Ambjörn Naeve and combining 2 troughs to get a point of light.
brian white
life and research
The important part for me is
http://kmr.nada.kth.se/files/pointfocus/PointFocus/PointFocus-Discovery.jpg
The picture shows 2 parabolic dishes combined to make a point of light.
The first dish is not curved in the same plane as the second one and the
focal line of dish one has to be on the directrix of the second one.
I do not understand it fully (of course). I bring it up because of the
solar cooking implications, like the solar wheelbarrow in this picture
http://kmr.nada.kth.se/files/pointfocus/PointFocus/PointFocus-wheelbarrel.jpg
and because I think it can be used to design a better solar cooking
dish. (Or a better solar cooking combined trough if you wish). And
perhaps trough collectors that do not need to be perfectly pointed. That
could mean just tracking or readjusting every 40 or 50 minutes rather
than every few minutes.
Even though you lose efficiengy by having 2 troughs instead of 1 dish,
being to get a hot point of light simply by bending probably has lots of
advantages due to its simplicity. Also, for one axis polar mounted
devices, it might be that the 2 trough device would be easier to adjust
for seasonal change. (Move the 2nd trough to keep the focus in the same
place, perhaps?) as the seasons change.
For solar cooking, a point of light is not necessary. But it would be
extremely useful to maintain the light on the cooking pot for much
longer than a parabolic dish does. Perhaps 2 combined troughs can have
their curves adjusted in experiment to achieve that? Then you "reverse
engineer" to find out what the corresponding dish looks like.
Please take a look and tell me what you think.
Brian White
dow
> http://kmr.nada.kth.se/wiki/Main/PointFocusis a webpage about naeve's
> life and research
> The important part for me ishttp://kmr.nada.kth.se/files/pointfocus/PointFocus/PointFocus-Discove...
> The picture shows 2 parabolic dishes combined to make a point of light.
> The first dish is not curved in the same plane as the second one and the
> focal line of dish one has to be on the directrix of the second one.
> I do not understand it fully (of course). I bring it up because of the
> and because I think it can be used to design a better solar cooking
> dish. (Or a better solar cooking combined trough if you wish). And
> perhaps trough collectors that do not need to be perfectly pointed. That
> could mean just tracking or readjusting every 40 or 50 minutes rather
> than every few minutes.
> Even though you lose efficiengy by having 2 troughs instead of 1 dish,
> being to get a hot point of light simply by bending probably has lots of
> advantages due to its simplicity. Also, for one axis polar mounted
> devices, it might be that the 2 trough device would be easier to adjust
> for seasonal change. (Move the 2nd trough to keep the focus in the same
> place, perhaps?) as the seasons change.
> For solar cooking, a point of light is not necessary. But it would be
> extremely useful to maintain the light on the cooking pot for much
> longer than a parabolic dish does. Perhaps 2 combined troughs can have
> their curves adjusted in experiment to achieve that? Then you "reverse
> engineer" to find out what the corresponding dish looks like.
> Please take a look and tell me what you think.
> Brian White
This is an old and quite simple idea. Naeve experimented with it more
than 30 years ago. I don't think he was he first. One simple parabolic
"trough" reflector brings light arriving parallel to its directrix to
a line-focus. The second mirror, placed between the first one and the
line focus, reflects the light to a point focus. To do this, the two
mirrors must be curved in perpendicular directions. Also, the focal
length of the first mirror must be longer than the second's. The two
mirrors must be held in a fixed orientation relative to each other.
That's why Naeve attached them both to a wooden "cradle". The entire
cradle, with the mirrors attached, can be moved around to track the
sun, or whatever.
The big advantage of this idea is that the reflectors are "simple
curves". Each of them can, in principle, be made by bending a plane
sheet of metal without any cutting, stretching or crumpling. This
contrasts with the process of making a three-dimensional paraboloidal
"dish", which requires more complex operations.
It would be possible to make the first mirror as a "complex parabola",
i.e. two or more parabolas side-by -side, sharing a common focal line,
but with their directrices angled relative to each other. As the sun
moves across the sky, the parabolas take turns to focus sunlight
approximately onto the focal line. Only one parabolic secondary mirror
would be needed. This kind of arrangement has the advantage that the
mirrors have to be realigned less often to track the sun, but the
disadvantage that only half, or less, of the mirror area is functional
at any time.
Unless the mirrors are very well polished, the loss of light caused by
using two mirrors instead of one would be significant. Under third-
world conditions, making curved mirrors that are well polished would
be very difficult. A friend of mine was recently in the Philippines
trying to develop solar cookers, and found fabricating them to be
almost impossible.
Good luck!
dow
Morris Dovey
> http://kmr.nada.kth.se/wiki/Main/PointFocus is a webpage about naeve's
> life and research
> The important part for me is
> http://kmr.nada.kth.se/files/pointfocus/PointFocus/PointFocus-Discovery.jpg
> Please take a look and tell me what you think.
The design strikes me as uneconomical of mirror surface.
Some time ago, someone sent me either a photo (or a link to a photo)
he'd taken of a solar cooker design he'd seen in common use in (IIRC)
Nepal - and I saved a copy. See
http://www.iedu.com/DeSoto/Misc/SolarCooker640.jpg
It struck me afterward that the wings /could/ be identical, which would
allow making a single parabolic reflector in halves for an economy of
labor as well as mirror material.
These look as if they might be mylar over fiberglass...
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
dow
> brian white wrote:
> >http://kmr.nada.kth.se/wiki/Main/PointFocusis a webpage about naeve's
> > life and research
> > The important part for me is
> > Please take a look and tell me what you think.
>
> The design strikes me as uneconomical of mirror surface.
>
> Some time ago, someone sent me either a photo (or a link to a photo)
> he'd taken of a solar cooker design he'd seen in common use in (IIRC)
> Nepal - and I saved a copy. See
>
> http://www.iedu.com/DeSoto/Misc/SolarCooker640.jpg
>
> It struck me afterward that the wings /could/ be identical, which would
> allow making a single parabolic reflector in halves for an economy of
> labor as well as mirror material.
>
> These look as if they might be mylar over fiberglass...
>
> --
> Morris Dovey
> DeSoto Solar
> DeSoto, Iowa USAhttp://www.iedu.com/DeSoto/
My friend who went to the Philippines took a kit for building a solar
cooker. He hoped to be able to duplicate it using local materials. But
the humid climate destroyed even the commercially made kit. There were
parts made of cardboard, which disintegrated into mush.
He didn't find any satisfactory way of making reflectors out of local
materials.
However, the kit did incorporate a neat idea. The cooking vessel was
enclosed under a glass dome which acted as a greenhouse, substantially
increasing the temperature. In fact, a blackened vessel reaches useful
cooking temperatures if it's under a dome (e.g. an inverted glass
bowl) even if there are no reflectors to concentrate the sunlight. If
glass bowls grew on trees, there would be no problem with cooking by
sunlight. However, they don't, and obtaining them in remote areas is
not easy.
dow
amdx
"brian white" <br...@shaw.ca> wrote in message
news:NRTAm.130$Vs1...@newsfe08.iad...
> The important part for me is
> http://kmr.nada.kth.se/files/pointfocus/PointFocus/PointFocus-Discovery.jpg
I can't get this drawing to work in my eye/mind view, I understand the
concept,
but I don't think the drawing is correct. Anyone else see it as improperly
drawn?
(not that I could do better)
Mike
amdx
"amdx" <am...@knology.net> wrote in message
news:1cabc$4ae77a0c$18ec6dd7$10...@KNOLOGY.NET...
>
> "brian white" <br...@shaw.ca> wrote in message
> news:NRTAm.130$Vs1...@newsfe08.iad...
>> http://kmr.nada.kth.se/wiki/Main/PointFocus is a webpage about naeve's
>> life and research
>
>> The important part for me is
>> http://kmr.nada.kth.se/files/pointfocus/PointFocus/PointFocus-Discovery.jpg
>
> I can't get this drawing to work in my eye/mind view, I understand the
> concept,
> but I don't think the drawing is correct. Anyone else see it as improperly
> drawn?
> (not that I could do better)
> Mike
>
http://kmr.nada.kth.se/files/pointfocus/PointFocus/PointFocus-cyl-1+2-rays.jpg
dow
> "brian white" <br...@shaw.ca> wrote in message
>
> news:NRTAm.130$Vs1...@newsfe08.iad...
>
> > life and research
> > The important part for me is
>
> I can't get this drawing to work in my eye/mind view, I understand the
> concept,
> but I don't think the drawing is correct. Anyone else see it as improperly
> drawn?
> (not that I could do better)
> Mike
>
>
>
> > The picture shows 2 parabolic dishes combined to make a point of light.
> > The first dish is not curved in the same plane as the second one and the
> > focal line of dish one has to be on the directrix of the second one.
> > I do not understand it fully (of course). I bring it up because of the
>
> - Show quoted text -
The perspective of the drawing of the left-hand mirror is wrong. The
left side of the mirror is supposed to be further from the observer
than the right, so the near-horizontal lines should converge toward
the left. But in the drawing they converge toward the right, which
makes the mirror look like it's pointing the wrong way.
dow