Wind tunnel and CFD modelling of wind pressures on solar energy systems on flat roofs

Wagner & Co Solartechnik GmbH, Colbe, Germany, francois.grepinet@wagner-solar.com ABSTRACT: Design of solar energy mounting systems requires more knowledge on the wind patterns around these systems. To obtain more insight in the flow patterns, which cause the pres-sure distributions on the solar energy systems, a wind tunnel test and Computational Fluid Dy-namics analysis have been performed. In this study the average pressure coefficients, determined in the wind tunnel measurements, are compared with Reynolds Averaged Navier-Stokes calcula-tions. The comparison, based on the median of all observations over 6 wind directions and all pressure points, showed that the general pressure distribution is well predicted. Overall differ-ences were found of 39% for the Renormalization Group k-e turbulence model, 35% for a differ-ential Reynolds Stress turbulence model with wall reflection term and 35% for a differential Rey-nolds Stress turbulence model without wall reflection term. The largest differences are observed in the wake of systems that have a large spacing, which is due to an incorrect prediction of the separation zones and therefore the shielding effect of the solar energy systems. 1 INTRODUCTION The increasing growth of solar energy systems in Europe asks for appropriate design data. Flat roofs of large buildings are often used for placement of these systems. In this case wind loads are known to be the main cause for damages. Guidelines for these loads have been derived from a well-defined wind tunnel research performed at TNO and have been implemented in the NVN 7250 (2007) in the Netherlands, a first in its kind standard for solar energy systems integrated in roofs and facades. Although these guidelines provide a useful tool for designers, they also limit the design freedom. Obtaining more insight in the flow patterns which cause the loads on the so-lar energy systems could allow for better solar energy systems designs. To utilize CFD for this application, validation is needed. This study investigates the average wind pressures on solar energy systems on a building with a flat roof using wind tunnel measurements and CFD calculations. A comparison is made be-tween the Renormalization Group (RNG) k-e turbulence model and two versions of a differential Reynolds Stress turbulence model based on the proposal of Launder et al. (1975), both as imple-mented in the commercial CFD solver Fluent V6.3 (Fluent, 2006). The comparison provides in-formation on the influence of these turbulence models on the accuracy of CFD for the calculation of average wind loads on solar energy systems on top of flat roofs. Furthermore, the application of CFD provides information on the flow patterns around the solar energy systems, which helps to understand the origin of the pressures observed in the wind tunnel measurements.