Prediction of total solar irradiance on tilted greenhouse surfaces
Keywords:solar radiation, greenhouses, tilted surface, diffuse fraction, clearness index
Solar radiation is the driving force for the surface energy balance in buildings such as greenhouses. The greenhouses are generally tilted towards the sun in order to maximize the solar irradiance on the surfaces. Precise computation of the solar radiation received on these surfaces assumes an important role in the energy simulation. It is practical to calculate the total solar irradiance on the tilted surfaces based on the solar global and diffuse radiation intensities on horizontal surfaces. In this work, a south-facing thermal box inclined at 26.5° from the horizontal was used for solar radiation measurements. Additionally, the recorded solar radiation data were retrieved for the study location and used to develop an empirical correlation. The derived 4th order polynomial correlation related the diffuse fraction to the clearness index. The conversion factors for the beam, the diffuse and the reflected solar radiation components were essential in the prediction of the total solar irradiance on the tilted surface. The measured solar radiation data were then compared with the simulated total irradiance on the tilted surface. The model performance was assessed using both graphical and statistical methods. Overall, the diffuse-to-global solar radiation correlation has proved to be a useful technique providing reliable results. The locally calibrated data led to a clear improvement in the estimated total solar radiation. Generally, reliance on indirect techniques of solar radiation estimation is gaining importance especially for data-scarce regions where measurement is quite infrequent.