Studies on the Control Method of Optimal Heat Rejection Pressure in CO2 Heat Pump

Abstract

For the last two decades, numerous environmental problems have affected all throughout the world. Among these problems, global warming and ozone depletion is severely under attack resulted from the use of conventional synthetic refrigerants. Thus, many researchers have endeavored to utilize alternative refrigerants in order to overcome this problem. Above all, carbon dioxide, called CO2, is well appreciated and eco-friendly as an alternative refrigerant because it has low global warming potential (GWP) and ozone depletion potential (ODP), compared to the other synthetic refrigerant such as hydrochlorofluorocarbon (HCFC) or hydrofluorocarbon (HFC). Usually, transcritical cycle is built in a CO2 refrigeration cycle where CO2 is employed as a refrigerant owing to its low critical temperature. In addition, high-side pressure of the CO2 refrigeration system is much higher than that of the conventional refrigeration system. In this way, coefficient of performance (COP) which is a main index of the system performance is highly sensitive to high pressure, called heat rejection pressure, at the gas cooler. In conclusion, optimization of the heat rejection pressure is a must to study performance of transcritical CO2 cycle. In this study, a control method of optimal heat rejection pressure in CO2 heat pump is investigated. First of all, one control parameter is analyzed by various conditions of the system and proposed. In sequence, basic experiment was performed in the transcritical CO2 cycle with internal heat exchanger, changing the compressor operating frequency and expansion valve opening. Moreover, change of refrigerant charge amount had an effect on the system performance. As a result, maximum COP was appeared at the smaller opening when the compressor speed existed in the lower state. An optimal refrigerant charge amount was occurred at a certain condition. Results were confirmed with suggested control method by conducting control experiment. The point where controlled parameter and current COP of a steady-state system are encountered called an operating point. Error was calculated with the difference between the value of parameter and maximum system COP. It was presented that if various conditions with setting control parameter were capable to use for real-time control in a transient–state system on the basis of result analysis in this study.