A set of indices to assess the real performance of daylight-linked control systems

The installation of Building Automation Control Systems (BACs) in general is an effective action to achieve relevant energy savings. Commercial BACs, installed in residential or small offices, often include functions of lighting control, acting as Daylight-Linked Control Systems (DLCSs). Nevertheless, because system performance is strictly dependent on different parameters, BAC's hardware and software configuration and as well as an inaccurate commissioning do not always allow a perfect execution of the desired tasks; therefore, the system could not work as expected. Moreover, it is well known that energy saving potential is specifically related to the variability of the light environment. Although many methods to assess it are available, these are mainly based on average data, tabular factors, and climate/lighting time series embedded in weather files used in detailed simulations and, in general, they adopt as a benchmark the performance of ideal control systems. In this paper, actual performances of a daylight-linked control system have been evaluated by a set of indices which, in general, can be calculated, after measurement, during the commissioning stage or in periodic monitoring of the system. In particular, these indices take into account the excess and the deficiency of illuminance over time with respect to a target set point that a lighting system should provide and the related energy consumption. The indices have been tested using data measured in a laboratory set up where a commercial daylight-linked control system, working not close to an ideal one, is installed and has been evaluated for different end uses, operating schedules, control strategy (dimming and ON/OFF) and daylight conditions. Finally, useful relations between system performance and environmental conditions have been found.