What About Parabolic Solar Cookers?

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So what is a parabolic solar oven?

Strictly, a parabola is a two-dimensional shape, which could be drawn on a flat piece of paper. The three-dimensional shapes discussed below are paraboloids. The distinction is like the one between circles and spheres. However, use of the word "parabola" for a three-dimensional shape is common. The authors of most of the following material have used it that way. Likewise, they have used the adjective "parabolic" in place of "paraboloidal". Readers should understand these words accordingly.

The sun's rays are collected using a reflective surface like a parabola or curved surface. The main difference between a parabolic solar oven and other solar cooker designs is that the parabolic shape focuses the sun's rays into one point; called the focal point. A small focal point means a greater concentration of solar energy. With a parabolic solar oven, it is possible to cook food at the same rate that food is cooked in a conventional oven.

Parabolic geometry is well known, and it was probably the very first type of solar cooker. In this category, there are maximum variations. The reason for its popularity was the focus which was much better and sharper than that of other types of reflectors, but at the same time it was very sensitive to even a slight change in the position of the sun and hence the use of such reflectors means constant tracking.

Parabolic reflector diagram

It is very important to learn about the geometry of the parabola. There are many a reviews, but the prolific programmer mathematician Mr. Mike Scirocco, from California, recommends the paper presented in Solar Energy, of 1978 by Dr. M. Srinivasan et al. Mike has taken lot of trouble to convert the text into a web page. This is not all. Mike has a small program which can give you a lot of dimensions on parabola. You just have to enter the diameter and depth of the parabola and you get a whole lot of data. Mike seems to have taken my suggestion seriously and is planning to add more data display boxes which would tell a interested person as to the diameter of flat circle of sheet from which a parabola of desired focus and diameter are to be constructed. Visit Mr. Mike Scirocco's site to download his excellent program. It is freeware. http://mscir.tripod.com/parabola/

One disadvantage of a parabolic reflector is that the solar energy is concentrated in a very small area, too small to be desirable for most cooking purposes. Moreover, this area moves quickly across the cooking container, unless the parabola is adjusted frequently. Parabolas with focal lengths of 1 to 3 meters (3 to 10 feet) are most useful for solar cooking. The focal length is the distance from the center of the parabolic surface to the focus. The solar image diameter is about 1/120 the focal length. For focal lengths in the range above, the image of the sun will be 8 to 25 mm (1/3 to 1 inch). As the earth rotates, the image moves 87 to 260 mm (10 image diameters) in 20 minutes. Building a parabolic solar oven really isn't as difficult as it may seem, once the basics are first understood.

There are many popular solar cookers in use today, including the Sun Oven, Sports Solar Oven, and the Cookit, to name but a few. However they all adopt either the solar box or solar trough design. Very few designs are actually parabolic, those that are, are generally quite expensive, compared to the more popular designs. The box solar cooker for example, is simply a cardboard box with reflective panels and a pane of glass which traps hot air in a box.

Why build a parabolic solar oven?

Building a parabolic solar oven will result in a higher power output than other solar cooker design. Remember, the smaller the focal point, the higher the cooking temperature. Water can be boiled in seconds using a sufficient sized parabolic cooker. However, due to the high level of accuracy required in the manufacture of parabolic cookers, a minimal focal point is very difficult to obtain.

Very high cooking temperatures can be obtained with a parabolic solar cooker, thus drastically reducing the time taken to cook food and to boil water. The box design solar cookers generally take several hours to cook food.

To summarize, building a parabolic solar cooker offers faster cooking times than other more conventional solar cooker designs. However, bear in mind that parabolas need more frequent adjusting, pointing to the sun.

Often known as, and called concentrator cookers, these are usually a bit more complicated to construct due to the design and necessary materials, but still are quite common in some areas of the world.

Cooking is accomplished more rapidly, but more frequent adjustment of the concentrator is needed for maximum effectiveness. It is often necessary to use them under the supervision of experienced hands since they can cause harm if used improperly.

Building a parabolic solar oven really isn't as difficult as it may seem, once the basics are first understood. There are many popular solar cookers in use today, including the Sun Oven, Sports Solar Oven, and the Cookit, to name but a few. However they all adopt either the solar box or solar trough design. Very few designs are actually parabolic, those that are, are generally quite expensive, compared to the more popular designs. The box solar cooker for example, is simply a cardboard box with reflective panels and a pane of glass which traps hot air in a box.

How to Build a Parabolic Solar Cooker

Several methods are available, one easy and ready-made solution is to use a satellite dish. Then line the inside of the dish with a reflective material such as aluminum foil, or better still Mylar. Anodized aluminium could be used, even small segments of mirror may not work because the focal point would become 'scattered' as the mirror and anodized ally would not give a true parabolic shape.

After researching and testing countless parabolic and general solar cooking designs, the following conclusions have been made:

- First of all, work out what size parabola you want to build. As a rule of thumb, every one square metre of reflective surface equates to 1kw of solar energy. This isn't quite the case for a paraboloid due to its shape, but the rule will suffice. A one metre diameter parabolic which has been engineered to a high spec will pump out more than enough heat to boil water in a few minutes.

- 800Watts or more of solar energy is sufficient for cooking your solar food.

- Establish a focal point. Let's assume you are building a one metre dish. A suitable focal point would be approximately 20cm as in Figure 1. Figure 2 shows a focal point of 25cm which wouldn't produce the same results as the focal point of 20cm. This is due to factors such as wind, stand design, and cooling effects. With the cooking vessel situated at a focal point of approximately a third of the way down from the top of parabola, this will ensure sufficient wind protection, as well as an area where the heat is 'captured' and retained.

It is also important to choose the right cooking pot for the cooker. It has to be dark in colour and placed in the correct position, namely in the middle of the focal point. It also needs to be oriented to the sun and placed on a small stand that sits at the point of focus for the oven.

Parabolic solar cookers can work for 60 to 90 minutes unattended. If you find that the food is not cooked or that another pot has to be heated, then simply adjust the dish to point to the sun every 15-20 minutes or so. then the sun rays will 'bounce' on the top and sides of the cooking vessel, and not from underneath as they should do.	One final note, if the focal point is too low or too shallow, i.e. a very deep parabola as in Figure 3,	Figure 4 shows the opposite of a deep parabola. This design is workable, providing a relatively large diameter dish is used. Factors such as the wind and insulation of cooking vessel must be taken into account for this audacious design.

Many satellite dishes have a focal point just outside the depth of the dish. This is due to ease of manufacture and optimal threshold for capturing frequency bands.

Suitable materials for building a parabolic solar cooker.

- Mylar - used in hydroponic set ups.

- Reflective chrome vinyl, highly reflective material which also comes with an adhesive backing. For more details visit solar cookers reflective vinyl section. http://www.solreka.com/chrome-mirror-reflective-sign-vinyl-1-metre.html

- Aluminium foil - cheap and cheerful, however creases and tares can play a large part in the build. Foil is a great starting point as it's cheap and readily available.

Solar Parabolic Trough Cooker

Some parabolic cookers are limited in the quantity of food that is possible to be cooked at one time since they usually have only one pot that is suspended in the center of the path of highest solar energy concentration, but as mentioned earlier they can cook faster. These cookers have been used on a large institutional scale in China for many years along with a few other countries.

Why Use a Parabolic Shape?

When a three dimensional parabola (i.e. a paraboloid) is aimed at the sun, all the light that falls upon its mirrored surface is reflected to a point known as the focus. If a black cooking pot is placed at the focus it will absorb the light's energy and become very hot. A satellite dish is an example of a parabaloid that can be made into a cooker. Parabolic Solar cookers heat up quickly and are used like a standard stovetop range to sauté or fry foods, boil water, or even bake bread. They can also be used to generate steam, power sterling engines, crack water to produce H2 gas, and even plasma matter.

It is easy to see in today's world that this shape is successful in its use. The parabolic shape can be seen in satellite dishes, radio towers, and yes, even in solar cookers around the world. It is simple to say it works, and just as simple to understand how it works.

Diagram 1 shows the unique reflecting properties of the parabola .You can vary the height of the focus of the parabola by altering the equation of the curve and thus changing the relative shape of the parabola.

Diagram 1

Diagram 2: Focus Height

Once you have decided which curve and 3D representation you wish to model your parabolic solar cooker you can copy the 2D curve image onto a transparency.

That image can be projected onto a wall where it can be traced and used to create a template.

A solar parabolic cooker can reach extremely high temperatures, very quickly thus producing the possibility of burning the food if left unattended for any length of time.

But if used properly the parabolic will produce great cooking results and replace the need to turn on your stove top kitchen range burners.

If a parabolic is too small in size it will not produce sufficient heat for the most effective cooking results and the opposite is true if it is too large; the heat therefore would be too intense.

A mid sized solar burner has a diameter of almost five feet, which produces a high output of energy (heat) for very efficient cooking, yet is not too large in diameter that one would have much trouble reaching the cooking plate. The focus point of the sun's rays comes together at about thirty inches from, and above, the middle of the parabola. Each of the six rigid, molded steel panels that form the parabola is covered with an adhesive backed reflective vinyl which is quite durable, yet easily repaired or replaced if ever damaged or worn. The reflective film on this solar cooker is the key, or means to the parabolic being able to concentrate the energy from the rays of the sun.

The parabolic is supported by a circular stand (base) and has an easily adjusted crank extension arm to move the elevation tracking of the parabolic up and down, and the whole cooker swivels on its base, allowing for side to side tracking of the sun. Unlike the solar ovens and panel cookers, there is no need for a "special" pot or pan. The high temperatures of the parabolic are such that you can use the shiny steel pots and pans that are more common and you will not have to worry about reflecting the sunlight away from the cooking vessel.

In fact, with a parabolic it is more recommendable to use the heavier gauge steel pots, because the thinner variety will scorch and burn your food more easily, especially the thin aluminum, which is also known for health related issues and concerns.

Some Quick Statistics " Shallow Parabola " Focus length 60 cm. (23.62 inches) " 1500 watts power on sunny day " Focus area 1.5 square meters (16.15 sq ft) " Focal spot temp 824 F.+- (440.56 C) " Diameter 1.5 meters across (4.92 ft)

http://www.solarcooker-at-cantinawest.com/best_solar_cooker_solar_burner.html

So now you have a good understanding of what a parabolic cooker is and the main concepts needed to start building your very own parabolic solar cooker.

Here are some great links which will help you with your journey for building you very own parabolic solar cooker:

Parabolic Solar Cooker kits http://www.solreka.com/parabolic-solar-cooker-kit-deep-dish.html

Parabolic Solar Cooker Templates http://www.solreka.com/parabolic-solar-cooker-template-120cm.html

Reflective Chrome Vinyl http://www.solreka.com/chrome-mirror-reflective-sign-vinyl-1-metre.html

www.solarcooking.org

http://solreka.com/blog/solar-cooking/how-to-build-parabolic-solar-cooker/008

How to make the parabolic AMSI solar cooker. ARBA Minch Solar Initiative. http://home.germany.net/100-441770/amsi2.pdf "Parabola Design Wood Model",

ARBA Minch Solar Initiative. http://home.germany.net/100-441770/amsi-model.html

Butti, Ken & John Perlin. A Gold Thread: 2500 Years of Solar Architecture and Technology. Cheshire Books, Palo Alto. 1980 Eric Weisstein's World of Mathematics - 1999 CRC Press LLC, 1999-2003 Wolfram Research, Inc. http://mathworld.wolfram.com/Parabola.html

Genesis. Dynamic Design: A Collection Process, The Concentrator" http://www.genesismission.org/educate/scimodule/CollProcess/CollProcess_pdf/ConcentratorST.pdf

Kim, Yun K. & Li-Yan Zhu. "A Parabolic Solar Cooker for Unattended Cooking." http://solarcooking.org/unattendedparabolic.htm

N8PPQ, Parabolic Reflecting Antennas" - http://www2.gvsu.edu/~w8gvu/geo/geo.html

http://www.solarcooker-at-cantinawest.com
http://www.cantinawest.efoodsglobal.com/