Thermodynamic optimization of ejector actuated refrigerating cycles

A cold generation system featuring a Rankine cycle powered refrigeration cycle actuated by a supersonic ejector was theoretically investigated in view of the thermo-fluid-dynamic optimization of the working fluid characteristics. The ejector model was validated against well established performance charts relating to water. A reference system was considered in which a Rankine cycle at moderate top temperature delivers its expansion power by means of an ideal turbine to an ideal compressor of a refrigeration cycle. Two main optimizing variables were ascertained: the fluid critical temperature and the complexity of the fluid molecule. The best performance of such reference cycle is around 80% of that of an ideal fully reversible, Carnot cycle based, system (COP of 2.0 for t(E,PC) = 150 degrees C, t(E,RC) = 5 degrees C, and t(C) = 35 degrees C). As easily predictable the ejector compression introduces severe losses mainly due to the normal shock and the mixing of the motive and of the driven fluid. Overall COP for the above quoted temperatures decreases from 2.0 (reference cycle) to 0.4-0.7. The optimization of the working fluid showed that comparatively low critical temperatures are favoured and that a fluid complexity similar to that of CH5N or CH2Cl2 gives the best performance. A detailed losses analysis explains this behaviour. In particular at low reduced temperatures the theoretical gain related to the better shape in the T-S plane of both the power and the refrigeration cycle is more than offset by the higher ejector losses due to the stronger normal shock needed to cope with an increased pressure ratio. Notwithstanding an extensive fluid screening we did not succeed in finding a fluid that could be considered optimum from all points of view including ambient and safety issues. However, a number of traditional (non-zero ODP) chloro-fluoro-carbons and of new (zero ODP) refrigerants were found that yield, on the whole, a satisfactory performance. Provided calculated COP will be confirmed by experimental testing, ejector powered refrigerators could compete with absorption systems in many applications. (C) 2007 Elsevier Ltd and IIR. All rights reserved.