Photo courtesy of Visual China

Heating and cooling buildings is a huge source of carbon emissions. So can the humble window become the next outlet for green technology?

Generating electricity with glass

At the end of 2019, employees of Ubiquitous Energy in Redwood City, California, gathered in a conference room made entirely of glass windows. The new glass windows in front of them not only presented the spectacular mountain scenery and beautiful sky in northern California, but also doubled as photovoltaic cells to power the company's lighting, computers and air conditioning.

After years of research and development, the company has achieved a technological breakthrough - power-generating glass. The secret lies in the addition of multiple organic polymer layers between the glass sheets, some of which are completely transparent, while others absorb invisible infrared and ultraviolet rays. When light passes through the glass, the electrons between the polymer layers form an electric current, which is collected by thin wires in the glass.

¡°It¡¯s a bit like a transparent computer monitor running in reverse,¡± says Veeral Hardev, the company¡¯s director of business development. In other words, whereas a monitor uses electricity to light up individual pixels on its screen, the glass generates electricity at different locations as light passes through it.

Currently, the glass generates about one-third as much electricity as conventional rooftop photovoltaics for a given light level, and transmits about half as much light as regular glass.

Hardev says those numbers are enough to make the glass a practical product, and his company hopes to significantly increase its transmittance. As for the lower efficiency, he points out that windows cover a larger area than roofs, making up for the lack of efficiency with area. ¡°You can use both, but windows will generate more electricity,¡± he says, adding that the biggest challenge is to expand the area of windows, which are currently less than 0.19 square meters, to about 4.65 square meters.

Clearer than glass

The window revolution is long overdue. As cities around the world rediscover their love of skyscrapers, gleaming towers have become fixtures, but the glass on them has barely advanced.

Controlling the temperature of buildings is a huge challenge, with 18% of U.S. energy dollars going to heating and cooling buildings. Lawrence Berkeley National Laboratory estimates that about $20 billion worth of heat is lost through windows in cold weather, while even more is lost through windows in air-conditioned buildings in summer. In short, more than half of the money spent on heating and cooling buildings is wasted because it leaks through windows.

Mackinac Technology, based in Michigan, is developing a coated plastic sheet that can be placed on the surface of ordinary glass, improving the glass's insulation and heat reflection properties without affecting the clarity of the view. The plastic sheet traps air in the middle, improving the glass's insulation. The coating lets visible light through but reflects infrared light (which carries most of the heat). John Slagter, the company's CEO, said the invisible coating reduces the light reflected from the plastic surface, which in turn increases the light transmittance of the window and the clarity of the interior.

By installing the coated plastic sheet directly on the existing window frame, the thermal insulation performance of single-layer or double-layer glass can be doubled. At the same time, it is very light and will not significantly increase the weight of the window. Slagter said that this new material has successfully passed the test on the window of Calvin University in the United States. Thanks to partial funding from US government agencies, this plastic sheet will be promoted and tested in a larger pilot project before it is officially put into use in 2022.

The battle of light and dark

But sometimes, high clarity is not an advantage, especially for south-facing buildings, where strong sunlight shines into the room through the windows. Michael McGehee, a materials science researcher at the University of Colorado Boulder, said: "Although strong light can increase the temperature of a room, people don't like to work in a sunny environment because they may not be able to see the computer screen clearly. Usually they choose to draw the curtains, but this will lose the scenery outside the window and cannot enjoy the other benefits of sunlight."

To alleviate the glare of sunlight, McGehee's team has long been committed to improving "electrochromic" windows (using an external electric field to make the material undergo a stable and reversible color change), controlling the brightness of the window through switches, filtering out severe glare, and adjusting the amount of light entering the room to a comfortable state. Based on this, the window designed by the team includes an indium tin oxide layer containing platinum and a nickel oxide layer, and a lithium solution is filled between the two layers. When a low voltage is applied to the two-layer structure, they act as electrodes to generate an electric field, and the lithium ions in the solution migrate and adhere to the nickel oxide layer.

Although lithium is transparent in solution, it becomes translucent when covered with a nickel oxide layer. "You only need to cover the electrode with a 10-nanometer-thick layer of lithium to block most of the light," McGehee said. He added that such windows are like "sunglasses" for buildings. In addition, the degree of sunlight blocking can be adjusted in stages by changing the voltage.

If these results are successfully commercialized, windows will become an important part of environmentally friendly cities in the next 10-20 years. This will be a solid step towards zero carbon footprint for mankind.

Translation: Chao Xujia

Source: Scientific American

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