Balancing the Energy Efficiency Benefits of Glazed Surfaces: A Case Study

Daylighting design is not only dimensioning glazed surfaces to provide sufficient natural light to an occupied space but also a method of analyzing how this can be achieved without unwanted effects, such as gains and losses of heat, glare, and variations in daylighting intensity at various indoor distances and levels. The case study presented in this paper highlights the energy consumed due to a group of windows with a large glazed area in an existing building located in a temperate continental climate area. The energy consumption results from supplementary artificial lighting required to maintain adequate illumination for indoor activities and to counterbalance heat loss during colder periods are evaluated. The analysis performed by modifying the glazed surface led to the identification of an optimum value of window dimensions for minimum energy consumption. The results of the case study highlight the fact that the evolution of the total energy consumption, evaluated as the sum of the energy consumption due to additional heating/cooling and the artificial lighting required to compensate for the reduction in natural light, is strongly influenced by the dimensions of the glazed surfaces, as well as the minimum level of lighting imposed by the regime of activities carried out in the building. Thus, the outcomes obtained in the research show that at lighting values below 500 lx, the total energy consumption is directly proportional to the glazed surface. From values of 500 lx for the illuminance level, the total energy consumption drops from 2730 kWh/year for a window height of 230 cm to 2399 kWh/year for a height of 110 cm, after which it starts to rise again, reaching a value of 2786 kWh/year for a height of 30 cm. This phenomenon is also found at values higher than 500 lx; accordingly, for an imposed lighting of 1000 lx, the minimum total consumption is identified at a window height of 150 cm. The case study presented in this paper clearly highlights a complex relationship between the height of the glazed surface and the energy consumption required to compensate for heating or cooling and the reduction in natural lighting. Lower window heights reduce heat loss or gain but also correspondingly increase the energy consumption of artificial lighting.