Dual-pressure absorption cycles: The second law and working fluids

Gas-fired absorption cycle technology has great potential for providing cost-effective, efficient, and environmentally-friendly space conditioning. Implemented as a natural gas heat pump for heating and cooling, absorption holds promise for increasing fuel utilization efficiency, reducing peak summer electrical demand, levelizing gas consumption, decreasing CO{dollar}sb2{dollar} production, and providing year-round space conditioning using natural refrigerants.; Of the many advanced absorption cycles, dual-pressure cycles are best-suited for residential applications due to their simplicity. In this study, five dual-pressure absorption cycles were compared on the basis of coefficient of performance (COP) and heat exchange requirements. These cycles were modelled as direct-fired by natural gas combustion and used ammonia/water as the working fluid pair. The absorber, generator, and solution heat exchange (AGS) cycle and the generator-absorber heat exchange (GAX) cycle provided the highest COPs while requiring the least total heat exchange. The second law was employed to determine the sources of greatest irreversibility in the cycles. For all cycles, the combustion gas-to-generator irreversibility was significantly greater than the irreversibility for any other component process.; The evaporator pressure in absorption cycles is not fixed by the evaporator temperature, in contrast to vapor compression cycles. Therefore, the evaporator pressure was optimized, i.e., the pressure which provided the highest COP for fixed operating conditions was used for each analysis. This studied also determined that the location of the minimum pinch point on the GAX heat exchanger was controlled by the methods of heat release on its absorber side.; The performance of the AGS and GAX cycles was studied using three fluid property models, i.e., curve-fit experimental data for ammonia/water (EL), ideal solution model (IS), and the Peng-Robinson equation of state model (PR). The IS and PR models required only minimal fundamental thermodynamic property data for the two pure components. This allowed investigation into the influence of each fundamental property on cycle performance, providing insight into desirable properties for new absorption fluid pairs.