The capture area is the area of
sunlight that you'll be capturing. If you just sit a black
jar out in the sunlight then the area of the jar exposed to
the sunlight is the capture area. If you have a box with
a window then the window is the area of sunlight you'll capture
and no more. If you have reflectors then it gets more
complicated.
Capture area of a standalone black jar.
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Note that sunlight doesn't always come directly from the
sun. I also reflects off of clouds, snow, water and other
reflective surfaces. For most solar cookers this won't change
the area of the hole, the capture area, that the sunlight has
to enter through. An exception to this would be a standalone
black jar when light can hit it from all sides, though unless
the reflective surface is something highly reflective like snow
or water then it won't make much difference.
Capture area with reflectors
Often people think the capture area is the surface area of the
reflectors, but it isn't. To bring this point home, the
diagram below shows a ridiculously long reflector on the
left along with its capture area. It should be clear that the
shape of the walls of the reflectors don't affect the amount
of sunlight captured compared to the two reflectors to the
right of it.
All the following have the same amount of capture area.
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Note that a badly designed reflector, possibly combined with other
reflecting surfaces, can reduce the benefit of a large capture area
by reflecting the sunlight right back out of the solar cooker.
The following photo shows an early stage of the design of
my cone solar cooker,
illustrating that with large rectangular reflectors
a lot of the sunlight would have been lost. Shortly after realizing
this I designed a cone for it instead.
Path of sunlight through a badly (and partially) designed solar cooker.
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Finding the capture area using a shadow
If your cooker doesn't have any reflectors on it but instead
consists of just a window on a box then the capture area is
the area of the window (see rightmost cooker in the diagram of
different capture areas above.)
But if your cooker has reflectors then point the solar cooker
directly at the sun, like you would want to when cooking (see the
diagram below.) Put a flat surface behind the cooker for the shadow
of the cooker to fall on. This surface must be facing
the sun directly, like your cooker is. The shadow made by the
reflectors on the surface is the capture area. You can make
measurements of the shadow for working out the capture area.
Using a shadow to determine the capture area.
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Capture area calculation example
When I built my cone solar
cooker, at one point during the design I had to figure out if the
cone would be adding a significant benefit to the cooker. The cooker
already had a large reflective bowl and so a cone reflector may not
have been needed.
Cone solar cooker showing the cone and bowl.
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Showing just the bowl.
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Calculate the capture area of
just the bowl. The capture
area for the bowl is a circle and the formula for the area of a
circle is the radius of the circle multiplied by itself multiplied
by pi (pi is always 3.14). So you'll need to measure the radius
of the bowl (the radius is the distance from the center of a circle
to the edge of the circle as shown below.)
Bowl capture area calculation.
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Calculate the capture area of the cone. The capture area
for the cone is the circle made by the top of the cone. Once
again, the formula for the area of a
circle is the radius of the circle multiplied by itself multiplied
by pi (pi is always 3.14).
Cone capture area calculation.
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Calculate how much bigger the cone capture area is than the
bowl capture area:
cone capture area divided by the bowl capture area = 314 / 132 = 2.4 times the area
or in diagram form...
I figured that 2.4 times the capture area would be worth it so I went
ahead and made the cone.
