The angle and size of solar funnel is interdependent. The idea is you want to reflect the suns energy into your solar box oven, that would have otherwise fallen outside the box.
This is a cross section of the funnel and part of the oven. The solid vertical lines represent the inside of the solar oven, and x is the inside measurement. We will assume the oven is square. The height of the funnel, y, you’ll have to pick based on convenience, really. The larger it is, the more sun it collects, but the harder it is to carry around. So we know x and y, we have to calculate the angle. The angle should be such that sun hitting the top of the funnel is reflected into the oven, if it’s too shallow, reflected sun will hit the reflector on the other side and is just wasted. Too steep and we’re not collecting as much sun as we could.
The formula for working out the angle is:
You’ll want to round up a degree or two just to allow for tolerances.
See the solar funnel calculator post which will calculate the angle for you (aren’t we kind)
One side of your funnel should look like.
Some examples to check if you’re punching the right buttons. If x = y, say they are both 50, cos theta = 0.5, inverse cos 0.5 = 60. So the inside angle on the diagram to the left is 180 – 60, which equals 120 degrees.
If y is twice as large as x, say y = 100, and x = 50. cos theta = 0.366, inverse cos 0.366 = 68.5 degrees. So round up to 69 (or 70) and 180 – 69 = 111 degrees
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One Comment
is there anywhere online where there is a step by step process on how to find the formula for theta?
I have been trying to figure it out and im not sure how to get to the final formula you gave us.
thank you