The cooling effect of air on flat plate solar energy collectors

Abstract

Use of solar energy devices requires that they be exposed to receive maximum solar radiation. Unfortunately, the intensity of solar radiation incident on these devices varies throughout the day with the position of the sun in the sky hence making these devices to under-perform. Moreover, high temperature associated with the heating effect of the much needed solar radiation also destroys solar PV cells whose optimal operation temperature is 25oC. This is contrary to solar thermal energy collectors whose performance improves with increased temperature. To improve the performance of flat plate solar PV and thermal energy collectors, it is important to determine their optimal angle of orientation with respect to solar radiation and wind. To address these challenges, it was important to achieve the study objectives which were to investigate cooling effect of air with a view to improving the performance of flat plate solar energy collectors, determine the dependence of wind-related heat transfer coefficient (hw) on angle of incidence of wind onto the flat plate solar energy collectors and to develop the currently lacking single expression of (hw) for the full possible range of angles of incidence of wind onto the flat plate solar energy collectors from 0o to 90o. To achieve these study objectives, it was important to understand the effect of air velocity (u), angle of incidence of wind (𝜃i) and temperature variation on the performance of these devices. To study the effect of these variables on the performance of flat plate solar energy devices, a laboratory experiment was set up having a flat plate inside a wind tunnel to collect data on cooling effect of air at different angles of inclination of the plate (𝜃p). Velocity and temperature at different angles of orientation of the plate were measured using thermo-anemometer and digital data logger respectively. Data collected under steady state condition were used to determine wind-related heat transfer coefficient (hw). The dimensionless numbers associated with this study were Nusselt (Nu), Reynolds (Re) and Prandtl (Pr). These are correlated by Nu = 𝛾 (RePr) n which links (hw) with 𝜃p, 𝜃i and u. To show variation of (hw) with (u) and (θi), graphs of (hw) against (u) and (θi) were plotted by OriginPro. The values of (hw) under different conditions were used to determine suitable orientation of flat plate solar energy devices with reference to wind and solar radiation, for their optimum performance. The study found that in a relatively low air velocity, the value of (hw) was insensitive to incident angles of wind onto plate. However, as air velocity increased, the variation of (𝜃i) with (𝜃p) strongly influenced the plate’s steady state temperature which in turn influenced the magnitude of (hw). The incident angles of wind yielding low and high values of (hw) were found suitable for optimum operation of solar thermal and PV energy collectors respectively. The study recommends that the optimal tilt angles of solar PV panels and solar thermal panels would be 0o and 45o respectively relative to wind. The results of this study will serve as reference for