Evaluation of Lightshelf Daylighting Systems for Office Buildings in Hot Climates

This paper presents part of an on-going research project in the College of Architecture at Texas A&M University. This research investigates how lightshelf daylighting delivery systems can manipulate sunlight and daylight both in terms of thier light and heat by shading view apertures below the shelf to reduce solar heat gain and glare and by reflecting light deep into the space through the daylight apenure above the shelf. 11 also investigates how to provide view with good interior lighting in terms of light levels, distribution, and glare. Evaluation of these systems are based on two different experiments. The fust uses scale-models for daylighting evaluation. Methodology of the research is presented as well as results and evaluation for part of the first experiment. The second experiment will use computer program simulations for energy evaluation that include reducing lighting and cooling loads and shaving peak loads, especially, when used with selective lowe glazing for office buildings in hot climates. INTRODUCTION In office buildings, consumption of energy for arlificial lighting alone is averaging 60% of the building's total consumption of electric energy [8,15]. Therefore, the reduction in the utilization of electrical lighting will, first, reduce the building's electrical lighting load and, second, reduce internal heat gain in the building which is generated from the use of electrical lighting "heat-of-light" . This heat gain reduction decreases the building's cooling load. especially in hot climates where the maximum cooling load is usually concurrent with maximum daylight availability. Moreover, this reduction in energy consumption will shave the peak demand during the key rate periods in these buildings and, therefore, additional saving in operating costs can be achieved. However, improper application of daylighting may offset these savings by producing one or more of the following: solar heat gain problems, excessive glare, and reduce view to the outdoors. Therefore, daylighting delivery systems should provide good illumination performance and good views of the outdoors without excessive glare as well as achieve acceptable energy performance. Office buildings are often located in hot climates where solar radiation availability is enormous, and therefore are subjected to major heat gain, excessive brightness and discomfort glare problems. In such regions, the best design approaches that are utilized today to control these problems are using tiny windows, tinted glass and heavy shading devices that successfu! ly exclude direct sunlight. However, these reduce the potential of achieving one of the main objectives for having a window, that is, view. In addition, unless these design approaches are modified, they tend to reduce the penetration of daylight, especially in interior spaces further away from the window, so much that artificial light is needed all day. Large office buildings which are internally load-dominated can shift the appropriate design strategy for the buildings to cooling, perhaps eliminating heating needs entirely. Therefore, this results in high operational cost due to the increase in consumption of electricity for using artificial lighting, and for using a bigger load to cool the added heat gain that is associated with the artificial lighting. Daylight can replace electric lighting for most of the typical working day in most building types if the building is designed to allow daylight to reach most of the interior. However, daylighting is hard to achieve in most office buildings by simply applying windows around the perimeter of the building for side-lighting. Even when these windows are from wall to wall, this approach is unable to supply daylight to large portions of a building especially deeper into rear spaces away from the windows. In other words, daylight is badly dismbuted in these spaces, because the horizontal illuminance levels in the rear spaces are significantly low when compared to the ones in spaces near the window. In hot climates, this bad lighting distribution becomes more prominent when simple shading controls such as overhangs, horizontal or vertical blinds are added. Therefore, the benefit from daylight to reduce the use of electrical lighting is limited to a small area near the window. In addition, the luminance of internal surfaces of such spaces is excessively high (bright) near the window and low (dark) away from it. Therefore, ESL-HH-94-05-04 Proceedings of the Ninth Symposium on Improving Building Systems in Hot and Humid Climates, Arlington, TX, May 19-20, 1994 there will be a potential of glare discomfon for occupants in such spaces when excessive brightness from the window comes into their field of view [I]. Moreover, the wide range in illuminance levels and luminance levels (uneven daylight disuibution) is common with side-lighting in office buildings [4,51, however, it becomes worse with the presence of the partial height partitions that obslruct the daylight penetration coming from the windows. There are different ways to deal with problems associated with side-lighting as presented above. One of the ways to approach this problem is to modify the wall fenestration by adding different kinds of design treatments that will enhance the performance of h e daylighting delivery system (when refening to the daylighting delivery system, we are considering a fixed system that does not require mechanical or motorized equipment). These design treatments include lightshelf, lightscoop, fixed mirrored louvers, prismatic materials and holographic films as shown in Figure 1. In this study, the daylighting delivery system will be considered for side-lighting, shown in Figure la , and will consist of a wall fenestration with lightshelf as a design treatment. Figure 1. Daylighting delivery systems for side-lighting that consist of a wall fenestration with a fixed design treatment such as (a)lightshelf, (b)lightscoop, (c)prismatic panel, (d)holographic films and (e)fixed mirrored louvers. The lightshelf configurations can be identified as: internal, external, and combined as shown in Figure 2. The combined lightshelf is intended to provide shade, shielding the occupants from the direct glare of the sky while reflecting sunlight and skylight off its surface onto the ceiling, and increasing the illuminance level at the rear of the space. Therefore, the objective of the lightshelf is to manipulate sunlight both in terms of its light and heat. If designed properly, a lightshelf should redirect sunlight or diffuse daylight onto the ceiling (based on the reflectance of the upside surface of lightshelf), enhancing lighting conditions in the space and improving the disuibution of light and reducing glare VI. (a) Double opening lightshelf