Abstract
The natural refrigerant carbon dioxide (CO2) offers new possibilities for design of flexible, efficient and environmentally safe mobile heat pumping systems. As high-efficient car engines with less waste heat are developed, extra heating of the passenger compartment is needed in the cold season. A reversible transcritical CO2 system with gliding temperature heat rejection can give high air delivery temperature which results in rapid heating of the passenger compartment and rapid defogging or defrosting of windows. When operated in cooling mode, the efficiency of transcritical CO2 systems is higher compared to common (HFC) air conditioning systems, at most dominant operating conditions. The investigation showed that higher refrigerant pressure drops could be accepted for multi-row heat exchanger designs when high cooling capacities are required, compared to single row multi pass concepts. The refrigerant temperature profile (gradient) of thermal counter current flow arrangements (physical co-current flow) resulted in increased local temperature differences. The (penetrating) air left the heat exchanger ‘together’ with the refrigerant at the lowest evaporation pressure, i.e. low temperature approach values can be achieved. The airside temperature is less uniform in multi-pass single row heat exchangers at high cooling capacities, i.e. refrigerant side temperature gradients occur, due to the increased refrigerant side pressure drop. A number of final conclusions can be drawn from the theoretical and experimental investigations, which finalized the PhD work of the writer on ‘Compact Interior Heat Exchangers for CO2 Mobile Heat Pumping Systems.