Application of a coelostat daylighting system for energy savings and enhancement of indoor illumination: A case study under clear-sky conditions

This paper examines the effectiveness of coelostat daylighting systems for enhancing the indoor visual environment of open-plan offices. A coelostat consists of a movable mirror capable of tracking the sun and a fixed second mirror, which, acting together, create a single beam of sunlight for daylighting. A series of computer simulations were carried out using TracePro and Radiance where the relevant IES output (polar candela distribution plots) of the former was exported to the latter for photometric analyses. The results reveal that coelostat daylighting systems can significantly enhance not only the visual environment but also the energy efficiency when sufficient daylight is introduced by simultaneously applying side and top daylighting (ST) schemes. When only daylight admitted through windows is considered, daylight availability varies from 0.12% (at 10 a.m., summer solstice) to 3.19% (at 12 p.m., winter solstice). In contrast, the highest daylight availability of 3.41% is observed at 12 p.m. on the day of the winter solstice when side and top lighting are considered in addition to daylight admitted through windows. Electricity savings of 1.78 kWh are estimated over a period of one hour around noon, that is, a half an hour before and after noon. A comparison between the on-site measured data and simulated values showed that they were in good agreement, confirming the reliability of the computer modeling developed in this work. The approach described here could be extended to analyze daylighting performance of buildings with similar solar features, such as light wells.