The Effect of Window Dimensions on Lighting and Thermal Energy in Office Buildings in Hot Arid Climate

Office buildings are one of the largest consumers of energy in the building sector. There are several conservation opportunities in these buildings in terms of heating, cooling and lighting that if applied, peak loads in each area will be reduced significantly. One of the main energy consumers in office buildings is lighting systems, as providing sufficient lighting in office spaces is one of the main foundation for increasing employees’ satisfaction and productivity at the same time. Since Iran is located in a region benefiting from high annual solar radiation, it is viable to maintain the required illuminance level through proper design of windows and benefit from natural daylight. A review of the literature reveals that defining a proper window-to-wall ratio (WWR) will help to conserve heating and lighting energy. Several studies have introduced various WWRs for different cities of Iran, but none is focused on Sabzevar office buildings. Therefore, this article aims to introduce optimal window-to-wall proportions in an office room in Sabzevar, a hot arid climate, in order to reduce heating load during the cold period of the year and provide the least lighting energy using natural light simultaneously. To determine the optimal ratio of windows in office buildings in Sabzevar and build a proper model of an office room as a representative of office spaces in the city, 26 office buildings in the city were studied and measured. Next, the model with proper dimensions was selected. Afterwards, to carry out energy simulation, a weather file was adopted and validated against Sabzevar weather station data. The research was carried out doing parametric simulation by Grasshopper software. The window dimensions of an office room, in the hot and dry climate of Sabzevar were then simulated parametrically and optimized through a genetic algorithm. The feats of the design elements such as the exterior envelope of the model were selected based on literature review and measurable parameters. On the other hand, artificial lighting was defined according to existing standards and mathematical equations presented there. Finally, the optimal dimensions of the window for an office room were presented in four main directions. In this regard, 90 modes for the south façade, 254 modes for the north façade, 131 modes for the east façade and 126 modes for the west façade were simulated and compared. After simulation and analysis of different scenarios, it was concluded that the lowest total energy consumption with a window-to-wall ratio of 63% for the south wall, 47.6% for the north wall and 31.7% for the east and west walls will be obtained. Additionally, the effect of applying shading devices regarding the energy consumption of the model was studied. For this purpose, optimal shading angles introduced by the national building code of Iran were applied to the model under examination. It was found that shading will reduce cooling energy in the warm period of the year, however it will cause an increase in heating load during the cold season. It was also concluded that the best shading for windows in the city is a movable one.