Imagine a city where towering buildings covered in glass windows surreptitiously harvest energy from every corner of the non-visible light spectrum to provide occupants with both a completely clear view and clean energy.
Well, you aren’t the first to imagine such a future. Solar windows have fascinated scientists and engineers for almost half a century. The trick of solar windows is that they need to absorb non-visible light rays, like the UV spectrum, but leave the visible spectrum untouched. They then need to convert that non-visible light into a longer wavelength, trap it inside the window, and guide it to the edge, where a solar panel is ready and waiting, embedded in the window’s frame.
Creating a solar window without any optical distortions or color is one thing. But the real sticking point is creating one that is actually scalable and able to be mass-produced using normal industry manufacturing processes.
“Those are the difficulties but we’ve overcome them already,” ClearVue Technologies Executive Chairman Victor Rosenberg told pv magazine Australia.
ClearVue Technologies now has an industry-standard, 3.5 x 1.6-meter solar window that can harvest light with 3.3% efficiency. It delivers a minimum of 30 watts per square meter, while maintaining 70% transparency.
ClearVue Technologies aims to boost that efficiency by up to 5% over the next 18 months with the help of researchers at the ARC Centre of Excellence in Exciton Science. Three of the center’s researchers are currently developing nanoparticles to transfer non-visible light rays in ways the scientists described to pv magazine Australia as something of a “breakthrough.”
ClearVue Technologies is far from the only company in the world developing solar windows, but Rosenberg believes they are the only ones with a completely clear and industry-sized window on the market. Rosenberg said the ambition is to integrate solar windows into the construction industry, where they could be used in office blocks, skyscrapers, bus shelters, airports, and greenhouses. “Wherever you see glass,” Rosenberg said.
The company is a partner of the ARC Centre of Excellence in Exciton Science. “Our contribution will essentially be ‘souping up’ the window they are already developing,” researcher Nick Kirkwood told pv magazine Australia.
Nanoparticles, or “quantum dots,” have really been solar window’s missing step, preventing the vision of a clear solar window from becoming a cold, hard reality.
“Making very high quality nanoparticles is a relatively recent development, having the control over these synthetic methods to make them well,” Kirkwood said.
The job of nanoparticles is to absorb the light of the sun and then re-emit it inside the window at a different wavelength. The first part of that job, the absorption, is called the “quantum yield.” It refers to the percentage of photons the nanoparticles emit relative to how much they absorb – the higher the number, the more is emitted.
Alice Chen is leading Exiton’s nanoparticle research for ClearVue, building on the work of her colleague, Tim Warner. She has managed to get her nanoparticles to reach a quantum yield of 80%, but that number tends to drop down to around 60% when scaled.
“These are really chemical reactions. We cook up these nanoparticles in beakers in our lab and so we can vary the temperatures and the times and what we add in – and then scaling that up is an art form,” said Kirkwood.
The second job of the nanoparticles is to re-emit the sucked-in light at a different wave length, known as a “stokes shift.” Again, the bigger the stokes shift, the better. You don’t want one nanoparticle to spit out light only for its neighboring nanoparticle to suck it right back up. Rather, you want all the other nanoparticles to ignore it, letting it refract along unimpeded to the side of the window. Here Chen has had another win, achieving a “breakthrough” gap between the absorbed and emitted lightwaves.
The sun’s re-emitted light is then “wave guided” to the edges of the window. How this part works is really in the same principle as an optical fiber. As long as the angles are quite flat, it will keep bouncing.
But, of course, there’s a snag. The angle at which light is re-emitted is always random, so there’s a chance it will just go straight back out the way it came. Generally you lose about 25% of the light hitting a window.
“This is one of the challenges we have to work with,” Kirkwood said.
While nobody has yet figured out a way to engineer chaos out of the universe, researcher Tim Warner has been looking into the effects of changing the nanoparticles’ shape and how this effects the angle light re-emits. If you use a nanoparticle that’s shaped like a rod, rather than a sphere, you have more control over which way the light is going to go, Warner explained.
This little kernel of knowledge hasn’t been integrated into the work the center has been doing with ClearVue yet, because they haven’t managed to get their rod-shaped nanoparticles transparent. But it’s certainly possible, bringing paths to higher efficiency well into view.
“It’s just this process of communication and iterative improvements until it works,” Kirkwood said.
To continue reading, please visit our pv magazine Australia website.
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