...performance potential transcritical carbon dioxide (C[O.sub.2]) cycle with linear compressor is explored for medium-temperature refrigeration systems, which have a return air temperature range from -2[degrees]C to 6[degrees]C, through C[O.sub.2] cycle simulation using compressor efficiencies measured. In addition, the design of the heat exchanger for such systems was practiced. The simulated results of the transcritical C[O.sub.2] cycle performance showed that the cooling capacity was 235 W at 40.5[degrees]C ambient temperature and the coefficient of performance was 1.31 at 32.2[degrees]C ambient temperature. The heat exchangers were then designed using a verified heat exchanger design software. As a target application, a 200 liter effective volume refrigerator cabinet was selected, and its refrigeration load was evaluated by using load calculation software. When the C[O.sub.2] linear compressor with suitably designed heat exchangers was applied to the selected refrigeration cabinet operating at medium temperature refrigeration conditions, the simulated results showed that the designed C[O.sub.2] system based on a linear compressor could provide sufficient refrigeration capacity.

INTRODUCTION

Extensive research and development have been performed on carbon dioxide (C[O.sub.2]) technology over the last two decades since the revival of C[O.sub.2] research by Lorentzen and his colleagues in the early 1990s (Lorentzen and Pettersen 1993; Neksa 1994; Pettersen and Skaugen 1994; Pettersen et al. 1995). Among the many investigated applications, there are several successful C[O.sub.2] technology applications: C[O.sub.2] residential heat pump water heaters, C[O.sub.2] environmental control units, C[O.sub.2] automotive climate control systems, and C[O.sub.2] commercial refrigeration systems. C[O.sub.2] heat pump water heaters have been successfully commercialized and introduced exclusively in Japan since 2001 (Endoh et al. 2006). Meanwhile, Cutler et al. (2000) developed a C[O.sub.2] environmental control unit using a 10 kW cooling capacity C[O.sub.2] reciprocating compressor; they reported that a cooling capacity of 10-13 kW and a corresponding coefficient of performance (COP) of 2.3-3.5 could be reached for outdoor ambient temperatures of 27[degrees]C to 41[degrees]C. Additionally, major automobile companies and their suppliers have developed vehicles with C[O.sub.2] automotive climate control systems to provide main cooling and heating or auxiliary heating. Manzione et al. (2006) reported that these C[O.sub.2] automotive climate control systems demonstrated better cooling performance than the current climate control system. Last, C[O.sub.2] commercial refrigeration systems have developed more efficiently. Rohrer (2006) compared the performance of the bottle cooler while using a 600 W input variable-speed two-stage rotary type C[O.sub.2] compressor; he reported that the energy consumption of the C[O.sub.2] system was 30% lower than that of the R-134a baseline system at 32.2[degrees]C ambient temperature. Suss (2005) reported a performance potential of the reciprocating type C[O.sub.2] compressor having 1 kW refrigeration capacity; in his research, 18% energy savings were found as compared to the R-134a system under the European climate.

As summarized, most previous studies have focused on the application of C[O.sub.2] technology to large refrigeration capacities greater than 500 W. Three major international food and beverage companies initiated the Refrigerants, Naturally partnership (RN 2004) to replace hydrofluorocarbon (HFC) refrigerants with natural refrigerants and to reduce the contribution to global warming from the food and drink industry and its supply chain. The partnership's focus is to develop equipment using alternative refrigerants while maintaining high efficiency. Moreover, the number of small-capacity systems that have refrigeration capacities less than 500 W is growing faster than systems of other capacity ranges, especially in developing countries. Therefore, C[O.sub.2] linear compressors with a small capacity range and high efficiency may have great potential in producing energy-efficient refrigeration equipment. The objective of the current study is to provide design examples of the heat exchangers and to investigate the system performance of a medium-temperature C[O.sub.2] refrigeration system of small capacity and with a linear compressor, which has a return air temperature range from -2[degrees]C to 6[degrees]C.

CYCLE MODELS

To model the performance of the transcritical C[O.sub.2] cycle, a previously developed computer model of the C[O.sub.2] refrigeration cycle (Hwang and Radermacher 1998) was modified in two steps. First, the compressor efficiencies...

NOTE: All illustrations and photos have been removed from this article.



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