# How to design solar reflectors for solar cookers

You can design many solar reflectors for solar cookers, and other things that need reflectors, without doing any mathematics. In fact you can often get by knowing just two things:

1. If you are pointing directly at the sun then the sun's ray are arriving straight at you across your entire reflector.

2. The angle that a sun's ray reflects off your reflector is the same as the angle that it arrived at.

This is also true if you're designing a parabolic solar cooker (see below), funnel solar cooker, CooKit, Pavarti, or any kind of solar cooker that has reflectors.

A helpful rule of thumb, shown below, that works for many designs is that the only sun's ray you need to worry about is the one that reflects off the top of your reflector. All sun's rays that reflect lower down will hit your target surface somewhere as long as the top ray hits your target on the opposite side. This is made use of in the calculators below.

## Using multiple reflector angles

Solar box oven cookers cannot always be tilted to face the sun if the sun is low in the sky. Doing so may cause the cooking pot to tip over, or if the pot is on a swivel base, it may already be at it's maximum swivel angle.

To handle this situation you'd tilt the oven as much as you can. Then, as shown in the following diagram, you'd put the top reflector on a very steep angle and the bottom reflector on a very shallow angle. Note that the angles are adjusted below such that the angles that the sun's rays reflect off the reflectors are the same as the angles that the sun's rays arrived at the reflectors. This is just as is decribed at the very top above as the number 2 thing to know.

A handy tool for measuring angles is a protractor. If you don't have one the you can download and print one from here.

## Improving the sunlight capture area

The capture area is basically the amount of sunlight you're capturing and then concentrating using the reflectors. It's basically the size of the hole made by the top of the reflectors. If you're not familiar with this concept the click here for a page all about sunlight capture areas.

So does simply making the reflectors longer while keeping the angle the same as shown in the following diagram improve the capture area? The answer is no, since the sun's rays that strike the top of the reflector will eventually reflect back out without hitting the target area and the cooking pot in a solar cooking oven in this example.

So you have to change the angle as well as the reflector length (see the following diagram.) Two methods are given below to show how to do this. However, you can see that as you increase the reflector length, and hence the total height of the reflector, more and more, the improvement you get in capture area is less and less.

This is because of the uniform shape of the reflectors and/or because of the location of the target. If you really want to increase your capture area a lot then a better way is to start moving your target in front of the reflector as shown in the following diagram. Compare the capture area of the parabolic reflector below, 1097 square inches, to the capture area of rightmost reflector above, 603 square inches. Note: they are both drawn with the same scale.

A calculator for working out your capture area for some types of reflectors without doing any math is given below.

## Two methods for figuring out solar reflector angles, lengths, ...

 A popular thing to do is to make your reflector length the same as your target width. In that case the optimal reflector angle is around 60 degrees and the above rule of thumb regarding the top ray will just happen.

Two methods are suggested below for figuring out your own parameters. You can either:

• make your reflector by eyeballing things and getting a very accurate result as talked about next, or
• you can use some of the calculators given further below.

### Method 1 - Eyeballing to get optimum reflector angle

Simply put, you pretend that your eye is the sun and you look at your reflector from the sun's perspective, as shown below. If you see your target surface then that means the sun will too. Look straight down at what is reflected in the top of your reflector. Keep adjusting the reflector angle until you see the opposite part of the target area reflected into your eye. Once you see that then you know the rest of the reflector is good too, as discussed above. If it helps, put a brightly colored object in the target area, like the red ball shown below.

I used this technique very successfully on my cone solar cooker. The following photos and video show in great detail how I did it.

### Method 2 - Some helpful calculators (no math needed)

The calculators below are for finding the optimum reflector angle, an angle that will cause a reflector to cover the entire target area. This works for designs where you need only make sure that the sun's ray that hits the top of the reflector reflects to the opposite side of the target.

#### Square target versus rectangular target

If your target is square, i.e. all sides are the same length, then you need only use one of the calculators, depending on what information you have available, reflector length or vertical height.

If you target is a rectangle, i.e. shorter in one direction and longer in the other, then you need both calculators. That's because the reflectors will be at different angles and have different lengths in order to cover the entire target area as this diagram shows.

So if your target is a rectangle,

• if you have a reflector length in mind already, start with the first calculator and give it the reflector length and the length of the longer side of the target. This will then tell you the optimum angles and the vertical height. Then go to the second calculator and give it that vertical height and the length of the shorter side of the target. It will then tell you the optimum angle and a shorter reflector length.
• if you have a vertical height in mind already, start with the second calculator and give it the vertical height and the length of the longer (or shorter) side of the target. This will then tell you the optimum angles and the reflector length. Then go to the second calculator and give it that reflector length and the length of the shorter (or longer) side of the target. It will then tell you the optimum angle and a different reflector length.

### Calculator for optimum reflector angle - given target width and reflector length

This optimum reflector angle will reflect to the entire target surface area.

#### Step 1. Input values below and click on Calculate...

Width of target surface: (inches)
Length of reflector: (inches)

#### Step 2. Use one of the following optimal angles...

Optimal angles for reflector are: Outer = degrees, Inner = degrees.
Vertical height of reflector is = inches

#### The formulas used:

The original source of the above formula for the Inner angle is David Omick's Solar Oven Design webpage.

### Calculator for optimum reflector angle - given target width and vertical height

This optimum reflector angle will reflect to the entire target surface area.

#### Step 1. Input values below and click on Calculate...

Width of target surface: (inches)
Vertical height to top of reflector: (inches)

#### Step 2. Use one of the following optimal angles...

Optimal angles for reflector are: Outer = degrees, Inner = degrees.
Reflector length is = inches

### Calculator for capture area

This will calculate the area of sunlight you'll be capturing.

#### Step 1. Input values below and click on Calculate...

Two sets of values are asked for, A and B, in case you have a rectangular target. If your target is square then just repeat the values.

Width A of target surface: (inches)
Width B of target surface: (inches)
Reflectors A length: (inches)
Reflectors B length: (inches)

If you're taking the following angles from the calculators above then make sure you're using Outer angles, as shown in the diagram on the right.

Angle A: (degrees)
Angle B: (degrees)

#### Step 2. Calculated capture areas...

Approximate capture area = square inches
(no corners)

These reflectors don't extend to the corners because any sun's ray that started at a corner would just reflect from reflector to reflector until it left altogether. It wouldn't hit the target area.

Approximate capture area = square inches
(with corner pieces)

Corner pieces are added that do reflect sunlight to the target.

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