Solar cooker

Cooks also in cloudy weather - forests will be spared - the greenhouse effect slows down?
There have been solar cookers in existence before, but not enough attention has been paid to their development. The solar cooker is the only environment-friendly cooker in village communities in the developing countries, and its introduction would mean that forests would be spared and desertification would be prevented. The model presented here is more than a cooker; it can be a frying pan and also the producer of electricity for the family's 12 V battery, which would make possible the small-scale use of electricity, for example, in lighting fixtures, TV- sets, mobile phones, computer with internet and perhaps small refrigerator.

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The clock turning the mirror (top left-hand corner)

The cooking utensil (in the middle)

1 setting of the season (in the base)

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The mirror and how to direct it

The mirror of the solar cooker should be 0,5 - 1 sq m in size, and it can be, for example, of bright-coated aluminium or it can be chrome-coated etc. The mirror can be larger, too, but then it may have to be designed to be easily accessible in cooking situations, for example, by making a big, asymmetric notch in it. A large mirror makes the device correspondingly a lot heavier, more expensive and sensitive to wind. A mirror larger than one square metre is not necessary, except when the cooker is mostly used in cloudy weather.

The mirror must follow the sun's course in the sky during the day for which it must have a spring-operated clock at the other end of a turning axle. The turning axle is in north-south direction. The mirror must be suspended so that it won't cause a great torsional moment to the turning axle in order that the clock would manage to turn the mirror. The spring must be of solid build. When the cook comes to the cooker in the morning, he/she winds the spring and sets the mirror in the direction of the sun. If the sky is overcast, he/she sets the mirror in the right direction according to the hour hand. The spring of the clock must be as strong as the spring of a grandfather's clock which lasted a century. The spring must not be allowed to get rusty in rainy weather. There cannot be any weather-sensitive plastic parts.

The idea of a battery-operated clock should be forgotten, because in disaster area, for example, expensive batteries are not always at hand. Furthermore, it is not in accordance with the idea of environment-friendly development. Instead, one might perhaps think of a solar-powered electric clock. It would give energy at about one watt's power to the turning mechanism, also taking care of the right direction in the morning. Perhaps the clock would even take care of the setting of the season. One must, however, be careful not to make the equipment too technical. This would make the price too high. Different versions of the cookers can of course be manufactured for different communities, the wealthy ones too, but will the right model always find the right place?

In addition to the turning axle, there must be another axle for the mirror, because the altitude of the sun changes with the seasons as well. In diagram 1 above, the suspension of the mirror is practical and therefore it causes a minor directing error. The error is unimportant because of the large size of the cooking utensil. As the altitude of the sun changes 10 -15 degrees, i.e. perhaps at an interval of one month (depends on the season), the fastening cotter (1) must be moved to the next notch, which will alter the inclination of the turning axle. The direction does not have to be exact at all. The practical suspension is exact enough between the tropics and will work passably everywhere.

The mirror itself of the solar cooker is the shape of a paraboloid mirror with the cooking utensil in the focal point. Looking from above, the mirror can take other forms than the circle, a rectangle even. Likewise, the mirror can consist of more than one piece, which have been placed on a plane. The design can be fairly free. The mirror does not have to be of precision work - small irregularities and lumps do not matter. Nevertheless, the mirror is still the most important, and expensive, part of the device and the following dangers threaten it: 1. The mirror gets dirty, e.g. by spilled soup. 2. Sandstorms tarnish it. 3. A hard wind threatens to disconnect the whole mirror. 4. It may get stolen. 5. In addition You can break the spring of the clock by accident.
You must polish the mirror from time to time and You need sunglasses.

The mirror can be cleaned but this may cause tarnishing. A thin plastic foil could protect it. After use it can be temporarily covered. The whole cooker is mounted firmly with bolts in concrete, a large rock, log frame or the stock of a tree, in a place sheltered from the wind. It can be placed differently, too, according to the season: one place for the summer, another for the "winter", under a shelter or even behind windows. The cooking shelter can also be modified, so that a part of the roof can be taken out or moved. This way the cooker itself does not have to be moved at all. The cooker should have holes against whirlwinds and in case of spilling soup. To support the mirror, it could have reinforcements for added rigidity. Solar cooker can also be placed to apartments balcony or to the roof like a satellite dish.

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Diagram 2: Suspension of the cooking utensil

The cooking utensil must stay upright with all positions of the mirror, so it must have a cardanic suspension (diagram 2). The utensil must be easily changeable. This means that there will have to be a stand made of wire, and the utensils have to be of the same type and size (2 litres). They will have to be dark-coated, fairly round-bottomed "pots" with a handle, designed specifically for this device. Being round-bottomed is inconvenient, so the cooking utensils require a special mount on the table, or the pot itself has a protecting wire mesh.

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Diagram 3: Dewar pot

The cooking utensils can be of many kinds. One example is the vacuum-cased Dewar pot (diagram 3). Its exterior is glass and the interior is a black-coated steel pot with a vacuum between them. The Dewar pot has its use when the soup has to be kept warm in cloudy times or in the dark. At its highest, the cooking power given by the mirror is less than a kilowatt, but in cloudy weather and even in rainy weather the mirror can heat up at 50 - 100 watt's power, which corresponds to the smallest power of an ordinary electrical hot-plate. Water heats up to boiling temperatures in a Dewar pot even if the sun was invisible, as long as it's directed towards the sun. The warming up is caused by long-wave heatrays that are able to penetrate the clouds. The glass has to be selective, i.e. the quality of the glass must be chosen in a way which lets through short-wave heatrays, but holds back only the rays with the longest wavelength (> 8000 nm). A Dewar pot breaks easily and is perhaps not even completely safe, that's why it may have to be protected with a wire-mesh stand. Maybe the glass can be replaced by a suitable type of plastic.

Another kind of utensil can be a pressure cooker, which differs from a standard utensil in having a dark exterior and a somewhat round bottom. It may be possible to combine these two types by making a pot which is both a pressure cooker and a Dewar pot. In addition, inside it can be placed a vessel which divides the Dewar pot in three parts. This makes it possible to make three dishes at the same time.

The frying and browning of food requires the temperature of at least 200 ^oC, and that is not always possible. At noon the sun shines onto the bottom of the frying pan, and then the temperature may be high enough in a small area to make it possible to fry food or bake bread. The solar cooker requires its own type of frying pan.

Diagram 4 and 5: Photocells

Production of electricity

A special kind of pot is one that produces electricity. One cooking utensil can be of a type which has an exterior covered with photocells (Diagram 4). The photocells can be multi-layer cells which make possible rather a good efficiency (20 - 30%, and the record is 60% by Richard Schaller and Victor Klimov, Los Alamos) . They can also have a thin layer of amorphic silicon. They are then cheaper to make, with an efficiency of only 5 -15%. The efficiency of GaAs cells is 20% at their best. The solar cooker's thermal efficiency is a lot greater than the electrical efficiency (60 - 80%). When the photocell pot produces electricity, it can simultaneously heat water.
Air does cool photocell too (Diagram 5). The photocell is probably cheaper to make and do not burn down perhaps so easy. A propeller helps perhaps cooling like in a prosessor of computers.

Importance

The basic equipment uses basic technology with aluminium, stainless steel and brass as materials. As an example, the clock is a strongly built spring-clock. The clock should wear normal use for decades. It must have a winding apparatus for the spring, an hour hand and a locking switch.
Many regions on Earth have become desolate because of the gathering of wood, for when trees disappear, the soil is exposed to the scorching sun. The comprehensive introduction of the solar cooker might reduce desertification and its consequences: migrations of peoples and the greenhouse effect. The device looks inconsequential, but its importance may be great. The costs will remain relatively small in mass production. The system works well in summer cottages and garden parties for an environmentally conscious user in the western world. The cooker can be modified for use in outings , it can be a basic version with a folding mirror, like in an umbrella.

Tapani Hakonen

Äänekoski Finland Europe