Effects of Parapets on the Performance of Unglazed Solar Collectors
The useful energy output of a solar thermal collector is influenced by various factors, such as wind velocity. Elevated wind velocity results in substantial heat loss caused by wind, thereby affecting the performance of the collector. Solar thermal collectors that lack insulation or a glass covering (unglazed solar collectors) are especially susceptible. Considering the placement of these collectors on rooftops and their exposure to wind, it is imperative to examine methods for minimising this impact. Studies have demonstrated that perimetric parapets can modify wind loads on roofs and the structural support system of solar panels and collectors. Insufficient attention has been devoted to examining the influence of parapets on collector heat loss, as well as the optimal placement of collectors on roofs with respect to parapets and their effect on heat loss. Consequently, the focus of this study was directed towards examining the impact of parapets on the localised velocity surrounding roof-mounted unglazed solar collectors, as well as the subsequent heat loss.
The quantification of this effect was accomplished using Computational Fluid Dynamics (CFD) simulations validated through wind tunnel experiments. The aerodynamic properties of the roof were investigated across three distinct scenarios, namely those with high parapets, low parapets, and no parapets. The results of the study indicated that when perimetric parapets were not present, the vortex formed on the rooftop was located in closer proximity to both the leading edge and the surface of the roof. In contrast, the inclusion of parapets resulted in the elevation of the vortex above the surface of the roof. In each instance, the velocity at the central region of the roof exhibited higher velocities, while the velocity zones adjacent to the leading and trailing edges of the roof and parapets displayed lower velocities.
Measurements were taken at different roof mounting locations, representing 25%, 50%, and 75% of the roof length, for collector tilt angles of 5°, 20°, 40°, and 60° at different parapet heights and wind incidence angles (0°, 45°, 90°, 135°, 180°). The results of the simulations indicated that there was a direct correlation between the increase in tilt angle of unglazed solar thermal collectors and the corresponding increase in heat loss, irrespective of the height of the parapet. Additionally, the study demonstrated that lower parapets led to higher levels of collector loss compared to parapets with greater perimetric height. The research revealed that increased collector tilt angles resulted in a decrease in heat loss for the following row of collectors, which were positioned further from the incoming flow in rooftop solar arrays. In all instances, it was observed that the leading row of collectors positioned at the edge of the roof experienced a greater heat loss compared to all subsequent rows of collectors.
In summary, the results of this study demonstrated the aerodynamic impacts of parapets on the heat loss of roof-mounted standalone and array unglazed solar thermal collectors. Based on the acquired insight, a set of correlations was formulated to enable designers and architects to consider the influence of parapets on the convective heat loss of unglazed solar thermal collectors. In a more comprehensive context, the study showcased the importance of mitigating the impact of velocity on unglazed solar collectors, thereby enabling its use in various solar thermal applications. This is particularly noteworthy given that unglazed solar collectors are cost-effectiveness.