Technical Potential for Thermally Driven Mobile A/C Systems 2001-01-0297
Aqua-ammonia absorption refrigeration cycle and R-134a Vapor jet-ejector refrigeration cycle for automotive air-conditioning were studied and analyzed. Thermally activated refrigeration cycles would utilize combustion engine exhaust gas or engine coolant to supply heat to the generator. For the absorption system, the thermodynamic cycle was analyzed and pressures, temperatures, concentrations, enthalpies, and mass flow rates at every point were computed based on input parameters simulate practical operating conditions of vehicles. Then, heat addition to the generator, heat removal rates from absorber, condenser, and rectifying unit, and total rejection heat transfer area were all calculated. For the jet-ejector system, the optimum ejector vapor mass ratio based on similar input parameters was found by solving diffuser's conservation equations of continuity, momentum, energy, and flow through primary ejector nozzle simultaneously. Then, similar to the absorption system, various parameters were computed and preliminary sizing of nozzle and diffuser throats was also presented. For the absorption system, generator high-grade energy requirements were compared to the engine's exhaust heat energy. It was concluded that the exhaust heat is enough to activate the generator to produce cooling power up to 1.4 tons of refrigeration. On the other hand, the jet-ejector system generator heat requirement, which could be either from engine jacket low-grade coolant or high-grade exhaust gas, was compared to waste heat energy from both sources. It was found that the engine jacket coolant is superior to exhaust heat energy in generator activation. The engine jacket coolant contains enough energy to activate the generator for all of the engines considered while the exhaust heat energy was able only to produce cooling loads up to 1.4 tons of refrigeration. Based on the analysis, it is clear that both systems are possible future candidates as a replacement to vapor compression system. However, the jet-ejector system might be advantageous and more promising method over the absorption system when considering factors such as waste-heat energy availability, complexity, toxicity, mass, leakage, refrigerant choice, reliability and COP.