Natural convection in building-integrated photovoltaic systems: a computational study

摘要

One of the very significant challenges in building-integrated photovoltaic (BIPV) systems is the overheating of the PV cells. The double-skin configuration, which allows natural ventilation to provide passive cooling of the PV cells, has been used as a cost-effective means of solving the difficulty. This research has been undertaken to improve the understanding of the flow and heat transfer phenomena in the channel formed by the double-skin thereby enhancing the cooling of PV cells by natural convection.A novel in-house LES computer code has been proposed to model the buoyancy-driven flow. Based on comparison with experimental data, it is shown that LES with the subgrid-scale Vreman model (VM) is able to capture the large-scale turbulent structures, whereas the Smagorinsky model (SM) yields unphysical results due to its more dissipative formulation. Several dynamic variants of SM have also been found to give results that do not agree with experimental data because of the ad hoc clipping procedure. A comprehensive parametric study has been performed using VM with varying channel widths, convective heat fluxes, inclination angles, and external disturbances. It is demonstrated that the average heat transfer is enhanced as (i) the convective heat flux or channel width increases, (ii) the inclination angle from the horizontal plane increases, and (iii) substantial amount of disturbances are present in the ambient. In each case, the augmentation of heat transfer is consistently due to turbulence production, thereby suggesting a promising outcome to adopt the findings listed above for the passive cooling of PV systems. The effect of including radiation in the numerical model has also been found to yield better prediction of wall temperature, thereby improving the comparison between numerical results and experimental data. Dynamic variants of VM have also been proposed and validated by conducting LES of natural convection in a rectangular cavity. It is found that disturbances resembling Tollmien-Schlichting waves occur in the transition zone to trigger the breakdown of the laminar flow. The comparison of the LES results with experimental data from the literature shows that VM with ‘global equilibrium’ can capture the coherent structures correctly, whereas the standard VM and VM with Germano yield postponed transition. This suggests that much finer grid is desired when using the latter models to better capture the weak transitional boundary layer.