Half a croissant, on a plate, with a sign in front of it saying '50c'
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DIY solar BBQ

cooking with stuff around the house
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Get an old umbrella (bigger the better) Sew a bunch of discarded compact disks, like the ones AOL sent you in the mail, onto the fabric inside the umbrella. Be sure the CD have the shiny side up. Find the focal point on the Umbrella handle. Build a grill at that point. Take it out on a sunny day and heat your weiners!
the great unknown, Jan 18 2007

(?) solar cookers http://solarcooking.org/
[nth, Jan 18 2007]

solarbbq http://www.users.bi.../cooker/cooker1.htm
I tried to make a test model of one of these out of cardboard & aluminum foil, but mine didn't work very well. [Zimmy, Jan 19 2007]

Umbrella with aluminum sheet metal?. http://solarcooking.../plans/barbeque.htm
I was looking for something else, though. [Zimmy, Jan 19 2007]

[link]






       Are umbrellas parabolic?
angel, Jan 18 2007
  

       //Are umbrellas parabolic//   

       Yes, all of the unbrellas i've seen seem to at least approximate a parabola.   

       CD's however, are not so good. The refract, as can be seen from the ranbow effect you get when looking at them. How about using mylar film?
webfishrune, Jan 18 2007
  

       They may refract, but I can still see myself in one just like a mirror. I was thinking of using household junk, so Mylar may not be around. How about old can lids? may need polish.
the great unknown, Jan 18 2007
  

       Baked. Many good plans at link. They really do work and I use one myself (homebuilt). Aluminum foil is very versatile.
nth, Jan 18 2007
  

       I'd suggest that solar cookers were widely known to exist. The large majority are home-built from household junk. The particular choice of junk (the umbrella) is novel at least, but I don't think you'd get a high enough temperature with an umbrella-sized cooker and CDs. I'll go do some maths and find out.   

       [edit] Interesting. If insolation is taken as 1kW/m2, then a 1m umbrella will focus about 800W onto a focus which must of necessity be the size of a CD. That gives an insolation at the focus of 36kW/m2 assuming 50% reflectivity of the CDs, which suggests that the focus will reach equilibrium about 350C. That's plenty high enough.
david_scothern, Jan 18 2007
  

       How did you determine equillibrium temperature? Did you omit a whole swag of variables and calculations from your anno for the sake of brevity, or do you have some cunning way of easily computing equillibrium temperature?
Texticle, Jan 18 2007
  

       I'm working on a satellite dish and sisalation version as we speak
BunsenHoneydew, Jan 19 2007
  

       [BunsenHoneydew], I remember reading about how quickly you can damage your eyes on some of the larger reflectors - be careful.
Zimmy, Jan 19 2007
  

       Thanks, all. Looks like it may be worth giving this a try. I seen in the link that people already made umbrella & foil cookers. I'm not even sure that sewing the CDs will hold, any other suggestions, like glue?
the great unknown, Jan 19 2007
  

       [Texticle] Find a figure for insolation as watts/m2, calculate the cross-sectional area of the umbrella to get the total amount of solar power the umbrella is receiving, reflect a percentage of this off the CDs to a focus that must be no smaller than the CDs themselves, then use a black-body radiation calc - Power = 5.67E-08xT^4, where T is in Kelvin - to find the temperature at which the focus will give off all of the power it receives. A bit crude, but hopefully close.
david_scothern, Jan 19 2007
  

       also, keep in mind that unless the CD's are overlapped, like fish scales, it may not cover the entire surface & be less effective.
the great unknown, Jan 22 2007
  

       Indeed. They should be overlapped; I don't see any inherent disadvantages in doing so.
david_scothern, Jan 22 2007
  

       don't forget the catch at the bottom for all the fat and juices exuding from the meat when cooking... Gravy.. mmmm
krewl, Jan 22 2007
  

       /36kW/m2 from an umbrella of any kind. Thats 10 kettles !/   

       You have 3.6kW kettles? We top out at 2.4kW here. Maybe "Two Cups of Coffee" isn't such a fanciful idea afterall.
Texticle, Jan 22 2007
  

       //how quickly you can damage your eyes// Mmm. I'm thinking you could pretty quickly give yourself third degree burns at the focal point. Mine's a 2 meter dish, and should be parabolic to roughly a 1cm2 scale, and I've seen midday insolation figures locally of 1kW/m2, so that's about 10kW at the focus.
BunsenHoneydew, Jan 23 2007
  

       [BunsenHoneydew] a 2m dish has an area of pi square metres, so you're receiving 3.14kW. Focussing that into 1cm2 would give you 31,400kW/m2. That should cook... a bit?
david_scothern, Jan 23 2007
  

       Mmmmyes. I used the diameter as the radius, silly me. And I threw in an 80% figure for reflectivity. And I went for the raw kilowatts rather than kW/m2 because, unless I'm melting steel, I probably won't want to have my target exactly at the focus. But yes it should cook up a snag a treat. I have vague plans to do some steam turbine electric thingy with it.
BunsenHoneydew, Jan 24 2007
  

       Not mmmbrella, the YUM-brella! :P
the great unknown, Feb 05 2007
  

       Churn.
Texticle, Feb 05 2007
  

       We had to do something like this in Algebra 2 with the parabolas, except it was cardboard and tinfoil, still in perfect condition, worked well, i think. (I didn't eat the hotdog, don't like them.)
krigre55, Dec 04 2007
  
      
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