Application of the Peltier sub-cooled trans-critical carbon dioxide heat pump system for water heating – Modelling and performance analysis

•Peltier subcooled trans-critical CO2 cycle is analyzed for heat pump water heating.•The analysis involves thermo-cycle, convective heat transfer and Peltier equations.•Results show an optimal Peltier heating factor exists to obtain the highest COP.•This optimal value increases as the inlet water temperature condition increases.•Increasing the Peltier device size increases COP but towards a converged maximum. Although the Peltier sub-cooled trans-critical CO2 cycle concept has been applied for refrigeration, which typically involves discharging the heat into ambient air, this system is rarely considered for heat pumping purposes. Therefore, this research aims to expand the scope of the Peltier sub-cooled trans-critical CO2 cycle into heat pump water heating where the generated heat is uniquely discharged into water at temperatures progressively higher than ambient. The heat flows between the CO2 and flowing water are modelled as Nusselt based convective heat transfers where a 1D model is imposed to the direct gas cooler to improve simulation accuracy. Moreover, important but often neglected characteristics such as Peltier device size and Peltier heating factor (PHF) will also be analyzed. Results indicate that the PHF has an extremely strong influence on the overall system’s coefficient of performance (COP). Specifically, an optimal PHF value exists as a trade-off between the benefit of sub-cooling and the losses due to reduced CO2 mass flow rate, the latter of which caused reductions in the convective heat transfer coefficient and the direct gas cooler’s heating capacity. In the meantime, although larger Peltier device sizes improves the system COP, the improvement will converge towards a specific maximum.