Daylighting efficacy: A new metric for solar radiation utilization and daylighting design

Appropriate daylighting design can create a comfortable indoor lighting environment, however, the objectives of providing adequate daylight and controlling solar gain often conflict. Existing daylighting metrics effectively evaluate the daylighting performance of buildings, but no connection has been established between daylighting and the solar gain it brings. To address this issue, this paper proposes a new metric, daylighting efficacy (DE), which is defined as the ratio of the luminous flux obtained through daylighting on the whole horizontal work plane to the solar radiation received by the daylighting system (lm/W). The calculation equation for DE is established accordingly, and its application is explored in a reference office using the parametric design approach. Variations of the reference room's orientation, glazing property and configuration, and window-towall ratio are then examined to assess their impact on DE with explanations provided for the underlying causes. Additionally, three innovative daylighting systems are evaluated using DE to demonstrate its suitability for assessing buildings with various daylighting systems. Finally, a comparison between DE and Daylight Factor (DF) is presented to highlight that DF cannot serve as a substitute for DE. The reasonable range for DE will require determination through the combined efforts of future researchers.