Model development and evaluation of global and diffuse luminous efficacy for humid sub-tropical region

Daylight has a significant role for energy conservation in buildings. Luminous efficacy models have been applied by many researchers to estimate the illuminance level from available irradiance data. In the present study, the characteristics of global (Kg) and diffuse (Kd) luminous efficacies are reported based on measured data for the humid sub-tropical region of New Delhi, India. Four existing efficacy models have been compared based on the measured irradiance and illuminance data. Statistical analysis indicates that the existing models performed poorly for the selected location. The performance of the models was significantly improved when the optimized coefficients were adopted using the measured data. Global and diffuse efficacy under clear, intermediate, overcast and all sky conditions, using existing and developed efficacy models were computed and compared. A good agreement has been exhibited for variation of efficacies with the measured values for different sky conditions from cloud to clear skies. Variation of global and diffuse efficacy for sky (clear, intermediate and overcast) w.r.t. solar altitude angle and sky clearness index has been presented. Finally, mean monthly global (Lg) and diffuse (Ld) illuminance were also estimated using the original and optimized version of efficacy models to provide impression of deviation of the models.