Working fluids selection and parametric optimization of an Organic Rankine Cycle coupled Vapor Compression Cycle (ORC-VCC) for air conditioning using low grade heat

Abstract

This paper presents the thermal performance of several combinations of working fluids in the Organic Rankine Cycle (ORC) powered Vapor Compression Cycle (VCC), for the domestic air conditioning. Seven working fluids, R245fa, R123, R134a, R1234yf, R1234ze (E), Butane and Isobutane were considered and a total of forty nine candidates were analyzed. The objective was defined to provide air at 15°C to a space whose temperature was desired to be at 24°C, as the outdoor temperature varies from 30°C–40°C. The hot water at 100°C and 1.5atm was considered as the heat source. With pressure ratios, COP, mass flow rates and the ratio of COP to pressure ratio as the performance indicators, Isobutane gave the best performance in the standalone VCC. For the ORC as the prime mover for the VCC, by considering the system thermal efficiency, cycle pressure ratios, mass flow rates and expander outlet volumetric flow rates as the criteria, R134a was found to be the best candidate. Henceforth R134a ORC- Isobutane VCC was the best combination. And by optimizing the system parameters of the VCC condenser sub cooling, its condenser temperature and the ORC condenser pressure, this combination gave a COP of 0.219, with dry air at the VCC evaporator inlet and 0.281, with 50% relative humidity air at the VCC evaporator inlet.