Two-phase flow, oil separation, developing flow
The last decade has seen a substantial increase in Organic Rankine Cycle system installations for low temperature waste heat power recovery. The availability of HFC245fa has played a major role in this recent surge in ORC systems since it allows the use of existing HVAC hardware (heat exchangers and compressors) to be used as ORC components (turbines, boilers and condensers) with minimal redesign. The environmental drawback of HFC245fa is its relatively high GWP value of 950. The advent of a number of new low-GWP refrigerants for HVAC duty which GWP values in the single digits will affect the choice of future working fluids for next generation ORC systems. This paper will analyze the potential of some of these new refrigerants as working fluids for ORC systems and compare the results against existing ORC systems using HFC refrigerants. The analysis uses a REFPROP-based in-house developed computer program that for given heat source and heat sink input data and the selection of a working fluid, predicts anticipated ORC efficiency and performs initial component sizing calculations. The analysis accounts for pump and turbine inefficiencies, heat exchanger approach and pinch point temperatures as well as piping pressure drops as experienced in existing installations. It also allows off-design performance analysis, a feature that is important for ORC pay-back calculations given the sensitivity of ORC power output to variations in ambient conditions.