The effect of the daylight zone on lighting energy savings

Abstract

Daylighting is the cornerstone of low energy building design. Therefore, the adoption of a daylight - harvesting lighting control system can substantially increase lighting energy savings. These are strongly depended on the determination of daylight zones (DZ) which in turn help to identify the lighting loads that must be controlled separately. The Daylight Zone (DZ) is defined in ANSI/ASHRAE/IES Standard 90.1-2016 as the floor area substantially illuminated by daylight. However, the DZ is defined differently in several standards and building codes. Lighting design plays also an important role since it determines the number of luminaires in the daylight zone and thus the resulted energy savings due to the daylight harvesting techniques. Each one of these luminaires inside the DZ should be dimmable and must be controlled by either a stand-alone photosensor or one photosensor per control zone. In order to investigate the influence of the DZ in lighting energy savings several simulations were conducted in five (5) office spaces - part of the reference office building used to determine cost optimal energy performance for office buildings in Greece. DZ area was estimated using three (3) different definitions according to a) EN 15193.1 as implemented in Greek regulation of Energy Efficiency in Buildings, b) CEN Technical Committee 169/WG11 'Daylight', and finally c) using dynamic daylight metrics for typical working hours in all four cardinal orientations. Results indicate that due to the differentiation of the DZ depth as this is calculated by the aforementioned methods, there is an associated variation of the calculated lighting energy savings. The extend of the DZ varies between 30-100% of the total area in each office when DZ is calculated using the geometrical method, 50-100% when daylight factors are used and 30-60% with dynamic daylight metrics. The number of luminaires within the DZ vary between 30-100% in the same space depending on the calculation method, the geometry of the room and the Window to Floor Area. As already mentioned, lighting energy savings for the examined test spaces vary, since these are strongly depended on method for the calculation of the DZ's depth from 61% to 89%, 53-72% and 69-91% correspondingly.