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Author Topic: Can a water-filled transparent sphere act as a solar collector?  (Read 12019 times)

Offline cpu42

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Would a transparent sphere, completely filled with a refracting liquid such as water, operate effectively as an omnidirectional solar heat/light collector, and would it be more effective in both cases when the bottom half is silvered as a mirror? Thank you!



[mod edit - subject altered to turn this into a question - please endeavour to do this in future to help us keep the forum organised, thanks. CS]
« Last Edit: 07/06/2008 14:08:58 by chris »


 

lyner

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I see what you are after, cpu42. It would be a very handy thing to stick on your roof.
You want it it to focus light onto a small volume in the middle, whatever direction it came from.
However, there is a problem. The refractive index of glass / water is not high enough to focus the light inside the sphere. You would need a substance with a refractive index probably twice that of water to focus light into a region inside the sphere. To get it to focus light from all directions near to the middle, you would need an incredibly high refractive index. Such materials don't really exist - even diamond has a refractive index of only 2.4
A curved mirror is lighter and cheaper and has a shorter focal length compared with its radius so it is easy to get a focus 'in close' so it is a better solution for the problem - except that you have to point it at / near to the Sun all the time (servos needed).
You don't need to focus the light in order to get solar heating, though. You just need the filling to be black and to absorb the energy. (With insulation all around, of course).


There is an equivalent problem with Wind Turbines. The conventional type needs a vane to keep it pointing into the wind but the 'vertical axis' type works from whichever direction the wind blows.
 I don't think they are as efficient tho.
 

Offline cpu42

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I had a suspicion that might be the case (re: the refractive index) and was wondering if a large, plastic Fresnel lens, heat-curved and applied to the top surface of the sphere, might overcome this? Optical quality is clearly of no consequence here! I'm assuming the mirrored base would increase the effective focal length by around 50%.

Thanks for your response - much appreciated.
 

Offline cpu42

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I ought to add that my second choice of a refractive medium would be something akin to isinglass.
 

Offline RD

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I've heard this story before about sunlight being focused be spherical fishbowls starting fires, it may be an urban myth...

Quote
The most unusual object I have come across that was thought to be responsible for causing a fire in this way was was a goldfish bowl filled with water and goldfish.
http://www.fire.org.uk/punbb/upload/viewtopic.php?id=101

From this example of a sunshine recorder, the focal point is at a distance less than the radius of the sphere from the surface of the sphere...
http://en.wikipedia.org/wiki/Image:Campbell-Stokes_recorder.jpg


« Last Edit: 07/06/2008 20:08:22 by RD »
 

Offline cpu42

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Interesting. While efficiency is always a goal, sufficient accumulation of heat to drive an LTD Stirling engine, employing cheap, DIY materials, would be a wonderful thing. That's (basically) what I'm after.
 

Offline RD

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Interesting. While efficiency is always a goal, sufficient accumulation of heat to drive an LTD Stirling engine, employing cheap, DIY materials, would be a wonderful thing. That's (basically) what I'm after.

I have seen old central heating radiators reused in reverse as solar panels.
lightweight radiators made from pressed sheet steel, salvaged from a junk yard, painted in waterproof black paint,
were put on the roof of a garden shed, with the warmed water used to increase the temperature of a nearby greenhouse.
[the electric pump required to circulate the warmed water was also solar powered]

Before building a solar collector you should do the energy calculations first.
e.g. how much solar energy is there at your latitude ?.
also bear in mind that your collector will be far from 100% efficient.
 

Offline cpu42

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I live in North Wales, in the UK. I'm not looking for energy-yielding solutions specifically for my own domestic use, though. I'd like to think that there are solutions, like solar cookers, available for all. With so much "free" energy available, even low-efficiency yields can make a difference (IMO) especially in the current "Energy Crunch" situaton.
 

Offline RD

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Wales is included on this solar energy graph (in January = worst case scenario)...
http://www.ecn.ac.uk/Education/warm__solar_radiation.htm
It peaks at about 100 Watts per square meter, but only averages about 10-20Wm2 over a 24Hr period, in January.

If all the energy from a couple of DIY solar panels (as described above) could be extracted (i.e. 100% efficiency)
they could continuously power a single 20Watt lightbulb.

In Wales I think wind power would be a greater source of renewable energy.
« Last Edit: 07/06/2008 21:27:29 by RD »
 

Offline cpu42

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It's going slightly off-topic, here. While I appreciate that Wales isn't that overly-blessed with sunlight, the thread remains how to devise an omnidirectional solar collector.
 

Offline RD

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You don't need to focus the light in order to get solar heating, though. You just need the filling to be black and to absorb the energy.

So a black sphere would function as an "omnidirectional solar collector".

If you have enormous patience you could wrap black plastic tubing in a tight spiral around a soccer ball, then pass water through the tubing to transfer the heat. Oh, from a soccer-ball sized collector you'd be lucky to get a couple of Watts in the UK, maybe 10 Watts in Africa.

http://simple.wikipedia.org/wiki/Image:Insolation.png
« Last Edit: 07/06/2008 23:21:30 by RD »
 

lyner

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Of course. It would present the same 'collecting area' from all directions.

In practical terms, a flat collector would be a lot cheaper and would not miss out on a lot of the available energy, which is at its maximum when the Sun is 'highest'.
A >2m sphere would look wierd on every roof of every house.
 

Offline Madidus_Scientia

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Did you know you can make a fire with a condom? fill it with water to make a sort of balloon size sphere then use it to focus the sun like a magnifying glass.
 

Offline Bored chemist

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"I've heard this story before about sunlight being focused be spherical fishbowls starting fires, it may be an urban myth..."
I don't know about fish bowls but my dad used a round bottomed flask full of water as a cigarette lighter. (He was in Africa at the time so the sun was probably stronger).
I think the idea of using a sphere as a lens to concentrate light on a collector in the middle is an intresting idea.
Can anyone calculate how high the refractive index would need to be?
I thik it's the same as asking how high it would need to be for a hemisphere with parallel light falling on the round surface to focus the light onto a point at the centre of the flat surface.
Of course. turning the idea round and using a hemishperical mirror  would be easier but then you would need to keep moving it to point it at the sun.
 

Offline cpu42

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using a hemishperical mirror  would be easier but then you would need to keep moving it to point it at the sun.
Exactly so, but that introduces a power-consuming control device that's potentially a point of failure, and possibly a danger by misdirecting the concentrated heat.
 

lyner

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Well, then, I think you will have to think of another system.
Photovoltaic cells are pretty effective for driving motors / producing motion. The beauty of them is that they work on the quantum level, extracting high grade energy rather than 'just heat' which needs to be 'concentrated' to obtain high temperatures for working a heat engine.
You could possibly work on some heat-operated pointing system in which  pointing error results in some differential heating of , say, some gas bulbs, which could move the focus to where you need it. FAiling that, a clock mechanism, powered by solar cells - or even by your stirling engine, could point your main energy collector in the right direction - the direction is very predictable.
 

Offline ukmicky

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If you go to turkey and other Mediterranean countries flats and hotels have oil drums filled with water on theirs roofs which they use as heat collectors and supply all the hot water during the day and early evening.
 

lyner

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Oh yes, indeed; they are great value.
But they don't produce the high localised temperatures which solar reflectors can. I believe that's what cpu42 was after.
 

Offline cpu42

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I've had an idea. I'll conduct an experiment, using only cheap, passive materials, and let everyone here know.
 

Offline qazibasit

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is it in anyway possible?
 

Offline cpu42

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It may be. It may be down to materials, and their properties. I don't know (yet).
 

Offline qazibasit

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why dont u just use a large conven glass and can fill it with water to give the effect of moving light.
 

Offline qazibasit

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*convex
 

lyner

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I've had an idea. I'll conduct an experiment, using only cheap, passive materials, and let everyone here know.
Put two litres of water in a black bin bag and leave it in the garden, in the sun, on a foam or equivalent insulating surface. You will be surprised at how warm the water gets. That's the best value.
 

Offline cpu42

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Is there a straightforward formula that expresses the specified area of a surface receiving (varying strengths of) sunlight to be concentrated to a specified, much smaller area, please? For instance, were a twelve-inch receiving area concentrated to, say, half-an-inch, on an ambient, continuous 25C sunny day, what would be the temperature rise in the target one-inch area?

Variables: the amount of concentration (surface area to target area ratio) and the "sunniness" of the time of day, spread from dawn til dusk.

Thanks in advance, and apologies too - I know I ramble!
 

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