Discomfort Glare Index for Automated Blind Control
Abstract
Since using daylight in buildings is expected to be part of the solution for global warming, controlling the amount of light and heat is important. Venetian blinds are the most-widely-used shading system in office spaces. Automated blinds control systems which are admissible for the Comprehensive Assessment System for Built Environment Efficiency (CASEBEE, 2008) point system, have been widely used in office buildings in Japan. Compared with offices in European countries, there are two significant reasons why automated blind control is required for offices in Japan. The first one is the climatic condition and the second is the type of office, which is typically “open-plan”. Japan belongs to the temperate zone with four distinct seasons, but its climate varies from cool temperate in the north to subtropical in the south. Central Japan has hot, humid summers and short winters. A simulation study using climate data was carried out to show the difference in annual air conditioning load for a model office building in Tokyo and in five European cities: Rome, Zurich, Paris, Frankfurt and London (Roh and Udagawa, 2002). It is shown that the annual space cooling load is much greater than the space heating load in all cities, because the space cooling load of office buildings is affected by solar radiation through windows and internal heat gains. Although, in the European cities, the cooling effect of the fresh air introduced as ventilation requirement contributes to reduce the cooling coil load of air handling unit, such effect cannot be expected in Tokyo. Controlling solar radiation through windows is usually the most effective strategy for energy savings in Tokyo Open-plan offices are so common in Japan that it is difficult to find private office spaces. Surveys of 779 open-plan-office occupants in five large Canadian and US cities showed that open-plan office occupants who were more satisfied with their environments were also more satisfied with their jobs. It was suggested that the physical environment has a role in organizational well-being and effectiveness (Veitch et al., 2007). In common automated blind control systems, the blind status (slat and blind up/down position) depends on the daylight information obtained via rooftop measurement and calculated solar position. The amount of light from dimmable electric lighting which is determined by occupancy sensor and illuminance on the desk has effects on the heating/cooling load. The control system can significantly reduce energy for lighting and cooling. In the early stage of automated blind control, the operation was often changed from automatic control to manual control by workers who had not been satisfied (Iwata and Tokura, 2002). It was learned that the blind slat angle should be decided not only to cut direct sunlight but also to prevent discomfort glare. To evaluate discomfort glare from daylight, there are evaluation methods using images of luminance distribution, whose validity has been examined (Iwata and Osterhaus, 2010). However, it is difficult to apply these glare evaluation methods to automated control blind in an open-plan office, which has large windows as well as a large floor area. Apart from the basic problem of calculation of glare indices, e.g. the determination of glare source and background, there are problems, e.g. the distribution of the workers’ desks (observation points), the view directions and zoning of the blinds. The purpose of this paper is to address these problems in applying the existing evaluation methods to automated blind control.