Compact Interior Heat Exchangers for CO2 Mobile Heat Pumping Systems
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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. Several issues were identified for the design of compact interior heat exchangers for automotive reversible CO2 heat pumping systems. Among theses issues are: • Refrigerant flow distribution • Heat exchanger fluid flow circuiting. • Air temperature uniformity downstream of the heat exchanger. • Minimization of temperature approach. • Windshield flash fogging, due to retained water inside the heat exchanger. • Internal heat conduction in heating mode operation. • Refrigerant side pressure drop. In order to provide a basis for understanding these issues, a calculation model was developed, a test facility was established, and different prototype heat exchangers were experimentally investigated. The test facility, installed in the laboratories of SINTEF/NTNU, provided a high degree of flexibility with respect to operating conditions and modes during the experimental investigation of different heat exchangers. Air and glycol can be applied as heat source and heat sink of the heat pumping system. The deviation between the measurements of the heat exchanger capacities was in the range of ±4 % with independent heat balances. The investigation showed that higher refrigerant pressure drops could be accepted for multi-row heat exchanger designs (see Part A of Fig. SC.1) when high cooling capacities are required, compared to single row multi pass concepts (see Part B of Fig. SC.1). The refrigerant temperature profile (gradient) of thermal counter current flow arrangements (physical cocurrent 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.