New multifunctional solar window generates energy by reflecting light

Higher energy efficiency buildings

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).

Solar Energy Scientist at TNO: Roland Valckenborg: ‘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, called the ZIEZO (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.

It offers three operational modes:

1. The blinds are stored in the top box (no boost)
2. The blinds are down and in horizontal position (partial boost)
3. The blinds are down and 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.

Operating principle

25% percent energy boost

Next to TNO the research group consists of the TU/e, glass company Pilkington Nederland and W/E adviseurs. For the past year they tested 12 small versions of this window at the outdoor facility SolarBEAT in Eindhoven and installed 6 full-size windows at Pilkington’s office in Enschede.

On sunny days, it increases energy by up to 25% with the most reflective blind, compared to a window with only one sided solar cells without blinds On average, over the whole year, the energy boost is about 13%. The results of the system were recently published in a paper for the solar energy journal SolarRRL.

Next steps

In the near future the group will further study the impact of the active shading device on the visual comfort and thermal control inside the 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 (weekdays vs. weekends), 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.