Abstract
Recent studies have shown that expansion work recovery by use of a two-phase ejector offers significant opportunity for improving the COP of refrigeration systems, especially transcritical CO2 systems. However, performance improvement is known to be less significant at off-design conditions that are not as favorable to the ejector cycle. This paper investigates methods for controlling the high-side pressure of a transcritical CO2 ejector cycle. The use of an ejector with an adjustable motive nozzle throat area as well as a simple expansion valve upstream of the ejector to actively control high-side pressure are investigated and compared experimentally. Proper control of evaporator flow rate for different conditions is also investigated, and the penalty for not actively controlling the evaporator metering valve is also investigated. The control methods are investigated over a range of capacities and ambient temperatures in order to see the performance penalties to a transcritical CO2 ejector cycle when it is not controlled properly.
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