Daylighting and its implications to overall thermal transfer value (OTTV) determinations

The overall thermal transfer value (OTTV) and daylighting are two approaches controlling building energy use. In Hong Kong, although OTTV calculations are mandatory in the submission of building plans for approval, daylighting credits are not included in building envelope designs. To promote energy-efficient building designs we use the computer simulation tool, DOE-2, to illustrate the energy performance of a generic commercial building due to various daylighting schemes and OTTV designs. The year-round energy expenditures and loads are determined from the simulation results. Analysis is carried out in terms of the reduction in electric lighting requirement and the cooling penalty due to solar heat. Regression techniques are conducted to correlate the annual incremental electricity use with OTTV and daylighting aperture (DA) (product of window-to-wall ratio (WWR) and light transmittance (LT)). Contours of equal annual incremental electricity use for different building envelope parameters are developed. Important features for daylighting schemes are highlighted and implications for OTTV designs are discussed.