TU Delft scientists develop optical filter to improve IBC solar module performance

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Scientists at Delft University of Technology in the Netherlands have designed a simplified optical filter for the passive thermal management of interdigitated back-contact (IBC) photovoltaic panels.

“The proposed design is tailored to IBC solar cells. However, the technology approach can be applied to any solar cell technology,” the research's lead author, Juan Camilo Ortiz Lizcano, told pv magazine. “We aimed to develop a filter that could work on a solar cell architecture that already presents good thermal behavior. The approach also works on other cell technologies, like Al-BSF [aluminum back surface field]. For example, our framework can predict if rejecting only sub-bandgap radiation could bring energy yield improvements to these solar cells.”

The research team analyzed whether simple and lossy designs could still provide energy yield benefits when accounting for degradation mechanisms. “We want to highlight the importance of considering degradation when studying the feasibility of a cooling solution aimed at solar modules,” Ortiz Lizcano added. “Additionally, this dual reflectance principle is also interesting for multi-purpose designs, like providing cooling and color to a PV module, an idea we hope to explore in the future.”

The research team developed the optical filter with two materials, one with a high refractive index, silicon nitride (SiNX), and another with a low refractive index, silicon dioxide (SiO2). It deposited the materials sequentially in pairs and calibrated their thickness depending on the wavelength value. “We selected these two materials because they significantly differ in their refractive indexes and are non-absorptive in the range solar cells produce electricity (300 – 1200 nm),” Ortiz Lizcano stressed. “A significant difference between the refractive index of the materials translates into lower layer counts to achieve high reflectance values on a wide wavelength range.”

The scientists benchmarked the optical filter with a control device based on titanium oxide (TiO2) and magnesium fluoride (MgF2), as well as with a solar cell without any filter. “Although TiO2 and MgF2 offer advantages from an optical perspective, depending on its deposition conditions, the use of MgF2 in multilayer systems can have reliability issues,” Ortiz Lizcano said. “Since our aim was practicality and simplicity, we selected two materials widely used in the PV industry that have proven reliable stability in multilayer stacks.”

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Through a computational model framework, the researchers conducted a series of simulations aimed at measuring the effect of the filter on the operational cell temperature. They assumed the filter to be applied on bifacial IBC solar cells from SunPower Maxeon, and the cells to operate in two different locations, Delft, in the Netherlands, and Singapore.

The analysis showed that the filter may reduce the temperature of the cell in Delft by 2.20 C and that of the cell in Singapore by 2.45 C. The optical losses for the cells were found to be 9.76% and 10.01%, respectively, and these losses were not found to be compensated by the cooling provided by the filter. “However, looking at long-term reliability, the thermal filter extended the lifetime of a PV module by 2 years in Singapore and 1 year in Delft,” the scientists stated. “With degradation effects accounted for, the extended lifetime provided by the thermal filter in Singapore yields an overall positive result in terms of DC energy yield in both scenarios, with relative gains of 3.12%.”

Ortiz Lizcano explained that the research work has been so far limited to modeling and experimental validation, including degradation mechanisms. “We have yet to assess the economic impact of the solution thoroughly,” he added. “Other researchers estimate that a filter consisting of eight layers for PV applications will have an increased cost of $1.7/m2, how this cost will increase with filters with higher layer counts is our next step to assess their suitability.”

The passive cooling tech was introduced in the study “Practical design of an optical filter for thermal management of photovoltaic modules,” which was recently published in Progress in Photovoltaics.

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