Energy and reliability optimization of a system that combines daylighting and artificial sources. A case study carried out in academic buildings

The necessity to reduce energy requirements of lighting systems should bring among engineers a more mature and conscious vision while planning and this means that their main goals should be the visual comfort and a decrease in energy and maintenance costs. Therefore this paper examines the case study of a classroom located in the Faculty of Engineering of Sapienza University in Rome and, to evaluate the possibility to combine natural and artificial light, it focuses on the realization of a new lighting system. This new solution is formed by LED sources and control systems able to modulate the power absorbed by every single lamp (to adapt, according to the hour and day, to the conditions of the natural light). In order to meet the EN 12464-1, the process of incorporating natural and artificial light must occur while guaranteeing both the minimum levels expected for the average illuminance Em [lx] and the uniformity coefficient (Emin/Em) and this is the reason why a simultaneous exertion of artificial lights and a variable shielding system of glass surfaces activated through a motorized electronic control unit is required. Thanks to the software DIALux Evo 5.1 a 3D model of the classroom was reproduced and validated in order to simulate the combination of natural and artificial lighting and to verify if the results complied with the regulations. This was followed by an analysis concerning: the reliability of the system (through the examination of the MTBF - Mean Time Between Failure) and energetic and economic aspects (through the software ecoCALC 4.5.4). The novelty of this study is represented by the fact that in order to obtain the highest results in each of the three fields examined, it is possible to avoid the exertion of those negative feedback control-type systems (requiring high installation and maintenance costs due to sensor devices), thanks to pre-programmed logic control systems based on the data obtained with the simulations by prediction softwares. In this case study the solution suggested, while using a pre-programmed control logic, presents a MTBF of 1205 h (about twice of a negative feedback system solution) with payback periods that justify the higher costs presented by the electronic characterizing the control logic system with respect to traditional plant solutions.