Theoretical analysis and experimental test of a CO2 household heat pump dryer

Carbon dioxide is regarded as an optimal working fluid for heat pump dryers. The transcritical cycle well fits the closed-loop drying process which requires humid air dehumidification and reheating according to high temperature lift . In this paper, the transcritical CO2 process is compared to the traditional subcritical R134a cycle. The comparison is carried out with a theoretical analysis which takes into account the drying process with definite temperature approach values at the heat exchangers. The study considers optimal upper cycle pressure for the transcritical cycle and optimal refrigerant subcooling for the subcritical cycle. The results of the analysis show for both fluids the energy performance of the thermodynamic cycle as a function of the operating conditions in terms of temperature and mass flow rate of the drying air. Experimental tests, conducted on a first prototype are reported as well; the results give a positive assessment for CO2 as the working fluid for heat pump dryers.