Science Project: Solar Cooker (help)

[mike22cha]

Well I got to do a Solar Cooker project for my science class, and wanted some input. I need a plan by Thursday, which I kinda got an idea but may need some help.
Ok here's the requirements for the solar cooker:
-30 cm wide
-40 cm long
-20 cm deep
-Must reach 80 degrees Celsius (while empty) in 30 min. (we get extra credit if it gets over 80degrees celsius by 30min).
Here's a link to a R-value table: http://www.coloradoenergy.org/procor...f/r-values.htm
I'm not looking to cheat or anything, just want some input on my ideas.
I'm thinking of getting a cardboard box for the oven. I want to leave the top open to atleast a 45 degree angle to reflect the sun. I'm thinking the best thing to heat it up is to get a cardboard box then cover it with foil.

Maybe I should build the box and reflectors out of something else and cover it with foil?

Just need some input on materials.
Oh and it can't be over 15$ to build (unless parent and teacher approval) which I could ask to go over a little, but I don't want to go over that much.

 

[mfp1016]

Styrofoam would be a much better insulator than cardboard and as such would provide a much more viable oven.

 

[alix2.0]

i had to make one for science a couple years back, i used a styrofoam bait box thing lined with foil and spray painted the outside black. worked real nice. help to keep the box as closed as possible to keep heat in, just open it enough to reflect light in.

 

[mfp1016]

By the way, I doubt styrofoam can withstand 80 centigrade, but I was trying to point your towards using a better insulator.

 

[darknes]

You want to trap in heat, but yet reflect the sunlight into it. I would cover the top with a piece of glass or plastic to trap the heat inside, much like a car on a sunny day.
The inside could be lined with aluminum foil to help reflect.

 

[alix2.0]

Originally Posted by mfp1016
By the way, I doubt styrofoam can withstand 80 centigrade, but I was trying to point your towards using a better insulator.
mine did okay, i dont remember too much damage. some had a little smoke coming out of theirs though.

 

[mike22cha]

Originally Posted by Darknes
You want to trap in heat, but yet reflect the sunlight into it. I would cover the top with a piece of glass or plastic to trap the heat inside, much like a car on a sunny day.
The inside could be lined with aluminum foil to help reflect.
I like this idea.
I found a stash of boxes at a place I help my mom clean so any size would be ok. I can't have it to big, just big enough to hold the pan (which are the min. deminsions), the bigger means the longer it takes to heat. I think I'll paint the outside of the cardboard black maybe put a coat on the inside too, then put foil over the inside. I'm going to get some of that long thin circular pieces of wood at homedepot, and cut them to an according size, so that it can be hooked onto the box easily, then removed to hold the reflectors in place and also hook them reflectors to the ground. I'm also thinking of weighting this sucker down with some gallon jugs of water you get at walmart for like .69$. I'll get some kind of drawing up, because that's what we have to show our teacher too.

 

[darknes]

I took several classes in heat transfer and thermodynamics and college, so I'll give you a few things to think about.
There are 3 ways that heat can be transferred between matter: conduction, convection, and radiation.
Conduction is the transfer of heat between bodies, such as if you hold a piece of ice in your hand the heat transfers from your hand to the ice. Heat always conducts from warmer to colder, so with your oven the heat will conduct through the walls to the outside. In your case, you want to minimize conduction, so you want to use materials with the lowest thermal conductivity (or inversely, the highest R value) to insulate the oven from the environment.
Convection is a little harder to explain, but is the transfer of heat from a solid to a moving liquid or gas (such as if you blow across some hot food to cool it off). In your case, convection won't play much of a part unless it's windy outside. If there's lots of wind, the wind will cool off the outside of the oven, and increase the speed of conduction through the walls. Since you cannot stop the wind from blowing, the best case would be to create a double-walled oven with a pocket of air between the two walls (probably not practical for your project

)
Radiation is where all your heat will come from (the sun). To maximize the heat produced, you want to line the inside with a material with a very low emissivity (such as aluminum foil). A material with low emissivity reflects the heat away from it, where a surface with a high emissivity will absorb your heat (such as black paint). You also want to maximize the area of reflection to maximize the heat reflected. Geometry also plays a large role in radiation. If you think about the reflectors on MH bulbs for your tank, they are shaped in such a way to maximize the reflection from the bulb to your tank (round, concave surfaces are ideal).
Hopefully that wasn't too much information, lol.

Theoretically, you can write up an equation to calculate the ideal dimensions and angles to use based on the day of the year, your latitude, solar azimuth, declination, and hour angle, but that would be too much work.

 

[mike22cha]

Ya we learned about the transfer of heat last year and we're reviewing it right now. What degree would maximize the reflection?

Originally Posted by Darknes
I took several classes in heat transfer and thermodynamics and college, so I'll give you a few things to think about.
There are 3 ways that heat can be transferred between matter: conduction, convection, and radiation.
Conduction is the transfer of heat between bodies, such as if you hold a piece of ice in your hand the heat transfers from your hand to the ice. Heat always conducts from warmer to colder, so with your oven the heat will conduct through the walls to the outside. In your case, you want to minimize conduction, so you want to use materials with the lowest thermal conductivity (or inversely, the highest R value) to insulate the oven from the environment.
Convection is a little harder to explain, but is the transfer of heat from a solid to a moving liquid or gas (such as if you blow across some hot food to cool it off). In your case, convection won't play much of a part unless it's windy outside. If there's lots of wind, the wind will cool off the outside of the oven, and increase the speed of conduction through the walls. Since you cannot stop the wind from blowing, the best case would be to create a double-walled oven with a pocket of air between the two walls (probably not practical for your project

)
Radiation is where all your heat will come from (the sun). To maximize the heat produced, you want to line the inside with a material with a very low emissivity (such as aluminum foil). A material with low emissivity reflects the heat away from it, where a surface with a high emissivity will absorb your heat (such as black paint). You also want to maximize the area of reflection to maximize the heat reflected. Geometry also plays a large role in radiation. If you think about the reflectors on MH bulbs for your tank, they are shaped in such a way to maximize the reflection from the bulb to your tank (round, concave surfaces are ideal).
Hopefully that wasn't too much information, lol.

Theoretically, you can write up an equation to calculate the ideal dimensions and angles to use based on the day of the year, your latitude, solar azimuth, declination, and hour angle, but that would be too much work.

 

[elfdoctors]

I used to be a beekeeper and I made a solar wax melter which worked very well. (I made over 40 pounds of pure beeswax until I ran out of old honeycomb). It was made with wood sides which had been painted black. I had a piece of plexiglass which sealed the top very well and kept the heat in. It would get hot enough that I could get burned on the metal tray which let the melted wax drip off of the honeycomb.
The oven would work best with the clear surface at an angle which would collect the most direct sunlight. I looked it up and found that the sun only gets to about a ~43 degree angle at noon in Oklahoma in October.

 

[mike22cha]

Originally Posted by elfdoctors
I used to be a beekeeper and I made a solar wax melter which worked very well. (I made over 40 pounds of pure beeswax until I ran out of old honeycomb). It was made with wood sides which had been painted black. I had a piece of plexiglass which sealed the top very well and kept the heat in. It would get hot enough that I could get burned on the metal tray which let the melted wax drip off of the honeycomb.
The oven would work best with the clear surface at an angle which would collect the most direct sunlight. I looked it up and found that the sun only gets to about a ~43 degree angle at noon in Oklahoma in October.
43 degree angle huh, sweet man. THanks a lot, that helps tremediously. Is that at noon? So I'll tilt it to make a 43degree angle then. Our teacher showed us how the sun moves and how big of angle change that makes, but if I can get somewhere close, then it'll be ok. Oh and we used the plastic magnifiers on projectors to catch cardboard on fire using heat from the sun.

 

[mike22cha]

 

[mike22cha]

these plans were posted for Lil' Tanker
if anyone has comments that's fine
I got my solar cooker oven to 180 degrees F in 15min

 

[stdreb27]

Look in a boy scout survival book. I used to have one that had instructions on how to make one. I don't remember how they built it, but i'd lean towards using a pit for an oven. If you have good clay dirt...

 

[mike22cha]

Originally Posted by stdreb27
Look in a boy scout survival book. I used to have one that had instructions on how to make one. I don't remember how they built it, but i'd lean towards using a pit for an oven. If you have good clay dirt...
Well it has to be portable and able to get in school and placed outside of school. I got it to 98 Celcius Monday, and we're retesting Thursday. 80 Celcius is a C, so I think I'm going to get a B or A-.