Performance and total warming impact assessment of pure fluids and mixtures replacing HFCs in micro-ORC energy systems

This study proposes a comprehensive evaluation of the actual greenhouse effect related to the operation of ORC systems in the kW scale. The method is derived from the TEWI (total equivalent warming impact) concept for refrigeration systems, since it includes both direct and indirect contributions to the greenhouse gas emission related to the ORC system. A comparison between traditional HFC-134a (R134a) and some of its low-GWP replacements has been performed, accounting for the effect of the operating fluid leakage during system operation, but also for the indirect contribution associated to the lower performance that can be achieved using more sustainable working fluids, such as hydrofluoroolefins (HFO). Alternative fluids that have been tested are two pure compounds (R1234yf and R1234ze(E)), and four mixtures (R134a-R1234yf; R-134a-R1234ze(E); R515A; R430A). A semi-empirical lumped-parameters model has been employed for simulating the behavior of the ORC system. For the model validation, the experimental data collected on a reference 2-kW ORC test bench with R-134a have been used. The model was then applied to investigate the performance of the system working with alternative fluids. The results show that the indirect emissions associated to HFOs may lead to higher values of total equivalent CO2 emissions, with respect to the employment of R134a as working fluid. The main factors affecting the environmental evaluation, such as emission factors, fluid leak rate and R134a concentration in the mixture, can be decisive and are discussed in this work.