Energy Performance of Low Charge, Central Type, Dual Stage NH3 Refrigeration Systems in Practice

摘要

Development of low charge NH3 refrigeration systems is taking place throughout the world for various applications, predominantly for refrigerated storage and packaged solutions to chill liquids and/or condition refrigerated spaces. These developments are initiated by a global phase-down of high global warming (GWP) refrigerants of the hydro fluorocarbon (HFCJ type. This phase-down is a direct result of the "top-down" emissions reduction agreement resulting from the Conference of the Parties (COP 21) in Paris, December 12, 2015. Ammonia refrigerant offers improved vapor compression cycle efficiencies in comparison with most other refrigerants. The energy performance improvements associated with the application of ammonia refrigerant in combination with other energy efficiency engineering techniques such as state-of-the-art superheat/quality (SH/X) expansion control, advanced low charge evaporator technology, extensive integration of variable frequency drives, two-stage compression, low oil carry-over compressors, low friction loss pipe lines, and genuine central plant concept have not been the subject of widespread investigations and reporting. Where compliance costs are directly proportional with NH3 inventory, favoring multiplexing as opposed to central plants is tempting for stakeholders. This, of course, risks sacrificing energy performance in return for reduction in NH3 inventory. For signatories to the COP 21 agreement, this is neither in the national interest nor is it in the commercial interest of plant owners if compliance cost increases outweigh energy cost reductions. This paper describes the energy performances of several refrigerated distribution centers with storage volumes of approximately 10,000 to 50,000 m~3 (353,000 to 1,766,000 ft~3). The performance evaluations are based on the electrical energy consumption as measured by the electrical energy providers over representative periods of time. All systems are serviced by central, state-of-the-art low charge, dual stage NH3 refrigeration systems. In the case of one plant the contribution of the photovoltaic panels to the energy requirement of the facility as a whole is shown on a month-by-month basis. An energy performance comparison is also made between two refrigerated distribution centers with a volume of approximately 10,000 m~3 (353,000 ft~3), but serviced by two different types of ammonia refrigeration systems. In one case the plant is a single-stage economized dual screw compressor based system with gravity flooded refrigerant feed. In the other case, the plant is a central, low charge NH3 dual-stage system with speed controlled semi industrial reciprocating compressors. Other features of the two facilities include general warehouse designs that are more or less identical. The energy performance comparisons are again based on the electrical energy consumption as recorded by the electrical energy provider over one calendar year.