Determination of the Most Accurate Horizontal to Tilted Sky-Diffuse Solar Irradiation Transposition Model for the Capital Cities in MENA Region

Abstract

Accurate solar radiation data is essential in designing, evaluating and optimizing solar energy systems. The meteorological recorded data is the mainly source of the solar irradiance data. Since solar irradiance incident on a specific tilted surface is not frequently recorded, the horizontal to tilted solar irradiation transposition models (HTTM) use to convert the measurable components of solar irradiation (global horizontal, sky-diffuse and ground reflected) to global tilted solar irradiation with high accuracy. The importance of the solar transposition model is in determining the optimum tilt angle of solar energy harvesters which is one of the important design parameters for maximizing solar radiation incident on the solar collectors. This paper introduces a statistical procedure to figure out the transposition model that is closest to the real model in the MENA region without needs for measured data. Also it provided a summary of optimum tilt angles and transposition models that recommended by local researchers for specific locations in MENA region. This study showed that the transposition models depend on the angle of inclination of the solar collector in addition to the location. The study identified models with deviation rates about 3% for most cities, which is an engineering reasonable percentage, and this encourages the authors to recommend this approach to determine more accurate transposition models for wider regions of the world. On the other hand, it showed that all models failed to achieve an acceptable deviation rate for high tilt angles especially vertical surfaces, which have great engineering applications. The authors advise researchers to take care when adopting a transposition model that has been validated at low tilt angles to apply it to high tilt angles and building façades. The study is also reveal that, the reduction in total annual global solar irradiation is not exceed than 1% due to the offset of tilt angle from the optimum angle for all considered transposition models and for all sites.