Windows of the Future: Glass That Produces Solar Energy

Windows will heat us, perhaps very soon. Innovative transparent coatings, the latest achievement in renewable energy technology (RET), will turn glass into a solar panel without losing its original function. A new player in this rapidly developing market comes from China. Scientists from Nanjing University have created a new transparent coating, a colorless and one-way diffractive solar concentrator (CUSC) that can be applied to any standard window and convert it into an efficient solar panel. Chinese scientists have already developed a small prototype that, if installed on all windows in the world, would create terawatts (a unit of power ten times greater than a watt) of green energy.

What is special

Solar concentrators are designed to direct solar energy to the side of architectural glass. The one-way diffractive solar concentrator directs a portion of the photons of sunlight onto the window glass, where mounted photovoltaic cells then convert them into electrical energy, while other light passes through. Existing solar concentrators in photovoltaic systems integrated into buildings have drawbacks, such as being less efficient due to multi-directional waveguides, not being of sufficient quality as they change color or become cloudy, and not being compatible with existing architectural glass.

The newly developed diffractive solar concentrator (CUSC) uses multilayer cholesteric liquid crystals (CLC) to selectively direct sunlight to the edge of the window where photovoltaic cells are embedded. The CLC film can be perfectly integrated into architectural glass and significantly reduce the number of required photovoltaic cells, making it practical and cost-effective in the construction of integrated photovoltaic systems while respecting aesthetic and economic parameters.

– The design of the CUSC is a step forward in integrating solar technology into the environment without sacrificing aesthetics. It is a practical and scalable strategy for carbon reduction and energy self-sufficiency. The CLC layers with lateral periodic alignment in the submicron range allow for broadband and one-way wave propagation within the glass. This is a highly efficient platform for capturing solar energy, whose advantages are high aesthetics and economic viability. A prototype with a diameter of 2.54 centimeters powers a 10-megawatt outdoor fan. A standard two-meter-wide window can concentrate solar energy fifty times. This design is expected to enable global green energy supply at the terawatt level and an annual reduction in carbon emissions by one billion tons, contributing to sustainable development, said optical engineer Wei Hu, professor and researcher at Nanjing University.

Optimizing Natural Resources

This invention, for which there is already excellent infrastructure in urban environments – a large number of glazed skyscrapers across world cities – could significantly contribute to energy savings. With the rapid growth of the population and fast urbanization in cities, more and more skyscrapers and glazed high-rise buildings are emerging. The increasing population and accelerated economic development require greater energy consumption, and green policies from the strategy eliminate thermal and nuclear power plants as they pollute the environment; regarding them, there is also a risk of nuclear waste radiation. Therefore, renewable energy sources are favored, but they are not without drawbacks.

Namely, renewable photovoltaic systems, wind farms, and hydroelectric plants occupy large areas and must be located near suitable sources.

In addition, centralized energy supply increases losses during transmission. This is why newly discovered photovoltaic systems that include capturing solar energy in building facades have attracted intense attention to zero-energy buildings. Modern architecture is characterized, among other things, by large glass windows, which allow for better living and working conditions. Large glass surfaces on skyscrapers and other high-rise buildings already serve to optimize natural lighting and heating, and the installation of photovoltaic technologies on architectural glass would enable eco-friendly buildings and sustainability.

Overcoming Challenges

However, existing photovoltaic techniques – amorphous silicon cells, organic photovoltaic systems, gallium arsenide, dye-sensitized solar cells, and perovskite (a mineral with a special crystal structure) – hinder the replacement of architectural glass due to opacity and fragility. To address these issues, solar concentrators have been developed for lateral concentration of solar energy and its capture by photovoltaic cells attached to the side of architectural glass.

Fluorescent materials such as organic dyes, polymers, quantum dots, perovskites, or carbon quantum dots are embedded in the glass to form a waveguide for light, and materials for its scattering, part of the incident light that is scattered into the waveguide and collected by thin photovoltaic cells, are attached to the edges of the glass.

Despite improved adaptability on existing windows, such solar concentrators still face several critical challenges: the waveguide can only collect light in the direction of propagation that exceeds the critical angle for total internal reflection (TIR), but omnidirectional fluorescent illumination and light scattering limit the efficiency of solar concentration. Furthermore, limited absorption and fluorescent bands relative to the solar spectrum reduce efficiency and induce colored transparency, causing the glass to fail to meet aesthetic criteria as it appears visibly cloudy, which hinders applications requiring an unobstructed view, and embedded functional layers cannot be added to existing windows.

Additionally, photovoltaic cells are required on all edges of the architectural glass, making them complicated and unprofitable to produce, thus new techniques need to be developed to overcome these barriers.

So far the best

According to Hu, the newly developed coating surpasses previous options in transparency, adaptability, and efficiency. When applied to a window, it transmits 64.2 percent of visible light and maintains 91.3 percent color accuracy, while functioning by directing part of the sunlight towards the solar cells. The material is made from cholesteric liquid crystals (CLC) that allow the necessary interaction with light as it passes through.

By layering different CLC layers, the coating can cover the entire light spectrum. It is crucial that only one polarization of light, one of the multiple ways in which light travels as a wave, is ‘captured’ and diverted to be converted into energy. This allows the window to retain its original function.

– By engineering the structure of cholesteric liquid crystal films, we create a system that selectively diffracts circularly polarized light, directing it into the glass waveguide at steep angles, said optical engineer and co-author of the research Dewei Zhang.

In tests applying green laser light, the color to which human eyes are most sensitive, 38.1 percent of the available energy was captured and converted, which is currently the highest possible for this technology. Although the overall efficiency of tests applying more realistic conditions and the full light spectrum was only 18.1 percent, scientists have already developed a prototype of two and a half centimeters using a coating that collects enough energy to power a small fan, so scaling it up to full window size could produce a large amount of electricity.

Since the coating can be applied to ordinary windows with minimal modifications, Chinese scientists expect their discovery to be commercially viable, but further work is still needed. They claim it can be refined to improve the stability and production of the coating, and the efficiency of energy conversion – the total amount of incoming solar energy converted into usable electrical energy – needs to be increased as well. Currently, this efficiency is low, at only 3.7 percent.

Necessary Improvements

Scientists assert that improving materials and processes is necessary to increase production. They intend to work on enhancing the efficiency of broadband access, polarization control, and adapting this technology for agricultural greenhouses and transparent solar screens.

In this regard, it is most important to optimize the design for a wide-angle, one-way, and colorless transparent solar concentrator and ensure perfect coordination with the application of high-efficiency solar light capture that meets both aesthetic standards and economic needs.