Investigation on a Novel Adiabatic Absorber

Hot water supplies account for approximately 30% of energy consumption in residential sectors of Japan; it is therefore important that the energy efficiency of water heaters be improved. Absorption heat pump water heaters are expected to provide greater efficiency than conventional water heaters that use gas. However, the absorption systems have larger volumes and are not widely accepted in the market. In this paper, we propose the use of adiabatic absorbers to replace conventional shell-and-tube or plate-fin-type absorbers to downsize absorption heat pump water heaters that use H2O/NH3. The adiabatic absorption system consists of two components: sub-cooler and adiabatic absorber. For the proposed absorption system, the solution from the generator first flows through the sub-cooler and is sub-cooled by tap water, and then flows into the adiabatic absorber to complete absorption. The absorption performance, as well as the volume of the adiabatic absorber, is important in order to obtain greater efficiency. In this study, three types of absorbers are proposed: atomizing nozzle, spreading tray, and packed-column absorbers. These three types of absorption system are modeled and their absorption performances are analyzed. In modeling the atomizing-nozzle-type absorber, the breakup position of liquid film and the diameter of generated droplets are discussed on the basis of the instability theory of liquid film. In analytically modeling the spreadingtray-type absorber, the breakup length of the liquid column and generated droplet diameter are discussed. In the case of the packed-column-type absorber, the effective interfacial area of dumped packing is discussed. From the calculation results, design guidelines are developed and the expected sizes are compared.