Advanced refrigerating plants based on transcritical cycles working with carbon dioxide for commercial refrigeration

Supermarkets and hypermarkets require a huge amount of energy to maintain chilled and frozen food in product display cases and cold storage rooms and thermal comfort in the whole building. Systems exploited require very large refrigerant charges for operation and suffer significant leakages. The challenge for advanced systems, based on equipment which reduce TEWI, suggests the proposal of carbon dioxide: a natural working fluid. This paper deals with a feasibility study of a refrigerating system for a big hypermarket in Sicily based on a three-stage transcritical cycle working with carbon dioxide which is compared with an alternative system composed of two cycles operating as a binary refrigerating plant: a top cycle working with propane and a bottom cycle working with carbon dioxide. The cold is produced at two temperature: - 15°C and -35°C. In both alternative systems, cold at -15°C will be delivered to display cases, cold stores etc. by means of a secondary loop into which circulates a refrigerant fluid (e.g. an ethylene-glycol solution), while cold at -35°C will be delivered by another loop into which circulates directly liquid carbon dioxide. Main results obtained demonstrate the effective feasibility of plants which seem suitable for commercial refrigeration, bearing in mind that is of capital relevance the best operating efficiency of the secondary loop system. This depends mainly on thorough design and building of the secondary loops, especially for those which op erate at lower temperature.