Ground reflectance estimation by means of horizontal and vertical radiation measurements

► Albedo is estimated from radiation measurements on vertical and horizontal surfaces. ► Two solving techniques are investigated: the direct and the inverse approach. ► The Tikhonov regularization method is introduced to deal with correlated variables. ► Albedo is obtained for clear skies from the isotropic model and diffuse correlations. The importance of an accurate estimation of the ground reflectance is well-recognized in energy balances involving solar radiation. Despite there are methods to estimate it directly it is still a hard task in highly reflective environment or in urban areas. In those areas it is also hard to find measurements of the ground-reflected component of the solar radiation. However, data coming from solar energy installations could be employed to estimate the local ground reflectance. To achieve this goal a method to estimate the ground reflectance from global radiation measurements in several orientations is needed. In this work the question of the estimation of the ground reflectivity from measurements of the global solar radiation on orientations typical from the energy installations is addressed. Two different approaches are presented to solve the same problem. First, by direct solution of global radiation models the ground reflectivity is deduced from the measurements (known as direct problem approach). In second place, from physical hypothesis about the behavior of the solar radiation such as isotropy and diffuse correlations the ground reflectance is approximated from experimental data (known as inverse problem approach) at the points where reality is closer to these conditions. In this framework, it is shown the need of mathematical tools such as Tikhonov regularization. The two approaches have been tested by means of the data produced at the CIEMAT’s test facilities from the Energy Efficiency in Building R&D Unit (UiE3) at Plataforma Solar de Almería (PSA) located at the South-East of Spain. Approaches are tested on data coming from several pyranometers on the rooftop of a building. Cross-validation is performed with measurements coming from an albedometer. It is shown that the inverse technique can solve the problem by taking near two weeks of measurements of global solar radiation on the horizontal and vertical surfaces. With the same set of experimental data the direct technique is unable to estimate the ground reflectance even with the inclusion of a third measurement on other surface.