Experimental investigation on thermal performance of an actively cooled light shelf

Abstract

Although the energy efficiency of light shelves has been studied, the thermal effects due to reflected solar radiation from light shelves have yet to be adequately investigated in previous research. The key research questions are whether this radiation results in additional heat load in buildings in tropical climates and how this can be reduced. In this work, the performance of an exterior aluminium light shelf is studied experimentally using a scaled prototype. The total amount of heat transmitted by the light shelf is about 40 W/ m2, which is close to the average thermal transmission of the envelope of a low-energy building. Experimental data analysis has revealed two components of the radiant heat transmitted by a light shelf: a) a direct instantaneous reflection of infrared radiations and b) secondary long-wave radiation resulting from the heating up of the light shelf material. The secondary radiation is a significant part of the overall heat transmitted by the light shelf. The light shelf was actively cooled by circulating water through hydronic radiant capillary mat tubes to reduce this component, and the radiant heat transmitted was measured. These experiments showed that an actively cooled light shelf could considerably reduce the secondary radiant heat transmitted through a clerestory window. A significant decrease in secondary heat transmission within a range of 3.8–5.9 W/m2 was observed when compared to an identical light shelf that was not cooled. This reduction is about 10% of the total heat transmitted by the light shelf.