Analysis of energy savings of three daylight control systems in a school building by means of monitoring

• The performances of 3 different daylight control systems are compared by monitoring.
• Lighting energy savings in 3 neighbouring classrooms differ significantly.
• Energy savings are caused by initial dimming and dimming by daylight harvesting.
• The measurements stress the importance of correct installation and commissioning.

Daylight control systems, which automatically adjust the artificial light levels depending on the daylight penetration, can result in substantial energy savings. However, their energy saving potential cannot be estimated accurately because it depends on several building and system parameters, climate conditions, occupant behaviour and type and commissioning of the daylight control system. The objective of this paper is to compare the energy saving potential and operation of different daylight control systems in school buildings. One year monitoring has been carried out simultaneously in 3 neighbouring classrooms, equipped with a different type of control system. The active power and the electric energy consumption of the artificial lighting were measured continuously on a minute-by-minute basis, as well as the occupancy of the classrooms and the global irradiance outside the building under an unobstructed horizon. Momentary visual comfort assessments were carried out in the classrooms.Although all classrooms have comparable occupancy and identical building characteristics, differences between the annual energy savings of the different daylight control systems are found to be significant: the total annual energy savings varied from 18% to 46%. Under the given conditions, the open loop system with the outward facing daylight sensor was noticed to yield the largest while the closed loop system with centrally positioned sensor produced the smallest savings. However, it has to be made sure that the energy savings are not at the expense of the visual comfort. The performance of the systems regarding both energy savings and visual comfort is related to the operation and the initial commissioning. An in-depth analysis of the monitoring campaign is discussed to explain the differences in energy savings and visual comfort. The energy savings due to the implementation of a daylight control system are divided into on the one hand dimming due to daylight penetration and on the other hand initial dimming to compensate for the over dimensioning of the lighting system and to take into account constant illuminance control.