ASHRAE Journal - February 2009 - (Page 24) System Type Low-Temperature HFC DX Baseline (Dual-Group) Low-Temperature CO2 Secondary (Single-Group), 10°C (50°F) Subcooled Liquid Low-Temperature CO2 Secondary (Dual-Group), –1°C (30°F) Subcooled Liquid Low-Temperature CO2 DX Cascade (Dual-Group) HFC Charge kg (lb), Percent Change 680 kg (1,500 lb) Direct Effect (Emissions) tons CO2, Percent Change 10,283 ton Indirect Effect (Energy) tons CO2, Percent Change 3,470 ton TEWI tons CO2, Percent Change 13,753 ton 272 kg (600 lb), –60% 866 ton, –92% 3,354 ton, –3.4% 4,220 ton, –69% 272 kg (600 lb), –60% 272 kg (600 lb), –60% 866 ton, –92% 3,203 ton, –7.7% 4,069 ton, –70% 866 ton, –92% 3,265 ton, 5.9% 4,131 ton, –70% Table 2: Refrigerant charge and TEWI versus system type for Atlanta climate conditions. of the low-temperature subcooler and evaporators. The analysis shows that low-temperature secondary coolant systems can save 3% to 12% compared to the low-temperature HFC DX baseline system depending on climate and level of subcooling. Additionally, the low-temperature DX cascade systems can save between 5% and 11% depending on the same subcooling and climate factors. If the minimum condensing pressures for the DX systems are allowed to float to the same minimum level of 10°C (50°F) level as the DX HFC baseline system, the differences between the system types becomes smaller though still favorable for the CO2 systems. Energy consumption for the secondary coolant systems saves 0% to 5% compared to the low-temperature HFC DX baseline system, and the cascade system saves 3% to 4% depending on the climate under investigation. Regarding the impact of including the heat gain analysis in the modeling, the analysis was repeated for the Atlanta climate conditions but without including the variation in load caused by the piping heat gain. Figure 8 compares the low-temperature system energy both with and without the heat gain impacts. A different comparison of the systems can emerge when the heat gain is ignored: loads on all the system types are underpredicted and the CO2 cascade system becomes an unfavorable alternative compared to the other system choices. Analysis of the systems without the heat gain would provide a misleading view for comparison of the technologies. Along with energy consumption, a comparison of refrigerant charge and total equivalent warming impact (TEWI) was performed. Table 2 shows the results of this analysis and the benefit to equivalent carbon emissions of all the CO2 system types. The HFC charge of the DX baseline system was based on actual field experience, and a conservative estimate of 60% reduction in HFC charge was used for the CO2 systems. Field experiences also indicate that better reductions can be achieved but depend on condenser type and type of heat reclaim that is used. Direct and indirect impacts of TEWI are shown for each system. As the energy consumption of the four systems analyzed is very close, the impact on total TEWI is primarily a result of the reduction in HFC charge. A value of 0.619 kg CO2 per kWh electricity generated was used for calculating the indirect impacts. This is an average value for the state of Georgia as Atlanta climate conditions were used for the analysis. However this is close to the U.S. average rate of 0.606 kg CO2 per kWh.4 Refrigerant leakage rate for the HFC direct expansion system was assumed to be the current U.S. average of 25%. Leakage rates for the HFC systems confined in the mechanical room were assumed to be 5%. An end-of-life recovery rate of 95% was used for all system types. Future Systems www.info.hotims.com/23932-27 Low-temperature CO2 secondary systems went into full commercialization in 2008 with many new installations under way. Extension of the technology to medium-temperature applications is viable and offers several performance improvements February 2009 24 ASHRAE Journal http://www.wrightsoft.com http://www.wrightsoft.com http://www.info.hotims.com/23932-27
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