CO2 transcritical refrigeration cycles: Potential for exploiting waste heat recovery with variable operating conditions

An analysis of CO2 transcritical inverse cycles is presented, for investigating the exploitation potential of the heat rejected from the gas cooler, usually considered as a not recoverable waste heat. The cycle is analyzed in the cooling configuration during the summer season, when there are contemporarily a high temperature of the heat sink, normally over 30 °C, and a high temperature waste heat at the gas cooler, possibly exceeding 100 °C. The latter is usually rejected to the environment, but some useful applications could be analyzed, where a not negligible fraction is somehow recovered. Here a second cooling machine, thermally activated, which adds further cooling capacity to the traditional vapor compression one, is considered. A thermodynamic analysis was performed, with CO2 evaporation temperature varying between -5 °C and 5 °C, compressor discharge pressure between 74 bar and 150 bar, and gas cooler outlet temperature variable between 30 °C and 45 °C, investigating the influence of the main thermodynamic parameters on the cycle performance by means of the software REFPROP. The depicted model allowed performing a wide sensitivity analysis of all the above mentioned thermodynamic variables, identifying the best operating conditions of the carbon dioxide transcritical inverse cycle, in terms of COP index and waste heat exploitation. © Published under licence by IOP Publishing Ltd.