Thermodynamic optimization of heat transfer process in thermal systems using CO2 as the working fluid based on temperature glide matching

Abstract

For thermal systems using CO2 as a working fluid, an isobaric heat transfer in the supercritical region is a variable temperature process. When CO2 is near the pseudo-critical point, its specific heat changes clearly with temperature, which results in an increase in the average temperature difference and thus, an increase in entropy generation. A thermodynamic optimization method for a heat transfer process based on temperature glide matching is proposed in this paper. This method can allow the temperature difference to be uniform and near the minimum value. The heat transfer process in the gas cooler of a CO2 heat pump water heater and the recuperator of the supercritical CO2 (S-CO2) Brayton cycle are optimized based on this method. Using this concept, a CO2 heat pump water heater can simultaneously obtain a certain amount of warm water without simultaneously reducing the hot water output; in addition, for S-CO2 cycle, the exergy efficiency of the regenerator is improved, and the thermal efficiency of the system is thereby improved.