


New solar dish from MIT concentrates sunlight intensely enough to melt steel
















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RE: Mythbusters
By masher2 (blog) on 6/21/2008 11:32:08 PM , Rating: 3
> "You forget, the technology to create curved mirrors did not exist so the point is mute"

You don't need a curved mirror to build a parabolic reflector; you simply need a large number of flat reflective segments, arranged on a parabolic surface...something which the Greeks certainly knew themselves.

RE: Mythbusters
By TheBaker on 6/22/2008 12:35:02 AM , Rating: 2
Yeah, but if you don't use curved mirrors you get a "stack" of flat panels of light, rather than a true parabolic reflector.

That is, the energy is focused into a (for example) 12" x 12" square rather than a 1" x1" (or smaller) area. The energy in the large square would make something really hot, while that same energy in the small square would be 144 times more powerful, and would burn things to a crisp. Make it a true parabola and the energy is condensed to a point source. Eventually, you get what these kids at MIT came up with.

They didn't do anything new, they just did it with more precision and efficiency than before.

RE: Mythbusters
By masher2 (blog) on 6/22/2008 12:46:14 PM , Rating: 5
Even a mathematically perfect parabolic reflector won't focus to a light to a single point. The minimum focus size is defined by many factors: aperture (size) of the reflector, wavelength of the reflected light, visual size of the sun or other psuedo-point source, etc.

So in reality, nothing is a "true"parabola. If a stacked mirror such as this contained, say, 1000 flat segments, the intensity at the focus could be as strong as 500X normal solar flux....certainly hot enough to ignite wood.

RE: Mythbusters
By blue7053 on 6/26/2008 6:48:06 PM , Rating: 2
The embarassing thing about this is the number of people who are amazed by it. 'The kids' at MIT didn't do it, some guy in Iowa did it and showed them where to buy 12' mirrors. There is absolutely nothing new here except the knowledge of the melting point of steel. Even there, they don't enumerate it. They just wave in the general direction.

RE: Mythbusters
By achintya on 6/22/2008 1:54:23 PM , Rating: 3
Yeah, but after a point, if you scale the piecemeal 'parabolic' mirror up it slowly starts achieving the shape of a true parabola. Basic geometry. Even a polygon with enough sides starts looking something like a circle. Think of a polygon with over 20 sides. Looks enough like a circle anyways. Most new large telescopes are made the same way by assembling many small straight pieces.

RE: Mythbusters
By someguy743 on 6/22/2008 3:39:16 PM , Rating: 2
Wow, this new solar thermal technology really could be revolutionary. I'd like to know what the "solar to grid conversion efficiency" number is for this solar thermal dish. Stirling Energy already makes a larger sized one that looks similar. It set the record for efficiency at 31.25%:

If Raw Solar's solar dish is a lot more efficient than Stirling Energy's AND it is a lot cheaper to manufacture and deploy in the field then these MIT guys might be sitting on a gold mine. Big Coal and the other fossil fuel industries won't like this one bit. :) It really could reach and exceed the price per kilowatt that coal is now. Now ... if only the solar thermal industry can get some really cheap energy storage technology to come out, they've got the world by the tail.

Cheaper energy storage and cheaper transmission lines are all that's standing in the way of solar thermal, photovoltaic and wind from becoming the dominant energy sources in the next 20 years ... in a super efficient and flexible "smart grid". I bet it's going to happen. The dream might soon be realized. Inexpensive, inexhaustible, non polluting clean energy that will NEVER run out and cannot be manipulated nearly as much by the Wall Street schemers and Big Oil, etc. :)

RE: Mythbusters
By masher2 (blog) on 6/22/2008 3:54:04 PM , Rating: 3
> "Stirling Energy already makes a larger sized one that looks similar. It set the record for efficiency at 31.25%:"

Concentrating solar cells already exist which have efficiencies in excess of 40%. They're still far more expensive than coal or nuclear.

RE: Mythbusters
By Denithor on 6/23/2008 9:00:44 AM , Rating: 2

...which is exactly why it likely won't happen for several years.

RE: Mythbusters


RE: Mythbusters
By blue7053 on 6/26/2008 7:11:25 PM , Rating: 2
I don't know whether this is new or not, nevertheless I worked it out 20 years ago. Showed it to a country neighbor once, he said, "Oh yeah. I read about that in a magazine."

Forming the Greek parabola:
Start with a 4x8 sheet of plywood, drive a nail in lower left corner of the long side.
Tie one end of an 8' string to the nail, double the string, (now 4') and put your pencil in the loop of the string.
Hold one end of the string at the nail and the other end with the pencil at the upper left side of the plywood.
Move the free end of the string along the bottom side of the plywood and the pencil to the right, maintaining a right triangle relationship between the nail, the pencil and the free end of the string.
As you move the pencil and the free end of the string to the right, you will be drawing a curved line describing the point of the opposite side and the hypotenuse. As the opposite side of the triangle moves away from the nail, the opposite side diminishes as the base and the hyoptenuse increase. The relationship amoung the sides remains constant.
When you end up with your 8' string at the lower right corner of the plywood, you will have half a parabola described across the sheet of plywood. The focal point is the nail.

RE: Mythbusters
By lco45 on 6/23/2008 4:07:22 AM , Rating: 2
A stack of flat mirrors is the same as one curved mirror, if you know your calculus.

A curve is just a bunch of straight lines, with the length of each line tending towards zero.

The greeks understood calculus, although not to the degree the Newtonians took it. Google "squaring the circle" for info on an early greek take on calculus.


RE: Mythbusters
By masher2 (blog) on 6/23/2008 11:24:17 AM , Rating: 2
More precisely, a curve can be considered a set of infinitely short line segments. There is a difference between a stack of flat mirrors and a curve...but that difference tends to zero as the size of the mirrors decreases.

And 'squaring the circle' isn't calculus. It's the equivalent problem to finding the area bounded by a curve, a problem which can also be solved by integral calculus..

