This article outlines the design of a simple solar cooker for use in the tropics. It is relatively simple to build with readily available materials. It is based on the ‘Heaven’s Flame’ design by Maria Telkes. The design will have to be adopted to have a sloping glass front depending on the latitude of the place of use.
Materials Required
1/2 inch plywood: We are going to build this cooker out of wood. This will give us a robust product that will last many years. We will need 5 pieces. One of size 23 x 23 inches to form the base, Two of size 24 x 12 inches to form the front and back and two of size 23 x 12 inches to form the two sides.
Cooking tinfoil: A few rolls of this will be required and will be used for lining the inside of the insulation and the reflector panels. If required we can also use this in-between layers of the cardboard insulation to increase the efficiency of the cooker.
Cardboard Packaging: Its best to use discarded cardboard packaging for this as it should be freely available. Just make sure it’s clean and does not have packing tape or other plastic or food material stuck to it.
Glass Pane: For a basic cooker a single pane of glass (22 x 22 inches) will be required. A second piece or a double glazed piece of glass will significantly increase the efficiency of the cooker.
Black Metal Plate: This will form the base of the Solar Box Cooker. It will attract the sun’s radiation and conduct it to the metal cooking containers. This needs to 19 x 19 inches to fit in the base of the cooker.
Black Duct Tape: A roll of this will be required to tape the glass in place.
2 metal hinges: To provide a door at the back to introduce food inside the cooker.
An Oven Thermometer: This will be quite useful to monitor the temperature within our Solar Box Cooker.
Assorted screws and nails: To assemble the Solar Cooker.
Glue: To stick the tin foil onto the insulation and the reflectors.
2
Black metal pans with lids: For cooking a couple of dishes at the same time.
1/4 inch plywood: This will be used to construct the reflectors. We will require 4 pieces each of size 24 x 24 inches.
8 metal brackets: For mounting the reflectors.
Assorted nuts and bolts: To mount the reflectors onto the metal brackets.
Tools required
Either a hand saw or an electric jig saw for cutting the plywood
Hammer
and screw driver to assemble the box
Scissors
to cut up the aluminium foil and tape.
Craft
knife or scalpel to cut up the cardboard packaging
Making the wooden box
The completed wooden box is shown here. Some designs of Solar Box Cookers use the glass lid to access the inside for getting food in and out. The problem with this is that a lot of heat is lost every time one opens the cooker to check on the food. For this reason in this design, we are going to put a door in the rear of the cooker.
Either nails or screws can be used to assemble the box.
Adding the insulation
Use the cardboard packaging to make up the insulation. This is added to the inside of the front and side panels. If the insulation is prepared to the right size, there is no need to use nails or glue to hold this in place.
Our design of Box Cooker is going to use 2 inches of insulation to make sure that the heat remains trapped inside for as long as possible. The cheapest and safest way to do this is to use layers of cardboard packaging. Because of the high temperatures that can be achieved in the oven one has to be careful of using certain types of insulation as they may release toxic gases when heated.
To increase the efficiency of the cooker one should use layers of tin foil and cardboard. The shiny side of the tin foil should face the inside of the cooker.
The insulation around the door is slightly offset to make sure that heat is still trapped inside the oven when the door is closed. This insulation needs to be glued or nailed to fix it to the door.
The black metal base
This is an important part of the Solar Box Cooker. Being black, it receives radiation directly from the sun, or bounced off the reflectors. It retains the temperature and conducts it to the cooking container. If you cannot get hold of a black metal then an aluminium sheet can be painted black. Just make sure that you use a non-toxic paint. Otherwise there is a danger of toxic gases being released due to the high temperatures within the oven.
This is an important part of the Solar Box Cooker. Being black, it receives radiation directly from the sun, or bounced off the reflectors. It retains the temperature and conducts it to the cooking container. If you cannot get hold of a black metal then an aluminium sheet can be painted black. Just make sure that you use a non-toxic paint. Otherwise there is a danger of toxic gases being released due to the high temperatures within the oven.
A 2 to 3 mm thick aluminium plate works best. One can use a thicker metal plate which can retain the heat for longer, but this will take longer to initially heat up. The photo below shows a painted aluminium plate in position.
Fixing the glass pane
Now we are ready to add the glass pane. This should fit nicely on the shelf formed by the insulation as shown below using black duct tape.
Please make sure that there is no tape where the top of the door meets the glass, to allow it to be opened and shut.
Adding the reflectors
The main cooker is now ready. All we have to do now is to add the reflectors. The design shows 4 reflectors being used. In practice just a couple will be more than adequate in the tropics unless there is a requirement to cook more food or cook things quickly.
The 4 reflectors are each 24 x 24 inches square and made from 1/4 inch plywood. Each reflector has tin foil glued onto the side facing the cooker. The one on the side with the door has a notch cut out to stop it from interfering with the door when it is opened and closed. The reflector panels have pre-drilled holes to match up with the brackets. The butterfly nuts allow quick assembly and disassembly.
When the reflectors are assembled the completed Solar Cooker looks like this.
Adopting for latitude
The information below is from the Apprevecho Research Centre.
“This depends on the latitude where the Solar Cooker is being used. The higher the latitude the wider the difference between the angle of the Sun in Summer and Winter. For Sri Lanka, which is at a latitude of 10 degress North of the Equator, the position of the sun can be worked out as follows.
In Summer this is calculated by subtracting 10 degrees from 90 and adding 23.5. This gives 103.5 degrees. For the winter Sun position, subtract 23.5 from this which gives 80 degrees. So although we think that the sun is vertically above us in the sky it actually changes position by + and - 12 degrees from this from June to December.
The most
effective position for the Solar Cooker is for the glass to be perpendicular to
the sun’s rays.
Given this there are marginal gains to be had by tilting the solar cooker towards the South in the winter months and the North in the Summer months. Practically this is probably not worth doing. It is more useful to tilt the box towards the morning and afternoon sun when cooking over a long period.”
Using the Solar Box Cooker
This design of Solar Cooker works like an oven. This will require a change of habit when one is used to cooking on top of a fire. However the time and money saved in buying firewood or kerosene will make this worthwhile.
The Solar Box Cooker is great for cooking a meal of Rice and Lentils. Here is a step by step guide on preparing a simple lunchtime meal for 2 to 3 people.
1. Soak a cup of lentils in water overnight.
2. Around 9 AM place the cooker in the sun
with an oven thermometer inside. Assemble the reflector panels (2 or 4 as
required) and make sure the door is closed.
Leave the oven to warm up. (We will need the oven to reach a temperature
of more than 100 degrees C before we can start cooking).
3. Start preparing the food around 10 AM.
Take an onion, a few tomatoes and a sweet pepper. Chop finely and put in a
black pan. Add a few cloves of garlic, salt and turmeric for flavour. Drain the
water from the lentils and add to the pan and add half a cup of water. Mix
well.
4.
Wait till the oven has reached a
temperature of more than 100 deg C. Put a lid on the saucepan and place in the
oven.
5. Wash a cup of rice thoroughly and add
to a black pan and add a cup of water. Cooking in a Solar Oven requires less
water than when cooking on top of a fire.
6. Put a lid on the saucepan and place
alongside the lentils.
7. After about an hour and a half both rice and lentils should be perfectly cooked and
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