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in heating, when the load exceeds the maximum capacity of the system, it is assumed that electric resistance heaters supply supplementary heat with an efficiency of one. Finally, as discussed in Chapter 4, an internal heat exchanger is assumed to be present on the R744 system, but not on the R410A system. 5.3 Results The load/capacity and efficiency results based on the above approximations are shown in Figure 5.3, for a 1 kW heating load at –10oC and a supply air temperature of 40oC. In cooling mode, the displacement of the compressor is capable of matching load requirements over most of the outdoor temperature range, and cycling is only necessary below an outdoor temperature of 30oC. In heating, however, varying the compressor displacement meets the load only in a limited portion of the temperature range, with cycling being required above an outdoor temperature of 5oC, and supplementary heating being required below an outdoor temperature of –4oC by R410A, and –8oC by R744. The efficiencies of the two systems are approximately equal, except where the supplementary heating is required and there is a sharp reduction in efficiency. Below –20oC, the heating is supplied predominately by the supplementary heat source. Additionally at this point, for the 60oC supply air case, the refrigerant discharge temperatures begin to approach 200oC and the working limits of the compressor lubricant become a consideration. Figure 5.4 shows the annualized energy efficiency for the three regions plotted in Figure 5.1 as a function of the outdoor temperature at which the heating load is 1 kW, for a heating supply air temperature of 40oC (the capacity and efficiency for a 1 kW load at –10oC was shown in Figure 5.3). In Dallas/Ft. Worth where cooling loads dominate, R410A is considerably more efficient. In Seattle, where the cooling requirement is relatively small and the majority of the heating load can be met without supplementary heat, the efficiency of the two cycles is nearly equal. In Chicago, R744 show a very slight advantage, except when the outdoor temperature for 1 kW load is low, representing a well-insulated environment. 1.2 1.0 0.8 0.6 0.4 0.2 0.0 10 8 6 4 2 Figure 5.3 Capacity load curve for sample system, 40oC supply air in heating Heating Load Cooling Load -20 -10 0 10 20 30 40 50 Outdoor Temperature (C) Supply air temperature: 40 C R744, Cooling R744, Heating R410A, Cooling R410A, Heating -20 -10 0 10 20 30 40 50 Outdoor Temperature (C) 45 Efficiency Load and Capacity (kW)PDF Image | Comparison of R744 and R410A
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