Evaluating the effect of trees on UHI mitigation and reduction of energy usage in different built up areas in Cairo

Abstract

Cairo experiences higher temperatures than its neighbouring rural and suburban areas. This is due to the lack of vegetation and the wide scale use of sealed surfaces. This phenomenon is known as Urban Heat Island (UHI). This leads to the greater use of air conditioning in indoor spaces during the summer time and the electricity bills are higher. This study seeks to find the best urban vegetation ratio in order to reduce the buildings' energy demand through mitigating UHI and enhancing thermal performance in high and low density built up areas in hot and arid climate. Two areas in Cairo were chosen (Imbaba - 65% urban density, and officers residential in Elsalam district - 23% urban density) with different sky view factor values. The two areas were compared in terms of the cooling effect of 30% trees, 50% trees and 70% grass scenarios using ENVI-met. Then the meteorological outputs for each scenario were used in DesignBuilder in order to calculate the energy consumption. This study shows that, while trees can reduce daily air temperature by 0.2–0.4 K in very high density built up areas, they also increase air temperature by 3 K in low density built up areas due to humidity and evaporation. Therefore, trees are effective at lowering air temperature and reducing energy usage in very high density built up areas, but ineffective in low density built up areas. Furthermore, trees reduce PET in both densities by 1–3 K.