Materials:
- Burner
- Nichrome wire (gauge)
- Cement
- Cardboard (or other structure)
- Fishing line (string will work fine)
- Heat resistance wiring (gauge)
- Solder
- Duct tap
- Inner Structure
- Cement
- Chicken wire (wire mesh)
- Pot (the one that will be used)
- Spacing material (ex: cloth, thin cardboard, etc.)
- OR
- 50X8 inch aluminum sheet metal, rolled into cylinder
- Spot weld, rivet, fastener, etc.
- Outer Structure
- Bricks
- Fixture-
- 2 X 4 wood, flat piece plywood
- Mud
- Straw
- Clay
- Fixture-
- OR, local village mixture for bricks
- Bricks
- Counter-top
- Plywood (size to cover hole and brick raise)
- Cement
- Sand
- Cement working tools
- Wire mesh
- Saw (to cut the plywood)
- Lid
- Same as countertop
- Handle
- Handle
- Rice hulls
- Corn husk
- Straw
- Grass
- Etc.
First Step (the hole):
The first step was digging a hole which was approximate 3 ft. by 3 ft. and 12 inches deep. Basically, the hole should allow 10 inches of insulation on all sides of the pot, which is the minimum insulation for the desired thermal resistance. The sides of the hole was supported by mud/clay.
Second Step (making straw clay bricks):
Afterwards, we made straw bricks by mixing mud/clay with straw with a 1:1 ratio. We made a fixture made from plywood and 2×4 to compact and form the mixture into bricks. They were then cut using a saw to roughly 8 inch sections.
Then bricks were then laid out and stacked around the hole so that the total height of the cooker was about 2 feet.
Step 3 (counter-top):
To make the counter-top, we found a flat piece of plywood that could cover the top of the cooker. We cut a hole in the middle and we add two more holes to allow space for the hands to reach into the cooker. The top surface of the plywood was then covered with chicken wire mesh. A thin layer of cement and sand mixture was spread on the top surface of the counter. We smoothed the top surface by spraying water and flattening the surface with a trowel. As the cement was drying we would spray the top surface of the cement with water, otherwise the counter would crack.
Step 4 (inner structure):
There are two methods for making the inner structure:
1) The first one is using a cement mold similar to the one used in Prototype 3. However, we did not have a bottom to the mold, otherwise it was just a thin cement mold of a cylinder. Afterwards, a circular piece of chicken mesh was attached to the bottom of the mold in order to keep the straw away from the burner. We decide to go with this technique in order to reduce the amount of thermal mass in the cooker, since we decide thermal storage will not be an option for this iteration of cookers. As for the rest of the inner structure, a vinyl roofing material was found at the farm and was used to extended the cylinder to the top of the cooker. This prevented straw from falling into the inner structure.
2) The second method is rolling an aluminum sheet into a cylinder as wide as the pot and as high as the counter-top. This is the recommended method since there will be relatively no thermal energy stored by the aluminum sheet.
Step 5 (nichrome burner):
We used our Custom Inexpensive Burners for Third World Countries.
Step 6 (assembly):
A ceramic dish was placed at the bottom of the inner structure so that the cement burner is not in direct contact with the insulation. See picture below.
The picture below shows how the cooker was assembled.
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| Pablo Arroyo adding insulation (straw) around the cooker. |
The picture below shows the final layout of the PV cooker.
Results:
The first experiment consisted of boiling 4 liters of water using a 3.4 ohm burner. This burner was used because we were considering the average maximum power per day obtained from the PV panel rather than the peak maximum power we can achieve from the solar panel; therefore, we needed a higher resistance. This is explained more in our webpage on selecting the optimal resistance, click here. The results are shown below.
We started our test around 11:00 am and ended a little over 5 hours of testing. Using a watt meter, the cooker varied between 60-70 watts of electrical input power. For every minute the water would warm up about a quarter of a Fahrenheit, which is acceptable considering that we are using a 100 W solar panel to warm up 4 liters of water and about 5 pounds of cement (inner structure), totaling about 13.8 pounds of thermal storage. Our last temperature reading was 163.7 degrees F. We ended here due to the fact that the shadows from the trees started to cover our solar panel, and the trend began to resemble an asymptotic behavior rather than a linear behavior.
For our second experiment, we decide to cook some food. We had about 4 liters of chili containing tomato, onion, beans, sauce, and ground turkey. The ground turkey was cooked separately for proper cooking and then inserted into the chili. We decide to lower our resistance of our burner to 3.1 ohms.The results are shown below.
The 4 liters of food warmed up a bit faster than the previous experiment (heating up the 4 liters of water). Also, the cooker electrical power input varied between 65 to 75 watts, which is possibly due to the fact that we lowered the resistance of our burner. We started cooking around 10:00 AM. We ended after 6 hours cooking with our end temperature being 174.7 degrees Fahrenheit. The disparity in our graph between 100 and 150 mins was caused by us opening the cooker in order to stir the food just in case it would burned. It did not burn, but we did lose 10 degF.
Lastly, we did one more test so we can compare this cooker to our previous cookers. We used the same burner with 3.1 ohms. Again, we had about 65 to 75 watts of electrical power. Our results are shown below.
