Urban Heat Island Mitigation and Life Cycle CO2 Reduction by Installation of Urban Heat Island Countermeasures

In Japan, the air temperature in big cities has been increasing rapidly since the 1980s. This phenomenon is referred to as the "urban heat island (UHI) phenomenon." UHI is an environmental problem that results in increases in energy consumption due to the increased cooling demand, and in unfavorable conditions for human health. Various local activities against UHI have been promoted after the enactment of the Outline of Countermeasures to Urban Heat Island in March 2004.

Many countermeasures against UHI have been developed so far in order to decrease the air temperature. If such countermeasures are installed, the air temperature during summer will decrease, and therefore the energy consumption required for cooling will also decrease. On the other hand, if such countermeasures are installed, the air temperature during winter will decrease, and therefore the energy consumption for heating will increase. In addition, a lot of energy will be consumed for the construction and the operation of such countermeasures. Thus, the installation of countermeasures can cause an increase in CO₂ emissions.

Global warming is also one of the most important environmental issues. The Japanese government has enacted the Goals of the Kyoto Protocol Target Achievement Plan in April 2005. In order to meet the goal, even when UHI countermeasures are installed, it is required that there should be no significant increase in CO₂ emissions. From the viewpoint of global warming, it is very important to evaluate not only changes in CO₂ emissions for air-conditioning demand of the building by UHI countermeasures but also increases in CO₂ emissions for the construction and the operation of UHI countermeasures.

This study evaluated both the changes in the urban air temperature and life cycle CO₂ (LCCO₂) emissions resulting from the installation of various UHI countermeasures using annual meteorological and building energy models (AIST-MCBM) and the life cycle inventory analysis (LCI). AIST-MCBM is a model developed by combining the 1D canopy meteorological model and the building energy use model. This model can evaluate the year-round air temperature and annual energy consumption. LCI is a life cycle assessment (LCA) methodology to calculate the environmental emissions from each stage of products or services and evaluate their total emissions.

In this study three major UHI countermeasures such as photocatalysts, solar reflective paint (SRP), and greening were evaluated. We did not evaluate other countermeasures such as energy-saving technologies, since these countermeasures have few UHI mitigation effects.

In the results, all the countermeasures mitigated the urban air temperature during summer. The averaged air temperature at 1400LST in August was decreased by -0.54 × 10^-3 °C/m₂ in the unit of installation area by installation of photocatalysts to sidewalls of buildings. Similarly, it was decreased by -1.83, -0.24, -1.10, and -0.31 by SRP (rooftop), SRP (sidewall), rooftop greening, and sidewall greening, respectively.

Annualized LCCO₂ emissions in the unit of installation area were also calculated. The annualized LCCO₂ from the construction and the operation of photocatalysts (sidewall) was 4.29 kg-CO₂/y/m₂. Similarly, those of rooftop greening and sidewall greening were 3.22 and 2.82, respectively. The change in the annualized LCCO₂ from air conditioning of the buildings was -4.25 kg-CO₂/y/m₂ by installation of photocatalysts. Similarly, those of SRP (rooftop), SRP (sidewall), rooftop greening, and sidewall greening were 0.13, -0.29, -4.60, and -1.68, respectively.