Thursday, September 15, 2011

Solar Bottle SuperHero



Solar Demi is a man who travels around installing solar bottles in order to bring light into homes in some of the poorest neighborhoods in the Philippines.



It's amazing to me that there are places where even minimal lighting is a luxury. I'm glad to see that basic physics principles are bringing a little sunlight into the dark days of some impoverished Filipinos. (Of course, if you feel like sending a donation to the cause, you can help spread the light and the love a little further.)

I've seen a lot of speculation about how, and even whether, solar bottles can light a room so well. The fact is, they do work, and they rely on some pretty nifty physics to do it.

To begin with, cutting a hole in your roof will let some sunlight in. Even a small hole in a darkened room can make a big difference. There are, however, some problems with a simple hole.

Besides letting in the rain and insects, you only get as much light as falls directly through the hole. As Solar Demi knows, you can do much better than that with some grassroots engineering. Putting a water-filled container in the hole collects more light due to two fundamental optical effects: Snell's Law and total internal reflection.


When light moving through the air runs into a denser material like water, it changes direction. That is, the light beam kinks where it encounters the water surface so that it's traveling more directly down into water. That's very helpful if you're trying to get more light to go down through the hole.

Despite the help that Snell provides, some of the light will still be on a path to the opposite side of the cylinder. A portion of the light is trapped in the cylinder because of simple reflection, which can happen anytime light passes from one transparent material to another. However, during much of the day, the light will strike the cylinder walls in such a way that lots of light will be reflected down toward the hole. This is called total internal reflection. Light reflected this way will bounce back and forth as it travels down through the water column, much like signals passing through an optical cable.
A laser beam trapped in a plastic illustrates the effect well.

So, without a solar bottle, this is all the sun light you can expect to collect.

After Solar Demi gets through with your roof, this is how much light you'll gather.

Once the light is in the room, the bottle offers another major benefit. If you only had a hole, or even a small glass window, then sunlight passes straight through, and illuminates a spot on the floor or wall, like this . . .


Because of multiple reflections and different paths caused by Snell's Law refraction and internal reflection, light spreads out as it passes through the water column. That's what made the bottles appear to glow in the video of Solar Demi doing his stuff.


In the first of these two sketches, sunlight passes directly through hole. In the second sketch, I've added a few of the paths the light can follow as a result of the water column. There are actually an enormous number of possible paths the light can take, which results in an apparent overall glow, much like a light bulb.

All in all, it's a simple, clever, cheap, and environmentally sound lighting solution. In fact, I'm so impressed that I think I'll go donate right now.

33 comments:

  1. what is the use of the bleach?

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  2. bleach keeps any algae from growing within

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  3. Can the design be improved to increase luminance. Ex wider diameter container or longer length bottle or glass container etc.

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  4. I'm sure the design could be improved. Adjusting the parameters you mention would likely change how well the system works in fairly significant ways. I also have to wonder whether it makes more sense to have the bottle cap at the top or at the bottom, and whether the bottle should be filled completely, or perhaps only part way. I'd love to see someone do a few experiments testing these things and anything else they can think of to improve the performance of the solar bottles.

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  5. thanks for explaining the physics behind it! (: was wondering how it works (:

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  6. Some little think are so bright !

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  7. simple bright light for a brighter simple life

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  8. does it mean that the solar bottles do not work at night?

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  9. Perhaps if the moon is very bright, or there's ambient light around from streetlights and signs, you might get a little light out of a solar bottle. But they are actually designed to gather daylight.

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  10. I think 'cap on top' is here to stay because even the tiniest leak would empty out your bottle eventually and make it bear to refill. If a 'cap on top' system leaks, you can put on a tiny bit sealant and then refill from the top without drilling even MORE holes in the bottle.

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  11. You're probably right. I'd still like to know how much the cap affects the brightness. Maybe a soda manufacturer could make bottles with transparent tops to improve solar bottle performance. It might make a nice PR move, as well as helping people out.

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  12. thanks for explaining it. it helped

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  13. will the solar bulb cannot do the multiple reflections as the sunlight is vertical go into the bottle?

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  14. For this system, total internal reflection (TIR) will only occur within the PET bottles' walls. TIR happens when light strikes the boundary from a higher refractive index material to a lower one at an angle greater than the interface's critical angle. In this case, the PET plastic bottle has the highest refractive index (n=1.575); both water (n=1.33) and air (n=1) have lower indices of refraction. Total internal reflection can therefore only occur within the walls of the PET plastic bottle, but not within the water column.

    Has anyone calculated the entrance cone of light that will be directed to the interior via the PET bottle? Thanks.

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  15. Can this system work with glass bottle instead of plastic? If an aluminum foil is placed in the north side of the installed bottle would it be more efficient?

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  16. Someone sent me this link to a clip that suggests the whole thing is a scam. Does anyone have an opinion on this matter?

    http://www.youtube.com/watch?v=C_Du8ItvCOA&feature=watch_response

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  17. The clip looks convincing enough, but I tend to trust that MIT students know their stuff.

    http://www.youtube.com/watch?v=gYPBr0kutk4

    Even if it's no better than a hole, at least it keeps out more rain than a hole would. Also, where is someone in a slum supposed to get a piece of transparent difusser?

    In any case, we're going to get a light meter and try the experiment for ourselves, right here at the American center for Physics.

    Experiment time!!!

    Stay tuned . . .

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  18. this is something to do with the amount of bleach, mang demi used 1 liter of water and 3 tablespoon, does the amount of water and bleach affect to the brightness of light produced? and if i have a small dog house about 40 cm width and 60 cm length, is it ok if i will just use 500 ml,how much bleach will i use?

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  19. Can this be improved to help in Urban city Where like in apartments.
    Like, Fitting this light receiver at terrace which collects and transfer light through optic fiber to the respective floor.

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  20. The light which is given out is too less how can we improve the quantity of light.....It does not lighten the room its very dim..

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  21. try clear flakes or reflectors inside the bottle glue them or whatever also maybe quartz or tin of some kind maybe it might bounce the light in more directions or use Lk gell it should be the same concept i think idk just a thought and i say this because any clear liquid or bottle you hold up to the light it does it u can do different colors for decorative purposes by using food coloring Lk i did this at my grandfathers house his tool room is dark even in mid day so i put these in and now we have light everywhere now

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  22. No. It cannot work at night. It transfers daylight from above the roof to the room below.

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  23. Its a great idea and am very impressed. But if it only works at night whats the point ? Maybe to light up your basement or a cave. Maybe they used this idea in Egypt ?

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  24. why are we using only distilled water?

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  25. It's for people with no windows. I thought that much was obvious.

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  26. Is there a good way to use these in colder climates? I intend to use them to add some light to an off grid basement, yet the fear of freezing is a concern. Would adding salt negate the reflective properties? would these also assist with hydroponic growing? An underground grow room would allow a much earlier start for my tomatoes. Do green bottles (Mountain Dew) work as well?

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  27. can we use some other liquid instead of water which will give more efficient results ?

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  28. Distilled water and bleach are used to retard the growth of algae. There is no additional refraction with their use.

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  29. does these stuff work at night because i am doing this project, if i show my teacher she will ask me questions then she will turn off the light

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