Solar oven for cooking
 |
| This article is part of the Food theme. Alimentation |
A solar oven or solar cooker is a cooking appliance that harnesses the longwave infrared radiation (Heat Energy) from the sun. It can be used to cook food thanks to solar energy, a renewable energy that is environment friendly ! The oven, like solar panels, is highly dependant on solar input so cooking on a cloudy day or in high northern latitudes(because the suns energy is more diffused than at the Equator) will not produce great results.
Solar cookers often combine the technique of concentration and of greenhouse effect.
Globally speaking, the energy resources that are most commonly used to cook food are coal, natural gas, electricity and wood. Using any of these energy resources has an impact on global climate:
- Coal and natural gas are finite fossil fuels that generate greenhouse gases.
- The electricty used for powering electric burners depends on the area you live in : it may be produced from, natural gas or nuclear power plants.
- Wood is a closed circuit carbon product so it does not contribute to greenhouse gases as long as new trees are planted to replace the tree that was used for burning. There are also concerns about local air quality issues because of particulate and carbon monoxide emissions from burning the wood.
This is why a solar oven may prove to be the a more eco-friendly solution to cook food.
Contents |
[edit] Frequently asked questions
[edit] Why doesn't everybody use them?
There are several reasons to that. First and foremost, most people in the world aren't even aware that the sun can be used for cooking. As soon as they realize it can, they universally become enthusisatic about it, especially in areas where it is difficult to find wood to cook.
Several conditions must be respected so that a solar oven can be used by people in developing countries on a daily basis. The most successful projects are the ones that answered essential needs, where the weather was best adapted and whose promoters wisely adopted an attitude of long term transition. A good example of this kind of project is the work done by "Solar Cookers International" at Kakuma refugee camp, Kenya.
[edit] At what time of the year can the sun be best used for cooking?
In tropical regions and in the mild areas along them, solar cooking is generally possible all along the year. In northern countries such as Canada, it is efficient when the sky is bright, except for the three coldest winter months.
Broadly speaking, the sunniest periods correspond to those of crops, which means that home-grown or local seasonal products can always be cooked with solar energy. Hence a double saving : no fuel for cooking, nor for transporting goods!
[edit] Is the temperature high enough?
121°C (248°F) is hot enough to cook many kinds of food. One must bear in mind that water cannot be more than 100°C (212°F) hot. This implies that food that's being cooked in water cannot be hotter either. Most cook books will indicate higher temperatures so that the food will be cooked and brown quicker. Cooking food in a solar oven takes longer but, as the sun rays directly hit the lid of the pan, the food will turn brown just as in a traditional oven.
[edit] Generalities
- The first solar oven that we know of was invented by Horace de Saussure, a Swiss naturalist who tested it as soon as 1767.
- A solar oven can be used during almost 6 months in northern countries and all year round in areas closer to the equator.
- Such ovens won't work when it's cloudy, so alternative solutions must be available.
- Cooking food will take about twice the time it would traditionally, so the food has to be put to cook early.
- As the cooking process is slow, the food will look different. Also burns are less likely.
- Mind the risk of burning oneself while handling the saucepan or other vessel ; even if the cardboard or the sides are not hot, the inside of the oven is a furnace.
- Don't use products that are or can become toxic with the heat when you make your own oven (e.g. glue, paint or adhesive tape).
- Don't use your solar oven for food products that must be cooked either very quickly or at very high temperatures.
- Place the food in very dark dishes or vessels so they absorb the heat best.Aluminium containers will also grow hot quickly. Transparent glass ovenware will work fine too, cover with the transparent lid or use clingfilm.
[edit] Savings generated from using a solar oven
According to the FAO, African people will daily need 1 to 1.5kg of wood each (2 to 3 pounds) to cook their food, this contributes to deforestation and desertification in some vulnerable areas. The average European inhabitant depends on fossil fuels for more than 80% of his needs, directly or indirectly as he consumes electricity or gas that are bound to become more and more scarce, and so more and more expensive.
The latest carbon study on the solar Alsol K14 barbecue (parabola), (see here: [1] The solar cooking atlas) shows such an aluminium or steel parabola will generate 39 kg CO2 equivalents (6 stones) during its life cycle. This life cycle takes its manufacturing into account, not only the extraction of the raw materials, the several transportations but also the greenhouse gas emissions the whole process induced until the end (reuse, recycling or inert waste). These 39 kg CO2 equivalents correspond to the amount of emissions produced by 15kg (2.3 stones) of charcoal used for a traditional barbecue according to the manufaturer [2] since 5 to 7 kilos (10 to 14 pounds) of fresh wood must be burned to produce 1kg of charcoal.
Alcan, who supplies the "Solar surface" aluminium to make the Alsol K14, indicates that the reflectors will last for more than 15 years when used daily in a developing country, this means huge quantities of wood are saved.
It's easy to figure out the impact considering the average African family is composed of 6 persons and that, according to the FAO, each individual will need 1.25kg (2.8lbs) of wood everyday :
so 7.5kg (19.6lbs) per day for every African household and 2737.5 kg (431st) a year for every family
The mass combustion of most types of wood is : 80% of organic matter among which there'll be 50% of carbon, 43% of oxygen, 5% of hydrogen, 2% of azote. The remaining 20% are made of 2% of mineral matter and 18% of water, these do not burn but produce ash.
Given 12 grams (0.42 oz) of carbon produce 44 grams (1.55 oz) of carbon dioxide while burning, we can deduce that :
50% of carbon * by 80% of organic matter = 400 grams (14 oz) of carbon emissions for every kilo of burnt wood
400 grams (14 oz) of carbon * 44 grams (1.55 oz) Co2 / 12 grams (0.42 oz) of carbon = 1.47kg Co2 (3lbs) for every kilo of burnt wood
2737.5kg (431st) of wood * 1.47kg Co2 (3lbs) Co2 = 4 024,125 kg (634st) Co2 per year for each family
Photos and calculations at the solar cooking atlas : [3]
[edit] How to get the highest possible temperature in a solar oven
If your oven is not properly postioned, it won't capture the heat needed for a good cooking. The best way to make sure the solar oven is correctly positioned is to watch the shadow it projects on the ground.
The window of the oven should directly face the sun. Knowing how to ideally positison the oven requires a bit of training. The light projected by the reflectors is not a reliable indication. The best way is to watch the shadow behind the oven. It must be similar on the right and on the left.
Adjusting the orientation of the oven every 30 minutes is key to maintaining high temperatures. However, that doesn't have to become an obsession. For instance, it's quite possible to put frozen food in the morning, they'll be perfectly cooked by 4 pm. Still, prefer ready to cook products.
In Mediterranean areas, the best period for cooking is between 10 and 15.30 in Summer and between 11 and 14 in Winter.
[edit] Angle of the oven relatively to the sun
It's necessary to position the oven upwards or downwards according to the season as the angle of the sun relatively to the earth changes, the difference between the winter solstice and the summer solstice is about 30 degrees. Most ovens are designed to be used in summer and have to be raised backwards in winter. As on any occasion, the reliable indication is that of the shadow behind that must be as small as possible and perfectly symmetrical.
Whether it be a home made oven or an manufactured one, using it requires experience and time. So till you get to manage the various parameters, you'd better make sure you have alternative ways of cooking your food. So, at the beginning, even if you have to finish the cooking in a traditional oven, you'll have to experiment and train.
For all that, as soon as you're used to using it, it becomes real fun! It's also a good activity for children who'll empirically learn about this astrophysical phenomenon : you won't put the oven at 9 in June at the same place as in July at the same time (unless you have a huge garden without anything that would mask the sunlight). Whatever the season, the oven will have to be reoriented during the cooking, a simple tip not to get burned or to spill food is to place the oven on a board so you just have to slide it smoothly (make sure the board is level).
[edit] Before first use
The very first thing to do before using the oven to heat it without any food inside. Thus the glue and the joints will be dried by the heat. Besides, once the oven is quite hot, the plastic or glass lid must be removed to let out the steam and prevent the formation of moisture. On first use, put a thermometer inside the inner box and check the temperature every 30 minutes.
After the first half-hour, it should be 80°C (180°F.) depending on the insolation, of course. By the end of the first hour, the temperature should near 100°C (225°F.). If the inner box doesn't reach these temperatures on a sunny summer day, try to improve it and mind the joints are well insulated.
[edit] Repairing
If the oven doesn't reach 100 à 120°C (225 à 250°F) within an hour, the first thing to check is that the heat doesn't escape from the oven. As a matter of fact, hot air being lighter, it tends to try to escape. Then check the orientation of the oven, adjust the reflectors to concentrate as much solar light as possible onto the mirror. The angle between the sun rays and the reflector is a key factor.
If a lot of steam escapes from the container, insulate it by placing a glass (pyrex) bowl on top if it. Hold your hand about 5cms (2in) above the glass lid of the oven, if you can feel heat coming out from the corners, stop the leaks with some adhesive tape. If the heat comes out from the sides, add stuff between the two boxes and fix the joints. Make sure the oven is in the full light of the sun, away from the shadow of anything.
[edit] Basic models
[edit] Box style solar cookers
The main advantage of this type is it can cook large quantities of food very slowly and uniformly. There can be variants as to the number of refelectors or the inclination towards the surface of the ground.
[edit] Panel ovens
This new trend has been launched by Roger Bernard in France. This type is made of several flat panels that concentrate the sun beams onto the a vessel put inside a plastic bag or a glass bowl. Its advantage is it can be built in about one hour is costs almost nothing. In Kenya, it's being built for the Kakuma Camp refugees for only 2 US $.
[edit] Parabolic cookers
As a rule, they're made of concave panels that make the sunlight focus onto the pot. The advantage is the food is cooked almost as soon as in a conventional cooker. The drawback is they're very difficult to build, have to be moved regularly to stay in the axis of the sun, they can cause burns or harm the eyes if not correctly used. There is another solar cooker based on the concentration sytem and which is quite safe. It is made of a concentrator with a very specific shape and a table, thus the cook is totally protected. It also prevents the shadow from being projected onto the concentrator. (see the Devos cooker )
[edit] Plans
[edit] A solar cooker made of a pizza box
This example with a pizza box is adapted from different sources and can be modified according to your needs.It's powerful enough to melt chocolate, bake biscuits or even brioche rolls.
What you'll need
- A clean pizza box
- Black cardboard
- Aluminium paper
- Transparent plastic (a sheet of stratified plastic will be perfect)
- Non-toxic glue (see "Glue"), adhesive tape, scissors, a rule, a whiteboard marker
- A wood or straw stick
Building plan
Use the rule and the marker to draw a square on the lid of the pizza box, let 2cms (0.8in) on each side. Take the scissors (or a cutter) to cut the lid on only 3 sides. Let the back side as it is. |
|
Use the cutter to be able to fold the cut part to the back. Prepare a strong of aluminium to adapt it to the vertical part. Stick it inside the square, mind it's very smooth without any crease, glue or air bubbles on its surface.
Prepare a transparent sheet (plastic, plexiglas..) of the same dimensions as those of the cut part plus 1 cm (0.4in) to stick it below. Glass can do too but is more difficult to stick. Stick this sheet inside the box, use adhesive tape (preferably black) to prevent it from opening. Mind it's totally airtight. |
|
Cover the inside of the box with aluminium paper and stick it as carefully as before.
Now cover the bottom of the box (inside) with thick black paper, the sides remaining covered with aluminium. | |
|
Close the lid of the pizza box and maintain it with a wood or straw stick so that it reflects in it and that as much solar heat as possible gets inside the oven.
Here you are, your oven is ready. You can put a thermometer inside to check its power. |
[edit] A mini oven made of a shoebox
Here is another example of solar ovens rather suitable for individual portions.
What you'll need
- A shoebox
- Thick cardboard
- Black paper
- Aluminium paper
- Transparent plastic (a sheet of stratified plastic will be perfect)
- Non-toxic glue (see "Glue"), adhesive tape, scissors
- An iron shaft
Building plan
|
Cut the front side of the shoebox. Leave 2 à 3 cm (0.8 to 1.2in) at the bottom to keep it strong enough. Cover with black paper. |
|
Cut a disk from this cardboard part. Then cut the latter into 2 half-circles. |
|
Cut a rectangle in the cardboard the width of which will be equivalent to half the circumference of the circle. Use adhesive tape to fix it to the 2 half-circles to transform it into a container. |
|
Cover this inside of this container aluminium paper. Mind not to make any crease or glue or air bubbles. |
|
Fix the dish (and the food) to the box with the iron shaft. Put the device in the sun and position it so that it gets as much heat as possible. Turn the food products to cook them to your taste. |
[edit] A portable family solar oven
|
Here is the example of a commercialial model that's easy to reproduce as explained below.
Two cardboard obxes of different size are needed as the smaller one must go into the larger one leaving a space of about 1 to 2 cm (0.4 to 0.8in oneach side. |
What you'll need:
- 2 cardboard boxes with flaps. A smaller one must go into a larger one.
- Newsprint or glasswool
- Black cardboard
- Packaging cardboard for the reflectors
- Aluminium paper or a mirror the size of the smaller box
- Transparent plastic (a sheet of stratified plastic will be perfect) slightly larger than the smaller box
- Non-toxic glue (see "Glue"), adhesive tape, scissors, a rule, a whiteboard marker
- A wood or straw stick
Building plan:
- cover the bottom of the larger box with crumpled newsprint or glasswool 1 to 2 cm (0.4 to 0.8in) thick.
- place the smaller box in the middle of the larger one. Stuff the empty spaces with the same materials as for the bottom without changing their shape, only they mustn't move.
- cover the inner sides of the smaller box with aluminium paper, mind it's smooth without any crease, glue or air bubbles.
- cover the bottom of the inner box with black cardboard that will absorb the heat.
- cover the inner flaps of the larger obx with aluminium paper, it must be smooth with no folds. You can make them bigger using the flaps of a bigger box.
- put the plastic sheet on the smaller box and close the openings with adhesive tape.
[edit] An easily transportable solar oven
[edit] See also
[edit] Internal links
[edit] External links
- Solar Cooking Atlas : Information about solar ovens and solar cookers
- IDcook solar cookers
- Plenty of building plans (multilingual)
- transforming sea water into fresh water with a solar convector
- a very comprehensive site on solar cooking
- environment friendly solar cookers in the Andes
- a blog on solar cooking : tests and recipies
