In keeping with the grand tradition of creating ways to warm my lunch at work with readily available office supplies, I present for your review, dear reader The Office Worker's Portable Solar Oven.
Step 1: The Situation
Due to circumstances beyond my control, my cubicle at work is close to both the break room and a small loading area on the second floor. Because of this I usually have a wall of boxes filled with copier paper situated somewhere behind me. Aside from this being a nuisance when it comes to concentrating on my job it also affords me the opportunity to scavenge as many copier boxes that a man can get his hands on.
This is a perfect situation to improve upon my earlier solar cooking ideas.
See here: http://www.instructables.com/id/Dashboard-Chilli/
Step 2: The Result
Behold! My dashboard cooking days are over. I present to you The Office Worker’s Solar Oven. This project was built completely with materials laying about the office, with the exception of tinfoil, which I picked up at the dollar store.
Step 3: Gathering the pieces
Above you can see all the precut pieces to form the main cooking chamber. I cut the box at an angle of 20 degrees to allow for maximum exposure to the sun. In my latitude the lowest elevation that the sun gets is 26 degrees above the Southern horizon.
Be sure to cut your box accordingly if you plan to make one.
Step 4: A little design consideration
I designed the oven to allow for one inch of insulation on each side of the box. To do this I placed slots in the inside liner. This is the back piece.
Step 5: More design consideration
These are the sidepieces, slotted as well. I also cut these with an angle of 20 degrees.
Step 6: Some Assembly Required
As you can see the slotted pieces fit together nicely.
Likewise the liner fits nicely into the oven box.
Step 7: The Insulated Floor
Once I was sure that everything fit together, I removed all the pieces and put a one-inch spacer in the bottom of the box to keep the bottom of the oven above a layer of insulation.
The insulation I used was just shredded documents I salvaged from the office shredder.
Step 8: The Reflective Surfaces
Once properly stuffed I rapped the bottom liner of the oven in tinfoil and installed it into the oven.
I then wrapped each of the side liners in tin foil and assembled them.
Everything fits together perfectly.
Step 9: More Insulation
A little more insulation around the sides.
Don't forget all the nooks and crannies.
Step 10: The Window
For the port that allows the sunlight in I basically just created an insert that would fit into a regular box lid with a hole cut into it. I used the same trash bags they use to hold all the shredded documents and stapled it to a piece of cut out box.
This when gets inserted into the box lid with a hole cut about the same size in it.
Viola! The Office Worker's Portable Solar Oven
Step 11: A Question of Efficiency, oh and a little math to.
This is all fine and dandy but what kind of power and efficiency are we talking about here. I mean can we bake bread, pasta, and stew?
I figured that for the first “light” of this oven it might be a good idea to run a test before I place a bunch of food in it and end up with a big mess and a half a dozen people laughing at my ruined lunch.
This will require some thought.
I will need to know a few things:
• How much potential wattage I can get out of a given area of sunlight
• How much wattage I AM getting out of a given area of sunlight
• How efficient is the set up based on these two values
To get potential wattage is easy.
The dimensions of the port that allows sunlight in are 40.64cm by 21.59cm. This equates to 877.4176 sq cm.
Each square meter of sunlight has a potential of 1000 watts of energy in it when it reaches the Earth. Each sq meter is 10,000 sq cm. So 1000 watts divided by 10,000 sq cm gives up .1 watts per sq cm.
So if we factor in the viewing area of the solar oven 877.4176 sq cm multiplied by .1 watts we get 87.74176 potential watts for the solar oven.
Potential wattage I have come to find out is often a pipe dream left for those that believe in endless amounts of power that can be conjured through a philosopher's stone at the stroke of midnight when moons are properly aligned. So I don’t readily buy into the thought of my copier paper box being able to harness 87 watts of power by merely being pointed at the sun.
I needed a way to measure the true wattage of this oven to be able to determine cooking times and more importantly what I could cook.
The easiest way to achieve this is to measure calories. This is done by multiplying the temperature change in Celsius by the mass of pure water heated in grams.
So I need to heat some water and measure the temperature change. However temperature change does not happen instantaneously, it takes time so that will need to be factored in down the road at some point.
Each calorie is equal to one degree centigrade increase in one gram of water. So with a given amount of water and the temperature change in the oven over a given amount of time, it should be enough to calculate its power.
Step 12: The Test Subject
For this I needed a small enough vessel for water. The container for the V8 I drink everyday will work fine. However I will need to paint it black to absorb the maximum amount of heat within the oven.
Here we see my test setup. The can has been painted black and filled with 163ml of filtered water. I plugged up the opening with a cork and stuck the probe of an oven thermometer in the center of this.
Step 13: The Test
And finally we see the entire test setup sitting out in the sun. The test consisted of leaving the entire apparatus in the sun for one hour and recording the temperature every moment.
This actually caused a small panic at my place of employment. Apparently the warehouse manager saw what looked to him as a strange device sitting out by one of the vehicles and it aroused a bit of concern. Lucky for me I managed to get a hold of him before anyone was called.
Step 14: The Numbers and Of Course Math
This is a graph of the progress.
The base temperature of the water was 77 degrees F. The time was 12:01 EST on 05/29/2008. The maximum elevation of the sun for that day was 71.8 degrees above the horizon. As can be noted there seemed to be a linear progression in regard to temperature increase. As I sat and observed I saw that the temperature went up one degree every two minutes. By the end of the hour the water was at 113 degrees. I would assume that it would have been higher if it were not for some clouds in front of the sun at the end of the hour.
To calculate the power of the oven I first needed to get the temperature change in Celsius.
77 F = 25 C
113 F = 45 C
Temp change = 20 C
163ml of water = 163 g
20deg * 163g = 3,260 Calories
Next we need to convert calories to joules. 1 Calorie = 4.1868 Joules.
3,260 Cal * 4.1868 = 13648.968 Joules
Now that we have the power in Joules we need to factor in the time to get wattage. Se we divide 13,648.968 Joules / 3,600 seconds (1 Hour)
3.971 Watts. Not very much. Hell even an Easy Bake oven uses a 100-Watt light bulb. But in the end one must remember that the heat is cumulative. The better the insulation the more heat will acquire and the better things will cook. It’s sad to report that out of 87 potential watts from the sun for the given area only a little fewer than 4 watts was produced. It would appear the overall the oven was only about 4.5% efficient
If nothing else this taught me a valuable lesson in the design considerations and power calculations necessary to build a more robust unit.
But will it cook food? Build one and see.