Potential Coefficient of Performance Improvements Due to Glide Matching with R-407C

Potential improvements of the Coefficient of Performance (COP) were investigated for R-407C at cooling and heating conditions of a residential heat pump. The study used CYCLE-11, a semi-theoretical vapor-compression cycle model, which was upgraded to simulate cross-flow, counter-flow, and parallel-flow evaporator and condenser with no restrictions on specifying superheated vapor and subcooled liquid regions. The COP benefit of using counter-flow heat exchangers in a low-lift air-conditioning application was found to be 7.1% when compared to a system with a cross-flow evaporator and condenser. For heating conditions (high-lift application), the predicted benefit was 3.6%. These results are representative of a water-to-water system, where counter-flow heat exchangers are suitable for implementing glide matching. Since air-to-refrigerant heat exchangers are not counter flow but rather cross-counter flow (in the best case), the potential to improve COP in a practical air conditioner is lower . The COP penalty caused by glide mismatch (parallel flow) is approximately twice as severe as the benefit that can be realized from glide matching. Aarea COP coefficient of performance c heat capacity ppressure Qheat sentropy Ttemperature Uoverall heat-transfer coefficient UA heat exchanger conductance Vvolume L1 difference NOMENCLATURE Subscripts: evap evaporator cond condenser hx heat exchanger p constant pressure v constant volume cr saturation line 1, 2, ... , i denotes a section of the heat exchanger INTRODUCTION The use of zeotropic mixtures as CFC and HCFC substitutes has brought about a discussion of performance benefits due to matching of the temperature profile of the zeotrope with the temperature profile of the heat-source and heatsink fluids. The matching of temperature profiles, also referred to as glide matching, results in smaller irreversibilities of the heat-transfer processes, which, in turn, results in an improved cycle efficiency. This concept is often graphically explained using · the Carnot cycle (for fluids with constant temperature evaporation and condensation processes) and the Lorenz cycle (for zeotropes), as shown in Figure I. The efficiency advantage of the Lorenz cycle corresponds to the reduction in the area between the refrigerant and heat-sink and heat-source fluids. 1 Contribution of the National Institute of Standards and Technology, not subject to copyright in the United States 2 Pem1anent address: Multibras S.A., Joinville-SC, Brazil. Multibras S.A. is a subsidiary of Whirlpool Corp.