Multi-Phase Rotary Pressure Exchanger as a Novel Compressor-Expander for Increasing Efficiency of Trans-Critical CO2 Heat Pumps
Authors: Thatte Azam 
Published: September 2023

Abstract
Heat pumps are one of the most energy efficient ways to supply heat and have traditionally used hydrofluorocarbons (HFCs) as refrigerants. HFCs, due to their high Global Warming Potential (GWP) are gradually being phased out globally and are being replaced by ultra-low GWP natural refrigerants like CO2. However, the optimal heat rejection pressure for CO2 heat pump for a given load return temperature is significantly larger than that for HFC based heat pumps. This results in a higher throttling exergy loss for CO2 heat pump system than that in an equivalent HFC based system and is one of the major reasons for reduced efficiency in CO2 heat pumps. Exergy analysis for the cycle suggests that the throttling losses associated with isenthalpic expansion across the high pressure expansion valve (HPV) over a large differential pressure are responsible for significant exergy destruction in CO2 heat pumps. To remedy this issue, this paper proposes a novel multi-phase rotary pressure exchanger (PXG) as a combined compression-expansion machine to recover pressure energy that would have otherwise been lost across the isenthalpic expansion and use it to compress the low pressure flash gas to the highest system pressure. This reduces the energy consumption of the system significantly. In addition to acting as a compressor, PXG also acts as an expander with relatively high isentropic expansion efficiency, thus increasing the amount of liquid produced after expansion compared to that produced by expansion through a traditional HPV. This further increases the COP of the cycle. In this paper, the gas dynamics taking place inside PXG and its effect on trans-critical CO2 heat pump cycle is investigated. The first law and the second law analysis establishes the thermodynamic limits on mass flow rate that can be compressed by PXG. A multi-phase species transport model is developed to gain insights into why PXG is capable of providing significant energy savings for trans-critical CO2 heat pumps. A multi-phase 3D CFD and species transport model of PXG provides insights into how the high pressure supercritical state of CO2 compresses the low pressure gaseous CO2 inside PXG as the two streams come in direct contact with each other. Experimental data on trans-critical PXG operation obtained previously is used to calibrate the PXG model and the PXG integrated cycle models. Key performance metrics like liquid content after expansion through PXG, amount of flash gas that can be compressed by PXG and the COP as a function of air source / ground source temperature and load return temperature are investigated. PXG can achieve the mass boost ratio of 0.27 and 0.4 at 50 C load return temperature for 37 bar and 48 bar receiver pressure respectively. COP lift and thus the energy savings provided by PXG are shown to increase with increased load return temperature as the density ratio between LP and HP inlet streams of PXG progressively increases. PXG is shown to provide as much as 40% COP improvement (COP Lift) for evaporator temperature of −10 C. COP lift is shown to increase from 30% to more than 50% with increase in the evaporating temperature from −30 C to 10 C. PXG is expected to provide a major stepping stone for developing ultra-low global warming, high efficiency and sustainable heat pump technology for applications in residential & industrial heating, waste heat recovery, geothermal heat pumps and energy storage applications.