DIY vacuum solar panel

Im wondering if it'd be possible to build a solar panel using a radiator painted black and encased in glass..i.e. like a double glassing unit. I know that if you tried to create a vacuum it would break due to the glass surfaces being crushed by the weigh of the atmosphere, but would it be possible or practical to support the glass from breaking i.e. by using hard spacers (made from a hard highly insulating material...can you suggest one?) placed at appropriate distances?

Steve

Reply to
Mr Sandman
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Why bother to evacuate the housing?

The major factor in the stated design is the 'greenhouse effect' pf the glass. [Sunlight at relatively short wavelengths - to which glass is transparent - look out of the window- is absorbed by the blackened radiator which becomes heated and emits radiation at a much longer wavelength to which glass is opaque Lots of radiation comes in; little goes out.

Putting the odd inch of vacuum between your 'black-body' (radiator and front glazing isn't going to do much. The infra-red wavelengths emitted by the blackened radiator can traverse a vacuum - they can't traverse the glass.

Behind your radiator; place "SHINY" reflective foil - ensure that any emitted radiation goes back into the box- and place foam insulation 'behind' the shiny foil. IOW; direct your attention at stopping radiation (Heat escaping from _BEHIND- the solar gatherers.

IMHO

Reply to
Brian Sharrock

Yes - it's very possible - did it myself with a couple of old 6ft x

2ft rads about mumble years ago

Works fine - the diisadvantage over a proper flat-plate collector (copper sheet with wiggly copper tube soldered to it) is that the volume of water is much greater in the 'radiator' version.

This means that the overall panel is quite heavy, and the thermal mass of the panel is quite high - so it takes longer to heat up.

Having said that, it's very cheap to produce, especially if you can get the rad 2nd-hand.

As to evacuating the unit - no need. Most designs used to provide a row of small air vent holes along the bottom of the unit to allow condensation to escape. If you leave the door to a greenhouse open, the greenhouse still warms up. Same effect.

The challenge with one of these units is making it sufficiently weather / rot-proof.....

Regards Adrian

Reply to
Adrian

On Wed, 27 Feb 2008 07:21:39 -0000 someone who may be "Mr Sandman" wrote this:-

It's possible. However, one can also buy the more efficient vacuum tube panels for reasonable prices. The difference between these and flat panels is large in the winter and late autumn/early spring. During the more summery months there is less of a difference.

The first three panels on are £220, £395 and £570 respectively.

Reply to
David Hansen

Just don't fill the system with water.. You put the rad higher than the header tank and let it drain when the pump is off.. as soon as the panel (no water) gets hot you can pump some water through to extract the heat. Letting the panel drain will also make it somewhat frost proof although the anti corrosion additives can do that too..

Reply to
dennis

Yesterday the wind caused a lot of dmage round here and it was bitterly cold but it was largely a cloudless day.

The water heated in our (commercial) panel gave us a large cylinder full of water at 44 C.

My point is that whatever you choose to do you'll be surprised and pleased at the result.

I believe that commercially made panels are more efficient than those you build yourself.

Something to remember if you use a radiator is the weight on your roof, you really must be sure that the supporting timbers will be strong enough.

Mary

Reply to
Mary Fisher

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sorts of ways of making them

Reply to
mogga

I think this is an excellent idea. Even in winter, the warming power of the sun streaming through my French windows heats the kitchen to around 22 degrees C by about 11 am, while the front of the house is still freezing cold.

I've thought about making some kind of heat exchanger, using standard copper pipe from B&Q. The amount of free heat being lost each day must be enormous, across the whole country. What I don't understand is how the heat gets concentrated in these various schemes, like a ground heat pump that sucks warmth from the earth. How *does* a heat exchanger work, exactly?

MM

Reply to
MM

On Wed, 27 Feb 2008 10:33:28 -0000 someone who may be "Mary Fisher" wrote this:-

Agreed. I checked and the system I am most familiar with put 6kWh into storage yesterday. Every building should have one.

Agreed. Vacuum tubes have the edge in cloudy conditions, but commercially supplied flat panels will do almost as well in bright sunshine.

Home made panels make an interesting DIY project, but will not get as much out of the sun as one built specially for the purpose.

Reply to
David Hansen

Same way as a fridge. Fridge moves heat from inside the cool cabinet to the radiating matrix on the back of the cabinet.

There are simpler schemes that might work for you - where simple fans and ducting are used to move warm air around from warm places to cooler places.....

Regards Adrian

Reply to
Adrian

I think "not being usefully used" would be better. The heat from the sky is not lost it just doesn't heat the bits one might rather have heated.

That heat is coming from the earth itself not solar heat. Go down a metre or so in this country an you'll find the temperature a pretty steady at around 10C. If you use a heat pump to remove that heat it will be replenished from the earth itself.

Heat "flows" from hot places to cold places. Have something hot next to something cold and the hot thing gets colder and cold thing hotter, the heat is "exchanged". Or did you mean "How does a heat pump work?"...

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Reply to
Dave Liquorice

I've thought about moulding a GRP housing to go on the roof thus avoiding the rot problems. Would GRP tolerate the likely temperatures in summer?

Reply to
1501

Well, yes, but I'm also think about getting warm water for free. At least hot enough for a shower, say. (I don't like it too hot!) Or for rinsing the dishes.

One thing about all the exorbitant fuel and water price rises is that it makes me want to look at alternatives really seriously now. For instance, a wood-burning stove. I have already started to syphon the bathwater out to use to flush the loos. In the summer I might syphon it out onto the lawn or flowerbeds.

Has anyone worked out at what point a small Honda generator would work out cheaper than paying NPower?

MM

Reply to
MM

seems to make sense. So would you use a single pane of glass or double glaze? is the trade off worth it? surly you would get more heat absorbsion from a black radiator than a load of black tubes encased in glass vacuums?

Steve

Reply to
Mr Sandman

I seem to recall that some of the commercial units are housed in grp. The operating temperature isn't the problem - you have to think about the possible temperature that might occur when / if there's some kind of failure with the pump / cooling water etc. (stagnation temperature)

Commercial units have a pressure release valve up on the collector panel that will blow when the water boils - but that just stops the system turning into a bomb !

There used to be a lot of info available from the Alternative technology people in North Wales -

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Reply to
Adrian

They can do better with a bit of effort as you are not trying to maximise what is already an exorbitant profit (for the seller). My home made one runs about 10% -20% more effectively than the one of the same size from the same manufacturer your conjoined twin uses (although it lacks the nuclear fusion device to generate heat out of nothing hers seems to have). The Solartwin is simply 3 passes of silicon plastic tube squashed between two thin aluminium sheets which are riveted together in between the tubes. It is placed on a sheet of Kingspan type insulation and has cheap conservatory roof type plastic glazing on the front.

The home made version has 6 runs of aluminium tube affixed to an aluminium backing sheet using low temperature brazing (sometimes called aluminium soldering but I'm not convinced) and the whole lot was anodised to give a matt black surface. It is in an insulated case with a double glazed glass front. Solar water heating isn't exactly advanced technology.

Reply to
Peter Parry

I'm assuming you're attempting to use the radiation captured for the purpose of 'heating' water (actually trying to cool down the 'black body' of the radiator) ... ?

Consider the glass as forming a diode - it'll posses different transmisitivity at different wavelengths, High at 'optical' wavelengths, which our eyes perceive, and low at infra-red wavelengths It's this 'diode ' effect which explains how solar radiation heats the central heating "radiator". But; glass also attenuates incident radiation . in extremis a very ,very, thick piece of glass would attenuate the visible wavelengths so much that it'd seem opaque!

It's a trade off!

Contemplate; you want a large aperture (size) to capture the solar radiation. say you use a piece of single glass one square (metre / yard/ what-ever) ... would your second piece if glazing be better used marginally increasing the blocking effect of the ir wavelengths -WHILE- decreasing the amplitude of the incoming visible wavelengths OR doubling the size of the aperture which is capturing the solar beams ?

BTW; when observing something through 'Night vision" optical devices it's amazing how glass in car windows , houses, etc. display as really , eally -BLACK-. It's the most instructive way I've seen to 'understand' the concept of glass's opacity to ir-wavelengths

AIUI; the glass tubes type of solar water heating relies on a slightly different mechanism to extract/concentrate the heat - each tube is heating up a metal block at it's individual head though which the water is passed.

Reply to
Brian Sharrock

On Wed, 27 Feb 2008 22:21:39 +0000 someone who may be Peter Parry wrote this:-

Excellent, abuse. Do keep it up.

Reply to
David Hansen

Never I expect. They're buying gas at wholesale prices and putting it into a very efficient system, whereas you're buying fuel at retail prices and putting it into an engine that's at most 50% efficient.

Reply to
Doki

If you want a vacuum system I'd try fluorescent tubes. You'll need to use metal pipe due to the high temps attainable. Foil on the interior underside acts as a concentrator, which hugely boosts the performance potential. Re vacuum I'd hook up a tiny pump as the seals are unlikely to be perfect.

To get any useful output you need to get water hot. Tepid isnt too useful. To get hot you need either strong summer sun or concentration. Concentration improves efficiency and extends the working season. And concentration is one of the main qualities of vacuum tube types. You can also make non-vaccum concentrating systems, and they work well too.

If you want a vacuum behind flat glass, you need some hefty glass. CRT TV screens are only strong enough when curve fronted and of limited size. The few CRT flat screens about use even thicker glass.

NT

Reply to
meow2222

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