EXPERIMENTAL INVESTIGATION OF CONTROL STRATEGIES FOR OFF-DESIGN OPERATION OF A TRANSCRITICAL CO2 TWO-PHASE EJECTOR REFRIGERATION SYSTEM FOR THE COLD CHAIN

The use of two-phase ejectors to improve the performance of CO2 refrigeration systems has been investigated extensively in recent years. Studies have shown that ejectors offer significant opportunity for COP and capacity improvement at conditions favorable to the ejector; however, performance improvement is known to be less significant at ejector off-design conditions. In this paper, methods for controlling the high-side pressure of a transcritical CO2 ejector cycle are investigated. The use of a simple expansion valve in series or parallel with the ejector to actively control high-side pressure is investigated experimentally and compared to other studies using an adjustable ejector. Control of the evaporator flow rate for different conditions is also investigated. Tests are performed at multiple capacities and ambient temperatures in order to investigate the performance improvement offered by an ejector cycle compared to an expansion valve cycle at off-design conditions with and without active control strategies.