It got me thinking, how hot can this really go? For example, it is known that if you add water (perhaps mineral oil would do similarly and be more permanent in an enclosure?) to the back of a fresnel lens, you can roughly double or triple the distance of the focal point. I wonder if one can combine multiple fresnel lenses in order to perhaps have a multiplicative effect on the heat level that the solar rays can output? Can this temperature of 3800ºF be raised to 1.5-2x this level by such an arrangement?
I don't think there's any reason to multiply the lenses. That's all the heat you could ever want.
But if you wanted to make it more useful, you'd put it some kind of enclosure, so it's not all blowing away on the breeze. I've been dreaming of a solar forge like his for a few years now. I picture putting the focal point inside of a high-temp box. It would need to be open on the top to let the sun in, and have partial openings on two ends to pass your workpiece in and through. Once you were collecting all that heat in one place, you could really accomplish any hot work you wanted. Me, I do some artistic/housewares kind of blacksmithing, and I used to make knives (it's been a couple years since I had time). The idea of forging over a silent, odorless, free-to-run forge... that sounds like heaven. So it only works on sunny days; that's ok. Worth it.
You'd build it different for smelting (big, well-insulated container, absolutely as hot as possible) vs. forging (longer, narrower container, ideally around 2500F) vs. glassblowing (wider container, somewhat cooler). But if he's recording 3800F (I don't think he said how he was measuring it), he doesn't need more heat. He just needs to catch it and control it.
Don't forget the difference between heat and temperature.
A wooden match can produce temperatures of well over 1000*F. But it's a trivial amount of heat.
A pan of boiling water will never exceed 212*F, so a comparatively low temperature, but orders of magnitude more heat.
So yes, at the focal point of a big Fresnel lens (I have one from an old projection TV), you can produce literally thousands of degrees F. But that represents no more heat content than the total diffuse BTU's hitting the front of the Fresnel lens.
If you want to melt a dime size piece of aluminum, it's awesome. If you're trying to heat your house, it's silly and hard to work with.
So it really depends on what you want to do with the heat/high temperatures.
We call ours the giant solar death ray because it will melt anything.
There is only one orientation (relative to the sun) that you can put a Fresnel lens, to get the smallest/hottest spot at the focal point.
If you have two of them, they will literally get in each other's way trying to put the focal points in the same physical space. You can't focus two Fresnel's on one spot. And I suspect that by adding liquid trickery to lengthen the focal length, you will almost certainly get a bigger and less focused spot. So even if you could manage to get two aimed at the same spot, I doubt you will get higher temperatures.
One way around that dilemma is a big mirror to redirect the light, and allow your 2nd fresnel to be at a significant angle to the primary fresnel. The mirror should be the same size as the fresnel.
But only trying it out in real life will answer your question, and I would like to see the results if you do that experiment.
Ours was pretty good sized and would melt aluminum, brass, bronze, steel (tiny pieces), ceramic, brick, dirt, sand, glass and every else we tried. But only tiny amounts, like 25 grams or less (S.W.A.G.)
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