Optimization study of heat pumps using refrigerant blends – Ejector versus expansion valve systems

Abstract

This article investigates tap water heating systems to highlight an ongoing debate. Some report that CO2-based transcritical heat pumps with an ejector have the best coefficient of performance (COP), while others report that blend-based refrigerant systems (without an ejector) are better. In the literature, however, these systems are only compared with conventional heat pump designs, and not against each other, making it difficult to conclude which design is the best. In addition, the outcome of combining the two modifications has not been explored extensively. This article investigates the performance of heat pumps using mixtures of CO2 and propane, with and without an ejector or a suction gas heat exchanger. It presents a novel method for modeling blend-based heat pumps with an ejector using an optimization approach and a minimum allowed temperature pinch in heat exchangers. A sensitivity study explores how the heat pump performance depends on operating conditions, ejector efficiency and the refrigerant blend. The sensitivity studies allow for the comparison of the heat pump designs. For example, the results show that it is inefficient to use CO2 and propane blends in systems with an ejector. A blend-based system with a suction gas heat exchanger was found to outperform a CO2-based system with an ejector if either the tap water is above 25 °C, the ejector efficiency is below 0.17, or the temperature of the heat source is reduced with more than 10 K when flowing through the evaporator.