Predicting vertical luminous efficacy using horizontal solar data

Daylighting is recognized as an important and useful strategy in the design of energy-efficient buildings. Daylight illuminance, particularly on vertical surfaces, plays a major role in determining and evaluating the daylighting performance of a building. Luminous efficacy approach is considered as a versatile and easily applied way to calculate outdoor illuminance. This paper presents an approach to estimate the vertical outdoor illuminance from computed vertical luminous efficacy based on the measured horizontal solar irradiance and illuminance data. Hourly data recorded from January 1996 to December 1998 in Hong Kong were used for the model development. The performance of the proposed model and two well-known anisotropic inclined surface models (Muneer and Perez) was evaluated against data measured in 1999. Statistical analysis indicated that the proposed model gives reasonably good agreement with measured data for all vertical planes. The proposed model can provide an alternative to building designers in estimating the vertical solar illuminance and irradiance where only the horizontal measurements are available.