Solar cooker

Solar cooker at a presentation on Ludwigstrasse in Munich, 2012

A solar cooker is a device that uses the energy of solar radiation to generate heat for cooking . The radiation is bundled in the focal point of a concave mirror .

construction

The parabolic mirror concentrates the sun's rays on a mostly matt black container in the focal area. The container absorbs sun rays, so its contents are strongly heated. The solar cooker is suitable for example for boiling water, for heating food, for cooking , frying , baking , grilling or frying, for commercial use in dyeing, for soap production, juice production, processing of natural fibers for weaving or as a small food stall in the open.

Executions

There are different designs of solar cookers. A solar cooker can consist of one or more mostly curved mirrors . The mirror can be made of polished aluminum sheet , mirrored foil or, for example, metal- laminated cardboard . In the focal point there is often a holder for a pot . In order to be able to follow the sun, the mirror and holder are usually pivoted in two axes.

Since solar cookers can be manufactured with relatively little material expenditure and do not consume fuel, they are seen as the solution to many problems, especially in poorer regions of the world. A solar cooker can be used for disinfection in regions with little clean drinking water .

Solar cooker
Solar cooker in operation
Scheffler
Solar cooking box
Solar cooker Alsol
Global Sun Oven
Scheffler mirror with secondary mirror and stationary hotplate

advantages

Independence from conventional fuels such as gas, kerosene and firewood.
No CO ₂ emissions
The overall temperature of the earth's atmosphere - unlike when burning fuels - does not increase.
There are no harmful fumes or smoke when cooking .
The set-up times for commissioning the solar cooker are short.
No burning of the food in solar cooking boxes , as the temperature usually remains below 200 ° C. For the same reason, the food does not and should not be stirred, because the cooking boxes should remain closed as much as possible during the cooking time.

Indirect advantages over the open hearth fire

The tree population can be protected. Even animal manure like cow dung must be no longer burned (which in many areas of the world dried as fuel is used), but is used in agriculture.
The daily collection of wood is shortened or eliminated, which relieves children and women in particular.

disadvantage

One hundred percent dependence on sunshine. If the sun is not shining, cooking is impossible.
Cloudy skies or too low a position in the sun make cooking difficult or even prevent it entirely.
The application is hardly worthwhile in too little or too short sunshine (e.g. in the evening). However, this only matters if there is no way to start the cooking process in time before sunset.
In particular, stoves with mirrors often have to be adjusted to the position of the sun, cooking boxes much less often.
Cooking with the solar cooker must practically always take place outdoors. Because of the wind, it usually takes longer and is more difficult to time than with gas or electric stoves. Comparable cooking times are only achieved with the parabolic cooker (from 1.4 meters reflector diameter). Solar cooking boxes can be set up in a corresponding direction in the morning so that the meal is ready in the evening.
Cooking habits often have to be changed, which can limit the acceptance of solar cookers.
Solar cooking boxes are unsuitable for certain dishes because of the low cooking temperature, e.g. B. for pasta and for grilling or crispy roast.
The costs for solar cookers (especially mirror cookers) are usually so high, even with local production, that they are unaffordable for the poor and energy-efficient stoves are usually the better alternative.

Studies have shown that solar cookers can usually only save around 30 to 40% of the firewood. This is less than improved cookers that save 50% and more. Nevertheless, they could possibly be used as an additional technology to save energy. In the meantime, inexpensive solar cookers are also available, but the other disadvantages still exist.

Instructions for use and cooking time

Diagram of the position of the sun

A solar cooker with a diameter of 1.40 meters easily achieves an output of around 750 watts . In good sunlight, boil three liters of water in 25 minutes. Matt black containers absorb the sun's rays best and thus speed up the cooking process. A lid of the same color can also accelerate the heating.

One stove can cater for up to 20 people if it is operated using a 12 liter pot.

The stove works from one hour after sunrise to one hour before sunset . Due to its high performance, it is effective even with relatively short direct exposure.

Food that has been parboiled can continue to cook in a warming basket. This is possible with rice, for example, which is why the solar cooker is often colloquially called a rice cooker.

If the focal length is less than 30 centimeters, the mirror must be realigned to the position of the sun every 15 to 25 minutes. Tracking is done in a few simple steps. The focal area is within the mirror, so burning and dazzling are largely avoided. For processing (stirring, filling, etc.), however, the reflector should always be swiveled so that the container (pot) is on the shady side.

Solar cooking box

Top view of the cooking area

The solar cooking box is a special form of the solar cooker . This is an insulated wooden box lined with sheet metal on the inside with an insulating glass window as the inner lid and an outer lid with a sun mirror mounted above it . This is a lid covered with aluminum foil on the underside to reflect the sun's rays into the inside of the box. It is attached to one of the upper long sides of the cooking box with hinges in order to be pivotable and thus adaptable to the respective position of the sun . The sheet metal with which the box is lined should also be colored black with a non-toxic, tasteless color in order to reduce the reflection of the sun's rays and thus have the highest possible amount of heat energy available for cooking. The inner insulating glass lid of the cooking box lets the heat radiation in, but not heated air out again. This makes the solar cooking box (apart from its manufacture or its components) an extremely inexpensive, ecologically sensible stove that could be of interest to developing countries that are already poor in wood. The solar cooking box was invented in 1767 by Horace Bénédict de Saussure .

A solar oven is a device for generating heat by means of solar radiation using different mirror constructions. Solar ovens exist in different designs, but mostly in box form.

commitment

The solar oven is a mostly portable device for heating, baking or cooking food and drinks. The smaller models are mainly used to prepare provisions for hikes, trekking tours, expeditions or longer stays in the great outdoors. The larger solar ovens are used in sunny countries to earn income by baking bread and cakes or roasting and selling peanuts. Today, solar-heated large kitchens are the most successful projects. For example, in the world's largest solar kitchen in Abu Road, Rajasthan (India), up to 18,000 visitors to a yoga center can be cooked using solar energy. Solar cookers are also a good way to show children how high the radiation output of the sun is.

power

All details relate to an irradiation of 1 kW / m ² with a clear sky and optimal positioning of the reflectors .

Since the sun shines directly on the glass plate and reflector (for example 665 watts), the calculated efficiency is around 37%, the remaining 63% is lost due to partial shading of the cooking space, convection , evaporative cooling , absorption and reflection . When using high-quality mirrors (for example anodized aluminum sheet) 95% of the radiation is reflected, when using aluminum foil, on the other hand, only 50%. The cooker space exceeds idle, i.e. H. without food, the 100 ° C mark within 2 to 3 minutes.

hazards

The solar oven reaches temperatures between 140 and 205 ° C when idling.
Make sure that pot holders or insulating gloves are used to avoid burns.
The eyes must not get into the reflection path or the focus area to avoid the risk of glare.
When opening, it must be expected that hot water vapor will escape from the cooking space. The container used should not be closed tightly so that steam can escape.

Similar devices

Solar melting furnace - melting furnace working on the same principle
Sun dehydrator - solar powered dehydrator
Solar collector - uses solar heat without focusing, but reaches similarly high temperatures

literature

Erdem Cuce, Pinar Mert Cuce, A comprehensive review on solar cookers . In: Applied Energy 102, (2013), 1399-1421, doi : 10.1016 / j.apenergy.2012.09.002 .
Rolf Behringer, Michael Götz: Cooking with the sun, solar cooking and baking in Central Europe . Ökobuch, Staufen near Freiburg im Breisgau 2008, ISBN 978-3-936896-39-8 .
Ursula Bremm-Gerhards: Chances of solar cooking boxes as an adapted technology in developing countries , Fort Lauderdale - Breitenbach, Saarbrücken 1991, ISBN 3-88156-521-3 .
Torsten Schneider: Cooking with the sun: conception of a simple solar cooker and implementation of a construction course. diplomica, Hamburg 2011, ISBN 978-3-86341-088-9 .