The Solar Funnel Cooker

Paul

https://wiki.lowtechlab.org/wiki/The_Solar_Funnel_Cooker

Dernière modification le 30/09/2018

Difficulty

Easy Easy

Duration

2 hour(s) hour(s)

Cost

15 EUR (€)

Description

Construction of the solar funnel cooker, safe, efficient, easy to build and to use.

Summary

chauffage, solaire, capteur, air, solar, heating, house, maison, habitat, Lowtech4Refugees en none 0

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Introduction

Very cheap, easy and efficient cooker.

Parabolic cooker can be dangerous (causing fires) and complicated to make. This cooker gave results comparable to the ones given by a "cookit" cooker and a solid wooden cooker.

French version : http://solarcooking.org/francais/funnel-fr.htm

Portuguese : http://solarcooking.org/portugues/funnel-pt.htm

Spanish : http://solarcooking.org/espanol/funnel-span.htm

Thanks to Tom Sponheim to let us share this great technology!

Materials

1. A piece of flat cardboard, about 800*1600 mm. (The length should be just twice the width. The bigger, the better.)
2. Ordinary aluminum foil or safety blanket
3. Three wire links-- or small nuts and bolts, or string to hold the funnel together. Here we used a wire made out of a water bottle
4. For a cooking vessel, I recommend a glass jar, type "le parfait" used here. The cooking jar (or vessel) should be painted black on the outside.
5. A block of wood is used as an insulator under the jar. I use a piece of 10cm square x 5cm thick.) One square piece of wood makes a great insulator.
6. A plastic bag is used to go around the cooking-jar and block of wood, to provide a green-house effect. 

Tools

*Cutter

*A glue such as white glue, and water to mix with it 50-50. Also, a brush to apply the glue to the cardboard (or a cloth or paper towel will do). Or, some may wish to use a cheap "spray adhesive" available in spray cans. 

Step 1 - Cut a Half-circle out of the Cardboard

Cut a half circle out of the cardboard, along the bottom as shown below. When the funnel is formed, this becomes a full-circle and should be wide enough to go around your cooking pot. So for a 125 mm diameter cooking pot, the radius of the half-circle is 125 mm.

Step 2 - Form the Funnel

To form the funnel, you will bring side A towards side B, as shown in the figure. The aluminum foil must go on the INSIDE of the funnel. Do this slowly, helping the cardboard to the shape of a funnel by using one hand to form creases that radiate out from the half-circle. Work your way around the funnel, bending it in stages to form the funnel shape, until the two sides overlap and the half-circle forms a complete circle. The aluminum foil will go on the INSIDE of funnel. Open the funnel and lay it flat, "inside up", in preparation for the next step.

Step 3 - Glue Foil to Cardboard

Apply glue or adhesive to the top (inner) surface of the cardboard, then quickly apply the aluminum foil on top of the glue, to affix the foil to the cardboard. Make sure the shiniest side of the foil is on top, since this becomes your reflective surface in the Funnel. I like to put just enough glue for one width of foil, so that the glue stays moist while the foil is applied. I also overlap strips of foil by about 2 cm. Try to smooth out the aluminum foil as much as you reasonably can, but small wrinkles won't make much difference.

Step 4 - Join side A to side B to keep the funnel together

The easiest way to do this is to punch three holes in the cardboard that line up on side A and side B (see figure). Then put a metal brad through each hole and fasten by pulling apart the metal tines. Or you can use a nut-and-bolt to secure the two sides (A & B) together.

Be creative here with what you have available. For example, by putting two holes about a thumb-width apart, you can put a string, twine, small rope, wire or twist-tie in one hole and out the other, and tie together. Here we used wire mae out of a plastic bottle as described in this tutorial :[1]

When A and B are connected together, you will have a "funnel with two wings". The wings could be cut off, but these help to gather more sunlight, so I leave them on.

Tape or glue a piece of aluminum foil across the hole at the bottom of the funnel, with shiny side in. 

This completes assembly of your solar funnel cooker.

For stability, place the Funnel inside a cardboard or other box to provide support. For long-term applications, one may wish to dig a hole in the ground to hold the Funnel against strong winds

Step 5 - Painting the jar

You must paint the jar in mat black. The darkest the better.

Step 6 - Final steps

At this stage, you are ready to put food items or water into the cooking vessel or jar, and put the lid on securely. (See instructions on food cooking times, to follow.)

Place a wooden block in the INSIDE bottom of the cooking bag. Then place the cooking vessel containing the food or water on top of the wooden block, inside the bag.

Next, gather the top of the bag in your fingers and blow air into the bag, to inflate it. This will form a small "greenhouse" around the cooking vessel, to trap much of the heat inside. Close off the bag with a tight twist tie or wire. Important: the bag should not touch the sides or lid of the cooking vessel. The bag may be called a "convection shield," slowing convection-cooling due to air currents.

Place the entire bag and its contents inside the funnel near the bottom as shown in the Photographs.

Place the Solar Funnel Cooker so that it Faces the Sun

Remember: Sunlight can hurt the eyes: Please wear sunglasses when using a Solar Cooker! The Funnel Cooker is designed so that the hot region is deep down inside the funnel, out of harm's way.

Put the Solar Funnel Cooker in the sun pointing towards the sun, so that it captures as much sunlight as possible. The design of the funnel allows it to collect solar energy for about an hour without needing to be re-positioned. For longer cooking times, readjust the position of the funnel to follow the sun's path.

It helps to put the Solar Funnel Cooker in front of a south-facing wall or window (in the Northern Hemisphere) to reflect additional sunlight into the funnel. A reflective wall is most important in locations farther from the equator and in winter. In the Southern Hemisphere, put the Solar Funnel Cooker in front of a North-facing wall or window to reflect additional sunlight into your cooker.

Step 7 - After Cooking

Remember that the cooking vessel will be very hot: Use cooking pads or gloves when handling! If you are heating water in a canning jar, you may notice that the water is boiling when the lid is first removed - it gets very hot!

Open the plastic cooking bag by removing the twist-tie. Using gloves or a thick cloth, lift the vessel out of the bag and place it on the ground or table. Carefully open the vessel and check the food, to make sure it has finished cooking. Let the hot food cool before eating.

Step 8 - Helpful Hints

1. Avoid leaving fingerprints and smudges on the inside surface of the cooker. Keep the inner surface clean and shiny by wiping occasionally with a wet towel. This will keep the Solar Funnel Cooker working at its best.
2. If your funnel gets out-of-round, it can be put back into a circular shape by attaching a rope or string between opposite sides which need to be brought closer together.
3. For long-term applications, a hole in the ground will hold the Funnel Cooker securely against winds. Bring the funnel inside or cover it during rain storms.
4. The lids can be used over and over. We have had some trouble with the rubber on some new canning-jar lids becoming soft and "sticky." "Ball canning lids" do not usually have this problem. Running new lids through very hot water before the first use seems to help. The lids can be used over and over if they are not bent too badly when opened (pry off lid carefully).
5. The jar can be suspended near the bottom of the funnel using fishing line or string (etc.), instead of placing the jar on a block of wood. A plastic bag is placed around the jar with air puffed inside, as usual, to trap the heat. The suspension method allows sunlight to strike all surfaces of the jar, all around, so that heats faster and more evenly. This suspension method is crucial for use in winter months.
6. Adjust the funnel to put as much sunlight onto the cooking jar as possible. Look at the jar to check where the sunlight is hitting, and to be sure the bottom is not in the shadows. For long cooking times (over about an hour), readjust the position of the funnel to follow the sun's path. During winter months, when the sun is low on the horizon (e.g., in North America), it is helpful to lay the funnel on its side, facing the sun.

Notes and references

http://solarcooking.org/plans/funnel.htm, by [E. Jones], Professor of Physics at Brigham Young University (BYU) and his team

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