How solar cookers work

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Below you will find information about how solar cookers work. The Solar Cooking Wiki, sponsored by Solar Cookers International, also provides an individual page for different countries where you will find news, NGOs, manufacturers, and individuals working on solar cooking projects in that country. Links to possible funders are also available:
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==How do solar cookers work?==
Most solar cookers work on the basic principle: Sunlight warms pots and/or food, which is used for cooking.
===Why do dark colored cook pots get hot?===
Light waves are absorbed by dark surfaces. When we see white it is because practically all of the light waves across the visible color spectrum are reflected away from the surface of the material. All those colors together combine to make white. Conversely, when we see black, all the visible light waves are being absorbed by the material, and nothing is being reflected away.
When they absorb this light-energy, molecules making up the dark colored cook pot get excited. Excited molecules have more energy to give, and we measure this excess energy as higher temperature. So, the dark colored cook pot gets hotter.
With a solar box cooker, some of the sunlight is reflected back out of the cooker from the non-dark colored surfaces through the glass enclosure. However radiation generated from a dark cook pot has a much longer wavelength than visible light, and can not escape through the glass, keeping more energy in the oven. A higher temperature builds within the cooking chamber. The insulated walls of the cooker also help to retain the energy collected by the dark cook pot. 
Below, is more on the basic science for solar panel cookers and solar box cookers. Another style of solar cooker is a parabolic solar cooker. They typically require more frequent reorientation to the sun, but will cook more quickly at higher temperatures as they can focus more sunlight on the cook pot, enabling them to be able to fry foods. Evacuated tube solar cookers use a highly insulated double-wall glass tube for the cooking chamber, and do not require large reflectors as they rely on their ability to retain high levels of heat.
	
		
		Fuel: sunlight
Sunlight is the fuel. A solar cooker needs an outdoor spot that is sunny for several hours and protected from strong wind, and where food will be safe. Solar cookers don't work at night or on cloudy days. 
	
Convert sunlight to heat energy
Dark surfaces get very hot in sunlight, whereas light surfaces don't. cooking is best done in dark, shallow, thin metal pots with dark, tight-fitting lids to hold in heat and moisture. With an efficient cooker, glass tops for pots can be used to avoid having to open pots for checking on the food. Baking is best done with a dark opaque lid.
A solar cooker needs an outdoor location that is sunny for several hours and protected from strong wind, and where food will be safe. Solar cookers don't work at night or on cloudy days, though during the best months for cooking, many foods can be cooked under intermittent clouds or a light haze, as long as food is put out early and there is definitely more sun than not overall. 
Retain heat
A transparent heat trap around the dark pot lets in sunlight, but keeps in the heat. These can be a clear, heat-resistant plastic bag or large inverted glass bowl (panel cookers), or an insulated box with a glass or plastic window (box cookers).
Capture extra sunlight
One or more shiny surfaces reflecting extra sunlight onto the pot, increasing its temperature more quickly and resulting in a higher final temperature.
*Parabolic solar cookers use a bowl-shaped reflector to focus the light more directly onto the cookpot, usually from below, and typically do not require a greenhouse enclosure to retain the heat. They can also fry and broil foods.
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===Converting sunlight to heat energy===
At its simplest, the sunlight-to-heat conversion occurs when photons (particles of light) moving around within light waves interact with molecules moving around in a substance. The rays emitted by the sun have a lot of energy in them. When they strike matter, whether solid or liquid, all of this energy causes the molecules in that matter to vibrate. This activity generates heat.
Dark surfaces get very hot in sunlight, whereas light surfaces don't. While food cooks best in dark, shallow, thin metal pots with dark, tight-fitting lids, there are many other containers that can also be used in a solar cooker.
===Retaining heat===
A transparent heat trap around the dark pot lets in the sunlight, and keeps the heat that is produced from escaping. This is a clear, heat-resistant plastic bag or large inverted glass bowl (in panel cookers) or an insulated box with a glass or plastic window (in box cookers). 
Light passes through the plastic bag or glass cover as a relatively short wavelength. Heat is reflected back as a longer wavelength, and does not easily pass back through the clear enclosure. This explains why cars left in the sun, especially those with black interiors, will slowly become hotter and hotter, even on days with low air temperatures.
Parabolic solar cookers typically do not require a heat trap, as the light from the reflector is tightly focused on the cook pot. They cook at higher temperatures, but require more frequent reorientation with the sun than box or panel cookers.
===Capturing extra sunlight energy===
One or more shiny surfaces reflect extra sunlight onto the pot, increasing its heat potential. Mirrors, aluminum foil, Mylar, mirror-finish metals, chrome sign vinyl, and other shiny materials have all been used successfully for solar cooking, depending on the type of cooker and the environment in which it will be used.
==Solar cooker types==
The three most common types of solar cookers are box cookers, curved concentrators (parabolics) and panel cookers. Hundreds — if not thousands — of variations on these basic types exist. Additionally, several large-scale solar cooking systems have been developed to meet the needs of institutions worldwide. Evacuated tube solar cookers are a more recent dvelopement in solar cooking. While very efficient, their use requires extra care.
===Box cookers===
Box cookers cook food at moderate to high temperatures and often accommodate multiple pots, typically taking between one and three hours to cook various foods. The cooking chamber walls are insulated using various nontoxic materials to maintain higher cooking temperatures. Worldwide, they are the most widespread. There are several hundred thousand in India alone. 
===Panel cookers===
Panel cookers incorporate elements of box and parabolic concentrator cookers, relying on a "greenhouse" enclosure to help retain heat and allow the light to reach the cook pot, and reflective panels to gather additional light. They are simple and relatively inexpensive to buy or produce. Solar Cookers International's "CooKit" is a widely used panel cooker, best suited for locations near to the equator. 
===Parabolic cookers===
 Parabolic solar cookers use a bowl shaped reflector to focus the light more directly onto the cook pot, usually from below, and typically do not require a greenhouse enclosure to retain the heat. The parabolic name refers to the shape of the curve of the reflector cross-section. 
They will require more frequent reorientation to the sun, possibly every 10 minutes, but they cook food more quickly at higher temperatures compared to other solar cookers, often reaching over . They also have the ability to fry foods. Generally parabolic solar cookers will need to be attended to more than box or panel cookers to avoid possibly burning the food at the bottom of the cook pot. They are especially useful for large-scale institutional cooking.
===Evacuated tube solar cooker designs===
Evacuated tube solar cookers use a double-wall glass tube for the cooking chamber. The space between the glass is created as a vacuum, providing excellent heat retention. While efficient, glass technology somewhat limits the size of opening of the glass tube, requiring smaller cooking pots set in a slender cooking trough. Extra care is required handling and using the glass tubes. Adding a cold liquid to a hot tube can cause thermal shock and tube breakage.
==Audio and video==
*May 2021:
::|400px|none|thumb|The basic elements and styles of solar cookers are explained by Dave Oxford of SLiCK
==See also==
*Sunlight 
*Glazing
*Greenhouse effect
*Principles of Solar Box Cooker Design - Mark Aalfs
*The Technology of Solar Cooking - Ed Pejack
*All solar cooker designs found on this wiki
*Introduction to solar cooking
*Why solar cook
*Where solar cook
*Health and safety
*Solar water pasteurization
*Solar calculations
*Varieties of Solar Cooker Devices and Uses - Shyam Nandwani
*:Category:Introduction to solar cooking
==External links==
* Parabola-Focus-Directrix: interactive parabolic curve calculator- Jose Antonio Gutierrez Guerra
* - Christelle Souriau & David Amelin (This is an excellent overview of solar cooking basics and simple solar cooker and dryer construction methods.)
*How Solar Cooking Works - HowStuffWorks
*The full text of the book The Expanding World of Solar Box Cooking - Barbara Kerr
*Evaluation of Several Original and Commonly Used Solar Cooker Designs - Dane Dormio and Steven Jones
*Rating Solar Cookers
*French: Evaluation des fours solaires
==Documents==
*June 2005: - E Vijayakrishna Rapaka
*June 2006: - E Vijayakrishna Rapaka