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Q: "the uses of sun power in cooking" ( Answered,   1 Comment )
Question  
Subject: "the uses of sun power in cooking"
Category: Reference, Education and News > Teaching and Research
Asked by: f_a_h_d-ga
List Price: $50.00
Posted: 07 Mar 2004 15:05 PST
Expires: 06 Apr 2004 16:05 PDT
Question ID: 314366
i need the uses .. or how to use the the sun power in cooking
Answer  
Subject: Re: "the uses of sun power in cooking"
Answered By: pafalafa-ga on 07 Mar 2004 19:26 PST
 
Hello  f_a_h_d-ga,


Thanks for asking such an interesting question.

We humans like to have our energy delivered in a big, concentrated,
steady flow -- enough to light a light a lamp, heat a bathtub full of
water, or cook dinner for eight.

The sun's a frustrating energy source, in that respect.  Oh, it's big
all right.  But solar energy is neither concentrated nor steady, and
devices that try to put solar energy to work are designed to overcome
these inherent limitations.  For solar cooking devices, the key
challenge is concentrating solar energy to reach sustained
temperatures suitable for cooking food.

People have been trying to cook with the sun -- with varying degrees
of success -- for more than a hundred years.  Scientific American
magazine carried an article in their June 1878 issue about solar
cooking in India.  Although the materials available for solar cookers
have changed remarkably since then, the basic design elements have not
changed all that much.

In essence, solar cookers use a variety of reflective devices to
concentrate the sun's rays from a broad area to a smaller area where
the cooking takes place.  The surfaces range from simple, flat
reflective areas, tp sophisticated computer-designed parabolic
reflectors.  Other design elements may serve to retain the heat, in a
greenhouse-effect sort of system, while others exist to protect the
people doing the cooking from any dangerous exposures to excessive
heat.

There are a few key, authoritative sources of information on solar
cooking on the web, and I've provided information on them -- and from
them - below.

If anything here is not clear -- or if you need additional information
-- please tell me before rating this answer.  Just post a Request for
Clarification to let me know what you need, and I will be happy to
assist you further.

Happy Cooking!

pafalafa-ga

========== 

The U.S. Department of Energy has information on solar cooking devices
compiled by the Office of Energy Efficiency and Renewable Energy, at
their web site:


http://www.eere.energy.gov/consumerinfo/refbriefs/db5.html

Generally, the EERE  classifies solar cookers into three categories: 
solar concentrator cookers, solar ovens, and combined (hybrid)
devices.

Here are some verbatim excerpts from the EERE site:

-----


--The main types of solar cooking devices are solar concentrator
cookers, solar ovens and box cookers, combined (hybrid) solar
concentrator/box cookers, and indirect solar cookers. Well-designed
models of these devices work effectively when the sun is shining. Each
type has advantages and disadvantages. No commercially available
design can store heat to cook with when the sun is not shining.
Consequently, a back-up cooking device is usually necessary.



--Solar Concentrator Cookers

These types of solar cookers use a device, usually a parabolic dish,
to concentrate and focus sunlight onto a cook pot or food item. A
parabolic dish is a concave, bowl shaped device with a reflective
material lining the inside surface of the dish. The dish focuses
sunlight onto a dark cooking pot, typically suspended or set on a
stand in front of the cooker. These devices provide immediate, intense
heat when exposed to direct sunlight.

The size of the dish, reflectivity of the lining material, and
intensity of sunlight determines the heat output. Because of cost and
operating considerations, concentrator cookers are usually about one
square yard or meter. A solar cooker of this size produces 400 to 600
watts, depending on atmospheric conditions (i.e. presence of clouds,
smog, haze, etc.). This is about half the maximum output of a typical
electric stove heating element. Consequently, cooking time is at least
twice that of conventional electric stoves.

Solar concentrator cookers have some limitations and disadvantages.
Because they require direct sunlight, they perform poorly during
cloudy or overcast weather. Many cooks might find that adjusting the
concentrator every 10 to 15 minutes to keep the sunlight concentrated
and focused on the pot is inconvenient. (To our knowledge, a reliable,
cost-effective automatic tracking mechanism has not yet been developed
for solar cookers.) Lightweight parabolic dishes may vibrate in windy
weather, reducing the amount of sunlight hitting the pot and
increasing the risk of the dish tipping over. Operating them can be
dangerous (exposing eyes or skin to the concentrated sunlight can
result in blindness or burns.

Small parabolic trough concentrators also exist. Unlike parabolic
dishes, parabolic troughs focus sunlight onto a line, rather than a
point. The cooking area extends along this focal line. Small units
have been manufactured for cooking foods such as hot dogs.


--Solar Ovens and Solar Box Cookers

A solar oven is basically an insulated box with an aperture that has a
transparent glass or plastic cover (glazing). Solar ovens use the
"greenhouse effect" to cook food. Short wavelength solar radiation
passes through the glazing, and strikes the interior sides of the oven
and the cooking pot. The interior sides of the oven can be either
reflective or black. If reflective, the sunlight reflects onto the pot
and heats it. If the sides are black, the sunlight is absorbed and
then re-radiated at longer wavelengths, heating the oven interior and
cooking the food. The glazing also helps to keep the heat in the box.

Solar ovens use both direct and indirect solar radiation. This allows
them to operate better than concentrator cookers under slightly cloudy
conditions. Also, because of their shape and weight, wind does not
affect their performance as much. They also retain heat longer.
Placing an insulated cover over the aperture glazing helps to reduce
heat loss and keep food hot if thick clouds block the sun for extended
periods. Solar ovens only require repositioning to face the sun every
30 minutes to an hour to maintain an adequate cooking temperature.

There are two major solar oven designs. The first, developed by Dr.
Maria Telkes and others, has exterior reflectors that surround the
aperture. The reflectors are set at a specific angle to focus
additional solar radiation into the oven. A variation of this design
has the oven in the form of an ordinary box, and the entire oven is
tilted to face the sun. A swinging tray that holds the pot can help
keep certain foods from spilling when tilting the oven. Oven
temperatures generally reach 400 to 450 Fahrenheit (F) (204 to 232
Celsius[C]) and are suitable for almost any kind of cooking except
frying. Because of these temperatures, tempered glass is better than
ordinary window glass for the glazing material. Materials for
insulating the sides of the oven must also be able to withstand these
temperatures and should not contain toxic binders. Because of design
limitations and expense of the materials, this type of
high-temperature solar oven is usually just large enough to hold one
large pot. Design variations affect performance significantly.

Another design, often called a solar box cooker, has only one exterior
reflector. The reflector is attached to the oven with a hinge, so that
it can be periodically adjusted to reflect the sunlight into the oven.
This eliminates the need to tilt the oven, although the entire oven
still needs to be oriented toward the sun regularly. Oven temperatures
normally reach around 250 to 300 F (121 to 149 C). Consequently, a
solar box cooker takes longer to heat up and cooks food slower than a
multiple-reflector solar oven.

A solar box cooker does have certain advantages. Most solar box
cookers can hold several pots. Because of the lower temperatures, they
can use less expensive glazing and insulation materials. For example,
ordinary cardboard and aluminum foil can serve as insulation. Solar
box cookers may be lighter and easier to carry. They are best for slow
cooking and baking, which does not require high temperatures. Because
of the lower cooking temperatures, food cannot burn as in
high-temperature ovens, although sufficient water is necessary to keep
foods from drying out.


--Combined Oven-Concentrator Cookers

A combined solar oven-concentrator cooker uses the best features of a
concentrating cooker and an oven. A parabolic reflector concentrates
and focuses sunlight into a glazed, insulated oven. The oven is set on
a frame above the reflector. This design reduces heat loss since the
cooking area is located within the oven. The bottom of the cooking pan
can become hot enough to fry food.

However, this hybrid design has many of the same problems as
concentrating models. For example, care must be taken to avoid eye
damage, and it is necessary to reposition the oven about every 15
minutes. Moreover, most of the hybrid designs have small ovens,
limiting the amount of food that can be cooked at one time.

==========

Another site that is an absolute must for learning about solar cooking
is the official Solar Cooking Archive at:



http://solarcooking.org/



This site is replete with information about plans for different styles
of solar cookers; technical information and how-to guides; field
reports of experience with solar cookers in different parts of the
world; and even a somewhat oxymoronic account of solar refrigeration.

They also offer an extensive and fairly up-to-date list of links to
other sites, suppliers of solar cooking equipment, groups that support
(and possibly fund) solar cooking efforts; recipes; and so on.

Some of the key items of interest at this site are:

-----

A very nice set of photos and drawings of different types of solar
cookers can be found here:

http://solarcooking.org/plans.htm

-----

An entire book on solar cooking, The Expanding World of Solar Box
Cookers, is posted online and is accessible free of charge at this
link:


http://solarcooking.org/kerr.htm


------

There is a good list of factors to consider when evaluating solar cookers:


http://solarcooking.org/rating.htm

that advises focusing on cost, convenience, safety, heating and
cooking capacity, durability and maintenance, wind stability, and
clarity of instructions.

----- 

A list of books on solar cooking is here:

http://solarcooking.org/books.htm


-----

Finally, probably everything you could want to know about solar
cooking -- and then some -- is contained in the comprehensive set of
links provide by the good folk at solarcooking.org at this site:


http://solarcooking.org/docs.htm



           
==========

Another online book-length report,  Review of Solar Cooking Designs,
can be found here:

http://members.tripod.com/~ashokk_3/solar1.htm

and provides a detailed account of more than 60 styles of solar cookers.


==========

Want to build your own solar cooker?  It's not that hard. 
Instructions for a pizza-box solar cooker can be found here:

http://www-ed.fnal.gov/ntep/f98/projects/nrel_energy_2/solarovens.html

You won't be grilling any burgers in it, but you might at least be
able to brew up a cup of hot (but not steaming!) tea.

Another home made solar cooking device can be found at this NASA site:


http://whyfiles.larc.nasa.gov/text/educators/activities/2000_2001/inclass/solar_cooker.html

this time based on an oatmeal -- rather than pizza -- box.  It
promises to cook up a couple of hot dogs, so it seems capable of
generating a respectable amount of heat.

==========

An effort to make use of solar ovens in the field is described at this
NASA Earth Observatory site (NASA measurements of solar radiation are
useful baselines for calculating the utility of solar cookers in
different parts of the world).  The site can be seen here:



http://earthobservatory.nasa.gov/Study/RenewableEnergy/renewable_energy4.html

and the article offers some useful perspectives on what might be
needed to get solar cooking devices in more wide-spread use in
developing countries, where abundant sunlight, and shortages of
affordable fuel make solar cooking a particularly desirable option:

-----


Baking in the Sun
    
"It is as unnecessary to use combustibles for fuel under a brilliant
sun as it is foolish to put a solar oven out in the rain," states
Darwin Curtis, a partner in Solar Household Energy, Inc. (SHE). "Yet
in many tropical areas of the world, more than 40 percent of families?
disposable income is spent on fuel for cooking. Sometimes more is
spent on fuel than on food."

 

...the entire effort to make solar ovens available to developing
nations--parts of the world where this technology is needed
desperately--was in the hands of governments, non-government
organizations (NGOs), and the United Nations..."The problem was much
too large to be dealt with by charitable organizations," says Curtis.
"The only way adequately to address the problem is to induce private
enterprise to get involved."

"Our objective is to increase by 50 percent the efficiency over
comparable ovens," Curtis explains. "The most widely affordable solar
ovens now in use cost $2 or $3. They are remarkably effective but they
are made of cardboard. We must use materials that can last a decade or
more. And the ovens must retail for less than $30."

NASA satellite data are helping SHE determine which parts of the world
receive sufficient sunlight to promote solar cooking. He reckons that
SHE should focus on places that receive at least 4 kilowatt hours per
square meter per day on average. Luckily, he says, the areas of the
world where many people in greatest need for this technology live have
solar insolation above 4, and as high as 6 kilowatt hours per square
meter per day.

So how do solar ovens work? The concept depends upon two things: a
means of converting solar energy into heat, and a means of trapping
this heat around a cooking vessel. Curtis explains this is done most
simply by using a black or dark-colored vessel surrounded by some sort
of transparent envelope. Sunlight passes through the transparent
envelope and strikes the dark pot, which absorbs the sunlight and
converts it into heat. The transparent envelope traps the heat to
achieve cooking temperatures.

"This is often referred to in literature, as ?the greenhouse effect,?"
Curtis says. "In cheap solar ovens, 250F (121C) can be achieved. In
the expensive models, 350F (177C) is not unusual."

==========

I hope this information meets your needs.  But as I said earlier, just
let me know if you would like any additional information.

All the best.

pafalafa-ga




search strategy:  Google search on:  "solar cooking" OR "solar ovens"
Comments  
Subject: Re: "the uses of sun power in cooking"
From: ac67-ga on 09 Mar 2004 08:49 PST
 
Here is a link to a couple plans for building your own:
http://www.motherearthnews.com/menarch/archive/goto.asp?article=050/050-032-01&ID=2318&Num=9

and: http://www.motherearthnews.com/menarch/archive/goto.asp?article=045/045-090a-01&ID=1990&Num=4

My brother actually built this second one when he was in high school. 
He had gotten the instructions from Mother Earth News, which is how I
knew where to look.  They have some additional plans here:
http://www.motherearthnews.com/menarch/archive/search.asp?search=solar+cooker&type=1&page=

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