Luminous efficacy of direct solar radiation for all sky types

Three models for the luminous efficacy of direct solar radiation on a horizontal surface, valid for all sky conditions, have been developed in the present work, and two of them are proposed. In all the models a function for the direct luminous efficacy for all sky conditions relative to the direct luminous efficacy for clear skies is obtained, using the function for the direct luminous efficacy for clear skies given in a previous work, and the brightness index as a new independent variable. One of the two models is obtained by fitting the values of the relative luminous efficacy as a function of the brightness index. The other proposed model is developed by obtaining in the first place models for direct illuminance and direct irradiance, both relative to the corresponding clear sky models, and then finding the ratio between the relative direct illuminance and irradiance models. Statistical assessment of the direct illuminance values estimated with the proposed models gives similar values for the MBE and the RMSE. However, the model obtained from illuminance and irradiance models is mathematically coherent. (C) 2001 Elsevier Science Ltd. All rights reserved.