A study on the real time optimal control method for heat rejection pressure of a CO2 refrigeration system with an internal heat exchanger

•A real time heat rejection pressure control method for CO2 heat pump is investigated.•The real time controller does not require preliminarily obtained correlations.•The characteristics of the real time controller are analyzed.•The possibilities of using the real time controller were shown in the paper.

Carbon dioxide (CO2) is one of the replacements of the synthetic refrigerants whose properties are good as a refrigerant with no ozone depletion and a very little global warming effect. However, its low critical temperature makes a CO2 refrigeration system to form trans-critical cycle and determination of an optimal heat rejection pressure becomes an important problem for the best performance. Until now, number of studies have been published which correlate preliminarily obtained data map with some major operating parameters. In this study, a generally usable real time optimal control method is introduced. This real time optimal control method does not require preliminarily obtained correlations and makes the system operate in a nearly optimal region. Additionally, its limitations which produce undesirable errors and bias of controlled value were also treated in this paper. Three causes of errors were discussed and it is helpful to improve the control method.