A Fresnell lens for heat

TwoGuns wrote in news:45217953-bb05-4711-85e7- snipped-for-privacy@h40g2000prf.googlegroups.com:

Just make sure you NEVER get anything close to the focal point. We played around with a 10" fresnel lens when I was a kid in New Mexico. Properly focused, it would melt firebrick.

Doug White

And I nearly set on fire one of the largest office parks in central NJ with one about that size, a couple of years ago, giving a demonstration of solar heat on our lunch break.

Man, they do have one heck of a hot-spot.

The Fresnel lens will concentrate the sunlight that is coming in the window, but will not increase the total heat coming into the garage.

I would think about some shutters that would be attached to the windows at the bottom with hinges. Line them with aluminum foil. During the day lower them so they reflect sunlight thru the window into the garage. At night raise them to minimize heat escaping thru the windows.

Dan

Hey D -

As others have said, the Fresnel lens concentrates power a little too much to be safe.

Here's an approach that'll work and won't torch the place if it is run dry:

Here it is, cleaned up and ready for commercialization: Do you like *lots* of coffee?

--Winston

This is what you need:

It is best if your sunlight isn't filtered by a window, especially a low-e window.

The Fresnel lenses from old overhead projectors is what you are probably talking about, eh? I think one would be fine to try out. The fact that they can light a 2X4 on fire when properly focused applies only when you focus finely on a flammable surface. Double the width of the spot and the intensity (and therefore the temperature) drops by a factor of 4, IIRC. The big issue as I see it is that you would have to arrange your coffee pot/lens holder so it will track the sun. I would not use any other materials than the coffee pot itself for the concentrated sunlight to impinge upon. You don't want to loose any energy through poorly conductive connections. As long as you have water behind the spot on the coffee pot where the rays impinge, you won't ever get the container above the boiling point of water. Safety: As others have said, you need an arrangement where the finely focused rays must not be allowed to fall on anything flammable. You will want to have a thermometer in the coffee water to gather some data on heating rate of a specific amount of coffee for future reference. And, of course, you must tell us what you found.

"THEY" usually figure that the bright sun at noon has a total power output on earth of about 1000 watts per square meter. That's about 1550 square inches. Your 10" X 10" square fresnel lens, at 100% efficiency (which it isn't) could transmit about(100X1000)/1550 or 65 watts, so don't expect to boil a lot of coffee. I think the "keep warm" heater on some electric coffee is about 300 watts.

Pete Stanaitis

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TwoGuns wrote:

What Pete said on the total power per square inch available.

The other problem is keep> Recently I moved into an apartment that has the apartment above the

A Fresnel lens can focus solar radiation to a small enough area to produce rather surprising temperatures, as others have noted. But it would take a while to make coffee. Solar radiation power density at noon at the equator is about 1000 watts per square meter, less at higher latitudes and different sun angles, and window glass will attenuate that significantly.

Even at 1KW/m^2 a 12" square Fresnel will only collect about 93 watts.

64 watts will raise about 500 ml (about a pint) of water at room temp up to boiling in about 30 minutes, if no heat is lost as some inevitably will be.

Perhaps you are patient about your coffee. With me it's more like "give me coffee, and no one will be hurt."

When my son was in 6th grade they had a science project about February or so: devise a way to collect and direct solar energy to raise the temperature in a bird house during a MN winter. They had those little thermometers like were once given away on desk calendars by businesses.

I'd used a surplus Fresnel from AxMan Surplus for eldest daughter's project so I wanted to think of something different for Kevin.

A trough parabolic reflector seemed like a good approach since some of my colleagues at work were working on real solar farms that used arrays of big parabolic trough collectors.

I calculated x and y points for a parabola and had Kevin plot those points on a piece of cardboard. He cut that out with snips and we used it as a template to cut some styrofoam formers into which we glued thin cardboard from the backs of tablets or beer cartons or something. We then stuck aluminum foil to the resulting parabolic trough.

Kevin had said that if he broke the thermometer they'd charge him a dollar for it. I said this contraption could conceivably pop his thermometer, might he be OK with that? Huge grin: oh yeah, he'd be just fine with that!

The trial day in February (in Fridley, MN) was cold, but clear and sunny. The thermometer in his birdhouse burst but his teacher somehow forgot to charge him for it.

He said it was a cool project because he'd never realized that math might actually have some practical utility.

He became an electrical engineer, go figure.

Since you don't need a point focus I think a circular reflector would work with the benefit that it doesn't need to track.

You may lose too much heat to the air unless the collector has a transparent door. Heat the pot and then measure its (exponential) cooling rate, I suspect in free air it won't be all that warm at the temp where it dissipates the 65 Watts. If you paint it black to absorb better it will radiate the heat away about equally better.

The equilibrium temperature of my solar water heater when the tank is empty is about 190F, full it's 130F. The concentration ratio is 3:1 but it's weathered and probably not too efficient. It saves me only $0.10 a week in hot water for laundry so it isn't worth an upgrade,

jsw

Now, that sounds memorable.

I think the project is highly risky. As the sun moves through the sky, the focal point will be in different places. Just as the shadow of a garden sundial moves. The risk is that the focal point will move off the coffee pot, and light the table (or something else) on fire. I'd want the project to be 100% supervised for the first couple days, so you're sure it's safe.

There was a fountain with a concrete pad around it, and a curb around that, and dry leaves had piled up against the curb. I pulled the lens out of my book bag to show some co-workers why they should take solar energy seriously. In about three seconds, I had the pile of leaves going. But they were really dry, and there was a breeze, so in a few more seconds I had a complete ring of fire going around the fountain.

It was kind of pretty, actually -- the fountain spraying water, surrounded by a ring of fire...

That's too bad. At least he didn't become a lawyer. ;-)

--Would love to mess around with one but have been unable to locate a large one. --Cautionary tale: Once upon a time these were available in little enclosures that looked like a suitcase with one side removed, lens occupying opposite side. They were used for some projection purpose, now obsolete. Anyway I worked at the Y in the print shop and someone carelessly left one near a window. I came in the next morning and smelled smoke. Sun shining thru window, thru fresnel lens had concentrated onto stacks of paper and a burned line traced the path of the sun across the sky. The only thing that saved us was focus being slightly off optimal.

Just this morning, I was making coffee in the company coffee pot, and the secretary commented, "Howcome everybody's drinking coffee these days?" I said, "It's COLD out there!"

I learned a trick with a drip maker - take out the pot, and put your cup on the hotplate part - fresh coffee in a hurry! You have to be careful when swapping the pot back into the spot - the drip can land on the hotplate and boil, which doesn't smell good at all. ;-)

Cheers! Rich

I once got one at the bookstore - "Full Page Magnifier". It's plastic, so the image is far from perfect - when burning stuff with it, the spot wasn't perfect, but it _did_ work. ;-)

Good Luck! Rich

I've lit a cigarette with reading glasses. ;-)

Cheers! Rich