DESIGN-THEORETICAL STUDY OF CASCADE CO2 SUB-CRITICAL MECHANICAL COMPRESSION / AMMONIA EJECTOR COOLING MACHINE

In this paper an innovative micro-trigeneration system composed of a cogeneration system and a cascade refrigeration cycle is presented. The cogeneration system is a combined heat and power (CHP) system for electricity generation and heat production. The cascade refrigeration cycle is the combination of a CO2 sub-critical mechanical compression refrigeration machine (MCRM), powered by generated electricity, and an ejector cooling machine (ECM), driven by waste heat and using refrigerant R717. Refrigerant R717 as the working fluid for ejector cooling cycle is selected in the present study. Effect of the cycle operating conditions on ejector and ejector cycle performances is studied. Optimal geometry of the ejector and performance characteristics of ECM are determined at wide range of the operating conditions. The paper also describes the theoretical analysis of the CO2 sub-critical cycle and shows the effect of the MCRM evaporating temperature on the cascade system performance. The obtained data provide necessary information to design a pilot small-scale cascade CO2 sub-critical mechanical compression / ammonia ejector refrigerating unit with cooling capacity of 10 kW for application in micro-trigeneration systems incorporating reciprocating internal combustion engines and gas micro-turbines.