ASHRAE Journal - February 2009 - (Page 23) System Type Low-Temperature HFC DX Baseline (Dual-Group) Low-Temperature CO2 Secondary (Single-Group) Low-Temperature CO2 DX Cascade (Single-Group) Low-Temperature CO2 DX Cascade (Dual-Group) Installed Copper Length m (ft), Percent Change 1690 m (5,544 ft) 753 m (2,472 ft), –55% 753 m (2,472 ft), –55% 809 m (2,655 ft), –52% Installed Copper Weight kg (lb), Percent Change 1147 kg (2,530 lb) 309 kg (681 lb), –73% 239 kg (527 lb), –79% 249 kg (549 lb), –78% Table 1: Installed length and weight of copper piping for various system types. Energy Consumption Compared to Baseline types. An additional configuration was added for the low-temperature CO2 cascade system with only one suction group to see if any significant installation savings could be obtained through combining the systems. Significant decreases in installed length of copper, from 52% to 55%, and dramatic decreases in installed weight, from 73% to 79%, are characteristic of the CO2 system types compared to the HFC baseline system. This indicates that the installation costs of the CO2 systems, once fully commercialized, can be expected to be lower than existing HFC systems due to both reduced labor and material costs. Regarding investigation of the CO2 cascade system with both one and two suction groups, it is clear that savings associated with a single suction group are quite minimal (7% less installed length and 4% less installed copper weight) and would not be significant compared to the higher energy consumption associated with the single suction group. System Performance Analysis 1.10 Energy Consumption Compared to Baseline Percent Difference Relative to HFC Baseline in Same Climate 1.00 0.90 0.80 0.70 0.60 0.50 1.00 –3.3% Low-Temperature HFC DX Baseline Low-Temperature CO2 SC, 50°F Liquid –7.7% Low-Temperature CO2 SC, 30°F Liquid –5.9% Low-Temperature CO2 DX, Cascade 0.944 Low-Temperature HFC DX Baseline –2.5% Low-Temperature CO2 SC, 50°F –7.2% –5.8% Low-Temperature CO2 SC, 30°F Low-Temperature CO2 DX 0.948 Low-Temperature HFC DX Baseline –8.4% –12.5% –11.8% Low-Temperature CO2 DX Atlanta Los Angeles Figure 7: Relative system energy consumption comparison. 1.05 1.00 0.95 1.000 Low-Temperature HFC DX Baseline 0.966 Low-Temperature CO2 SC, 50°F Liquid 0.941 Following the heat gain and piping analysis, an 0.923 0.922 annual energy analysis was performed on the three 0.897 0.892 0.90 systems under investigation. A bin analysis was performed based on weather data for three different 0.856 0.85 climate regions: Atlanta, Boston, and Los Angeles, using weather bin data in 2.8 K (5°R) increments. Figure 6 illustrates the temperature variations for the three 0.80 climactic regions and hourly occurrence per year. Minimum condensing temperatures were set 0.75 With Heat Gain Without Heat Gain at 21°C (70°F) for the low-temperature HFC DX system and 10°C (50°F) for the low-temperature Figure 8: Impact of neglecting heat gain. CO2 systems; the primary HFC portion of the CO2 systems would use electronic expansion valves that are well- system, the low-temperature CO2 secondary system, and the suited to handle large variations in differential pressure from low-temperature CO2 DX cascade system. Two variations of lower condensing pressures while the HFC DX system would the low-temperature CO2 secondary system were analyzed, one typically use thermostatic expansion valves that would not oper- with liquid subcooling from the medium-temperature system ate efficiently or would require costly readjustment to function providing +10°C (+50°F) subcooled liquid (the same as the well under these conditions. Analysis was also performed with low-temperature HFC DX system) and one with deeper liquid all systems operating with minimum condensing temperatures of subcooling providing –1°C (+30°F) subcooled liquid. The 10°C (50°F) for comparison. deeper subcooling provided by the medium-temperature system Figure 7 shows the results of the annual energy analysis for is a unique feature that the low-temperature CO2 secondary the three climactic regions for the low-temperature HFC DX systems can take full advantage of due to the close proximity Low-Temperature CO2 SC, 30°F Liquid Low-Temperature CO2 DX, Cascade Low-Temperature HFC DX Baseline Low-Temperature CO2 SC, 50°F Liquid February 2009 ASHRAE Journal Low-Temperature CO2 SC, 30°F Liquid Low-Temperature CO2 DX, Cascade Low-Temperature CO2 SC, 50°F Low-Temperature CO2 SC, 30°F Boston 23
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