Simulation and experimental investigation into diffusion absorption cooling machines for air-conditioning applications

The paper presents the development, experimental analysis and simulation of a solar heat driven ammonia/water (NH3/H2O) diffusion–absorption cooling machine (DACM). The designed cooling capacity of the machine is 2.5 kW for air-conditioning applications. The indirectly heated generator with its bubble pump is the main new feature of this cooling machine and it showed good performance for all prototypes constructed. A major challenge of the technology is the constant total pressure level in all components, which makes condensate distribution into the evaporator vertical tubes with no distribution pressure extremely difficult. As a consequence, the first prototype had problems in the auxiliary gas circuit, which only works with very low driving pressures and stops to work, when evaporation rates decay. In the second prototype, the auxiliary gas circulation was improved. However, the liquid condensate distribution into the evaporator remained problematically and the cooling power was limited to 1.6 kW maximum. Coaxial solution heat exchangers gave much better performance results than the initially chosen plate heat exchangers. In the third prototype the evaporator was optimised and the cooling capacity could be increased to 3 kW maximum. The maximum COP reached was 0.38. The diffusion–absorption cycle was modelled using an expanded characteristic equation of sorption chillers. The simulation model showed good agreement with the measured data.