C06 -- Novel Expansion Work Recovery Machine for Decarbonization of Residential and Industrial Cooling Using Low GWP Refrigerants

Decarbonization of residential & industrial cooling processes is one of the top priorities in fight against climate change. Per 2016 Kigali amendment to Montreal protocol, major world economies are working to reduce the use of hydrofluorocarbons (HFCs) in refrigeration, air conditioning and heat pump systems. CO2, due to its low GWP and non-toxic, non-flammable nature, is a strong candidate to replace HFCs, however, due to its lower efficiency, especially in warmer climates, its wider adoption as a low GWP refrigerant remains a challenge. Exergy analysis suggests that the throttling losses across high pressure expansion valve (HPV) are responsible for large exergy destruction and thus inefficiencies of the trans-critical CO2 system. To increase its efficiency, this paper proposes a novel “Rotary Gas Pressure Exchanger” (PXG) to achieve expansion work recovery and reduce external compression work. PXG achieves “free compression” in the first half of the rotor using the differential enthalpy extracted during simultaneous expansion inside the other half through direct contact fluid-to-fluid pressure exchange between a high pressure supercritical CO2 and a low pressure gaseous CO2. PXG placed between a gas cooler / condenser and a receiver compresses the flash gas without consuming any external mechanical or electrical energy. Thus PXG provides a “free pressure lift” upward of 60 bar (900 psi) for up to 25 % of the flow. In this paper, a novel PXG integrated cooling architecture and key results for PXG performance including pressure recovery coefficient, mass boost ratio and COP improvement are presented. The decarbonization potential of PXG is quantified and it is shown that it can reduce up to 34,000 kg of CO2 equivalent emissions annually in US (up to 100,000 kg in some cities) for every 150 KW cooling delivered. Additionally, PXG facilitates transition away from HFCs and towards CO2 as the low GWP refrigerant. This transition has potential to decarbonize building cooling systems by as much as 14,000 MT CO2_e per 300 KW cooling delivered. On the heating side, PXG based system can reduce emissions of a 10 MW district heating CO2 heat pump by up to 2800 MT CO2_e. Thus PXG provides a major stepping stone for decarbonizing building cooling and heating systems in USA and Europe.