Thermal and daylight performance of tilted glass as shading alternative in hot arid climate of Baghdad

Abstract

Solar gain mitigation in buildings is essential to achieve thermal comfort and reduce cooling load in hot arid climate. Opaque shading is a useful strategy to block solar radiation in excessively hot locations like Baghdad. However, such shading comes at the expense of daylight illuminance and the openness of the window. A review of previous studies showed a gap in investigating the potentials of the clear glass in reducing solar gain. Solar transmittance through glass is angular dependent. The transmitted radiation decreases with bigger angles of incidence following increased reflection and reduced solar intensity. This research investigated the effects of using glass slats tilted away from the sun as an alternative to conventional horizontal shading on thermal and daylight performances of southwest rooms in commercial buildings in Baghdad. Integrated Environmental Solutions-Virtual Environment simulation tool was used to conduct the study. Six simulation cases were formulated; the base case used the common 75 centimetres overhang, the horizontal shading case had an additional three 75 centimetres horizontal shading devices, and four cases of 50°, 60°, 70° and 80° tilted glass slats in addition to the overhang. The simulation was run for seven representative days during the predominantly cooling season from April to October. Reductions in the solar gain, air temperature and mean radiant temperature using the slats were significant, especially at lower solar angles. For example, on September 21st, the solar gain of the base case was reduced by 71%, 69%, 57% and 46% with the glass slats tilted at 50°, 60°, 70° and 80° respectively compared to 61% reduction through horizontal shading. On the same day, the average air temperature was reduced between the range of 2.2–3.3°C with the slats compared to 2.7°C with the horizontal shading; the mean radiant temperature was reduced by 2.3–3.4°C with the slats compared to 2.8°C with the horizontal shading. The daylight average illuminance was decreased by around 58%, 32%, 24% and 23% by using glass slats tilted at 50°, 60°, 70° and 80° respectively compared to around 78% decrease by using horizontal shading with an average illuminance of below 100 lux in most simulated hours. However, the glass slats failed to raise the low illuminance uniformity to the acceptable threshold due to higher illuminance levels near the window and lower illuminance levels at the rear part of the room, unlike the horizontal shading that reached the acceptable to the preferable uniformity levels in most simulated hours. This study showed that replacing the conventional opaque horizontal shading with tilted glass slats, especially at an angle of 60°, can have a better impact in improving the heat mitigation by solar radiation and increasing daylight availability in the study area. Furthermore, these improvements were obtained without compromising the openness of the window.