Using solar heated water to generate electricity

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Sonic132
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Using solar heated water to generate electricity

Postby Sonic132 » Thu Jul 28, 2011 9:25 pm UTC

Mods: I wasn't sure if a practical engineering type thread belonged better here in science, or in general. Feel free to move it if you like.

So I remember a while back I watched this video wherein someone was demonstrating something or another, and the part that caught my eye was that he used a flame to heat up a portion of a square (or maybe it was rectangular) set of tubes that was standing up vertical. The portion he heated up was one of the lower corners, and the tubes were full of water. Heating the tube heated the water which caused it to rise and start a current going up from where he heated it, to the right, then down, then back to the start.

The point? I want to do this on a larger scale, replacing the flame heating with something large and black and metal that would get very hot in the sun, and replacing the clear tubing the guy in the video used with either larger clear pipes or white pvc pipes. What I want to know is: would I be able to install a water turbine somewhere along the pipe and generate a useful amount of electricity with it? Or would the current be much too weak without using massive pipes?

Since I kinda suck at expressing my ideas in words I made a picture: http://img685.imageshack.us/img685/7606/conceptarti.png
Also don't mind the suckiness of the art: I made this in MSpaint on a laptop with a touchpad.
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Charlie!
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Re: Using solar heated water to generate electricity

Postby Charlie! » Thu Jul 28, 2011 11:31 pm UTC

The trouble with building this would be cooling the water off after it was heated up, so that it would make a cycle. The part you want to cool off would have to be high up, which means that you'd need to run a pump if you wanted water cooling, and I don't think this would generate enough electricity to justify that. Maybe just use a small white roof to keep the cool part in the shade, and consider making it wind through a radiator.

And yeah, the power generated would be pretty dang small, but it could be neat.
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Mr_Rose
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Re: Using solar heated water to generate electricity

Postby Mr_Rose » Fri Jul 29, 2011 12:12 am UTC

For cooling, what about running the "up" pipe behind a reflector/concentrator focussed on the "hot" area/tank?
Instead of just letting the extra light bounce off into the atmosphere.
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KittenKaboodle
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Re: Using solar heated water to generate electricity

Postby KittenKaboodle » Fri Jul 29, 2011 5:09 am UTC

Using convection is not going to be a very practial way to convert thermal energy to mechanical energy (and then to electricity) . As a novelty or demonstration such a device could be built, but for "a useful amount of electricty" I don't see it making much sense.
Photovoltaics ("solar cells") would be much more cost, size and weight efficient.
If you want something extreamely low-tech that could potentialy be built with indigenous materials it could be a posibilty (not that i know anywhere PVC pipe grows wild, and while lodestones and native copper, siver or gold exist, they are rare). But what low tech application would there be for a small ammount of electricty? You might be in some remote location and need to recharge your ipod or some other low power modern eletronics, but if you brought your ipod why didn't you bring a solar panel?

On the other hand getting your working fluid hot enought to boil can be an eccective method of converting thermal energy to mecanical then to electricty. You might take a look at this: http://en.wikipedia.org/wiki/Solar_ther ... collectors

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AvatarIII
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Re: Using solar heated water to generate electricity

Postby AvatarIII » Fri Jul 29, 2011 9:25 am UTC

all i can think is they already do heat fluid with solar energy to generate electricity, just not in the way you describe
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KestrelLowing
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Re: Using solar heated water to generate electricity

Postby KestrelLowing » Fri Jul 29, 2011 3:24 pm UTC

I think the main problem would be repeating it on a larger scale. Heat transfer is tricky (haven't actually taken the class yet - that's next semester) and something that works for a small amount of liquid might not work for a larger amount. Also, the actual force of the hot water pushing itself upwards probably is quite small and wouldn't be able to produce very much electricity at all.

I'm not positive, but I think the only force would be due to the difference in density of the water. The difference in density is quite small if you remain as a liquid so the force is very, very small, although I don't know the actual formula for finding it.

Another option that used to use the sun to create electricity is using a binary cycle. Reflectors heat up water, but the water is still a liquid. Then, this water is used to heat a working fluid with a lower boiling point that can be used to drive a turbine. These cycles aren't as efficient as higher temperature cycles, but it can be much easier to heat the water to hot instead of steam using solar power.

Gaelic rock
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Re: Using solar heated water to generate electricity

Postby Gaelic rock » Sun Jul 31, 2011 1:59 pm UTC

You could try running the water over a painted black surface under glass to heat it. Using multiple glass sheets to insulate the water, you could achieve a higher temperature, but at the cost of reflecting more solar energy, thereby making it less efficient overall.

Building a demonstrator would be fun, however, like Charlie! Said,
Charlie! wrote:the power generated would be pretty dang small

I think it would be more useful for heating water to shower in than trying to generate electricity. I don't think it would be at all efficient on a large scale.

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idobox
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Re: Using solar heated water to generate electricity

Postby idobox » Sun Jul 31, 2011 2:05 pm UTC

If you want to generate electricity from heating water, the easiest way is to use a parabolic reflector to concentrate light, boil water, and run the steam through one type or another of turbine. It will have a rather low efficiency, but a steam engine would look cool. A Tesla turbine is not very difficult to build and works quite well, or you could simply use a propeller from a toy boat, and use it as a generator.

For cooling, I think the best would simply be to release the steam/water out of the system, but you could build a closed loop with a suitable "cold source", like a river, or a large radiator.
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