Sammendrag
The pernicious effects of synthetic refrigerants on different environmental aspects leave natural refrigerants as the only alternative for vapour compressions systems. Among natural refrigerants, CO2 (R744) has become the preferred choice for commercial refrigeration at almost any location and climate. However, efficient R744 refrigeration systems for warm climates have a great level of complexity, implementing technologies such as mechanical subcooling or ejector that increase the investment costs. The goal of the novel hybrid configuration presented in this work is to simplify ejector-supported R744 commercial refrigeration systems while maintaining all the benefits of the ejector implementation in transcritical operation mode. Moreover, utilization of a low-pressure accumulator layout in the subcritical mode removes all the practical challenges related to the booster layout operating at low ambient temperatures. This solution is based on: (i) MT and LT compressor suction groups, (ii) non-superheated MT evaporation with increased evaporation temperature, and (iii) ejector utilization throughout the year. The ejector is actively operated as a high-pressure-control device at elevated ambient temperatures ('summer mode'), while it is passive and acts as a check-valve at lower ambient temperatures ('winter mode'). The experimental campaign performed proved that this novel system configuration is more energy efficient than booster systems with parallel compressors at any condition, improving the COP by around 40 % at the most extreme gas cooler outlet temperatures tested, i.e., 40 °C (summer mode) and 10 °C (winter mode).
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