Two-phase ejector enhanced carbon dioxide transcritical heat pump for cold climate

Abstract

Among the solutions to current energy efficiency and climate change issues, the use of natural refrigerants in mechanical compression heat pumps is an important measure for an effective response. However, the heat pump’s performance decline as the ambient temperature drops is well known. The use of the two-phase ejector under these conditions can be a simple and easy to implement way to address this problem.In this respect, a thermodynamics model to simulate a CO2 transcritical heat pump cycle integrating a two-phase ejector is first presented. The modeling allows implicit adjustments of the ejector geometry with varying operating conditions, to adapt its operation in the cycle for maximised overall efficiency. The interaction between the main cycle components is highlighted and one can clearly see that the optimal operation of the cycle does not necessarily coincide with that of its components.For the simulated ejector conditions, the results show the importance of maintaining a particular entrainment ratio (0.5–0.65) in order to avoid mass balance issues in the separator. With lower Tevap, an enhanced ejector efficiency is achieved by striking a balance between decreased entrainment and improved compression ratios. The results clearly show the need to control the ejector geometry in order to meet capacity variations. Thus, decreasing Tevap (from 5 °C to −20 °C), needs to reduce the throat and the mixing diameters by 36% and 22% respectively.Cycle simulations show a definite advantage with the ejector use in terms of COP and heating capacity for low evaporator temperatures. In this case, the compression ratio of the ejector improves, resulting in a higher compressor pressure suction, compared to the conventional cycle. Thus, at optimal gas cooler pressure and for an evaporator temperature of −20 °C, gas cooler and evaporator capacities as well as the cycle COP improve by 14%, 23% and 9% respectively. These improvements are associated with a slight increase in the compressor work (≈4%).