Thermodynamic analysis and experimental investigation 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 dehumidification and re-heating according to high temperature lift of the air stream. In this paper, the transcritical CO2 cycle is compared with a sub-critical R134a cycle. The theoretical analysis is based on fixed temperature approach values at the heat exchangers. The study considers optimal high pressure for the transcritical cycle and optimal refrigerant subcooling for the sub-critical cycle. The theoretical analysis investigates the energy performance of the thermodynamic cycle as a function of the temperature and mass flow rate of the drying air. The optimisation of the operating conditions for CO2 involves lower air temperature than in the case of R134a; this conditions can be satisfied by a suitable design of the appliance, whose thermal balance is achieved when the dissipated heat corresponds to the work spent by the compressor and the fan; the air temperature is a floating variable that adjusts its value to comply with the thermal balance. Experimental results, conducted on a prototype, give a positive assessment for CO2 as working fluid for heat pump dryers: a negligible decrease in the electric power consumption, with a limited (+9%) increase in the cycle time, is shown in comparison with the reference R134a heat pump dryer.