Thermodynamic assessment of trans-critical refrigeration systems utilizing CO2-based mixtures

Since the last decade of the 20th century, there has been increasing attention to the use of CO2 as a working fluid in transcritical refrigeration and cooling cycles. Transcritical cycles suffer from a heavy expansion loss, requiring specific means for its mitigation. Moreover, most applications for cooling or refrigeration are cooling a stream that is not changing phase, therefore has a finite heat capacity and does not well match with the evaporation curve of any pure fluid. In this work, to overcome these limitations (low COP and poor matching of heat exchanges curves) the potential of using a second component, mixed with CO2, has been assessed. For this purpose, several fluids have been investigated, from synthetic refrigerants, such as R1233zd(E) or R1234yf, to hydrocarbons, such as n-butane, or n-pentane and alcohols, like dimethyl ether. To investigate the effects of the addition of the second element in the CO2 transcritical cycles, several configurations have been investigated, from the base refrigeration cycle to the double evaporator ejector cycle. All examined mixtures show a COP improvement with respect to pure CO2. However, the mixture with R1234yf has the flattest evaporation curve and hence, in the case of a constant specific heat of the fluid to be cooled, gives the best result. The quantity of R1234yf that must be added to the mixture never exceeds 15%.