Effect of dedicated mechanical subcooler size and gas cooler pressure control on transcritical CO2 booster systems

•Mechanical subcooling increases the COP of CO2 systems at high outdoor temperature.•The gas cooler pressure of CO2 plants can be reduced with high subcooling degree.•Optimum subcooling degree and optimum cooling capacity of the subcooler are found. Dedicated Mechanical Subcooling (DMS) is one of the most investigated and effective strategies applied to increase the performance of CO2 commercial refrigeration systems in transcritical operation. Further performance benefits can be obtained by a reduction of the gas cooler pressure of the main cycle at transcritical conditions. In this work the most important parameters for the design and operation of such a system, i.e. the DMS cooling capacity, the subcooling degree and the gas cooler pressure, are considered and their effect on the annual energy use of the plant is estimated in warm and hot climate conditions by means of a validated model. DMS is also compared to the parallel compression scheme and subcooling performed through a water chiller dedicated to HVAC. DMS results to be the most effective solution among those investigated, and the choice of the best design and operating parameters allows further energy saving and cost reduction.