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Fresnel lens vs. reflectors Answered


I would like to make a solar meat BBQ, which is more efficient in space to heat ration. a Fresnel lens or reflectors pointing to where the food is?


7 Replies

Kiteman (author)2009-10-22

Problem: a lens is obscured by smoke and gets between food and chef.

Problem: a reflector is obscured by dripping fat, and shaded by cooking food.

Solution: Set a reflector or a lens to the side of the BBQ, and use a mirror and/or light-pipe to direct the beam at the food from a 45o angle, either from below or above.

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phddas (author)Kiteman2009-10-22

sure I could do that. however that would be another step which coasts efficiency.

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kelseymh (author)2009-10-22

They are equivalent, in the ideal case. 

You already understand that you cannot get more energy out than comes in in the first place.  The area of the reflector, or the area (aperture) of the lens, determines exactly how much energy is coming in.

There is no a priori relationship between absorption losses through a thin transparent material and absorption losses at a reflective surface.  Different materials, different interactions, different conditions. 

You need to look up two numbers:  the transmission coefficient for the glass or plastic used to make your lens, vs. the reflectivity for the metal you use for your mirror.  Whichever number is closer to 1 gives you your answer.

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phddas (author)kelseymh2009-10-22

indeed the smoke coming out of the food will reduce the transmission coefficient of a lens specially if the wind chooses to blow it onto the lens direction.
With the reflectors I have to worry about a drip tray which is easier to control than the smoke however the problem with reflectors is removing the food from the direction of the light so that no food shad shows on the reflectors not to mention finding the best reflectivity materials to use.

the location of the drip tray can be overcome with making the focal point's vertical axis away from the edge of the reflector.

The second problem is open for suggestions.

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lemonie (author)2009-10-22

Reflectors tend to find themselves underneath what you're cooking, and lenses on top. One gets covered in drips & spits, the other in smoke & spits. You'd find it easier to make a reflector from carboard & foil than a lense with a wide focal point, so I'd try a reflector first.
In energy terms you look at the surface capturing the light: i.e. if sunlight is x watts per square metre, how much sunlight does each design harness in terms of area?


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V-Man737 (author)2009-10-21

The Fresnel lens is going to be more efficient.
Using a Fresnel lens is more like cooking something in a microwave. To emulate a BBQ, you'd want the slow patience of reflectors to heat it underneath. And the grill. That's important too.

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phddas (author)V-Man7372009-10-22

a hypothetical question: given a Fresnel lens of the the same size and focal point as a reflector. which would produce more heat?

what I am trying to get to is this;
is the energy lost when the sun light passes through the lens equal to the energy lost when the rays are reflected?
if not, which and why?

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