Urban cooling and CO2 reduction potentials of mass deployment of heat pump water heaters in Tokyo

Although countermeasures against nocturnal urban heat islands that cause health hazards are required, few measures with significant cooling potential at night are known. An air-source heat-pump water heater (HPWH) absorbs heat from the atmosphere to produce hot water and simultaneously emits a cold exhaust. The mass deployment of HPWHs is expected to contribute considerably to the improvement of urban thermal environments and mitigation of CO₂ emissions. To assess these contributions, we conducted a case study in Tokyo using an urban climate and energy model. The average peak value of summer nighttime temperature drop from HPWH reached 0.31 °C at the urban scale and exceeded 1 °C in residential areas with high hot water demand. This substantial temperature impact is attributed to stable atmospheric conditions at night. Here, urban vegetation was shown to reinforce this cooling effect by further stabilizing the nighttime atmosphere. The use of the HPWH resulted in a significant direct CO₂ reduction of 41–47 % from water-heating origin, and only a marginal indirect CO₂ increase from the space-heating origin. Given that the HPWH efficiency improves under high temperatures, the benefits of urban cooling and CO₂ reduction can be optimized by seasonally switching operation times.