The interest in the absorption refrigeration systems continuously grows in many applications as they can exploit either waste heat or both traditional fuels and renewables (natural gas, sun, geothermal, biomass) as primary energy. Among several configurations of absorption cycles the GAX (Generator/Absorber/heat eXchanger) one plays an important role and allows to achieve good energy efficiencies of energy utilization with no relevant increase in system complexity. Here the analytical models of both GAX and GAX Hybrid (GAX-H) absorption cycles using ammonia–water mixture as working fluid are presented. Developed in Matlab R2010a, they simulate mass and energy conservation laws in each system component. The thermodynamic properties of ammonia-water solution are calculated based on the equations by Ziegler and Trepp, on the equations of state by Schultz and on the explicit relations for the bubble point and the dew point temperatures by El-Sayed and Tribus. The developed models were run to investigate the influence on the overall cycles’ performance of the generator, the condenser and the evaporator temperatures, of the absorber pressure and of the heat exchanger effectiveness. Results show the general trends of the efficiency of the cycles and allow the comparison between GAX-H and GAX cycles. When a pressure ratio of 1.5 between the absorber and evaporator pressure is maintained, the GAX-H COP (Coefficient Of Performance) is 50% and 32% higher than the conventional GAX cycle one operating with a degassing range of 0.2, 0.3 respectively.
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