In a recent article published in the journal PhotoniX, researchers from Nanjing University developed a new type of transparent and unidirectional solar concentrator.
Image credit: Pablo Rogat/Shutterstock.com
The study involved coating the architectural glass with specially engineered cholesteric liquid crystal (CLC) layers. These layers selectively guide sunlight towards the edges of the windows, where small photovoltaic (PV) cells convert this energy into electricity.
The experimental results showed that the device can power a 10-mW fan outdoors as a proof-of-concept. Modeling suggests that using it on a large scale could significantly reduce carbon emissions and potentially contribute to a terawatt-scale renewable energy supply. It is a promising, sustainable technological innovation in solar concentrators that keeps the look and clarity of the windows intact.
Introduction
The rapid growth of urban areas and population has led to increased high-rise buildings and demand for sustainable solutions to mitigate carbon emissions. Conventional power generation from fossil fuels, thermal plants, or nuclear plants poses environmental risks.
In contrast, power generation from renewable sources has its own drawbacks. Building-integrated photovoltaics (BIPV), such as solar energy-powered windows, offer an alternative to the existing solar energy harvesting technologies. They do this by embedding a photovoltaic system directly onto the architectural glass.
Among the existing BIPVs, luminescent-based solar concentrators use dyes or quantum dots to capture and re-emit sunlight. In contrast, scattering-based concentrators work by using a scattering medium.
Despite the positive results, these technologies face several challenges, including reduced efficiency from omnidirectional propagation, haziness, tinting, and complex changes that lessen compatibility with current architectural glass. These shortcomings have limited their adoption and remain a long-standing challenge for a solar technology that harvests energy without altering the aesthetics or transparency of modern glass windows.
The proposed CLC-based solar concentrators address these gaps by offering high performance and transparency benefits.
Current Study
The researchers developed a CLC-based solar concentrator that addresses the drawbacks of the existing BIPVs in terms of aesthetics and energy efficiency.
CLCs are one-dimensional chiral liquid crystals that selectively bend circularly polarized light within a specific wavelength range. The team stacked multiple CLC layers with varying helical pitches to broaden the reflection range and cover the visible spectrum.
One of their main innovations was submicron lateral periodic alignment through circular polarization holography. This was vital in ensuring that incoming visible light diffracted in a single direction and transmitted towards the glass edges via total internal reflection.
Samples were fabricated by sequentially coating CLC mixtures containing chiral dopants and photoalignment agent onto commercial architectural glass. Each CLC layer was polymerized under ultraviolet (UV) light for structure stabilization. The final stacked CLC films exhibited an average high visible transmission (64.2%) and almost neutral color rendering index (91.3), meeting the transparent consistency with architectural standards.
Silicon PV cells were mounted on the edges with masking tape to collect the concentrated light and avoid non-guided interferences. They fabricated small to large window-like panels as prototypes to demonstrate scalable deployment and performance.
Results and Discussion
Experimental tests and outdoor performance of CLC-based solar concentrators exhibited high efficiency and durability, and potential applicability as energy-harvesting windows.
From the prototype trial, a one-inch CLC-coated window panel successfully powered a 10-mW fan under direct sunlight, highlighting the ability to generate energy from a very small surface area.
The prototype demonstrated excellent visual transparency and optical efficiency of 18.1% and a power conversion efficiency (PCE) of 3.7%, confirming clear and natural views without visual distortions. The simulations for the standard two-meter-wide window panel showed a capacity of concentrating solar energy up to 50 times higher towards the edges.
This edge guide approach of CLC solar concentrators reduces the required photovoltaic material by about 75%, substantially reducing the material cost while maintaining aesthetics and high performance. Durability tests revealed that over continuous operation for 1500 hours, about 95% efficiency retention was achieved with inherent UV protection provided by the glass windows, thus further extending the lifetime.
The researchers note that future challenges include preventing UV-induced polymer aging and addressing polarization leakage in large-area devices. By focusing on transparency, efficiency, durability, and saving materials, this approach offers a valuable way to create energy-generating windows. These windows can make a real difference in sustainable urban energy systems.
Conclusion
Using the special properties of CLCs, the concentrator achieved high efficiency, clarity, and wide-angle light guidance while maintaining a neutral appearance. This overcomes the drawbacks of earlier solar window technologies.
These concentrators propose a construction-friendly solution that reduces installation and material costs by minimizing the photovoltaic material required and preserving the windows' transparency.
With further advancements in material design and roll-to-roll photopatterning for large-area fabrication, these concentrators could also be used in greenhouses and solar displays. The authors emphasize that scaling up will require optimizing layer design and fabrication methods, but this could change the way transparent surfaces help create sustainable, energy-efficient cities.
Download your PDF copy now!
Reference
Zhang, D., Guo, Z., Xu, C. T., et al. (2025). Colorless and unidirectional diffractive-type solar concentrators compatible with existing windows. PhotoniX, 6(1), 1–12. DOI: 10.1186/s43074-025-00178-3. https://photonix.springeropen.com/articles/10.1186/s43074-025-00178-3
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.