Two­-Stage Compression With Vapor Injection: A Study On A Disregarded Solution In CO2 Booster Applications For Supermarket Refrigeration

CO2 refrigeration has been largely implemented, especially in supermarket applications. Nonetheless, one solution that has been often disregarded in these applications has been the inclusion of Two­Stage compression cycles including vapour injection. Available literature has not yet compared these solutions with Booster applications. Thus, this paper presents the modelling of two solutions of Two­Stage vapour injection cycles in a Booster configuration application: Flash Tank vapour injection (FTVI) and Sub­cooled Vapour injection (SCVI). The adopted methodology is more focused on the thermodynamic cycle rather than detailed components modelling. An analysis of the optimal displacement ratio between compression stages has been conducted, exploring the
influence of this parameter on the optimum transcritical high pressure. It has been found that optimal high pressure does not vary considerably when changing displacement ratio and that, for different operating points, FTVI optimum ratio lays around 0.95, while for SCVI, sits around 0.85. The FTVI and SCVI cycles performances are compared with models of Basic Booster Cycle, Parallel Compression, Gas Ejector assisted Parallel compression and Liquid Ejector assisted Parallel Compression architectures. COP comparison for different ambient temperatures is performed, with each system working close to its optimum. The most competitive Two­Stage solution is the SCVI cycle, performing similarly to the the Gas Ejector cycle at high temperatures conditions, reaching a 21.5% against 23% COP increase, respectively, over the basic Booster at 44 °C ambient temperature. A sensitivity analysis is carried for different hypotheses. It is found that assuming higher compressor efficiency in all systems increases the comparative advantage of the Two­Stage cycles. In an ideal scenario, SCOP is computed for the climates of Helsinki, Strasbourg and Athens. Results suggest that the ejector cycles still are the best overall performing. However, this scenario assumes that Parallel Compression is always active, which is not often possible in sub­critical operation. A second ”realistic” scenario is defined, giving FTVI and SCVI cycles energy savings with respect to the basic Booster an advantage of 4.08% and 5.35% against 3.41% of the Gas Ejector system.