Daylight fluctuations effect on the functioning of different daylight-linked control systems

One of the problems preventing daylight-linked controls spread is users' reluctance in accepting them. Continuous electric light oscillations, determined by control systems, can annoy users. This problem is particularly relevant for switching systems and it is strictly dependent on sky conditions: the more variable the weather is, the more frequent the oscillations are. To deepen these issues, irradiance and daylight illuminance measurements were performed in a mock-up office. Percentage daylight fluctuations (PDFs) were calculated. Then, starting from measured data, the functioning of different switching systems (differential switching, switching with switching linked time delay, switching with daylight linked time delay and solar reset switching) was simulated, in order to investigate daylight fluctuations effect on controls performances. Simulations were repeated on varying dead band and time delay, to verify the effectiveness of techniques in reducing daily switching actions. Results demonstrates that when sky is clear, PDFs are generally comprised in the ranges [-5%; 0% [and [0%; 5% [both outdoor and indoor; whereas, when weather changes from a sky condition to another, fluctuations can be higher than 50%. Despite the choice of the switching technique is not univocal and its effectiveness strictly depend on specific indoor daylight availability, differential switching associated with a switching linked time delay turns out to be the strategy more adaptable to different cases. Moreover, not always the increment of the dead band extent or time delay corresponds to a reduction of the daily amount of switching actions.