Preliminary design of an oil-cooled supercritical CO2 heat exchanger for applications in high-temperature heat pumps
There is a pressing need for industrial decarbonization, particularly in the area of process heating and cooling. Heat pumps with thermal energy storage (TES) can supply thermal energy for industrial processes and provide the opportunity to reduce fossil fuel use, with a growing interest in employing CO2 as the working fluid in transcritical cycles to provide higher temperature heat. A key component impacting the overall performance of a CO2 heat pump system is the heat sink, in which supercritical CO2 rejects heat to a secondary fluid and interfaces with the hot TES. This work considers the preliminary design of the CO2 cooler (i.e., the heat sink), in which CO2 exchanges heat with thermal oil. In addition to developing a design with superior thermofluidic performance (i.e., high heat transfer and low pressure drops), the heat exchanger must withstand relatively high temperatures and pressures of up to 350 °C and 200 bar, respectively, on the CO2 side. The thermofluidic performance of a hot and cold channel pair is evaluated via CFD, with the necessary channel wall thicknesses being assessed to ensure the mechanical robustness of the design. CFD is also used to evaluate the flow distribution from the header into the channels on the CO2 side. The proposed channel design has an energy density of 5.6 MW/m3, with pressure drops of 0.129 and 0.101 bar for the CO2 and oil sides, respectively. The design is developed for later experimental characterization and integration into a heat pump test rig.