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1.1 Background Chapter 1: Introduction Interest in R744 (carbon dioxide, one of the earliest-used refrigerants,) has recently been revived as a result of environmental considerations and modern heat exchanger manufacturing methods. Investigating the use of R744 in residential heat pump applications is a part of comprehensive program in transcritical R744 system and component research, which began by comparing the performance of R744 and R410A in cooling mode (Beaver et al., 1999a). In this report experimental results comparing the performance of R744 and R410A in heating mode are presented, as well as a theoretical comparison of the maximum operating efficiency in both heating and cooling mode of R744 and R410A subject to comfort constraints and real cycle limitations. By including the effect of comfort constraints and using R410A as a baseline, the theoretical portion of this report differs from earlier R744 cycle studies (Lorentzen and Pettersen, 1993; Hwang and Radermacher, 1998; and Robinson and Groll, 1998). Much of the interest in R744 is due to the fact that it lends itself well to compactness. Several groups (including the ACRC) are investigating R744 for mobile applications, and it is possible that R744 systems could be marketed by the automo bile industry by 2005. Recent studies have investigated the use of R744 in environmental control units for the military (Robinson and Groll, 2000), in which R744 is advantageous in terms of portability, refrigerant handling and worldwide availability. In residential systems, especially in the United States, compactness is not a driving concern. Rather, the comfort of a conditioned space and efficiency of the system under a variety of conditions is of overriding importance. This report consists of four parts: first, experimental results comparing the first 3-ton R744 a/c system (RAC1) to a conventional baseline system in heating mode; second, a theoretical analysis of ideal transcritical cycles subject to comfort constraints in heating and cooling modes; third, an assessment of the effects of finite exchangers and a real compressor on cycle efficiency; and fourth, a comparison of annual efficiency as a function of climate. Supplementary analyses are included in the Appendixes. The experimental facilities used for the experimental heat pump comparison are described in Appendices A-C. Appendix D discusses various vapor compression cycle control and distribution options that would be necessary in implementing the assumptions included in the theoretical analysis. Finally, Appendix E contains a discussion of psychrometrics and sensible heat ratio as they relate to the comfort constraint in cooling. 1.2 Selection of baseline refrigerant The choice of the representative baseline system and refrigerant for the residential market is sensitive due to the variety of systems around the world. Pettersen et al. (1997) compared the simulated performance of an earlier R744 prototype with an R22 ductless split system designed for climate typical of Japan’s. We have chosen a system typical of those used in the USA: a split system with an outdoor compressor/condenser unit and a ducted indoor heat exchanger. Comparisons are made against R410A as it is arguably the most efficient, and widely used HFC refrigerant commercially available. For the experimental results, heat exchangers in the baseline R410A units are of a type commonly used in conventional systems: copper tubes and aluminum louvered and wavy fins. Due to higher operating pressures, the 1PDF Image | Comparison of R744 and R410A
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