The effect of urban evaporation on building energy demand in an arid environment

An open-air scaled urban surface (OASUS) was used to physically model the influence of urban structure on microclimatic conditions that affect the cooling requirements of buildings. The OASUS scale-model consists of an extensive urban-like building/street array constructed at an open site in the arid Negev region of southern Israel. Building rows are comprised of hollow concrete masonry blocks and have thermal and optical properties analogous to common local construction materials. Previous experiments with the scale-model were limited to "dry" conditions, with only negligible exchanges of latent heat. Considering that one of the main advantages of using the scale-model facility is to be able to control factors affecting microclimate, this study analyses the impact of adding moisture to the scaled "streets" between "building" rows, and gauges the impact of outdoor evaporative cooling on the energy demand of adjacent buildings. Measurements carried out during the summer month of August 2006 at the scale-model facility were used to obtain street canyon air temperatures, which in turn provided input for a dynamic energy simulation of indoor cooling loads in an actual building. The simulation model was calibrated with simultaneously measured data from a nearby residential building. Results suggest that the cooling factor in a street canyon is a direct function of the relative availability of moisture, with respect not only to horizontal area but also to the "complete" three-dimensional urban surface. In addition, simulation results of building energy demand show the importance of accounting for urban density when planning the disposition of vegetated surfaces for cooling purposes.