Step 1: The Situation
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
Step 3: Gathering the pieces
Be sure to cut your box accordingly if you plan to make one.
Step 4: A little design consideration
Step 5: More design consideration
Step 6: Some Assembly Required
Likewise the liner fits nicely into the oven box.
Step 7: The Insulated Floor
The insulation I used was just shredded documents I salvaged from the office shredder.
Step 8: The Reflective Surfaces
I then wrapped each of the side liners in tin foil and assembled them.
Everything fits together perfectly.
Step 9: More Insulation
Don't forget all the nooks and crannies.
Step 10: The Window
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.
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
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
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
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.