An Australian research team has developed a reflective coating strategy for semi-transparent solar cell-integrated roof tiles, aiming to enhance both thermal regulation and electrical performance in building-integrated photovoltaics (BIPV).
According to corresponding author Mohammad A. Alim, the study addresses the specific thermal, optical and structural challenges associated with roof-integrated PV systems, rather than window-based applications that dominate previous research.
“Unlike prior studies that focus mainly on window-integrated PV, this research addresses the unique challenges of roofing integration,” Alim told pv magazine. “Our development demonstrates a dual benefit by simultaneously reducing roof surface and internal temperatures while improving the electrical performance of the solar cells through temperature control.”
The researchers said the approach supports net-zero energy building goals by combining thermal and electrical optimization in a single system. Most previous studies have examined cooling coatings or PV performance separately.
Experimental setup
The team fabricated solar roof tiles (SRTs) using mortar slabs measuring 390 mm × 290 mm × 27 mm, cured for 28 days. Two layers of commercially available white reflective coating (RC) were applied. Semi-transparent cadmium telluride (CdTe) solar cells with transparency levels of 20%, 40%, 60% and 80% were then installed on top.
Initial testing was conducted under halogen lamps at an average irradiance of 228.1 W/m2. Outdoor experiments were subsequently carried out in Sydney from Dec. 4 to Dec. 7, 2023, using tiles with 20% and 80% transparency.
The team also conducted EnergyPlus simulations to evaluate building energy savings. A 511 m2 office building model was tested under climate conditions for Sydney; New York City; Karachi, Pakistan; Dhaka, Bangladesh; and Beijing. Simulations compared SRTs with 20% and 80% transparency against reference reflective-coated conventional roof tiles.
Thermal and electrical performance
The researchers found that reflective coatings reduced surface temperatures by 5.4 C to 10.8 C under laboratory and outdoor conditions. Higher-transparency tiles delivered stronger cooling effects.
In outdoor testing, white semi-transparent tiles with 80% transparency operated 8.5 C cooler than 20% transparency tiles and up to 20 C cooler than uncoated tiles.
The temperature reduction translated into electrical gains, increasing cumulative power output by 5.6% to 11.7%.
“Perhaps most unexpectedly, these significant thermal and electrical improvements were achieved using a simple, low-cost, passive coating,” said Alim. “This demonstrates that reflective-coating-based cooling can outperform more complex BIPV cooling strategies while simultaneously boosting energy generation and reducing cooling demand.”
EnergyPlus simulations showed that solar roof tiles (SRTs) with 20% transparency delivered modest improvements in building energy performance, achieving gains of 0.69% in Sydney, 0.38% in Karachi, 0.57% in Beijing, 0.73% in New York and 0.47% in Dhaka. When transparency was increased to 80%, however, the performance gains rose significantly, reaching 5.73% in Sydney, 2.99% in Karachi, 4.46% in Beijing, 5.85% in New York and 3.49% in Dhaka.
These findings suggest that transparency level plays a critical role in enhancing cooling performance and maximizing overall building energy savings across diverse climate conditions.
“Aside from the curved roof tiles, we also plan to investigate how roof tilt angle influences the thermal behavior and electrical performance of these semi-transparent roof tiles under real operating conditions,” Alim concluded. “In addition, future work will assess coatings with alternative colors and tailored spectral reflectance properties, enabling both performance optimization and greater architectural flexibility.”
The research's findings were presented in “A Novel Reflective Coating Strategy for Semi-Transparent Solar Cell-Integrated Roof Tiles in Energy-Efficient Building Design,” published in Energy. Researchers from Australia's Western Sydney University and Charles Sturt University have participated in the study.
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