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Is there a way to focus the sun (with a magnifying lens?) & heat a body of water??

Posts: 371

Location: Guerneville, CA

posted 10 years ago

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Don't laugh!

I want to magnify the sun, focus it on a fireproof medium surrounded by a moving lens pool such that it heats the water as it charges down into the ground. I could do it with a sealed pool but that contains the heat & spread of influence more. I'm thinking of trying it eventually in a colder climate like Montana. I don't really need to do that extreme a deal where I am now, but I'm looking for someone to support or tear up this idea.

Thanks!
MP

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Mike Cantrell

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Location: Mid-Michigan

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posted 10 years ago

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So, what's a "moving lens pool" ?

What are you building? What are you trying to accomplish?

Sam Barber

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Location: Missoula Mt

posted 10 years ago

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Fresnel Lens would help you focus the sun.

If no one from the future comes to stop you is it really that bad of a decision?

Peter Ellis

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Location: Fennville MI

posted 10 years ago

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Fresnel lens (good sized ones can be had from old projection televisions) will focus a scary amount of solar energy. Aim the mounted lens at your body of water.
Get fancy and use an arduino board and servo motors to track the sun.

But, as asked previously, what's the purpose? Might help us focus our thinking in the direction you are going

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kirk dillon

Posts: 68

Location: Maple City Michigan

posted 10 years ago

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Just something I saw about concentrating solar.... Let's say your trying to heat up a swimming pool with solar. If you use a Fresnel or any other magnifying lens, it only concentrates the square footage that it accepts. Lets say you cover the entire pool with Fresnel lenses. Each pin point of focus from each lens will be extremely hot, but the entire rest of the area covered by each Fresnel lens will be shaded. It would be easier and cheaper to just paint the bottom of the pool black and use the square footage of the pool to absorb the heat. Maybe you could paint your container(?) black instead of using a magnifying lens...
However, if you have a 12 square foot (3X4) Fresnel lens and your heating a 1/4 inch copper line with water running through it, then you can do some serious water heating. So I guess the idea is to use a big lens on a small amount of water.
I have burned into a log in just seconds and I watched a local guy burn a hole in the asphalt in 30 seconds with a Fresnel lens and I saw a video of a guy MELTING a padlock with one. They are extremely bright and you should definitely wear eye protection when using one. (welding goggles)

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Chris Kott

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If the focal point is deliberately placed behind or into the object you are trying to heat, the surface area being heated increases, because the beam of light is conical, with the closed end being the focus. A fresnel lens would still gather the same amount of light whether it is placed a yard or an eighth of an inch from the water's surface.

So theoretically, you could choose a dark object to put into the body of water, say, a dark rock, and then adjust the distance of the lens from the rock such that the beam where it hits the rock covers its whole surface area.

Also, there are plastic fresnel lenses made into sheets for the purpose of enlarging print. You just drop it over the page and it enlarges whatever it's on top of. The prisms that make it up are simply transparent plastic ridges that run parallel to eachother; the shape of the beam is a triangular prism, with the focus being a line, as opposed to a point. The largest I have seen of these are 50 lines per inch, and I have seen some experiments with PV that amplify the all-weather capability of the solar cells because they increase available light, so much so that there are overheating problems on sunny days during the summer.

-CK

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kirk dillon

Posts: 68

Location: Maple City Michigan

posted 10 years ago

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If the focal point is deliberately placed behind or into the object you are trying to heat, the surface area being heated increases, because the beam of light is conical, with the closed end being the focus.

If you're trying to say that the amount of light/heat touching water molecules increases then I would agree. The conical shape of the light is touching the water all the way down to the focal point.

A fresnel lens would still gather the same amount of light whether it is placed a yard or an eighth of an inch from the water's surface.

This is also true and that's why you cannot gain any MORE heat by focusing anywhere in particular. Every part of that conical shape that is touching the water molecules is losing it's heat to those molecules. By the time you get to the focal point, you've lost a bunch of it. Like you said, the lens gathers the same amount of light no matter how far it is from the surface.

So theoretically, you could choose a dark object to put into the body of water, say, a dark rock, and then adjust the distance of the lens from the rock such that the beam where it hits the rock covers its whole surface area.

This is also true but the larger the focal point, the lesser the amount of heat at that point. I can put my hand directly in front of the beam if the focal point is just an inch or so in diameter. It's warm, but not anywhere near hot.

Also, there are plastic fresnel lenses made into sheets for the purpose of enlarging print.

All Fresnel lenses are sheets that's what Mr. Fresnel invented. The ones in rear projection TV's are made to enlarge the picture. If I hold mine up and someone is behind it, it enlarges their image.
So I feel it still goes back to my previous post. If you are trying to heat up water then you need a large square footage of sunlight gathering surface and a small amount of water for the best results.........

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Chris Kott

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posted 10 years ago

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What Fresnel invented was a way to project light a distance with less material than a lens used for optics. I believe the first ones were employed by lighthouses.

The ridges on the Fresnel lens increase the light - gathering surface area and project the gathered light out the smooth side. So effectively, one side has a greater surface area. It acts as a light funnel.

If you want to avoid damage to anything being heated, you'd be advised to avoid the pinpoint focus. I am not seeing your perceived problems.

-CK

Peter Ellis

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Location: Fennville MI

posted 10 years ago

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Side stepping for a moment.

We all seem to be caught on the magnifying lens idea.

Parabolic mirrors are another option. Depending on what the objective actually is, you might be able to achieve it with something as simple as a bolt of white cloth, a staple gun and a dozen wooden stakes

Make a curved white sheet wall whose focal point is in the body of water.

Chris Kott

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posted 10 years ago

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Or there's the idea of making the pool bottom black, which is easy if it's dark silt sediment. Or if it's, say, an above - ground pool, make the outside black, too.

-CK

Josiah Miller

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Location: Willamette Valley-Marion County

posted 10 years ago

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Here's a rather lengthy pdf dealing with energy gain and loss on ponds that has some interesting information (esp chapter 3.6). It also has neat info on how biological factors like decomposing organics affect the heat of ponds (like how compost piles warm up).

http://etd.lsu.edu/docs/available/etd-0707103-142120/unrestricted/chapter_3.pdf

I think most "passive solar" heated ponds that there are examples of are highly salted, the salt forms a gradient with the saltier water on the bottom, and reflected light heats the more salinated water more. the salt gradient helps hold in the heat and make it more efficient collector. Might be a worthwhile thought if your just trying to heat a greenhouse or area with fragile plants, but if you wanted a productive pond system I doubt Salt is the way to go.

~Josiah

Kevin EarthSoul

Posts: 135

posted 10 years ago

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One thing to consider on passive warming of the pool is the sun's angle of incidence on the surface of the water. Light will go from penetrating the surface to reflecting off of it at a certain angle. This means that you'll only get passive solar gain when the sun is high. When the sun is low, it just bounces off the surface.

Redirecting sunlight coming in at a low angle down into the water by the use of reflectors is one thing to consider.

(Incidentally, and a little off-topic, I could see using this effect to aid with passive solar gain in the Winter. If a house were built for passive solar gain--- with windows on the South side--- that's all well and good, but additionally, constructing a reflecting pond on the South side would also be useful.)

Kevin EarthSoul (real, legal name)
Omaha, NE

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kirk dillon

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Location: Maple City Michigan

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Kevin EarthSoul wrote:Redirecting sunlight coming in at a low angle down into the water by the use of reflectors is one thing to consider

I've heard of a similar tactic of putting a dark rock earth bermed wall on the north side of a pond (in the northern hemisphere)to absorb the reflected heat and create a micro-climate in your garden for the night. The Bullock brothers Permaculture farm does this in Washington state.

Matt, is any of this helping? Can you give us some more of the details on what you're actually trying to do?

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