Rotary gas pressure exchanger for expansion work recovery & efficiency improvement in trans-critical CO2 heat pumps
Heat pumps with CO2 as the working fluid provide unique advantages and facilitates transition away from global warming hydrofluorocarbons (HFC) towards natural refrigerants with ultra-low global warming potential. However, the molecular structure of CO2 causes the heat rejection pressure to be much higher compared to HFCs, resulting in lower efficiency of the cycle at high gas cooler exit temperatures. Exergy and second law analysis indicates that largest fraction of exergy is destroyed during the throttling process over high differential pressure between heat source and sink. To increase exergetic efficiency, this paper presents a novel rotary gas pressure exchanger (RPX), which allows expansion work recovery using direct fluid-to-fluid contact acoustic pressure exchange between high pressure supercritical CO2 and low pressure gaseous CO2. RPX facilitates both, the compression and the expansion using acoustic waves generated in a compact high speed axially ducted rotary machine. RPX can compress a large fraction of the low pressure CO2 vapor for free without consuming any external mechanical or electrical energy, but rather through the expansion work recovery. This expansion work recovery, not only reduces the exergy destruction during expansion process but also provided free subcooling, thus reducing total system mass flow for same amount of total heat delivered or delivering more heat for same amount of compressor work consumed. Test results demonstrate more than 95% pressure recovery coefficient and no-pass through characteristic of mass transport through RPX. The paper presents a novel RPX integrated heat pump architecture that shows up to 16% COP improvement achieved through this reduction in exergetic efficiency of the cycle. Such an RPX integrated heat pump architecture has a great potential to facilitate efficient decarbonization of district and industrial process heat using ultra-low global warming refrigerants like CO2.