A new solar window designed by researchers in the Netherlands could revolutionize building architecture, offering a sleeker alternative to traditional rooftop solar panels and a way to offset growing energy demands by harnessing the energy in sunlight.

The window, which combines solar cells with reflective blinds, has the potential to replace standard glass windows in high-rise buildings, turning entire facades into energy-generating surfaces that could potentially be used to create energy-neutral buildings worldwide.

How the photovoltaic windows work

The Eindhoven-based researchers described the windows as a “next-generation advancement” on commercialized windows, which feature either blinds or solar cells, but not both. Their multifunctional windows not only provide shade from the sun, keeping buildings cool in warmer months, but also generate their own energy, helping to offset a building’s electricity costs.  

“To meet the stringent requirements for energy-neutral buildings, windows must turn into energy-generating elements,” stated Simona Villa, a solar applications scientist at TNO Energy and Materials Transition in Eindhoven.

The semitransparent windows are double glazed, consisting of two glass panes separated by a gap filled with a mixture of air and argon gas. Strips of silicon solar cells are laminated on the outer glass pane, facing the sky.

Importantly, the cells are bifacial, which means they can generate electricity when sunlight hits them from either side. Missed sunlight is reflected off the slats of the blinds embedded in the gap between the two glass panes and back toward the cells. 

“When the venetian blinds are deployed, besides providing shading to the interior of the room, they act as reflectors for the bifacial solar cells, redirecting part of the sunlight into the rear side of the cells and boosting energy generation,” explained Villa.

Blinds provide big energy boost

Over a nine-month period, the researchers measured the energy-boosting performance of 12 small-scale windows installed at the solar energy research facility SolarBEAT in Eindhoven.

On sunny days, when the slats were completely closed with the convex side facing outwards, they found that the energy generated when the blinds were deployed was 25% more than without blinds, with a 12% daily average.

“The electricity generated by the window is substantial enough to be directly injected into the grid via a connection to an inverter,” said Villa. “This allows the windows to actively offset the building’s energy consumption, contributing to its overall energy efficiency and sustainability goals.”

Aside from the position of the sun in the sky and weather conditions, the blind type and configuration have the biggest influence on the amount of energy produced. The amount of light the blinds reflect–– and thus the amount of electricity produced––depends not only on their tilt angle, which can be adjusted by the user, but their optical properties such as color and coating composition.

Villa and her colleagues tested three different commercial venetian blinds to incorporate in their design, with one model clearly outperforming the others. “The V95 blinds are superior because of their optical properties,” said Villa. They have a coating that improves the reflection of the aluminum-based slats, which are slightly curved and overlap when the blinds are fully closed.  

Current limitations and next steps

Photovoltaic windows are still on the expensive side and overall less efficient at generating electricity than solar panels. But standard solar panels could still be used alongside the windows.

“Because standard photovoltaic modules are ‘opaque’, they could be used for the opaque parts of the facade, but not to replace windows, which need to ensure a high degree of transparency,” said Villa.  

“The cost is still a challenge, but the company Pilkington is currently working on lowering the production costs of this window in order to be able to introduce it to the market,” Villa added. The researchers enlisted the window manufacturer’s help to assemble the prototype windows.

The main window components—the blinds and bifacial crystalline silicon solar cells—are already commercially available. But Villa says certain processes, such as laminating the silicon cell strips onto the windows, still need to be automated to streamline the manufacturing process and enhance scalability.

The researchers also mentioned that the measurement period did not include the peak summer months, which are naturally expected to be the most sunny. The windows are still being monitored, said Villa. “The focus is on quality, scalability, standardization, and plug and play.”

Reference: Simona Villa et al. Outdoor Performance Analysis of Semitransparent Photovoltaic Windows with Bifacial Cells and Integrated Blinds. Solar RRL (2024). DOI: 10.1002/solr.202400515