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• CO 2 refrigeration systems operate in both subcritical and transcritical modes. • Consistent control strategy can be independent of operation modes. • Average COP losses of typical systems with new control strategy are within −0.5%. CO 2 refrigeration systems operate in both subcritical and transcritical modes throughout a year. Different control strategies have been developed under different operation modes. However, this leads to complexity in implementation. In this work, we proposed a consistent control strategy (CCS) independent of operation modes for CO 2 refrigeration systems to provide stable operation in a simple way. This strategy is grounded on the fact that discharge pressure control in transcritical mode is essentially regulating refrigerant charge in a running system. The active pressure control can be avoided with fine refrigerant charge management, thereby employing the same control strategy as in subcritical mode. Through numerical simulations, the characteristics of optimum charge in three typical CO 2 refrigeration configurations were identified, which are single-stage system, two-stage system, and booster system. The optimum charges of three systems under different operating conditions all vary less than 22%, particularly the booster system fluctuates by 6% only. By adopting CCS, the average COP losses for three systems are all within -0.5% relative to their ideal optimum performance. Compared with re-optimized conventional multi-zone control, the maximum deviation is -0.3%. Therefore, the proposed new strategy can achieve comparable quasi-optimum performance, while significantly simplifying control scheme. It can be an attractive solution for small units which require simple and robust control. [ABSTRACT FROM AUTHOR]

CO 2 transcritical cycle has been widely promoted as a green energy utilization technology. Internal heat exchanger (IHX) is under consideration for cycle efficiency improvement. However, the interaction between IHX and compressor in CO 2 cycle has not yet been fully explored. In this regard, compressor operating boundaries are introduced as constraints to IHX applicability evaluation in CO 2 cycle. It indicates that considering compressor limits, IHX can either benefit or attenuate the CO 2 cycle performance, depending on operating conditions. By reducing optimal discharge pressure, IHX helps CO 2 cycle avoid the upper pressure limit, further improving performance by 5%. Over a wide range, however, IHX increases compressor discharge temperature, indirectly resulting in up to 50% performance degradation. The major factors affecting the duality of IHX in CO 2 cycle are compressor discharge temperature limit, IHX efficiency and motor heat loss. Finally, performance-oriented IHX design guidelines for mainstream single-stage CO 2 applications are provided. For automotive air conditioner, optimum IHX efficiency improves cycle performance by up to 30% in cooling mode. By contrast, IHX contribution is negligible in heating mode. For heat pump water heater and refrigeration system, both can benefit from IHX, particularly when compressor discharge temperature tolerance is high. • Applicability of internal heat exchanger (IHX) in CO 2 cycle is limited by compressor. • IHX leads to lower optimum high pressure but higher discharge temperature. • IHX design guidelines are provided for three mainstream CO 2 applications. [ABSTRACT FROM AUTHOR]