System analysis and optimization of a CO2 heat pump tumble dryer.

The use of heat pump technology in tumble dryers reduces energy consumption by approximately 50% compared to conventional vented and condensing dryers. At present R134a is typically used as refrigerant, although environmentally friendly alternatives exist. Dryers for semi-professional use require larger refrigerant charge compared to domestic dryers, which makes the use of flammable refrigerants challenging. CO2 appears to be a promising alternative. This study covers a detailed energetic analysis and optimization of a semi-professional heat pump tumble dryer with CO2 as refrigerant. The dryer consists of two closed cycles, i.e. the air flow and the heat pump cycle. The latter is equipped with an additional sub-cooler after the gas cooler to reject energy to the ambient air when necessary. The prototype of the CO2 heat pump dryer was developed by modification of a commercially available R134a dryer. Details of the configuration are given and experimental results are presented to illustrate the transient system behavior and overall performance. The experimental data is used to validate a detailed physics-based simulation model on both component and system level. Relevant energy flows to the environment through leakage and heat losses are evaluated by simulation. Interactions between heat pump and air flow cycle are discussed in detail. Consequentially, an alternative system configuration is presented. This includes a suction line accumulator and thermal insulation. Simulation results show an improved system performance in terms of energy efficiency and drying time. Finally, the CO2 heat pump dryer is compared with the corresponding R134a system.