Solar furnace - what coating, what insulation?

[dedeleco]

Basically agree with Hic but

the plastic sheet ....... does not insulate

is not exact, because it blocks the heat losses by convective movement of the air around which is a huge heat loss by chimney effect as when a house has its roof open and its windows open instead of simple closed glazing.
So a simple plastic essentially insulates without work.

Increasing the number of layers of transparent insulating walls decreases the amount of light that penetrates and therefore we gain significantly less, except special and expensive optical treatments on double and triple glazing.

The vacuum is much better, if with walls that reflect thermal infrared well ((but not so easily transparent for the sun), but more complex, especially to keep without a good vacuum pump, with the difficulty that a heated thermos bottle degasses the gases adsorbed on the surface in its insulating vacuum, which greatly reduces the insulation if the getter is heated.

It is much less tiring to increase the surface of the solar collector mirror with a little more cardboard with aluminum foil.


The bottom and top heating solution, good opaque insulator, is certainly very good and remains very simple with a very sufficient simple plastic film.

[fthanron]

In a different style - tube

http://blazingtubesolar.com/
I would have put photos but do not know if they are free.

@+

[hic]

Basically agree with Hic but

the plastic sheet ....... does not insulate

is not exact, because it blocks the heat losses by convective movement of the air around which is a huge heat loss by chimney effect as when a house has its roof open and its windows open instead of simple closed glazing.
So a simple plastic essentially insulates without work.

Increasing the number of layers of transparent insulating walls decreases the amount of light that penetrates and therefore we gain significantly less, except special and expensive optical treatments on double and triple glazing.

The vacuum is much better, if with walls that reflect thermal infrared well ((but not so easily transparent for the sun), but more complex, especially to keep without a good vacuum pump, with the difficulty that a heated thermos bottle degasses the gases adsorbed on the surface in its insulating vacuum, which greatly reduces the insulation if the getter is heated.

It is much less tiring to increase the surface of the solar collector mirror with a little more cardboard with aluminum foil.


The bottom and top heating solution, good opaque insulator, is certainly very good and remains very simple with a very sufficient simple plastic film.

hi dedeleco

You underestimate the effect of insulation

With the vacuum you can easily climb to 100 °
- to boil water which will limit the temperature,
unless you make a 130 ° pressure cooker

now if you plan to fry with 200 ° oil
see obtaining oven temperatures of 500 °. . . .


I lost the video showing the boiling with only white glass

Solar Evacuated Tube Vacuum Tubes DIY Test in the Sun
*** http://www.youtube.com/watch?v=1_NOT2VZW6E ***

DIY SOLAR TUBES EVACUATED VACUUM TUBE SOLAR HOT WATER Boil water with the power of
*** http://www.youtube.com/watch?v=SB9UkZe6 ... ure=relmfu ***

Glass, "plastic" and infra-red
Unlike common glass that blocks infrared,
the sheet of "plastic?" current allows 90% infrared to pass,
the tri-layer of "plastic" sheets with 70% transmission, should be a good compromise.

[dedeleco]

There are large, powerful solar power plants of this type with km of vacuum tubes at the focal point of a parabolic cylindrical mirror.

Here the central vacuum tube has a heat pipe or heat pipe makes it a form of thermo-siphon.
It is certainly very effective, even with cloud passages sometimes, and very studied!
http://blazingtubesolar.com/pb/wp_a8ed9 ... d9ce5.html
but the unknown price is not very low in return for this very effective technology.

but where to buy it and at what price? mystery! Not found.
It looks like an ideal realization very well made but not sold ??
If you break the tube, you must have a replacement.
But well built careful use over decades justifies the higher price for a very good solar oven very isolated, without errors.

If you tinker it yourself I am not convinced of the need for a vacuum, because it is enough to double the mirror surface to heat as much, drastically lowering the price, if you tinker it yourself (simplistic loop tube with oil in thermosiphon).

Read the 1867 pages of the solar ovens website:
http://solarcooking.wikia.com/wiki/Sola ... %28Home%29
http://solarcooking.wikia.com/wiki/Buy_a_solar_cooker
http://solarcooking.wikia.com/wiki/Blazing_Tube_Solar

The price cannot be found, probably blazing !!

[seb1000]

Hic, your suggestions support my project, that's exactly what I intend to do.

as i said my plastoc cardboard oven was a draft, but a convincing draft.

a note, however, about the parable:
we often tend to say that the parabola is great: it is, it is true since it allows to concentrate the light on a focal point or on a "focal line".
but the radiation picked up by a parabola is much lower than the radiation picked up by an identical surface perpendicular to the rays of the sun.
in no solar oven (to my knowledge) there is no mirror perpendicular to the rays of the sun; but the parabola (at least a part) is very inclined compared to the rays of the sun.
and therefore for a radiation of (for example) 700W / m², a dish of surface 1m² will never receive 700W (and it will reflect a little less) it will certainly only receive 500W, if that surface on the ground (or shadow ) is 0,7m².

[dedeleco]

in no solar oven (to my knowledge) there is no mirror perpendicular to the rays of the sun; but the parabola (at least a part) is very inclined compared to the rays of the sun.
and therefore for a radiation of (for example) 700W / m², a dish of surface 1m² will never receive 700W (and it will reflect a little less) it will certainly only receive 500W, if that surface on the ground (or shadow ) is 0,7m².

The parabola with a diameter of 1,1284m and a section of 1m2 for the rays of the sun returns all these rays of the sun stopped on 1m2 in its focus in the concentrated center.
So there is no loss for a circular dish that is perpendicular to the sun.

The dish is not tilted, but we have sometimes cut a piece of the dish in a bizarre shape to have the focus outside, which remains in the real center of the complete circular dish.

[Christophe]

Dede, do you have pictures of your montage?

In the books on solar ovens it is clearly indicated that concentrator ovens (i.e. parabolic, parabolic ...) will work less well in overcast weather than Ulog type ovens which capture diffuse radiation.

In a different style - tube

Completely feasible in self-construction, there is one on these photos: https://www.econologie.com/forums/cuiseurs-f ... 10742.html

The most difficult will be to find the tube (I do not know if the plexi would hold ...).

I continue my oven today, I started the chassis.

[fthanron]

@ Christopher

I refine the approach of the gentleman :-)

Tube + thermal storage (oil ... that reminds me of another post that; o)) + insulation of the walls. These three points seem entirely relevant.

Good realization to all.

@+

[dedeleco]

Tube suggestion:
One of those on commercial vacuum sensor for CESI usual and not expensive like with brico depot or other of better quality, by taking only one sold for replacement of broken tube.
It has a heat pipe at the end which heats the pan instead of CESI water.

With plexi, and plastics which is porous to gases, the vacuum is not preserved.
Only the glass is good.

[Christophe]

I continue my oven today, I started the chassis.

It advances, evolution of the day:



I made a subject dedicated to this "site": https://www.econologie.com/forums/four-cuise ... 10753.html