# How much surface area of mirror would you need to boil 16 oz of water in less than 5 minutes using concentrating solar?

I'm working on a coffee machine powered by the sun and need a little help with the mirror sizing. How much surface area of mirror would you need to boil 16 oz of water in less than 5 minutes using concentrating solar? Can I do this practically with a dish less than 1 meter square? If I get this to work I'll post it for sure.

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8 years ago
Starting here with 600 watts/hour/square meter at 40 degrees latitude during summer in five minutes you would have 50 watts/hour/square meter assuming 100 percent efficiency (not gonna happen). One watt/hour is aprox 850 calories (50 x 850=42,500) and each ounce about 35 grams (16 x 35=560), assuming a temp change from 15 degrees C to 100 degrees C (100-15=85) then (560 g)(85 degree change)= 47,600 calories needed. 47600/42500=1.12 square meters IF YOU CAN GET 100 PERCENT OF THE LIGHT TO REFLECT FROM THE MIRROR AND TURN TO HEAT (and you don't loose any off the unlit side).

I'd guess that a black metal container would need at least 3 square meters to allow for heat loss, reflected light, mirror errors, ect.
robbtoberfest (author)  jtobako8 years ago
Thanks!!!! I'm speechless. Thanks!!!
8 years ago
Be very careful. The good lenses will boil asphalt in 5-10 seconds (I've seen it, it was hard to believe). So it might even melt glass. Use like a magnifying lens: focus the sunlight to a point.
chuckr448 years ago
A 12 inch square fresnel lens should do it, and melt your plastic container to boot. Or did you want reflected solar light, not concentrated light? Try your bookstore for a page size "magnifying lens" which are often fresnel lenses, about \$10-20us.
robbtoberfest (author)  chuckr448 years ago
Sounds interesting and possibly easier to do than parabolic mirrors. I'll be tinkering with a lens by the end of the week. Thanks!
8 years ago
if the full page magnifiers fail here is a tid bit of dangerous information, most rear projection tvs use a giant Fresnel lenses for the screen(you can melt coins with them), just keep an eye pealed at your local recycling center.
robbtoberfest (author)  cokebottle tuque8 years ago
The word "dangerous" always puts a twinkle in my eye (with goggles) for some reason. LOL
gaiatechnician8 years ago
I have done tests where I heated up 7 liters of water at a time to boiling in a solar cooker. https://www.instructables.com/id/Make_a_quotcompoundquot_parabolic_solar_cooker/ and I have the results there. If you can plan your day well, and point your solar cooker in the right direction, there is no reason for the urgent 5 minute to the boil time, Must it be boiling for good coffee? 95C is much easier to get and maintain. 100c ic boiling.
For long period boiling of water, a parabolic dish is not a good shape because it needs to always point at the sun. Better shapes are a hemispherical dish and compound parabolic dish.
I will upload my first tests with modling software to the instructable to show this in the next 10 minutes! Brian
robbtoberfest (author)  gaiatechnician8 years ago
Ideally coffee should be brewed in about 5 minutes or less; that's why I'm going for the 5 minute time. Also I want to really display the power of solar energy with this project in a fun way.
8 years ago
Sure coffee should be brewed in 5 minutes. Does that preclude preheating the water?
robbtoberfest (author)  gaiatechnician8 years ago
Good point. I still want fast coffee. Once I smell the grounds, I hover around the brewer until it's done.