Yearlong evaluation of urban heat island countermeasures from the viewpoints of thermal environment mitigation and urban energy conservation

In large Asian cities where commercial and residential energy demands are concentrated, urban heat island effect causes the increase in cooling electricity demand. This increase rate of demand is estimated up to 3%/°C in recent years in the Greater Tokyo area, and about 1.6GW of new demand is required as the daily maximum temperature increases by 1.0 °C. This huge demand of summer electricity could create the additional waste heat, which would further intensify urban heat island. From the viewpoint of the reduction of anthropogenic CO2 emission to mitigate the global warming, this huge demand should be reduced through the control of the urban heat islands.

In order to analyze the above-mentioned interaction process between urban thermal environment and air-conditioning energy use on city-scale, and to evaluate the technologies for urban warming alleviation, we have been developed a new multi-scale numerical simulation system, which consists of a three-dimensional mesoscale meteorological model (MM), a one-dimensional urban canopy model (CM), and a building energy analysis model (BEM).

In this study, BEM was improved and validated for the yearlong numerical prediction of the air-conditioning energy consumption in buildings. We combined this improved BEM with CM towards the yearlong evaluation of urban warming countermeasures. The combined model, CM-BEM, was then applied to the two contrastive urban districts in Tokyo. One was Nihonbashi area, and the other Suginami area. The former consists of middle-rise office buildings, and the latter of low residences. The following results were obtained from numerical experiments, which were conducted to simulate the yearlong temporal variations in the near-ground meteorological fields and buildings’ air-conditioning energy demands in the both areas.

1.In Nihonbashi area, the simulated sensitivities of cooling and heating electricity demands to the air temperature were roughly consistent with actual regional sensitivities, which were estimated by using the yearlong actual electricity demand data in the area. This result indicated the applicability of CM-BEM to yearlong simulation.

2.The annual effects of several heat-island countermeasures were evaluated from the both viewpoints of the air temperature alleviation and the cooling and heating energy conservation. As results, several effective measures to mitigate summertime air temperature, such as albedo increase and greening of buildings’ surfaces, were predicted to result in annual increase in the air-conditioning energy demands in some cases. Those negative effects were attributed to the wintertime temperature decrease and it’s consequential increase in heating energy. The importance to assess the yearlong impacts of countermeasures on urban thermal environments and building energy use was clarified.