A critical approach to the determination of optimal heat rejection pressure in transcritical systems

In this paper the optimal energy efficiency and high cycle pressure problem in single-stage refrigerating carbon dioxide vapour compressor units operating in transcritical conditions is addressed. Literature approximated solutions to the optimisation problem are analysed and critically discussed. A numerical model for CO2 heat exchangers and refrigerant systems is developed. Different gas coolers are simulated in order to investigate the effect of literature simplifying assumptions on the optimal pressure determination: the analysis showed a strong sensitivity of the gas cooler outlet temperature from the secondary fluid temperature, from its capacity rate and from the heat exchanger geometry. Nevertheless it resulted that approximated solutions obtained considering the carbon dioxide gas cooler outlet temperature as an independent variable behaved better than solutions correlating it to secondary fluid inlet temperature. A commercial refrigeration plant and a heat pump water heater were finally simulated to verify their energy performances when applying literature approximated solutions in presence of an on-board compressor capacity and supply water temperature control respectively. While in the case of the commercial refrigeration plant no major penalisation resulted from the literature approximated solutions simplifying assumptions, the heat pump performance was strongly deteriorated, up to −30%, as a consequence of the huge variation in the water capacity flow rate resulting from the water temperature control. It was finally concluded that an approximated correlation should be critically evaluated before implementation; a real-time algorithm for determining the optimal (or quasi-optimal) pressure value could provide a more efficient and robust solution.