Cascade refrigeration system with inverse Brayton cycle on the cold side

• Published in: Applied thermal engineering 2017-12, Vol.127, p.986-995
• Main Authors: Giannetti, Niccolò, Milazzo, Adriano, Rocchetti, Andrea, Saito, Kiyoshi
• Format: Article
• Language: English
• Subjects: Ammonia; Brayton cycle; Cascade system; Cold; Cold storage; Cold stores; Cold-store refrigeration; Configurations; Design optimization; Design parameters; Evaporation; Heat exchangers; Inverse Brayton cycle; Optimization; Performance analysis; Refrigeration; Temperature; Working fluids
• ISSN: 1359-4311; 1873-5606
• DOI: 10.1016/j.applthermaleng.2017.08.067

•A cascade configuration is used to refrigerate a cold store and its loading dock.•On the cold side, an inverse Brayton cycle uses the cold store air asa working fluid.•The top cycle, that refrigerates also the loading dock, is an NH3 compression cycle.•The global system performance is comparable to conventional plants. Low temperature refrigeration of cold stores poses some specific issues: single stage, vapour compression cycles have modest COP at low evaporation temperature; cold evaporator surfaces require de-frosting and a fan for air circulation; a part of the refrigeration load may be delivered at intermediate temperature levels, e.g. for the cold store loading dock. Cascade system may improve the COP and add flexibility on the temperature levels and working fluids, but the problems related to the cold evaporator surface remain unsolved. The refrigeration system presented herein features a cascade configurationcombining a vapour compression cycleand an inverse Braytoncycle. Both cycles use “natural” fluids, complying with strictest regulations. The top cycle uses Ammonia in order to increase efficiency, while the bottom cycle uses air, which directly circulates in the cold space and hence eliminates the cold heat exchanger. A detailed thermodynamic analysis allows a complete screening of the relevant design parameters for an overall system optimization. The results show that, notwithstanding the intrinsic gap of efficiency suffered by the Brayton cycle, the proposed system features an acceptable global performance and widens the implementation field of this technology. This system configuration shows a COP 50% higher than the corresponding simple Brayton cycle at temperatures of the refrigerated storage of −50°C.