To improve the heat efficiency of gas water heaters, we studied a hybrid type water heater that combines a conventional gas-fired water heater and an adsorption heat pump cycle. With a target heat efficiency of approximately 120%-a dramatic increase over the 85% efficiency of a conventional gas water heater-this hybrid gas water heater (HGWH) is expected to be an energy-saving replacement for commercial users. The HGWH combines a boiler and a heat pump cycle that uses city gas as its heat source, which are operated either simultaneously or independently depending on the outside temperature, making year-round high heat efficiency operation. Considerations regarding the heat pump cycle have taken into account the adsorption cycle, the absorption cycle, and the vapor compression cycle, but our study focused on the adsorption cycle from the standpoint of overall priorities. There are two possible adsorption cycles: a dual single-stage heating cycle, which heats the water supply in two stages, and a single-stage cycle, which heats the water in one stage. We evaluated the feasibility of both systems using prototypes. We found that a compact system can be achieved by combining a single-stage adsorption cycle and a gas water heater, and the heat efficiency of this HGWH achieves 110% to 120%, which is greater than a dual single-stage heating cycle.
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