Luminous efficacy of direct irradiance: Variations with insolation and moisture conditions

Abstract

The present work is an experimental study of the relationship between the luminous efficacy of direct solar radiation and the following quantities: global and direct solar radiation, total water vapor content (as estimated from surface dew point), and solar zenith angle. This work will contribute to the development of a model to derive direct illuminance, a critical component in daylighting applications, from more commonly available direct irradiance. Direct luminous efficacy is defined as the ratio between the direct illuminance and direct irradiance. This varies with turbidity, water vapor content, cloud cover, and solar geometry. Insolation conditions were parameterized by global and diffuse irradiance and solar geometry in earlier work. We attempt here to account for atmospheric water content as an additional descriptor. This has an impact on luminous efficacy directly because of spectrally selective absorption and indirectly because of spectrally selective aerosol scattering. An important step of this work includes the determination of instantaneous total precipitable water (as a measure of water vapor absorption) from surface dew point temperature. The latter quantity is routinely measured, the former is not. The utilization of surface dew point temperature as an estimator of atmospheric water vapor content improves the determination of the visible radiation from solar radiation data. Experimental observations presented in this work are consistent with expected atmospheric physical processes.