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?

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?

you trying to say that multi foil doesnt work Ludite!

careful.....

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.

Google "concentrated photovoltaics" http://en.wikipedia.org/wiki/Concentrated_Photovoltaics

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)

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)

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.

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.

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).

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.

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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"