Hot water supplies account for approximately 30% of the energy consumed by Japan's residential sector. Therefore, it is important that the energy efficiency of water heaters be improved. Absorption heat pump water heaters are expected to be more efficient than conventional gas-powered water heaters. However, absorption systems have large volumes; therefore, they are not widely adopted in the market. In this paper, we propose to replace conventional shell-and-tube or plate-fin-type absorbers with adiabatic absorbers for reducing the size of absorption heat pump water heaters that use H_2O/NH_3. 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. Then, it flows into the adiabatic absorber for completing absorption. The absorption performance, as well as the volume of the adiabatic absorber, is important from the efficiency viewpoint. In this study, an atomizing-spray absorber with a swirl nozzle was proposed. This type of absorption system was modeled, and its absorption performance was analyzed. In modeling the atomizing-nozzle-type absorber, the mass flow rate injected from the nozzle, breakup position of liquid film and diameter of generated droplets, and drop size distribution were discussed based on both theory and measurements. From the calculation results, design guidelines were developed and the expected sizes were determined.
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