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Storing Daylight

This article was written by Jeremy Faludi in April 2007. We're republishing it here as part of our month-long editorial retrospective.

light.jpg Indulge me in a bit of science fiction for a moment, for a very simple product that could revolutionize all lighting. Today, if you wanted eco-friendly illumination, you would have solar panels generate power during the day to run your T8 fluorescent bulbs at night. But what if you could just store daylight itself and save it 'till later?

All lighting today ultimately uses power from the sun, whether it be direct solar power, wind power, or fossil fuels. An excellent fluorescent light (one giving 100 lumens per watt) converts electricity to light at about 15% efficiency . If it's powered by a solar panel that is 15% efficient (which is on the high end of affordable these days), you get a total efficiency of 2.25%. That's not even including the losses in the electrical system (converting from DC to AC, and storing in a battery or sending power to the grid and getting other power back). Terrible, right? That performance should be easy to beat.

Light in a Box

What if we could avoid the losses we get in converting sunlight to electricity and back into light? Can't we just store daylight in a box or an optical fiber and release it later? Unfortunately, no, not for hours on end. Every time light bounces off a surface you lose a bit of it, and since it travels at about one foot per nanosecond, it will bounce around a lot in any box you can reasonably fit on a continent. You can get 100% reflection in a prism, but then the material of the prism absorbs a little of the light as it passes through. Likewise with optical fiber, every foot of material that the light passes through absorbs some of the light, and again, the distance it would travel in several hours is big. (It only takes eight minutes for light to get to the Earth from the sun.)

You might think the new 'metamaterials' with a negative index of refraction could be the solution, but I doubt it; they are very lossy and dispersive. Likewise, photonic crystals are new and hip but unlikely to help. However, optical amplifiers have been around for decades, and are widely used in lasers and fiber optic networks. The ones that exist now only work on narrow bands of wavelengths (single colors) rather than a broad spectrum of visible light, and they require a fair amount of power, but perhaps these problems can be solved. Color should be easily dealt with: white-light LEDs have been made by combining red, green, and blue LED's; the same trick could make RGB optical amplifiers. As for power, significant gains need to be made, but if you can use less than you would in converting light to electricity and back (at 2% efficiency, remember), you're still ahead of the game. Even with perfect optical amplifiers, you still need your fiber loop or light box to have the least losses you can get--otherwise the light that gets absorbed will become massive amounts of heat that you need to deal with. But it could be used for cogeneration of heat for air and water.

You might point out that even if you did have a box holding a day's worth of light, you would need a system that could release it a tiny bit at a time instead of all at once as one blinding flash. This is fairly plausible, though--you just need a good switch that can turn on and off extremely fast, and the telecom industry have already been working on switches like this for years, as have optical computing researchers. Again, they require power, but with further development might be able to create an adequately efficient system.


There are rocks which will absorb light and re-emit it later: that's what phosphorescence is. How would you like to have the sun shining through windows onto your walls during the day, and have the walls keep shining through the night? Or maybe you'd rather install a window shade that cuts the brightness of the sun during the day and brightens your window at night? Yes, we're ultimately talking about glow-in-the-dark paint here, but not like the ones that exist today. Efficiency is already competitive--the best minerals re-emit almost 10% of the input light. The main problem is that currently, "long lifetimes" in phosphorescent materials are measured in seconds; we need ones that last hours. Then there's the question of color. Some people already don't install compact fluorescents because their light isn't "warm" enough; the best glow in the dark paint today provides colors that are Vincent-Price-like at best. Clearly a great deal of material science clearly needs to be done.

The semiconductor industry has done amazing things with doped silicon, getting it to hold high-energy electron states like you would need for long-lasting glow. The physicists among you might say "wait, storing light energy for that long is like a battery storing electricity for that long." But it's not. An electrical battery must both conduct charge into and out of itself easily, and store large amounts of charge without letting it leak out easily. A phosphorescent material uses incoming photons to kick electrons into higher energy states, but they stay in the atoms they started in. Granted, it has difficulties of its own, but there are many other marvels of material science today that we would not have thought possible even ten years ago.

So all you optical engineers, physicists and materials scientists out there, you've got some product development to do! It might be a wild goose chase, but it might be a breakthrough.

Things That Should Exist: Light Storage is part of our month long retrospective leading up to our anniversary on October 1. For the next four weeks, we'll celebrate five years of solutions-based, forward-thinking and innovative journalism by publishing the best of the Worldchanging archives.

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I love the Vincent Price reference!

Posted by: greensolutions on 17 Sep 08



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