Efficiency of materials for making a Solar Oven

by Harriet
( Harvard Mass.)

Hi. I'm an 8th grade student at the Harvard, Mass High School. For my science fair project, I am comparing the efficiency of aluminum, clay,and glass "solar ovens." I am predicting the aluminum will be the hottest and most efficient due to its ability to conduct heat, but am curious about a clay oven, which is made from more natural substances from the earth, and a glass oven, which is esthetically pleasing. In comparing heating times and cooking temperatures of all three ovens (that I'm building myself), do you recommend that I add any specific materials (to all three) or can you guide me in any way as I look at the three different materials? Thank you very much,

Harriet Grade 8, Harvard, MA.



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Thank you Harriet for visiting our site and for your great question!

I would like to invite any of our site visitors and solar cooking experts to contribute if they might in answering your questions. I too will add my own thoughts as well.

Nathan
Admin.



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Here are more of my thoughts:


I will start by saying that most commercially manufactured solar ovens use aluminum for the interior “box” portion of the oven. This is where the food will be placed when cooking. Also many solar cooks will place the food inside of an aluminum pot or pan which are usually covered by dark enamel such as the enamelware or speckleware cookery. Because the pots are dark in color and the metal is thin, this allows for the fastest most efficient transfer of heat to the food allowing for quicker cooking times.

Many people ask us if they can use cast iron in their solar ovens; and you can, but cast iron takes much longer to heat up thus extending the overall cooking time. Although once the cast iron and the food are heated, the cast iron cooking vessel will retain the heat much longer.
If you have ideal conditions and all day to cook, then cast iron will do alright.
But, you can always obtain quicker and just as satisfactory results by just using thin, dark colored cooking vessels.

As for aluminum as your overall material in the construction of your solar cooker you will do fine using it as your interior.
Some people will paint the interiors all black using high temperature BBQ paint so that the interior of the oven will absorb the heat which in turn is transferred to the cooking vessel by conduction.
Some people prefer to keep the interior a shiny, bright aluminum color with the principle in mind that the dark colored pot/pan will absorb more of the suns rays because the reflectivity of the interior of the oven will increase the amount of the suns energy (rays) bombarding the cooking vessel.
Many solar cooks are of the consensus that both kinds of interiors work well.

If you make the cooker out of aluminum then you will want to cover the whole aluminum box exterior with an insulation of some sort so that you do not loose all of the heat out through the box walls. This is the same way that the famous Global Sun Oven is designed, with a high quality insulation placed in the space between the inner aluminum interior and the out ABS Plastic exterior.


With regards to a clay solar cooker, you may want to use aluminum in conjunction with the clay, using the clay as the insulating exterior and the aluminum as the interior where the food will cook.
Of course this would not give you the two distinct material comparisons that you were suggesting in your letter of inquiry.

If you use clay only to make the oven then be sure to make it dark in color so that it will absorb the suns energy.
You may also want to mix the clay with some type of grass or straw to give it a better insulating quality as well as making it more solid, such as the process for making a Cobb Oven.
A dark clay interior should do well with heat absorption as well as conduction from the clay to the pot.

In regards to a glass cooker, I am not quite sure if I understand how the glass will be used.
Most cookers have a glass door or lid so that the suns rays can pass through and then the lid traps the heat preventing it from escaping.

Of course glass is not the best material for holding the heat though, whereas it will also conduct the heat out more quickly than other materials.
This loss of heat to a certain degree is present in all solar cookers that have glass or lexan covers since these covers are the only method of letting the sunlight into the cooker, but they will slowly allow the heat to escape as well through conduction and radiation.

If you are planning on making the whole box cooker of glass; sides, top, bottom etc., then you will probably have to insulate some of the cooker, possibly the bottom or sides so as not to loose too much of your heat.

There are people who have taken a dark pot with food and covered it with an inverted glass bowl, and then placing it inside of a reflector panel, have solar cooked their food.

This would be a very close example of a glass solar oven.

This method of making a solar cooker would probably be the least efficient of the three.

I hope you might find some of this information to be helpful in your solar cooker comparison project.

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Jan 30, 2011
Solar Career
by: Brad

I work in the power industry, so I know how little power density solar power actually has, and how little grid support (Volt-Amperes Reactive) it can contribute.

Solar for me, is a point-of-use, direct application power source. If I were to convert the BTUs used for cooking into a specified voltage, using a transformer and charge controller (which I have done) and then attempt to push it through long power lines, the eddy-current losses would make it completely unusable. If you're interested in learning about power versus politics, I'd suggest Robert Bryce

and his latest work, "Power Hungry: The Myths of "Green" Energy and the Real Fuels of the Future"

Like me, he's a liberal who thinks green energy is illogical. I've worked at steam plants, nuclear power plants, run waste water treatment plants, and now work in hydroelectric. My co-workers have worked solar, hydrothermal, coal, nuclear, etc. as well, and we know the limitations and benefits of all of them.

That being said, I approach every power use that I have in my personal life, first with power conservation in mind, and if I can afford it, employ an alternative energy (I owned a Prius when they first came out in 1999). Solar cooking is one of those "no brainers", and I think the more you research the subject and try solar parabolics, panel cookers, and solar ovens, the more you'll appreciate it. I could go on all day, so I'll stop here.

Jan 30, 2011
Thank you all so much!
by: Harriet

Thank you all so much!
I will respond in more depth after running my experiment and observing my data, for it's deadline is Wednesday. Many of your comments have already contributed to my experiment. Just curious, do you all work in the solar industry?...and would you recommend it? :D

thanks again!
Harriet

Jan 26, 2011
Inexpensive Boxes for Solar Cookers
by: Anonymous

Stopped by the wine store on the way home, just to research wine boxes (and accidentally bought a few Sauvignons, oops...) and they were selling exactly what you'd need for less than $2 (American) per box. Now you've got me thinking about building a wine box solar cooker, and I read what Nathan and Jim wrote (excellent summary Nathan) and I'd go with an aluminum interior and dark, lightweight cookware combination. The wood is just for weight support. Angled front cut, foamboard covered with aluminum tape reflectors. Wouldn't even bother with a glass front if I reached my desired temperatures. I would add one if I could not, because of wind or ambient temperatures (i.e. NYC).

Jan 26, 2011
Dark and thin for your cooking vessel
by: Jim La Joie


For cooking vessels, dark and thin are best. I use anodized aluminum pots which are naturally dark for most of my solar cooking.

Jan 24, 2011
Interesting Page for Solar Oven Makers
by: Brad

Harriet,

This page:

http://www.williamgbecker.com/MakeSolarOven.html

shows the process the builder went through and which features to put in, and which to leave out, while developing his solar oven. I thought you'd like to see this process. I still do not understand why he did not put a hinged door in the back for easier cooking vessel movements, or why he did not reflect his inner oven, if he's going to use black cookware, but he may have his reasons.

Not everyone has a woodworking shop (I certainly do not), so you may like this link, to PDF instructions on how to make a Box Solar Cooker from more ordinary materials:

Solar Box Cooker PDF File

Haven't made this particular one, but it was designed for Pacific Islanders to make, with lightweight wood, and other simple materials. You may be able to get your wooden box at a local wine shop for a few dollars, and the size may be just right, helping you skip many of the construction steps. A Hobby Shop will have the rest. I like this design, but think it should have an angled front, like the GSO.



and that first oven shown, so keep that in mind as you alter the Pacific Islander design. They may be more on the equator than we are, and are working with an overhead sun most of the day. In North America, we have the lower angle to consider.

Jan 24, 2011
A Lot to Think About
by: Brad

I think that the first thing you should do, is establish some sort of baseline of comparison. The use of masonry in heating and cooking could be that starting point. I'd go here:

http://www.heatkit.com/html/bakeoven.htm to gather the various types and styles.

Now that you've got a baseline, you can look to use those various materials you listed (glass, aluminum, etc.) in designing various sun ovens, and see how they match up. Here is a spreadsheet that covers a lot of those details. The first link is the explanation page:

Solar Oven Design Spreadsheets

This link takes you to the java script page:

http://www.webplaces.org/solaroven/spreadsheet.htm

I believe you can download the spreadsheet from this page:

http://www.webplaces.org/solaroven/maths.htm

That's a good start. Good luck.

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