Solar windows, also called photovoltaic or solar glass windows, function as windows while also generating electricity from the sun. Solar windows’ potential impact on world energy generation is enormous. They could be utilized in myriad applications, from greenhouse glazing, to replacing conventional windows in homes, to cladding entire skyscrapers. Solar windows are among the latest developments in building-integrated photovoltaics (BIPV) in which, rather than being applied over it, the photovoltaic element is incorporated into the building’s skin itself. In this article we’ll review what solar windows are, how they work, their advantages over conventional photovoltaics, and future prospects for this rapidly emerging technology.

What are Solar Glass Windows?
Solar windows look like regular glass windows but act as solar panels, generating electricity from the sun. Any glass surface – greenhouse glass, a window in homes and offices, and yes, even smartphones – thus has the potential to become an electricity generator. Researchers created the first transparent solar concentrator in 2014 and by 2020 researchers in the U.S. and Europe had achieved full transparency solar glass. Since solar windows can simply replace conventional windows, this technology holds the potential to turn any building in the United States, and the world, into a solar producer.

Solar Glass’ Advantage over Conventional Solar Panels
Solar glass windows have an advantage over conventional solar panels because they can be installed where windows typically go; on the side of buildings. This distinction is important because they can absorb the UV light or infrared light that bounces off surfaces and hits the windows from all directions. This means solar windows do not need to be angled towards the sun to catch the energy-giving rays. This will revolutionize solar energy as an entire building, from roof to windows, can be working daily to capture all of the light that hits it.

The key here is that the panels take up far less horizontal space than traditional solar panels. In cities with lots of buildings and limited horizontal space, setting up traditional solar panel installations is challenging. Solar windows, on the other hand, can be widely distributed, even in cramped cities. And the aesthetic benefits are obvious – you could turn nearly every surface of a building into a solar array and generate power right where you use it, without even noticing that it’s there.

Types of Solar Glass Windows
Just the way solar roof panels are currently being produced using differing technologies, solar windows are also being developed using different techniques. The two major types of transparent solar panels are partial (or semi-) transparent, and fully transparent units.

Semi-transparent Solar Windows
Semi-transparent solar windows collect near-infrared rays. To achieve this, the solar cells for semi-transparent windows have non-fullerene acceptors. Since non-fullerene IR acceptors have weak bonds, they’re easily destroyed by UV rays. For this reason, a buffer material is used to block them.

On hitting the solar cells, the IR rays are converted to an electric current. After that, a zinc oxide solar panel film is used to conduct the electrons to an electrode and later to an inverter of a solar PV system. From the inverter, the solar power can be stored in a battery bank or used to power devices in the building.

The efficiency of semi-transparent solar windows is in the 10% range. Researchers from the University of Michigan developed solar cells with 43.3% transparency and a record efficiency of 8.1%.

Fully Transparent Solar Windows
Although partially transparent solar windows are suitable substitutes for heat-reflecting tinted glass, they are not well suited for applications requiring clear glass. A fully transparent solar panel may not, by definition, absorb visible sunlight. However, researchers at Michigan State University used organic salts to absorb specific invisible wavelengths of light e.g. infrared and ultraviolet light, leaving light in visible spectrum free to pass through the medium. The light spectrum which is captured is then transformed and the material of the panel moves it to its edges, where stripes of photovoltaic solar cells convert it into electricity.

The efficiency of the fully transparent solar panels is currently about 1% with an estimated potential of 5%. Compared to the average efficiency of 12-15% for conventional solar panels, efficiencies of 5% are still quite low.

It’s vital to understand that when it comes to solar panel windows, efficiency of the panel is not the be all, end all. In practical terms, a less efficient solar window only means that the window has to be larger in size compared to the more efficient horizontal panel in order to generate the same amount of electricity. Once fully transparent solar panels get integrated into large windows in buildings, their lower efficiency is bound to be overcompensated by their potential scale of deployment.

Skyscrapers Could Become Solar Farms
Skyscrapers have massive amounts of glazing. The potential for these buildings to generate clean, renewable energy from the sun is enormous. For example, California-based solar window manufacturer Ubiquitous Energy has recently partnered with major window manufacturer Andersen Corporation with the goal of turning skyscrapers into “vertical solar farms” by the installation of solar windows. The Redwood City-based company expects to start manufacturing floor-to-ceiling, transparent solar windows at high volume in 2024.

Europe is Leading the Way in Solar Windows
As you might guess, solar windows are taking off throughout Europe, with solar glass currently in production in Italy, Germany, Belgium, Croatia, the United Kingdom, Romania, Lithuania, Switzerland, and Greece.

One company, Physee, based in the Netherlands, has rolled out a solar window hybrid called SmartSkin, described as a dynamic facade system combining photovoltaics and sensors with a self-learning building management system. Transparent windows fitted with sensors and solar cells in the frame generate electricity while also monitoring indoor and outdoor light, weather conditions, and air quality. An energy efficiency system interprets all data and sends control commands to different SmartSkin applications in order to optimize energy usage. This enables an annual 20% savings in the building’s total energy consumption. At present the company is in process of installing 15,000 units in office buildings throughout Europe.

The Future of Solar Windows
Considering the number of buildings already in existence or under construction, the potential of solar glass to generate renewable solar energy is enormous. There are six to eight billion square yards of windows in the United States alone, with another three billion square yards of glass installed around the world each year. With this much of glass surface available, transparent solar panel technology is estimated to have potential to meet about 40 percent of annual energy demand. This potential is nearly the same as that of rooftop solar. If energy storage is also improved in kind, and both of these technologies deployed in tandem, it is estimated that approximately 90% of electricity needs could be met.

Taken altogether, solar windows thus represent a leading edge in emergent solar technologies.

For Further Reading:
• The World Economic Forum’s article, “This technology turns windows into solar panels”, can be read at: https://www.weforum.org/agenda/2022/09/transparent-solar-panel-windows/
• “Solar window start-up aims to turn skyscrapers into vertical solar farms”, a news article presented by CNBC, is available at: https://www.cnbc.com/2022/01/12/ubiquitous-energy-aims-to-turn-skyscrapers-into-vertical-solar-farms.html
• DreamCivil.com has published their article, “Solar Windows: Features, Advantages, Disadvantages & Conclusion” at https://dreamcivil.com/solar-windows/