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164 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 Fig. 59. Distributed CO2 supermarket refrigeration system with central heat recovery. fluid in indirect systems. Further advantages of cascade systems include the greatly reduced low-temperature compressor sizes, the absence of a liquid pump, and fewer stages of heat transfer. With heat recovery, centralized all-CO2 systems may also have an interesting potential. The decentralized supermarket system described by Neksa ̊ et al. [174] uses CO2 as the only refrigerant in a system with heat recovery. Self-contained display cabinets each with CO2 refrigeration units are connected to a hydronic heat recovery circuit that heats service water and buildings (Fig. 59). By utilizing the transcritical CO2 process, it is possible to have a large temperature glide in the hydronic circuit, typically 50–60 K, and a correspondingly low volume flow rate and small pipe dimensions. Waste heat with high temperature (70–75 8C) is available for tap water and/or space heating. Excess heat is rejected to the ambient air by direct heat exchange. The system offers a very easy installation and gives the owner of the store a great flexibility in arranging and rearranging the cabinets. System simulations for a medium size supermarket have been carried out. Optimum hydronic supply and return temperatures to the cooling and freezing cabinets were identified. A comparison between the CO2 system and a conventional R-22 system with respect to the overall energy consumption of the supermarket for one year of operation in a southern European climate was carried out. The CO2 system was found to reduce the total energy consumption for refrigeration and heating by 32% compared to the R-22 system. Each CO2 unit can also be equipped with a condensing unit in order to reject heat directly to the shopping area when space heating is required. In the warm season with a heat surplus, the waste heat recovery circuit removes the heat. This concept reduces the power demand for the refrigeration units to the same level as for the baseline R-22 system, and the resulting overall energy consumption of the supermarket will then be further reduced. 8.9. Dryers Another interesting application of CO2 vapor compression cycle is heat pump dryer One example is the prototype of a commercial size dryer as shown in Fig. 60, which shows the schemes of the heat pump and the hot air circuit [175]. The auxiliary heat exchanger is located Fig. 60. CO2 heat pump dryer [175].PDF Image | CO2 Vapor Compression Systems
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