Manufacture of solar vacuum tube

[chatelot16]

like satellite TV dishes: the receiver is not in the center of the dish to provide shade but on the side

the boiler can be copiously insulated it does not mask the parabola

and even with a deflection angle close to 90 ° the boiler can be completely fixed: there is only the parabola which turns to follow the sun

[gegyx]

It is practically impossible to empty large panels given the thickness of the glass which you will have to use!

Anyway!
An ordinary glass of 4 mm, with spacers (studs) every 8 cm (maybe more), making a constant spacing between glass and bottom, could be suitable, right?

[bernardd]

the vacuum tube is not my preferred system: I prefer the large offset parabola and the boiler at home: it gives superior performance without the need for vacuum

To cover 20m2 of roof, it will have an effect :-)

The more the light is concentrated, the more the temperature increases and the more the conduction losses increase, especially in cold weather, when the heating is most useful.

And therefore the more the vacuum is useful in these conditions.

This is why I think rather of a parable by transmission, in the form of a fresnel lens.

I tried with a circular symmetry lens: focused on a vacuum tube, you have to choose them well, some do not resist :-)

And we could make 1m2 lenses forming a quarter of a circle, i.e. the concentration of 4m2 or 4kW on less than 1cm2 ...

I look forward to Remundo's light trap ;-)

But for hot water, too high a concentration is counterproductive, because no material resists: this is why a lens with cylindrical symmetry would be quite sufficient and could be produced by extrusion at very low cost.

[chatelot16]

to make a lid of a vacuum pot 10cm in diameter, I had to put much more than 4mm so that it does not break ... but it was ordinary glass: at the time I did not know the strength of tempered glass

anyway it will take a lot of support, rather lines than points: it risks making a lot of thermal bridge: with the vacuum tube there is no other contact than the 2 ends of the 1.8m tube

and then with flat glass assembled by seals it remains to make the vacuum pump! and to verify that it does not eat more energy than the benefit brought by the vacuum

front side in cold glass, back side in hot black sheet: with dilation it will curl in catastrophe

for vacuum the tube is really the best solution

another remark on the vacuum tube: the 2 glass tubes are welded together at one end: at the other end there is a piece of elastic metal which ensures the centering of the inner tube but lets it expand freely in length: because the pyrex may have a low expansion coefficient, the inner tube can rise to more than 200 ° and the outer tube remains cold!

[chatelot16]

But for hot water, too high a concentration is counterproductive, because no material resists

high concentration does not mean that the material to be heated is at home

for me it is the hole entered in the boiler which is at the focus: the smaller the hole the less light is likely to be able to come out the better the output: then the interior of the boiler has a sufficient surface not to suffer from an excessive temperature: with ordinary steel it works well at 300 ° c it could do better with higher materials, the insulation around the boiler does not have to be transparent: any rock wool is sufficient, no need empty

of course it does not have much interest in making hot water: it is to make steam, or operate a stirling

the adjustable dish cannot be used as a roof! you have to see it as an independent thing like a wind turbine

[Alain G]

It is practically impossible to empty large panels given the thickness of the glass which you will have to use!

Anyway!
An ordinary glass of 4 mm, with spacers (studs) every 8 cm (maybe more), making a constant spacing between glass and bottom, could be suitable, right?

Yes it could well resist with 121,6 kilo for 8 cm3 (maximum vacuum) but it is very difficult to keep the vacuum and consumption may not be worth it (cost).

[dedeleco]

It’s hard phosphorus !!
For the dilations one can insulate with double glazed glass modified on the edges with tight joints and numerous specific supports every 8cm to resist the pressure giving a thermos plane bottle instead of tube.
Pumping must use getters which trap and pump after degassing under hot vacuum (activated carbon, zeolite, etc.) !!
With a large getter size, we can hope to pump by heating them once a week, or even monthly, depending on the sealing materials (as many glasses and metals as possible).

For thermos tubes I am very skeptical about their real lifespan without pumping over years ??? !!!