Multifunctional solar windows produce energy by reflecting light

14 Nov 2024

TNO-led “ZIEZO” project creates double sided solar cells and Venetian blinds to raise electricity yield.

A new type of window had been developed and tested that produces electricity and simultaneously controls heat and light for indoor comfort. This has been achieved by researchers from the “ZIEZO” project, led by Dutch research laboratory TNO.

This window, described as “the first of its kind”, has special double-sided solar cells and built-in Venetian blinds that reflect sunlight. This arrangement directs more light to the cells’ rear side, increasing the energy yield whilst protecting against overheating.

TNO’s announcement states, “To achieve climate neutrality by 2050, buildings in the Netherlands need to become much more sustainable. Stricter energy performance standards for buildings are boosting the market for building-integrated solar, or photovoltaic, technologies (BIPV).”

Roland Valckenborg, a Solar Energy Scientist, at TNO, commented, “By effectively using sunlight and solar heat, this window can have a major impact on the energy efficiency of buildings. For a general medium sized office building these windows can reduce the CO₂ footprint of that building by at least 20%. It’s also commercially interesting because as far as we know this concept is not available on the market yet.”

Three modes

The new solar window, named ZIEZO (which stands for “Sunshade Including Electricity-Generating Solar Window”) window, consists of double-sided crystalline silicon solar cells strips and embedded Venetian blinds. When the blinds are down, they reflect sunlight onto the back of the solar cells, making more energy.

The new window arrangement offers three operational modes:

• Blinds stored in top box (no boost)
• Blinds down in horizontal position (partial boost)
• Blinds down in vertical position, blocking all sunlight (max boost)

Users can automatically adjust the blinds to focus on either generating more energy or controlling the room’s temperature and light – depending on season or local weather conditions.

Besides TNO, the research group consists of the TU/e (Eindhoven), glass company Pilkington Nederland and W/E Adviseurs. For the past year the group has tested 12 small versions of this window at the outdoor facility SolarBEAT in Eindhoven and installed six full-size windows at Pilkington’s office in Enschede.

On sunny days, the new design increases energy by up to 25% with the most reflective blind, compared to a window with only single-sided solar cells without blinds On average, over the whole year, the energy boost is about 13%, says TNO. The results of the system were recently published in Solar RRL journal.

In the near future the group will further study the impact of the active shading device on the visual comfort and thermal control inside a building. The project partners are also considering a follow-up project in a European context.

In addition, a new parameter tool is being developed to enhance building design by optimizing solar energy generation, indoor climate, and CO₂ savings through reduced heating and cooling needs. This is challenging due to changing sun positions and varying office occupancy patterns, along with individual preferences for light and temperature behind the ZIEZO window. Pilkington is also taking steps try to bring the product to the market.