Roof-Top Solar PV Performance Under Different Roofing Materials and Air Gaps in Tropical Climates.
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Date
2024-05
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Kenyatta University
Abstract
High temperatures negatively impacts the performance of Photovoltaic (PV) modules. This
is more pronounced on Roof-top installations where due to limited air spaces and the choice
of materials influence the natural cooling of PV modules. Regions experiencing high
temperature conditions such as the tropical African countries present a unique opportunity
to study the interactions between the PV module and the base roofing materials. This study
presents both simulation and experimental results of the thermal effects of roofing materials
and air gaps on the performance of rooftop-installed PV modules. Clay and concrete tile
roofs as well as various metal roofs commonly used in sub-Saharan Africa were studied.
The base material properties of the roofs were studied and the effects of roof pigmentations
were examined as well. Pigmentations of Iron (III) oxide (Fe2O3), Titanium dioxide (TiO2)
and Basalt which are three commonly used roof pigmentations were studied and compared
with PV heat interactions of unpigmented roofs. For the simulation study, the PV module
and the various roofs under investigation are designed on Solid Works software and model
designs are then exported to ANSYS workbench where modeling and simulations are
implemented on ANSYS steady-state thermal. The roof pigmentation study was
implemented in COMSOL Multiphysics software for transient thermal analysis. Surface
radiosity is investigated to further differentiate the heat transfer influence of the various
pigments and to underline the environmental impact of each of the models. The results
obtained are validated with field-based experiments carried out at Kenyatta University in
Nairobi, Kenya. Six different commonly used roofs namely; Decra roofs, Box Iron roofs,
Clay tile roofs, Concrete tile roofs, Orientile roofs, and Galvanized Iron roofing sheets were
specifically investigated. PV installation optimization was investigated for the various roof
materials and optimal air gaps for specific roof materials were determined, to guide future
installations. Results from the study provide relative PV performance on the six different
roof materials. Results from the study also show the role of roof pigmentations in PV heat
transfer. The clay tile roofing sheet revealed minimal deleterious effects on PV
installations, closely followed by the concrete roof tiles. Among all the metal roofs
investigated, the Orientile roofing sheet requires larger clearance for better performance,
followed by the Galvanized Iron roofing sheet and Decra. The PV module on the Box Iron
roof recorded lower temperatures when compared to those on the other metal roofs at
specific air gaps, consequently leading to higher performance. PV maximum temperature
across the different modules on the various roofs ranged from 64 °C – 50 °C, at
instantaneous times at the 20 mm airgap. This range difference decreased as air gaps were
raised to 100 mm. Open circuit voltage values ranged from 20.8 V – 21.1 V as airgap
increased on the best-performing module on the Clay roof. The modules on the other roofs
also revealed voltage gains at higher air gaps. Results also reveal the optimum airgap for
PV installations of Concrete, Clay and Box-Iron roofing sheet at 100 mm. while Orientile,
Decra, and Galvanized Ion roof materials have optimum airgaps at 150 mm, 140 mm, and
140 mm, respectively. Further, out of the three roof pigments investigated, the Fe2O3
pigmented roof gives the highest solar cell temperature. The unpigmented roof reveals solar
cell temperatures that are notably higher than the cell temperatures of all pigmented models.
From the three pigmentations, the Fe2O3 pigment gives the highest surface radiosity. The
unpigmented model displays a surface radiosity that is significantly higher than that of the
pigmented models. This study establishes that the type of roofing material will influence the
minimum air clearance necessary for the natural convective cooling of PV cells.
Description
A Thesis Submitted in Partial Fulfilment of the Requirements for the Award of Master of Science Renewable Energy Technology in the School of Engineering & Architecture of Kenyatta University, May, 2024
Supervisors:
1. Francis Njoka
2.Mathew Munji