The influence of the solar radiation model on the calcutated solar radiation from a horizontal surface to a tilted surface

Measured solar radiation data are most commonly available as total solar radiation on a horizontal surface. When using solar radiation measured on horizontal to calculate the solar radiation on tilted surfaces and thereby the thermal performance of different applications such as buildings and solar heating systems, different solar radiation models can be used.

The calculation of beam radiation from a horizontal surface to a tilted surface can be done exactly whereas different solar radiation models can calculate the sky diffuse radiation. The sky diffuse radiation can either be assumed evenly distributed over the entire sky dome and calculated as pure isotropic radiation or by anisotropic radiation models that also uses contribution from circumsolar radiation in the calculation or by anisotropic radiation models that apart from the isotropic and circumsolar contribution uses horizon brightening in the calculation.

The weather data are measured at the solar radiation measurement station, SMS at the Department of Civil Engineering at the Technical University of Denmark. In this study the weather data are combined with solar collector calculations based on solar collector test carried out at Solar Energy Center, SEC, Denmark.

With measured solar radiation on horizontal and the different solar radiation processing models the total radiation is calculated on differently tilted and oriented surfaces and compared with the measured solar radiation on the different surfaces. Further, the impact on the yearly thermal performances of a solar collector using the different solar radiation processing models is investigated.

The study shows that the isotropic diffuse radiation model is underestimating the diffuse radiation from south and overestimating the diffuse radiation from north, while the anisotropic models give a better estimate on the diffuse radiation from all directions.