International.
The recovery of expansion works by means of a two-phase ejector is an attractive option for improving the efficiency of commercial CO2 cooling systems. Transcritical CO2 systems have relatively large choke loss, meaning they offer the greatest opportunity to improve it with an ejector.
However, the use of ejectors in a system subject to various environmental conditions and capacity requirements presents a significant challenge, as ejectors are seen to deliver poor off-design performance or by reducing capacity conditions. For example, the COP can decrease by up to 17% from its optimum level under the given conditions if the refrigerant gas pressure varies by 0.5 Mpa.
In a paper presented at International Congress Refrigeration 2015, N. Lawrence and S. Elbel presented two different strategies for achieving higher ejector and cycle performance at different capacity conditions: variable geometry ejector and parallel arrangement of multiple ejectors.
They considered different ejector cycle options with two evaporation temperatures and different uses of the ejector. They showed with a theoretical model that using an ejector to discharge the medium-pressure compressors in a CO2 amplified cooling cycle can result in the improvement of COP by 15%, but using an additional ejector to discharge the LP compressors resulted in a small or no additional IMPROVEMENT of COP compared to the individual ejector cycle.
