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      CommentAuthorrogerwhit
    • CommentTimeJul 3rd 2008
     
    I'm on about "Trials & tribulations of a DIY solar project" in the summer 08 issue. The configuration of the test rig remains a total mystery to me, despite the photos. How does heat from the sun get inside the polystyrene box?? The article seems to be just anecdotal. But what's going on?
    • CommentAuthorsune
    • CommentTimeJul 4th 2008
     
    I agree - I think it was deliberate - I think the author wants to patent the method and make loads of £££'s, I think the article was so obscure to stop us copying the method.....? I also wondered about the mistakes in the mathematics in the original patent - that whole bit made no real useful sense to me without the context.

    I get the sense that it is basically some big mirrors, focusing onto a car radiator in an insulated, black on the inside, glass fronted box.....that's how some solar cookers I've seen are set up (without the mirrors): an insulated box painted black on the inside with a glass lid. Food, say potatoes, is placed inside and cooks in a jiffy (this was in Ladakh though so it might take longer here). So I guess you replace the potatoes or whatever with a car radiator full of water and link it to a coil in your tank....

    It obviously both got us interested though so top marks for that!
    • CommentAuthorjoe.e
    • CommentTimeJul 4th 2008
     
    I've been wondering about mirrors and refractors and things with relation to solar hot water and electricity. Mirror glass is cheap compared to PV and solar hot water panels, and it reflects out a decent percentage of the light that hits it - 80%, from memory. I'd have thought you could greatly increase the effective area of a panel with the judicious use of mirrors. Has anyone experimented with systems to throw more light onto a solar panel set-up of either sort? How about a system with a big mirror on a tracker that reflects onto a water heater of some sort, similar to those solar furnaces? Or side panels for PV systems that gather a bit more light in to the panel? Or would it turn out to be cheaper and easier just to fit more panels?
    • CommentAuthorludite
    • CommentTimeJul 4th 2008
     
    I'm not sure about mirrors joe.e They reflect light just fine, but they don't seem to reflect heat - or maybe that's my imagination.
  1.  
    you trying to say that multi foil doesnt work Ludite!

    careful..... :smile:
    • CommentAuthorludite
    • CommentTimeJul 4th 2008
     
    well, I've seen the runners on the london marathon being wrapped in bacofoil. . . . . I've even wrapped someone in it myself. . . . . I use tinfoil on the chicken when I roast it for dinner. . . . . . but you would have to work pretty hard to convince me that I could warm myself up just by looking at the reflection of a fire in a mirror without having the fire itself. . . . . . could I put a picture of a fire on my laptop - which is light after all - and warm myself?
  2.  
    The first time I stood in front of one of those new huge flat screen tvs, mein gotte! you dont need a fire place anymore, just turn the telly on!

    I couldnt believe it, if you go into a show room you can stand a couple of meters away from the wall of TVs and get a suntan.
    • CommentAuthorludite
    • CommentTimeJul 4th 2008
     
    yeah, BUT that's the heat coming out the back mate! not the light out the front:bigsmile:
    • CommentAuthorstephendv
    • CommentTimeJul 5th 2008
     
    Google "concentrated photovoltaics" http://en.wikipedia.org/wiki/Concentrated_Photovoltaics
    • CommentAuthorjon
    • CommentTimeJul 5th 2008 edited
     
    Hello Roger & Sune

    Missed this thread the first time, sorry have been exceptionally busy of late.

    The article was written back in February or March from memory but I had very limited space and couldn't explain how spherical works in one page so it became a relatively light work about how the test came to be. The current primary application is to help communities, largely on the equatorial zone, to community cooking and autoclave facilities: I don't intend to make a profit from doing this. We did take out a few patents on some later aspects of the arrangement, however, patents are a bit of a lottery and rarely worth what inventors (or the public) think they are.

    The full UK tests are yet to be done. I won't know it it is feasible to use this for solar heat and/or sterling generation in the UK until we have proper figures in (which won't be until late September as spherical isn't easy to arrange for high latitude summer collection and works best in the other 9 months of the year). Looks good so far though.

    I don't know if this will be picked up by any aid organisations: A few have shown interest so far but for it to become a go we probably need a large corporate sponsor for the initial frame.


    jon
    • CommentAuthorjon
    • CommentTimeJul 5th 2008 edited
     
    Incidentally, the mirrors certainly do reflect heat: When I did the second unfocused test in June, the excess back-reflection around the boiler managed to burn the water pipe insulation at the back in a few minutes (which is why I had to wrap the water pipe insulation in foil)
    • CommentAuthorjoe.e
    • CommentTimeJul 5th 2008 edited
     
    Posted By: luditewell, I've seen the runners on the london marathon being wrapped in bacofoil. . . . . I've even wrapped someone in it myself. . . . . I use tinfoil on the chicken when I roast it for dinner. . . . . . but you would have to work pretty hard to convince me that I could warm myself up just by looking at the reflection of a fire in a mirror without having the fire itself. . . . . . could I put a picture of a fire on my laptop - which is light after all - and warm myself?

    Someone who remembers more physics than me can add more detail to this, but... heat reaches you in different ways. Air is heated by a fire, and this hot air warms you. But there's also infra-red: radiant heat. The hot sunlight that reaches you through all those miles of space is hot because as well as visible light it contains infra-red. Infra-red is reflected by mirrors - think of those electric heaters with curved, shiny metal reflectors behind the element to throw the heat forwards. And google 'solar furnace' for an example of really, really powerful reflected heat.
    • CommentAuthorEd Davies
    • CommentTimeJul 5th 2008
     
    Posted By: luditeI'm not sure about mirrors joe.e They reflect light just fine, but they don't seem to reflect heat - or maybe that's my imagination.


    By "heat" I assume you mean infra-red radiation. Unfortunately the term "infra-red" covers a pretty wide section of the electro-magnetic spectrum with quite different properties.

    Visible light has a wavelength of about 400 nm (nanometres, or billionths of a metre) for blue light to about 700 or a bit more nm for red light. At slightly longer wavelengths than red light is "near" infra-red up to about 2000 or 3000 nm (or 2 to 3 µm, micrometres or millionths of a metre).

    Most of the energy hitting the atmosphere from the sun is in the visible and near infrared part of the spectrum with, of course, some in the ultraviolet to cause suntans and skin cancer, etc. I'm not sure about the exact proportions but I think it's about 2/3rds visible and 1/3 near IR. Though our eyes don't respond to near IR (very much, they do a tiny bit I've read somewhere) for most other purposes visible and near IR act in pretty much the same way. Near IR will be reflected by most mirrors - though I expect there are some that are less good at it than others.

    At much longer wavelengths than near IR is thermal infrared - radiation given off by warm objects such as people, badly built houses and so on. As a quick rule of thumb the peak wavelength of the radiation given off by an object is 3 mm divided by its temperature in kelvins. An object at around room temperature is at about 300 K so gives off IR with a peak around 10 µm - of the order of 10 times the wavelength of near IR.

    This difference in wavelengths is sufficient that you can't just assume that mirrors which work for visible and near IR light will work for thermal IR or vice-versa. Just because a mirror doesn't do a good job of reflecting the heat from a fire (at, say, 300°C so about 600 K and therefore with wavelength around 5 µm) doesn't mean it will not do a good job reflecting the near infrared part of the solar spectrum.

    Also, of course, a large proportion of the energy going into a solar thermal collector arrives in the form of visible light.

    (By the way, the temperature of the surface of the sun is around 5700 K so by the rule of thumb I quoted the peak wavelength should be around 526 nm - actually it's about 550 nm)
    • CommentAuthorjoe.e
    • CommentTimeJul 6th 2008
     
    I haven't read the article, but I'm interested in the concept. Is it normal mirror glass? And, how do you add a picture to your comment?
    • CommentAuthorAlbert
    • CommentTimeJul 6th 2008
     
    On the mirrors & IR radiation topic, I used to be involved with military standard thermal imaging. Basically they're TV cameras sensitive to body-temperature infra-red. Glass is opaque to that waveband so the lenses were made from solid germanium.

    Using surface-silvered glass would solve the problem. The reason most mirrors have glass on the front is to protect the silvering. An easy alternative would be aluminium foil glued to a substrate. As this could be anything, including wood, it would be easy enough to make decent sized convex mirrors.
    • CommentAuthorjoe.e
    • CommentTimeJul 6th 2008
     
    It would be an absolute bugger to get a good surface finish with aluminium foil on wood, though - that's the other point of mirror glass; you can lay on a very thin film of the reflective coating, and because the glass is very flat you get an excellent finish. But I seem to remember that the glass used in two-way mirrors is silvered on the face, not the reverse - is that the surface-silvered glass you're talking about, Albert? Pilkington used to do a few varieties with differing proportions of light reflected and passed through.
    If glass is opaque to body-temperature IR, I wonder what happens to the IR? Is it reflected off ordinary glass, or absorbed?
    • CommentAuthorEd Davies
    • CommentTimeJul 6th 2008
     
    Posted By: AlbertOn the mirrors & IR radiation topic, I used to be involved with military standard thermal imaging. Basically they're TV cameras sensitive to body-temperature infra-red. Glass is opaque to that waveband so the lenses were made from solid germanium.


    Short version of my previous post: solar radiation contains very little body-temperature infra-red compared with the amount of energy arriving in the visible and near-IR parts of the spectrum.

    Using surface-silvered glass would solve the problem. The reason most mirrors have glass on the front is to protect the silvering. An easy alternative would be aluminium foil glued to a substrate. As this could be anything, including wood, it would be easy enough to make decent sized convex mirrors.


    There isn't a problem to solve.
    • CommentAuthorjoe.e
    • CommentTimeJul 7th 2008
     
    Ed, do you know how easily the near-IR passes through glass?
    • CommentAuthorjon
    • CommentTimeJul 7th 2008 edited
     
    Hi Joe, Ed Albert


    As Ed says, I'm not sure the mirror problem exists except for places like Arecibo where low frequency collection may be required.
    http://en.wikipedia.org/wiki/Arecibo_Observatory

    The mirrors in the picture are just ordinary glass from IKEA which have had a little extra protection treatment applied to the rear and then fixed to flat frames on off-set timber stools to mimic a sphere.

    For Africa in particular, we would need a durable cheap reflector material (because wind and sand become more of a problem than rain) and rear mirrored glass seems to fit the bill well: At the equator, the mirror glass would essentially be a fixed bowl on the ground so it becomes very inexpensive to provide large arrays compared to the costs when the mirrors have to move (as they must do in all types of concentrating array other than spherical).
    • CommentAuthorjoe.e
    • CommentTimeJul 7th 2008
     
    So the object you're heating moves to track the moving focal point of the mirror array?
    • CommentAuthorjon
    • CommentTimeJul 7th 2008
     
    Yes, that's it joe: All the cost is essentially condensed into the receptor and the tracking along the arc.

    This has a great advantage because the velocity of travel along the arc is constant and so only needs simple timing devices to control movement rather than complex tracking.
    • CommentAuthorEd Davies
    • CommentTimeJul 7th 2008 edited
     
    Posted By: joe.eEd, do you know how easily the near-IR passes through glass?


    I don't have any specific figures but:

    1. Ordinary photographic lenses can be used to take near-IR pictures.

    2. Window glass doesn't heat up in bright sunshine anything like as much as a dark surface (when a substantial amount of the solar radiation is near-IR).

    3. IR was discovered by diffracting the near IR out of the solar spectrum using a glass prism.

    4. Near IR is used for telecommunications down glass optic fibres (tends to be rather special glass with only small and well controlled amounts of extra stuff in it but it's effectively many kilometres thick rather than a few mm).

    5. There's no particular reason to think that ordinary glass would somehow have the same rather arbitrary cut-off point as the human eye.

    By the way, this graph is well worth a look:

    http://en.wikipedia.org/wiki/Image:Solar_Spectrum.png

    The thing I like about this version is that the vertical axis (W/m²/nm) uses the same units as the horizontal axis (nm) so the power in a particular part of the spectrum is simply proportional to the area under the curve.
    • CommentAuthorEd Davies
    • CommentTimeJul 7th 2008 edited
     
    .
    • CommentAuthorjon
    • CommentTimeJul 8th 2008
     
    That's a great graph Ed
    • CommentAuthordickster
    • CommentTimeJul 14th 2008
     
    Just found this article.

    Thought I should mention that I slipped a redundant mirror behind my freestanding 10 tube array. No idea whether it improved things, but seemed an easy solution to "what do I do with this big,unwantd mirror"
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