Solar cooker

In Ghana, Zouzugu villagers like this woman prevent dracunculiasis and other waterborne diseases by pasteurizing water in a CooKit solar cooker

Sun Oven box cooker

A solar cooker, or solar oven, is a device which uses the energy of sunlight to heat food or drink to cook it or sterilize it. High-tech versions, for example electric ovens powered by solar cells, are possible, and have some advantages such as being able to work in diffuse light. However at present they are very unusual because they are expensive. The vast majority of the solar cookers presently in use are relatively cheap, low-tech devices. Because they use no fuel and cost nothing to operate, many nonprofit organizations are promoting their use worldwide to help reduce fuel costs for low-income people, reduce air pollution and slow deforestation and desertification, caused by use of firewood for cooking. Solar cooking is a form of outdoor cooking and is often used in situations where minimal fuel consumption is important, or the danger of accidental fires is high.

Many types of cookers exist. Simple solar cookers use the following basic principles:

• Concentrating sunlight: A reflective mirror of polished glass, metal or metallised film is used to concentrate light and heat from the sun into a small cooking area, making the energy more concentrated and increasing its heating power.
• Converting light to heat: A black or low reflectivity surface on a food container or the inside of a solar cooker will improve the effectiveness of turning light into heat. Light absorption converts the sun's visible light into heat, substantially improving the effectiveness of the cooker.
• Trapping heat: It is important to reduce convection by isolating the air inside the cooker from the air outside the cooker. A plastic bag or tightly sealed glass cover will trap the hot air inside. This makes it possible to reach similar temperatures on cold and windy days as on hot days.
• Greenhouse effect: Glass transmits visible light but blocks infrared thermal radiation from escaping. This amplifies the heat trapping effect.

Box cookers

A box cooker has a transparent glass or plastic top, and it may have additional reflectors to concentrate sunlight into the box. The top can usually be removed to allow dark pots containing food to be placed inside. One or more reflectors of shiny metal or foil-lined material may be positioned to bounce extra light into the interior of the oven chamber. Cooking containers and the inside bottom of the cooker should be dark-colored or black. Inside walls should be reflective to reduce radiative heat loss and bounce the light towards the pots and the dark bottom, which is in contact with the pots. The box should have insulated sides. Thermal insulation for the solar box cooker must be able to withstand temperatures up to 150 °C (300 °F) without melting or out-gassing. Crumpled newspaper, wool, rags, dry grass, sheets of cardboard, etc. can be used to insulate the walls of the cooker. Metal pots and/or bottom trays can be darkened either with flat-black spray paint (one that is non-toxic when warmed), black tempera paint, or soot from a fire. The solar box cooker typically reaches a temperature of 150 °C (300 °F). This is not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time. Food containing a lot of moisture cannot get much hotter than 100 °C (212 °F) in any case, so it is not always necessary to cook at the high temperatures indicated in standard cookbooks. Because the food does not reach too high a temperature, it can be safely left in the cooker all day without burning. It is best to start cooking before noon, though. Depending on the latitude and weather, food can be cooked either early or later in the day. The cooker can be used to warm food and drinks and can also be used to pasteurize water or milk.

Panel cookers

HotPot panel solar cooker

Panel solar cookers are very inexpensive solar cookers that use reflective panels to direct sunlight to a cooking pot that is enclosed in a clear plastic bag. A common model is the CooKit. Developed in 1994 by Solar Cookers International, it is often produced locally by pasting a reflective material, such as aluminium foil, onto a cut and folded backing, usually corrugated cardboard. It is lightweight and folds for storage. When completely unfolded, it measures about three feet by four feet (1 m by 1.3 m). Using materials purchased in bulk, the typical cost is about US$5. However, CooKits can also be made entirely from reclaimed materials, including used cardboard boxes and foil from the inside of cigarette boxes.

The CooKit is considered a low-to-moderate temperature solar cooker, easily reaching temperatures high enough to pasteurize water or cook grains such as rice. On a sunny day, one CooKit can collect enough solar energy to cook rice, meat or vegetables to feed a family with up to three or four children. Larger families use two or more cookers.

Solar tea kettle in Tibet

The HotPot is an advanced panel cooker design that includes a glass bowl with an inner black steel liner and a glass top. The panel has polished aluminium sections that fold flat. The HotPot has high thermal gain due to exploiting the greenhouse effect. The HotPot is being used in various field projects around the world.

Solar kettles

Solar kettles are solar thermal devices that can heat water to boiling point by relying on solar energy alone. Low-tech ones are used in the preparation of hot drinks, and to sterilize water. There are also fairly high-tech ones, using vacuum tube technology.

Cookers with paraboloidal reflectors

Theory and background

Parabola (red), showing focus (F), vertex (V), and rays of light (blue) being brought to the focus

Paraboloidal Solar Cooker. At high magnification, note segmented construction

Scheffler cooker. This reflector has an area of 16 m² (170 sq ft), and concentrates 3 kW of heat

If a reflector is axially symmetrical and shaped so its cross-section is a parabola, it has the property of bringing light that has come from a very distant source such as the sun, so the rays of light are effectively parallel, to a point focus. The diagram shows parallel rays QP being reflected and then converging to the focus (or focal point), marked F. The point V, where the curvature of the parabola is greatest, is called its vertex. For the focussing to be accurate, the incoming rays have to be parallel to the axis of symmetry of the parabola, the line that passes through V and F. The distance between V and F is the focal length of the parabola. If the axis of symmetry is aimed at the sun, any object that is located at the focus receives highly concentrated sunlight, and therefore becomes very hot. This is the basis for the use of this kind of reflector for solar cooking.

Strictly, the three-dimensional shape of the reflector is called a paraboloid. A parabola is the two-dimensional figure. (The distinction is like that between a sphere and a circle.) However, in informal language, the word parabola and its associated adjective parabolic are often used in place of paraboloid and paraboloidal. In this article, the strictly correct words are used, but readers should be aware that they will probably find the informal usage in other places, including other articles in Wikipedia.

The dimensions of a symmetrical paraboloidal dish are related by the equation: $\scriptstyle 4FD=R^{2},$ where $\scriptstyle F$ is the focal length, $\scriptstyle D$ is the depth of the dish (measured along the axis of symmetry from the vertex to the plane of the rim), and $\scriptstyle R$ is the radius of the rim. Of course, they must all be in the same units. If two of these three quantities are known, this equation can be used to calculate the third.

A more complex calculation is needed to find the diameter of the dish measured along its surface. This is sometimes called the "linear diameter", and equals the diameter of a flat, circular sheet of material, usually metal, which is the right size to be cut and bent to make the dish. Two intermediate results are useful in the calculation: $\scriptstyle P=2F$ and $\scriptstyle Q={\sqrt {P^{2}+R^{2}}},$ where $\scriptstyle F$ and $\scriptstyle R$ are defined as above. The diameter of the dish, measured along the surface, is then given by: $\scriptstyle S={\frac {RQ}{P}}+P\ln \left({\frac {R+Q}{P}}\right),$ where $\scriptstyle \ln(x)$ means the natural logarithm of $\scriptstyle x$, i.e. its logarithm to base "e".

The volume of the dish, i.e. the amount of liquid it could hold if the rim were horizontal and the vertex at the bottom (e.g. the capacity of a paraboloidal wok), is given by $\scriptstyle V={\frac {1}{2}}\pi R^{2}D,$ where the symbols are defined as above. This can be compared with the formulae for the volumes of a cylinder $\scriptstyle (\pi R^{2}D)$ and a cone $\scriptstyle ({\frac {1}{3}}\pi R^{2}D).$ Of course, $\scriptstyle \pi R^{2}$ is the aperture area of the dish, the area enclosed by the rim, which is proportional to the amount of sunlight the reflector dish can intercept.

Although paraboloidal solar cookers can cook as well as a conventional oven, they are difficult to construct. Paraboloids are compound curves, which are more difficult to make with simple equipment than single curves. Frequently, the reflectors are made approximately, using many small segments that are all single curves. They generate high temperatures and cook quickly, but require frequent adjustment and supervision for safe operation. Several hundred thousand exist, mainly in China. They are especially useful for large-scale institutional cooking.

Scheffler cookers

A Scheffler cooker (named after its inventor, Wolfgang Scheffler) uses a large ideally paraboloidal reflector which is rotated around an axis that is parallel with the earth's by a mechanical mechanism, turning at 15 degrees per hour to compensate for the earth's rotation. The axis passes through the reflector's centre of mass, allowing the reflector to be turned easily. The cooking vessel is located at the focus which is on the axis of rotation, so the mirror concentrates sunlight onto it all day. The mirror has to be occasionally tilted about a perpendicular axis to compensate for the seasonal variation in the sun's declination. This perpendicular axis does not pass through the cooking vessel. Therefore, if the reflector were a rigid paraboloid, its focus would not remain stationary at the cooking vessel as the reflector tilts. To keep the focus stationary, the reflector's shape has to vary. It remains paraboloidal, but its focal length and other parameters change as it tilts. The Scheffler reflector is therefore flexible, and can be bent to adjust its shape. It is often made up of a large number of small plane sections, such as glass mirrors, joined together by flexible plastic. A framework that supports the reflector includes a mechanism that can be used to tilt it and also bend it appropriately. The mirror is never exactly paraboloidal, but it is always close enough for cooking purposes.

Sometimes, the rotating reflector is located outdoors, and the reflected sunlight passes through an opening in a wall into an indoor kitchen, often a large communal one, where the cooking is done. And also all of the solar ovens make awesome toast!

Focus-balanced reflectors

Paraboloidal reflectors that have their centres of mass coincident with their focal points are useful. They can be easily turned to follow the sun's motions in the sky, rotating about any axis that passes through the focus. Two perpendicular axes can be used, intersecting at the focus, to allow the paraboloid to follow both the sun's daily motion and its seasonal one. The cooking pot stays stationary at the focus. If the paraboloidal reflector is axially symmetrical and is made of material of uniform thickness, its centre of mass coincides with its focus if the depth of the reflector, measured along its axis of symmetry from the vertex to the plane of the rim, is 1.8478 times its focal length. The radius of the rim of the reflector is 2.7187 times the focal length. The angular radius of the rim, as seen from the focal point, is 72.68 degrees.

Paraboloids made by rotating liquids

See also: Rotating furnace

Parabolic shape formed by a liquid surface under rotation. Two liquids of different densities completely fill a narrow space between two sheets of plexiglass. The gap between the sheets is closed at the bottom, sides and top. The whole assembly is rotating around a vertical axis passing through the centre.

Although paraboloids are difficult to make from flat sheets of solid material, they can be made quite simply by rotating open-topped containers which hold liquids. The top surface of a liquid which is being rotated at constant speed around a vertical axis naturally takes the form of a paraboloid. Centrifugal force causes material to move outward from the axis of rotation until a deep enough depression is formed in the surface for the force to be balanced by the levelling effect of gravity. It turns out that the depression is an exact paraboloid. If the material solidifies while it is rotating, the paraboloidal shape is maintained after the rotation stops, and can be used to make a reflector. Materials can be used such as epoxy resin, which solidifies by chemical change, and glass and candle wax, which are initially molten and solidify as they cool. Materials such as concrete, plaster, and pot-making clay have also been suggested, but they are not sufficiently fluid to take the paraboloidal shape exactly when they are soft. Vibrating them would make them more fluid, but this does not yet seem to have been tried. This rotation technique is sometimes used to make paraboloidal mirrors for astronomical telescopes, and has also been used for solar cookers.

The focal length of the paraboloid is related to the angular speed at which the liquid is rotated by the equation: $2f\omega ^{2}=g$, where $f$ is the focal length, $\omega$ is the rotation speed, and $g$ is the acceleration due to gravity. They must be in compatible units so, for example, $f$ can be in metres, $\omega$ in radians per second, and $g$ in metres per second-squared. The angle unit in $\omega$ must be radians. 1 radian per second is about 9.55 rotations per minute (RPM). On the Earth's surface, $g$ is about 9.81 metres per second-squared. Putting these numbers into the equation produces the approximation: $fs^{2}\approx 447$, where $f$ is the focal length in metres, and $s$ is the rotation speed in RPM.

Cookers with parabolic trough reflectors

Single troughs

Parabolic troughs are used to concentrate sunlight for solar-energy purposes. Some solar cookers have been built that use them in the same way. Generally, the trough is aligned with its focal line horizontal and east-west. The food to be cooked is arranged along this line. The trough is pointed so its axis of symmetry aims at the sun at noon. This requires the trough to be tilted up and down as the seasons progress. At the equinoxes, no movement of the trough is needed during the day to track the sun. At other times of year, there is a period of several hours around noon each day when no tracking is needed. Usually, the cooker is used only during this period, so no automatic sun tracking is incorporated into it. This simplicity makes this design attractive, compared with using a paraboloid. Also, being a single curve, the trough reflector is simpler to construct. However, it suffers from lower efficiency.

Using two parabolic troughs to simulate a paraboloid

It is possible to use two parabolic troughs, curved in perpendicular directions, to bring sunlight to a point focus as does a paraboloidal reflector. The incoming light strikes one of the troughs, which sends it toward a line focus. The second trough intercepts the converging light and focuses it to a point.

Compared with a single paraboloid, using two partial troughs has important advantages. Each trough is a single curve, which can be made simply by bending a flat sheet of metal. Also, the light that reaches the targeted cooking pot is directed approximately downward, which reduces the danger of damage to the eyes of anyone nearby. On the other hand, there are disadvantages. More mirror material is needed, increasing the cost, and the light is reflected by two surfaces instead of one, which inevitably increases the amount that is lost.

Experimental arrangements of this kind have been made, and have worked well. The two troughs have been held in a fixed orientation relative to each other by being both fixed to a wooden frame. The whole assembly of frame and troughs has to be moved to track the sun as it moves in the sky.

The Solar Bowl in Auroville

Cookers with spherical reflectors

The Solar Bowl is a unique concentrating technology used by the Solar Kitchen in Auroville, India. Unlike nearly all concentrating technologies that use tracking reflector systems, the solar bowl uses a stationary spherical reflector. This reflector focuses light along a line perpendicular to the sphere's surface and a computer control system moves the receiver to intersect this line. Steam is produced in the solar bowl's receiver at temperatures reaching 150 °C and then used for process heat in the kitchen where 1,000 meals per day are served.

Hybrid cookers

A hybrid solar oven is a solar box cooker equipped with a conventional electrical heating element for cloudy days or nighttime cooking. Developed primarily for use in first world countries where both solar energy and electricity useage is abundant a hybrid solar ovens can be a convenient and reliabile cooking appliance. They currently, however, lack the cost advantages of some other types of solar cookers, and so their use in third world countries where electricity or fuel sources simply do not exist has been limited.

A hybrid solar grill consists of an adjustable paraboloidal reflector suspended in a tripod with a movable grill surface. These outperform solar box cookers in temperature range and cooking times. When solar energy is not available, the design uses any conventional fuel as a heat source, including gas, electricity, or wood.

It has been recognized that solar cookers are limited to cooking on clear days. Moreover, most people want to eat hot food late in the day, when the sun is low or has already set. For these reasons, solar cooking advocates are recognizing the need for combining three devices for a total cooking solution: a) some type of solar cooker; b) a fuel-efficient cookstove; c) an insulated storage container such as a basket filled with straw to store heated food. Hot food will continue to cook for hours if it is stored in a well-insulated container. With this three-part solution, fuel use is minimized while still providing hot meals reliably. This concept is referred to as "integrated solar cooking" or the "integrated cooking method".

High-tech solar cookers

Although various ideas for high-tech solar cookers have been proposed, such as the electric oven powered by solar cells mentioned in the introduction, very few of them have progressed past the experimental stage to the point where they are used in practice. They are generally much more expensive than low-tech cookers, which precludes their use in third-world situations.

One exception consists of solar kettles that use vacuum tube technology. They use evacuated glass tubes to capture, accumulate and store solar energy needed to power the kettle. Besides heating liquids, since the stagnating temperature of solar vacuum glass tubes is a high 220 °C (428 °F), these solar kettles can also deliver dry heat and function as ovens and autoclaves. Moreover, since solar vacuum glass tubes work on accumulated rather than concentrated solar thermal energy, these solar kettles only need diffused sunlight to work and need no sun tracking at all. If solar kettles use solar vacuum tubes technologies, the vacuum insulating properties will keep previously heated water hot throughout the night e.g. the SK-TF, or the SunRocket Solar Kettle.

Operation

Solar oven in use

The different kinds of solar cookers have somewhat different methods for use, but most follow the same basic principles.

Food is prepared as it would be for an oven or stove top. Because food cooks faster when it is in smaller pieces, solar cookers usually cut the food into smaller pieces than they might otherwise. For example, potatoes are usually cut into bite-sized pieces rather than being roasted whole. For very simple cooking, such as melting butter or cheese, a lid may not be needed and the food may be placed on an uncovered tray or in a bowl. If several foods are to be cooked separately, then they are placed in different containers.

The container of food is placed inside the solar cooker, perhaps elevated on a brick, rocks, metal trivet, or other heat sink, and the solar cooker is placed in direct sunlight. If the solar cooker is entirely in direct sunlight, then the shadow of the solar cooker will not overlap with the shadow of any nearby object. Foods that cook quickly may be added to the solar cooker later. Rice for a mid-day meal might be started early in the morning, with vegetables, cheese, or meat added to the solar cooker in the middle of the morning. Depending on the size of the solar cooker and the number and quantity of cooked foods, a family may use one or more solar cookers.

The solar cooker is turned towards the sun and left until the food is cooked. Unlike cooking on a stove or over a fire, which may require more than an hour of constant supervision, food in a solar cooker is generally not stirred or turned over, both because it is unnecessary and because opening the solar cooker allows the trapped heat to escape and thereby slows the cooking process. If wanted, the solar cooker may be checked every one to two hours, to turn the cooker to face the sun more precisely and to ensure that shadows from nearby buildings or plants have not blocked the sunlight. If the food will be left untended for many hours during the day, then the solar cooker is often turned to face the point where the sun will be when it is higher in the sky, instead of towards its current position.

The cooking time depends primarily on the equipment being used, the amount of sunlight at the time, and the quantity of food that needs to be cooked. Air temperature, wind, and latitude also affect performance. Food cooks faster in the two hours before and after the local solar noon than it does in either the early morning or the late afternoon. Larger quantities of food, and food in larger pieces, take longer to cook. As a result, only general figures can be given for cooking time. With a small solar panel cooker, it might be possible to melt butter in 15 minutes, to bake cookies in 2 hours, and to cook rice for four people in 4 hours. However, depending on the local conditions and the solar cooker type, these projects could take half as long, or twice as long.

A low-cost thermometer has been invented to provide a reliable method for determining when the cooker has reached the temperature for pasteurization of water or milk (65 deg. C or 149 deg. F). This device is called the Water Pasteurization Indicator or WAPI.

It is difficult to burn food in a solar cooker. Food that has been cooked even an hour longer than necessary is usually indistinguishable from minimally cooked food. The exception to this rule is some green vegetables, which quickly change from a perfectly cooked bright green to olive drab, while still retaining the desirable texture.

For most foods, such as rice, the typical person would be unable to tell how it was cooked from looking at the final product. There are some differences, however: Bread and cakes brown on their tops instead of on bottom. Compared to cooking over a fire, the food does not have a smoky flavor.

Advantages and disadvantages

Solar cookers use no fuel, which means that their users do not need to fetch or pay for firewood, gas, electricity, or other fuels. Therefore, over time a solar cooker can pay for itself in reduced fuel costs. Since it reduces firewood use, the solar cooker reduces deforestation and habitat loss. Since there are about 2 billion people who are still cooking on open fires, widespread use of solar cookers could have large economic and environmental benefits.

Solar box cookers attain temperatures of up to about 165 deg. C (325 deg. F), so they can be used to sterilize water or prepare most foods that can be made in a conventional oven or stove, from baked bread to steamed vegetables to roasted meat. When solar ovens are placed outside, they do not contribute unwanted heat inside houses.

Solar cookers do not produce any smoke as a product of combustion. The indoor concentration of health-damaging pollutants from a typical wood-fired cooking stove creates carbon monoxide and other noxious fumes at anywhere between seven and 500 times over the allowable limits. Fire-based cooking also produces ashes and soot, which make the home dirtier. However, any type of cooking, including solar cooking, can evaporate grease, oil, etc., from the food into the air.

Unlike cooking over an open fire, children cannot be burned by touching many types of solar cookers, which are made from cardboard or plastic and do not get hot. Unlike all fuel-based cooking arrangements, these solar cookers are not fire hazards. However, solar cookers that concentrate sunlight, e.g. with paraboloidal reflectors, do produce high temperatures which could cause injury or fire.

Solar cookers are less usable in cloudy weather and at high latitudes, so some fuel-based backup heat source must still be available in these conditions. Also, solar cooking provides hot food during or shortly after the hottest part of the day, rather than the evening when most people like to eat. The "integrated solar cooking" concept accepts these limitations, and includes a fuel-efficient stove and an insulated heat storage container to provide a complete solution.

Many solar cookers take longer time to cook food than a fuel-based oven. Using these solar cookers therefore requires that food preparation be started several hours before the meal. However, it requires less hands-on time cooking, so this is often considered a reasonable trade-off.

Cooks may need to learn special cooking techniques to fry common foods, such as fried eggs or flatbreads like chapatis and tortillas. It may not be possible to safely or completely cook some thick foods, such as large roasts, loaves of bread, or pots of soup, particularly in small panel cookers; the cook may need to divide these into smaller portions before cooking.

Some solar cooker designs are affected by strong winds, which can slow the cooking process, cool the food, and disturb the reflector. In these cases it is necessary to anchor the reflector with string and weights.

Projects

Students perform an experiment, using a solar cooker built out of an umbrella

Michael Hönes of Germany have established solar cooking in Lesotho, enabling small groups of women to build up community bakeries using solar ovens.

Cardboard, aluminium foil, and plastic bags for well over 10,000 solar cookers have been donated to the Iridimi refugee camp and Touloum refugee camps in Chad by the combined efforts of the Jewish World Watch, the Dutch foundation KoZon, and Solar Cookers International. The refugees construct the cookers themselves, using the donated supplies and locally purchased Arabic gum, and use them for midday and evening meals. The goal of this project was to reduce the Darfuri women's need to leave the relative safety of the camp to gather firewood, which exposed them to a high risk of being beaten, raped, kidnapped, or murdered. It has also significantly reduced the amount of time women spend tending open fires each day, with the results that they are healthier and they have more time to grow vegetables for their families and make handicrafts for export. By 2007, the Jewish World Watch had trained 4,500 women, and had provided 10,000 solar cookers to refugees. The project has also reduced the number of foraging trips by as much as 70 percent, thus reducing the number of attacks.

Some Gazans have started to make solar cookers in order to cook their meals, due to a lack of cooking fuels due to the Israeli blockade. The cooker is made from cement bricks, mud mixed with straw and two sheets of glass. About 40 to 45 Palestinian households reportedly have started using these solar cookers.

Bysanivaripalle, a silk-producing village that is 125 km (78 mi) northwest of Tirupati in the Indian state of in Andhra Pradesh, is the first of its kind: an entire village that uses only solar cooking. Intersol, an Austrian non-governmental organisation, sponsored the provision of powerful "Sk-14" parabolic solar cookers in 2004.

See also

• Heliostat
• Kyoto box
• Liquid mirror telescopes
• Solar Cookers International
• Solar Cookers World Network
• Solar furnace
• Solar still
• Solar updraft tower
• Solar water disinfection

External links

• Solar Cookers World Network on Solar Cooking wiki, an external wiki
• Solar Household Energy, Inc
• Solar Cooking Atlas: Make a sun oven
• 60 examples
• The Solar Cooking Archive
• The Solar Bowl at Auroville
• Solar Funnel
• Infinitely large solar furnace
• Cornell University Solar Ovens Team Wiki
• Solar Ibex Project
• STC.UNM Solar Oven Plan
• Bolivia Inti Sud Soleil
• Solare Bruecke