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M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 163 hydrocarbons or ammonia; the advent of lightweight Table 3 Summary of capacity and COP for the different configurations [169] compact microchannel heat exchangers presents new opportunities for system optimization. Second, the world- wide availability of CO2 and freedom from HFC-related regulatory uncertainties fits well with the global nature of the transport refrigeration industry. Shipping containers must comply with regulations at all ports, and mass- produced trucks must be capable of operating at the farthest reaches of the ‘cold chain’ in rural developing countries where fluorocarbon refrigerants and trained recovery technicians may not be available. Preliminary test results on a prototype CO2 system for truck refrigeration gave COP data that matched equally sized systems using R-502 and R-507 [170]. A Danish study predicted the performance of refrigerating systems for transport containers, concluding with COP values that were 15 – 20% below those of R-134a systems, not including the effects of differences in compressor efficiency and refrigerant-side pressure drops [171]. Jakobsen and Neksa ̊ [172] conducted more detailed simulations including the effects of capacity control and varying compressor efficiency at varying compression ratio. The results showed very similar COP values in freezing mode for CO2 and R-134a over the full range of ambient temperature. In cooling mode, the excess capacity was much greater with R-134a than with CO2 due to differences in refrigerant properties. When the influence on COP by suction throttling or cylinder unloading was included, the estimated COP in freezing mode became slightly (3 – 10%) higher for the CO2 system than for the R-134a system. One problem with CO2 may be very high compressor discharge temperature for freezing operation at high ambient temperature. 8.8. Commercial refrigeration Commercial refrigeration systems for shops, super- markets, larger kitchens, etc. have large refrigerant emissions, and the energy use is in many cases high. Thus, there is a need for efficient, safe and environmentally friendly refrigeration systems. New concepts based on CO2 have been demonstrated for centralized systems using CO2 as a secondary heat transfer fluid or in a low-temperature cascade stage, and recently decentralized concepts with heat recovery have been shown. Some of these developments are outlined in the following text. Eggen and Aflekt [173] reviewed the possibilities for CO2: (i) as secondary refrigerant, (ii) as a primary refrigerant in a low temperature stage in a cascade system, and (iii) in all-CO2 centralized systems. They also presented a prototype CO2/NH3 cascade system built in Norway. A large number of secondary fluid systems are already operating in the Nordic countries using CO2 as a volatile secondary refrigerant. The safety aspects and good thermo- physical properties of CO2, leading to small pipe dimensions and good heat transfer, make it a preferable Averagea Peakb Design conditionc R-22 Mil-Std Capacity (kW) 9.91 COP 1.46 Capacity (kW) 12.9 COP 2.10 Capacity (kW) 9.60 COP 1.40 CO2 CO2 Basic Plus 7.62 8.20 0.96 1.03 11.62 11.47 1.34 1.54 6.75 8.47 0.97 1.03 a Average values across all sixteen test conditions (indoor: 32 8C/50 and 90% RH, and 26.7 8C/50 and 90% RH, outdoor: 52, 49, 43, and 38 8C). b The one test condition out of sixteen where the value was greatest (R-22 Mil-Std: at 26.7 8C/50% RH indoor and 38 8C outdoor, CO2 Basic: at 32 8C/50% RH indoor and 38 8C outdoor, CO2 Plus: at 32 8C/90% RH and 38 8C outdoor). c It reflects the current military rating point of 32 8C/50% RH indoor and 49 8C outdoor. Future Mil-Std design conditions will be raised to 32 8C/50% RH indoor and 52 8C outdoor. this type of system compared performance of CO2 and R-22 and found them roughly equivalent under some conditions, even if the CO2 unit used conventional fin-and-tube heat exchangers [36]. The first data from a microchannel-based CO2 proto- type ECU were presented in 2002 as shown in Table 3 [169]. The ‘CO2 Basic’ unit consists of an automotive reciprocating compressor, microchannel heat exchangers, an accumulator, and an expansion devise which was a hand-adjusted metering valve. The system was designed with a safety factor of four times the expected working pressure, which is common for current military systems. The design burst pressure was 62.0 MPa since the high- side system pressure was limited to 15.5 MPa. The CO2 Basic ECU did not perform as well as the R-22 Mil-Std ECU in terms of capacity and COP. The addition of internal heat exchanger (‘CO2 Plus’ unit) improved the capacity and COP but still fell short of the R-22 baseline. They speculated that the system capacity and COP could be further improved by using an appropriate compressor, and a change in fan type. These government-sponsored research efforts have augmented the already substantial industry-funded efforts to identify opportunities for improving system efficiency by developing advanced component technologies for the next generation of prototype residential space conditioning systems. 8.7. Transport refrigeration Research interest in CO2 has also been renewed in the area of transport refrigeration for two reasons The first relates to the relatively high density and capacity of CO2 at low temperatures, compared to alternatives such asPDF Image | CO2 Vapor Compression Systems
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