle efficiency. Finally we assess through an experimental thermal stability analysis that 200-250 °C could be a feasible working temperature limit for HFO-1234yf.