Editing Solar cooker (section)

== Parabolic or paraboloidal reflectors ==
{{see|parabolic reflector}}

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Parabolic solar cookers concentrate sunlight to a single point. When this point is focused on the bottom of a pot, it can heat the pot quickly to very high temperatures which can often be comparable with the temperatures achieved in gas and charcoal grills. These types of solar cookers are widely used in several regions of the world, most notably in China and India where hundreds of thousands of families currently use parabolic solar cookers for preparing food and heating water. Some parabolic solar cooker projects in China abate between 1–4 tons of carbon dioxide per year and receive carbon credits through the [http://unfccc.int/kyoto_protocol/mechanisms/clean_development_mechanism/items/2718.php Clean Development Mechanism (CDM)] and [http://www.goldstandard.org/ Gold Standard]. Some parabolic solar cookers incorporate cutting-edge materials and designs which lead to solar energy efficiencies >90%. Others are large enough to feed thousands of people each day, such as the solar bowl at Auroville in India, which makes 2 meals per day for 1,000 people.<ref>{{cite web|url=http://archive.auroville.org/research/ren_energy/solar_bowl.htm |title=The solar bowl |date=22 December 2014 |work=archive.org |url-status=bot: unknown |archive-url=https://web.archive.org/web/20141222040320/http://archive.auroville.org/research/ren_energy/solar_bowl.htm |archive-date=22 December 2014 }}</ref>

If a reflector is axially symmetrical and shaped so its cross-section is a [[parabola]], it has the property of bringing parallel rays of light (such as sunlight) to a point ''[[Focus (optics)|focus]]''. 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.

[[Paraboloid]]s are [[wikt:compound curve|compound curves]], which are more difficult to make with simple equipment than [[wikt:single curve|single curves]]. Although paraboloidal solar cookers can cook as well as or better than a conventional stove, they are difficult to construct by hand. Frequently, these reflectors are made using many small segments that are all single curves which together approximate compound curves.

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. (See [[Liquid-mirror telescope]].) 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.{{citation needed|date=August 2013}} This rotation technique is sometimes used to make paraboloidal mirrors for astronomical telescopes, and has also been used for solar cookers. Devices for constructing such paraboloids are known as [[rotating furnace]]s.

Paraboloidal reflectors generate high temperatures and cook quickly, but require frequent adjustment and supervision for safe operation. Several hundred thousand exist, mainly in China.{{citation needed|date=November 2012}} They are especially useful for individual household and large-scale institutional cooking.

[[File:Parabole de cuisson solaire Scheffler coccion solar cooking.jpg|thumb|upright=0.75|left|Scheffler cooker with area {{convert|16|m2|abbr=on}} which concentrates 3 kW of heat]]

A Scheffler cooker (named after its inventor, [[Wolfgang Scheffler (inventor)|Wolfgang Scheffler]]) uses a large ideally paraboloidal reflector which is rotated around an axis that is parallel with the Earth's using 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 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.{{citation needed|date=August 2013}}

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.{{citation needed|date=August 2013}}

[[File:focus-balanced parabolic reflector.svg|thumb|300px|An [[oblique projection]] of a focus-balanced parabolic reflector]]
Paraboloidal reflectors with their centers 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.{{citation needed|date=August 2013}}

=== Parabolic troughs ===
[[Parabolic trough]]s are used to concentrate sunlight for solar-energy purposes. Some solar cookers have been built that use them in the same way.<ref>{{cite book|author=Shah, Yatish T.|title=Thermal Energy: Sources, Recovery, and Applications|url=https://books.google.com/books?id=QdArDwAAQBAJ&pg=PT340|date=12 January 2018|publisher=CRC Press|isbn=978-1-315-30593-6|pages=340–|access-date=28 September 2019|archive-date=22 December 2023|archive-url=https://web.archive.org/web/20231222093202/https://books.google.com/books?id=QdArDwAAQBAJ&pg=PT340#v=onepage&q&f=false|url-status=live}}</ref> 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.<ref>{{cite book|title=Solar Tracking |author=Prinsloo, GJ |author2=Dobson, RT |name-list-style=amp |doi=10.13140/RG.2.1.4265.6329/1 |isbn=978-0-620-61576-1|pages=1 |date=2015}}</ref> 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 the design attractive, compared with using a paraboloid. Also, being a [[wikt:single curve|single curve]], the trough reflector is simpler to construct. However, it suffers from lower efficiency.

It is possible to use two parabolic troughs, curved in perpendicular directions, to bring sunlight to a point focus as does a paraboloidal reflector.{{citation needed|date=August 2013}}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.{{citation needed|date=August 2013}}

Compared with a single paraboloid, using two partial troughs has important advantages. Each trough is a [[wikt:single curve|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.

The two troughs are held in a fixed orientation relative to each other by being both fixed to a frame.{{citation needed|date=August 2013}} The whole assembly of frame and troughs has to be moved to track the sun as it moves in the sky. Commercially made cookers that use this method are available. In practical applications (like in car-headlights), concave mirrors are of parabolic shape.

=== Spherical reflectors ===
[[File:Auroville Solar Bowl.JPG|thumb|left|The Solar Bowl in [[Auroville]], India]]

Spherical reflectors operate much like paraboloidal reflectors, such that the axis of symmetry is pointed towards the Sun so that sunlight is concentrated to a focus.  However, the focus of a spherical reflector will not be a point focus because it suffers from a phenomenon known as [[spherical aberration]].  Some concentrating dishes (such as satellite dishes) that do not require a precise focus opt for a spherical curvature over a paraboloid. If the radius of the rim of spherical reflector is small compared with the radius of curvature of its surface (the radius of the sphere of which the reflector is a part), the reflector approximates a paraboloidal one with focal length equal to half of the radius of curvature.<ref>See [[Parabola#Focal length and radius of curvature at the vertex]]</ref>
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=== Vacuum tube technology ===
Evacuated tube solar cookers are essentially vacuum sealed between two layers of glass. The vacuum allows the tube to act both as a "super" greenhouse and an [[Insulator (electricity)|insulator]]. The central cooking tube is made from borosilicate glass, which is resistant to thermal shock, and has a vacuum beneath the surface to insulate the interior. The inside of the tube is lined with copper, stainless steel, and aluminum nitrile to better absorb and conduct heat from the Sun's rays. Solar cooking tube systems use a reflector to enhance the thermal energy capturing. There are models of tube cooking system in India which has energy storage devices installed at the bottom of the tube to store heat for cooking or heating food during night.

These vacuum tube solar cookers can cook a meal in as little as 20 minutes.<ref>{{Cite web|url=https://www.bostonglobe.com/lifestyle/travel/2019/06/11/here-there-and-everywhere/qy5a9nmIw3H8hIMYptskKJ/story.html|title=Here, there, and everywhere - The Boston Globe|website=BostonGlobe.com|access-date=2022-02-16|archive-date=2022-02-16|archive-url=https://web.archive.org/web/20220216164544/https://www.bostonglobe.com/lifestyle/travel/2019/06/11/here-there-and-everywhere/qy5a9nmIw3H8hIMYptskKJ/story.html|url-status=live}}</ref>

<gallery mode="packed">
File:Small solar cooker.jpg|A solar vacuum tube cooker
File:Professional Solar Oven.jpg|Professional Solar Oven
File:Solar oven glass tube cooking bread.jpg|Solar oven glass tube cooking bread
File:Solar_Cooker_with_Evacuated_Glass_Tube.jpg|Solar cooker with evacuated glass tube
</gallery>