Semi-transparent PV window based on passive radiative cooling coating

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Researchers from Hong Kong University of Science and Technology and Hong Kong Polytechnic University have designed and investigated a novel coated semi-transparent photovoltaic (CSTPV) glazing for building-integrated photovoltaic (BIPV) applications.

They explained that the novelty of their approach consisted of equipping the glazing with a highly transparent passive radiative cooling coating. “Highly transparent passive radiative cooling paints are also available now, which allow the solar radiation to transmit while keeping certain radiative emotive characteristics,” they stated. “The novel radiative cooling material holds the potential to mitigate urban heat island effects associated with carbon emissions and reduce energy consumption.”

The experiment started by manufacturing two samples, a CSTPV glazing and a glazing without coating (STPV) utilizing cadmium telluride PV modules. They have also compared them with traditional and vacuum windows, one being a 6.3 mm thick vacuum glazing and a 3.2 mm thick single glazing.

“The primary distinction between CSTPV and STPV is that CSTPV includes an additional cooling layer, while STPV operates with a glazing surface without any coating,” explained the academics. “The STPV glazing, being 40% of PV coverage ratio, measures 7.92 mm in total thickness. The inner side of the sample incorporates vacuum glazing, composed of two layers: a 3.2 mm low emission coated glazing on the front side and a 3.2 mm transparent glazing on the rear side.”

Following the construction of the samples, the team measured their electrical, thermal, and optical properties and inserted the data into software simulations. They assumed the glazings to be deployed in a south-facing hexagonal office with windows 1.5 m in width and height and a sill height of 0.8 m. The data used in the simulation was from Hong Kong. The results were also verified through outdoor experimental measurements.

“The results indicate that the CSTPV windows can significantly reduce the heat gain rate by about 15% and achieve excellent electricity generation, with an annual output approximately 3% higher than that of STPV windows,” said the research group.

“All samples measured have correlated color temperature (CCT) values within the range of neutral light and color rendering index (CRI) values exceeding 96, demonstrating satisfactory high-quality indoor lighting conditions,” they added. “Photovoltaic vacuum glazing features elevated CRI and CCT values, ensuring accurate light transmission without distortion.”

The results were presented in “The overall performance of a novel semi-transparent photovoltaic window with passive radiative cooling coating – A comparative study,” published in Energy & Buildings.

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