As an effort to prevent environmental problems caused by ozone depletion and global warming, alternative refrigerants are being developed and one of the candidates is carbon dioxide. CO_2 has no toxicity, flammability, environmental harming characteristics. However, owing to its low critical temperature, refrigeration cycle using CO_2 refrigerant inherently forms trans-critical cycle in the normal operating conditions. In this reason, a control problem of deciding optimal heat rejection pressure of trans-critical cycle occurs. In this study, an optimal control method of heat rejection pressure which is not necessary to use experimental correlation is proposed based on the graphical method. The method adopts some assumptions which include 'isothermal condition at gas-cooler outlet', 'constant specific compression work ratio condition' and 'constant evaporation pressure condition' while expansion valve generates small variation of heat rejection pressure. The error caused by those assumptions shifts the controlled heat rejection pressure away from its optimal point and degree of the shift was analyzed. A control variable for the method was proposed and the method was applied to a real CO_2 refrigeration system. According to the experimental results, the COP obtained by the optimal control method was fairly closed to the maximum COP and the controllability was also quite good.
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