UV solar radiation on a tilted and horizontal plane: Analysis and comparison of 4 years of measurements

In the global wavelength range (300–3000 nm), it is known that a plane with a slope equal to the latitude of the location, receives more annual energy than the horizontal plane, mainly due to the increase in direct irradiation on the interest plane. The UV (280–400 nm) spectra at the earth surface, has a larger component of diffuse and a minor component of direct solar radiation compared to the global wavelength range, therefore the increase in annual energy due to plane inclination should also be different. This work, analyzes 4 years of solar UV radiation measurements performed on tilted and horizontal planes located at the Plataforma Solar de Almería, Spain. The monthly mean ratio of tilted/horizontal solar UV irradiation varies with the time of the year, reaching values of 1.25 and 0.95 for winter and summer, respectively. The same ratio in the solar global spectra rises up to 1.70 and 0.85 for the same months. The annual UV solar energy increase on a plane tilted 37° and oriented towards the equator is around 3–4%, whereas is around 10% in the global spectra. In this way UV annual energy increase due to inclination and orientation of the plane is much lower than that for global radiation. Determination of a unique method to assess all possible inclinations and orientations, require simultaneous measurement of diffuse and direct UV radiation performed with radiometers of identical spectral response. Given the worldwide scarcity of these type of data, an empirical correlation that relates horizontal UV irradiation to that on a 37° inclined plane was determined. Monthly and annual tendencies of solar UV irradiation have been analyzed and compared with the solar global irradiation.