Studies in preliminary design of fenestration: Balancing daylight harvesting and energy consumption

Energy consumption is a significant issue that encompasses environmental concerns, financial aspects, code compliance, operations and maintenance over the lifetime of the building, and occupant comfort. One critical large impact on future energy consumption is the early decision making process during the design phase of a building regarding the choice of windows. One technique for saving energy is to harvest daylight; this lessens the amount of electricity needed for indoor artificial lighting while maintaining adequate lighting levels. There are trade-offs, however, as larger windows that allow more daylight can be detrimental in other areas, such as increased heating and cooling loads depending on the location of the building, the climate, and even the season as benefits and drawbacks change over the year. This paper explores balancing daylight harvesting and energy demands of fenestration in the early design stage for office buildings in two climate zones in California (CZ 06 and CZ 15). Window orientation, height, aspect ratio, window-to-wall ratio, lighting design level, and glazing are parameters that were analyzed by DIVA for Grasshopper and eQUEST. The simulation results provide guidelines for designers for reducing cooling loads and electric lighting use throughout the year. Results indicate that a window-to-wall ratio of 10%-30% is recommended for south facade and window-to-wall ratios of 30%-60% promise better energy performance on the north, and a window-aspect-ratio of 2/1 saves most energy load for all orientations for these climate zones.