On the Viability of Hydrofluoroolefin-R245fa Mixtures as Organic Working Fluids in a Low-Temperature Solar-Powered Organic Rankine Cycle (ORC)

Abstract To address the intermittent nature of solar and other renewable energy (RE) sources, an organic Rankine cycle (ORC) can be employed to facilitate steady production of power. Organic working fluid (OWF) mixtures in ORCs are of interest due to the possibility of reducing irreversible losses in the system and having a better sense of control over its environmental impact while not compromising on the performance benefits when paired with conventional fluids such as R245fa. Hydrofluoroolefins (HFOs) are considered as one of the components due to their zero ozone depleting potential (ODP) and very low global warming potential (GWP). This study evaluates the performance of binary zeotropic mixtures of R245fa and HFOs with available thermodynamic properties (R1234yf, R1234zeE, and R1234zeZ) as OWFs in a simple low-temperature solar-powered ORC. Specifically, the optimal compositions of each pair of HFO-R245fa mixtures within the specified operating conditions are identified via the performance indicators (1 kW net power output; thermal efficiencies; and GWP); and are compared against their pure components and known conventional OWFs. The turbine power outputs and efficiencies of the mixtures, determined from ASPEN Plus simulations, were observed to be competitive as alternatives for the pure components, and conventional fluids. At optimal compositions, R245fa/R1234zeZ (0.2619 mass fraction R245fa) was shown to be the most viable OWF blend with peak efficiency at 7.5545% and GWP of 274.2, accomplishing the target 1kW turbine power output, followed closely by R245fa/R1234zeE (0.2678 R245fa; 1kW power output; 7.4824% peak efficiency; GWP = 281), and R245fa/R1234yf (0.3589 R245fa; 0.9595 kW power output; 7.4338% peak efficiency; GWP = 372.2). It is recommended that more HFO-R245fa mixtures, as well as other complex blends, be explored and be subjected to further studies.