Predicting the Effects of Green Roofed Urban Buildings on Cooling Loads

Abstract

There is a growing use of living green roofs in some parts of the world to reduce heat gain into houses. The reduction of heat gain or loss into buildings automatically reduces energy consumption or combats urban heat island effect in cities, which also reduces energy consumption of buildings on a wide scale. This study was necessitated by the need to determine the potential reduction of cooling loads that can be realized by using living green roofs on building roof tops located in urban areas. The objectives set out to be achieved were to develop a computer model for predicting indoor air temperature in living green roofs, assess the potential reduction of energy consumption from using living greens and evaluate the differences between the indoor microclimate of a green roofed urban building and a bare roofed urban building. Two field scale urban like houses were constructed in Jomo Kenyatta University of Agriculture and Technology campus, Juja, Nairobi. One room had a living green roof and the other room’s roof was left bare. There was no any form of ventilation for the two models, both natural and mechanical. Sensors were placed within and outside the physical models to monitor microclimate parameters. The green roof was interchanged for the two houses and field observation continued. The total period of obtaining the data was from 28th April to 6th July, 2014. A new thermodynamic model called GRUBCLIM was developed to predict the indoor temperature of green roofed urban buildings. Modelling and simulation were carried out in SIMULINK. The model was validated with data measured at the field scale model site and simulated with another set of different of data. A correlation of 0.885 was met between predicted and obtained values. The energy savings potential of the green roofs were xviii determined based on the heat gain/loss in the green roofed urban building and the bare roof urban building. Results show that during the daytime, a mean heat gain reduction of approximately 63% was recorded for the house with the living green roof when compared with the bare roofed house. The indoor microclimates of both rooms, green roofed and bare roof urban buildings were investigated to determine the differences in temperature and relative humidity. A mean daytime value of 22.86 °C was recorded for the green roofed building while that with a bare roof recorded a higher value of 24.48 °C.