Assessment of HVAC load in light zones to determine energy efficient shading for tall office buildings of Dhaka

Abstract

This research addresses the compromises that can be struck, using the passive architectural feature of sunshades, when balancing daylight and the accompanying thermal gains. At present, daylight is being encouraged in tall office buildings all around the world and Dhaka, where this study is based, is no exception. The recent trend is of deep plan open offices, with extensive use of curtain glass envelopes, using large apertures. Daylight inclusion into the office interiors creates two types of luminous areas: light or daylight zone, and dark or artificial light zone. The daylight zone includes sunlight, and sometimes unwanted heat, increasing consequent energy consumption due to HVAC load. Excessive daylight can also create glare zone. However, if daylight can reduce the building's use of electric light and HVAC load, it can reduce peak energy use, as well as total energy consumption. External Shading devices can successfully be used, to address this issue of balancing luminous and thermal loads. A simulation study was undertaken to evaluate luminous and thermal performance of shading devices, used in tall office buildings of Dhaka. Ten selected fixed external shading devices, placed in different orientations, from a field survey were evaluated, during the overheated period of summer. 'ECOTECT' (version 5.50) simulation program with raytrace based 'Radiance' was used, for this evaluation, based on the light zone distribution and the HVAC load. The simulation results revealed that, the performance of aluminum louvered cornice type sunshade in South, is significantly superior to the other tested shading devices, in this orientation. This is also the preferred device in West orientation, where the HVAC load is the maximum. The gaps in this louvered sunshade, allows air movement, causing any trapped heat to dissipate from the building curtain wall, thereby decreasing the cooling load and increasing energy efficiency. It also helps to illuminate greater area with acceptable illumination level, up to greater spatial depth. In the east orientation, solid concrete cornice is more energy efficient, compared to the louvered one, for its significant usable area of acceptable illumination level. However, control over the glare zone, resulting from both these shading devices, can increase their efficiency. The results also prove that, most of the tested shading devices are better for east and west orientation, when the glare zone can be used, by adjusting window-observer-task relationships.