Absorption refrigeration, waste heat recovery, ammonia-water
Vapor absorption heating and cooling systems, utilizing heat input from different sources such as waste heat or natural gas, are attracting increasing interest in commercial and residential applications. Residential heat pump applications require compact heat exchanger geometries to ensure a small system footprint. Compact microscale heat and mass exchangers are developed and implemented here. These novel heat and mass exchanger geometries for different components of the system require evaluation at design and off-design conditions to characterize the individual component and overall system performance. This study presents results from experimental investigations of a small-capacity ammonia-water absorption chiller. The chiller comprises discrete heat and mass exchangers with novel design features, and is installed on a breadboard test facility. The absorber and condenser are directly coupled to ambient air and designed to operate at extreme ambient temperatures as high as 51.67Â°C. The microchannel evaporator is hydronically coupled. The desorber is direct-fired and coupled to a hot air-stream simulating a waste heat source. The system is designed to deliver 2.64 kW of cooling at a coefficient of performance (COP) of 0.55 based on the cooling duty and the desorber heat input rate. A steady-state model to specify various state-points in the system is developed. The experimental setup and details of heat exchanger geometries, measurement and instrumentation, and data analysis are presented. The overall system and component performance is evaluated to determine system limitations. The evaluation is conducted at lower ambient conditions of 35-40Â°C and the system model is modified to account for the corresponding changes. Results from baseline system operation at design mass flow rates of concentrated solution and heat input rates are presented. The performance of individual components is analyzed and compared to design predictions. Various operating conditions in the system such as the flow rate of concentrated and refrigerant solution, the heat source temperature and flow rates, and ambient temperature are varied to study the effect on the overall system and component performance. Â Â The results from this investigation demonstrate the potential of small-capacity absorption chillers using microchannel and direct air-coupled heat exchangers, and will guide the development of a packaged waste-heat-driven chiller operating at extreme ambient conditions.