Solar Parabolic Collectors

[Antman]

Someone has been playing around with solar collectors like parabolic mirrors for cooking in the backyard, parabolas or nearly so. What happens to the rays passing through the focus to radiate any direction, upto 180º. Depending on the perfectness of the parabola OR NOT, is there not a liklihood of dangerously high radiation, maybe millions of times greater than natural sunlight, that can be focused on small areas, hot spots, anywhere remotely in front of your dish, the closer you get to it, the very much worse it must be for you.
I expect someone with better knowledge than I of optics, to be able to discuss the inherent dangers, also taking into account that many of these mirrors are far from perfect, an example of their construction being petals of aluminium polished sheet bent into a rough parabola with dents, flat spots and bulges, and the pot might not be at the focus.
I suspect that these things are VERY DANGEROUS even in small sizes and any accuracy, and may be damaging to not only eyes or skin.

 

[rickharris]

There aren't any issues other than getting burned or damaging your eyes from looking into the focuses reflection of the sun.

Why are you thinking of building one? Sterling engines have often been sun powered.

 

[Antman]

So far my shop is 6 weeks away from having any machines in it, first I must get a little old Ford out my workspace. My son has quit his first job as an apprentice tool and die maker in a smallish shop with non safety anything, which is how bad employment is in this country. He has a good problem solving mind and instead of doing many of a part for some machines he never sees, he wants to see a working machine thru from start to finish. I’ve always wanted a machine shop and only now I guess I have the time. Somehow we have steered this past his Mother. Me too, if he doesn’t learn something, v.b., very bad.
I think my main interest will be mods to the lathe and the milling machine itself, low temperature differential engines and ultimately I.C. like 4stroke diesel.
I saw a 1,2m diameter solar cooker, made of 18 polished aluminium petals, in operation and my first impressions was that this was something potentially very dangerous. I think I did some of the maths and it looks like to me even at 10m it can hurt you bad.

 

[tel]

Solar cookers? That technology has been around for at least 30 years and probably considerably longer and I've heard no reports of injuries. I have a book here that dates from the mid 70's that gives the 'how to' on building one, along with how to produce an accurate parabola. Can't see a problem, frankly.

 

[Antman]

Basically the problem is the more accurate your mirror is the more dangerous it is to get you hand anywhere near the focus. Where do the rays go after passing thru the focus if not into your pot? And with a crudely built parabola won't there be hotspots of radiant energy quite outside the dish?

 

[ksouers]

Antman,
Please don't take this the wrong way, but maybe you are worrying about this reflector too much. As tel said, they've been in use for decades. Yes, absolutely, certain precautions should be taken. But you would do that with any heat source, wouldn't you? In this instance dark sunglasses are definitely in order. I wouldn't go sticking my hand at the focus, nor would I stick my hand directly in a flame.

 

[Antman]

Understand where I am coming from. Free non polluting energy is great and at upto 1 kiloWatt per square metre solar power is definitely usable for stationary power. The photons received come at us with the temperature of the Sun's surface. Here in Africa these solar cookers can find their way into the hands of less than gourmet chefs. Cooking is somewhat hands on, stirring the pot, seeing your stirfry saute nicely and taking your food out of the fire which is actaully far away behind you. I've been around arc furnaces and basic oxygen furnaces (the new take on Bessemer) and nothing has given me as bad a feeling as a flimsy 1.2m reflector.

 

[Maryak]

Can't see a problem, frankly.

Tel,

Me neither.

Best Regards
Bob

 

[Bluechip]

You don't see a problem in Oz, or even here in UK. Nothing to do with the weather either.

If you have some 700W of Solar power per sq. metre, and focus this onto a target of say 0.1 sq. metre you will get a 'target power density' of 7kW per sq. metre. If this exceeds the targets capacity to dissipate power it will get hot. Hopefully to the point we get useable energy from it, (energy ( joules) being watts x seconds).
Any solar power that does not hit the target will radiate outwards, but will be not significantly different to the sunlight being collected in the first place.

Different matter if you get anywhere near the focus.

If there was a problem with reflected sunlight, we would be cremated if we passed a shop window. This has never happened to me, even in UK. Or if it has, I've not noticed ..

Dave

 

[Antman]

My interest in parabolic reflectors stems from the fact that I want to get about a half kilowatt from a solar powered sterling engine, and only having seen a solar cooker made from 18 triangular petals of thin polished aluminium and held together with pop rivets and a circular rim, in operation I am definately having second thoughts until I can make a near perfect parabola. Consider a dent in a flimsy mirror just described. Say it has an area of 100 sq cm and this dent focuses to an area of a couple of sq mm somewhere outside of the dish. This spot of light has many times the intensity of sunlight. We are conditioned not to stare at the sun but the way we watch pots it's surprising we ever get a cooked meal or a cup of tea.

 

[Noitoen]

Here's an idea http://hackaday.com/2005/03/23/solar-death-ray/

 

[Noitoen]

Also an interesting project ;D

http://hackaday.com/2008/12/06/track-the-sun-for-home-lighting/

 

[tel]

As I see it, it is highly unlikely that our dent will be focused to any great degree, and the effect would be minimal. That is the problem with solar reflectors - keeping them focused on the sun - it has a tendency to move about, and unless you have computer tracking .....

 

[Tin Falcon]

If you are concerned about the dangers of a solar cooker stay away from oxyacetylene torches metal melting furnaces etc. Metal working can be dangerous to the misinformed.
Tin

 

[Kermit]

http://www.power-technology.com/projects/victorville/victorville6.html

 

[Antman]

Well I am sure there is a fair size fence around the place, and that's not only to stop people writing on the mirrors.

 

[Thad Swarfburn III]

Here's an interesting one, plastic water bottle in car in bright sunlight:

http://www.hoax-slayer.com/car-fire-water-bottle.shtml

No fires reported yet, but many have small burn marks in their upholstery! Just goes to show that as careful as you think you are, there are hidden dangers everywhere.

Matt

 

[tel]

Despite the popular myth, there has NEVER been a confirmed, recorded instance of a fire being started by shining through a bottle.

This snip from the above site gives the clue;

Thus, if the sun's energy is concentrated through a water bottle on to combustible material, then it is possible that fire could result. I conducted experiments using the same kind of bottle featured in the above photographs and I found that, by focusing the sun's rays through the bottle onto a thin plastic sheet, I could quite easily burn holes. The following photographs illustrate the results of one of my experiments. A hole was burned through the plastic after around 30 to 40 seconds of placing the bottle. A small indentation was also melted into the hard plastic tool-case that I used to support the bottle and plastic sheet:

The trouble is that the sun is constantly 'moving' from our perspective, and will not hold focus in one spot long enough to reach combustion temperature. Ever tried to light a fire with a magnifying glass? It ain't easy, even with the adjusted focus and suitable tinder.