|Figure 1. Appropriate Technology Class Straw Bale Prototype|
Our goal was to use this model as a starting point and to compare the results to the other prototypes. We used a electric range burner with a resistance of 16 ohms. The burner was hacked by using a Dremel to cut the burner in half and then connecting the halves in parallel as shown in Figure 2 and 3. Theoretically, the burner resistance should be 4 ohms; however, the measured resistance was 4.3 ohms. From using the graph in Figure 4, the ideal resistance is 3.25 ohms.
|Figure 2. Hacked Burner 4.3 ohms|
|Figure 3. Closer look at hacked burner.|
|Figure 4. Ideal burner resistance for specified PV power and voltage.|
We then went ahead and built a straw bale prototype. We added 1 liter of water to a pot and placed it in the straw oven. We measured the temperature using a Type K thermocouple and a thermo reader provided by Professor Glen E. Thorncroft from the Mechanical Engineering department. The results are shown below in Figure 5. The test was performed in the morning at 11:45 AM with a slight breeze and the solar panel was in direct sunlight.
|Figure 5. Test for straw bale cooker with 4.3 ohms.|
The cooker only reached around 165 degrees Fahrenheit before it started to reach an asymptotic behavior. Also, a linear trendline was applied to the data resulting in a R-squared value of 0.97 and suggesting the temperature rise was about one degree per minute. Keep in mind that this prototype does not contain any thermal storage, only the 1 liter of water as our thermal mass. As a results, several changes were made to our straw bale cooker. For our second prototype, the hole in the straw bale was covered with cement and a homemade a Nichrome burner was made in order to have find the ideal resistance.
2nd Prototype (Improvement upon straw bale cooker)
Our second prototype was made by improving the 1st prototype. The first improvement was covering the inside surface of the straw bale with a thin layer of a cement mixture. The mixture consisted of 3 substances: 1 part mortar clay, 2 part cement, and 5.5 parts of sand. This resulted in adding about 3 pounds of thermal storage to our straw bale. And to keep the cement attached to the inner walls of the cooker, a chicken wire mesh was place around the walls of the cooker. The cement also provided a tighter fit for our pot, therefore, inhibiting the air from producing a natural convection. A picture of our cooker is shown in Figure 6 and 7.
|Figure 6. Bottom half of straw bale cooker.|
The wires of the burner are guides through the bottom of the straw bale and attached to the PV panel. The hole for the wires is shown the Figure 6.
|Figure 7. Top cover for the straw bale cooker.|
As for the burner, an inexpensive burner was developed for third world countries. The link will direct you to the webpage,
Custom Inexpensive Burners for Third World Countries. To burners were made: one 2.9 ohms and the other 3.5 ohms. Both burners were tested on the same prototype. The burners used were the burners designed in the first iteration.
|Figure 8. Temperature versus time using the 2.9 ohm cement burner.|
|Figure 9. Temperature versus time using the 3.5 ohm cement burner.|
Our results reveal that the 2.9 ohm resistor heated the 1 liter of water faster than the 3.5 ohm resistor. Also, we did not bring the water up to a boil, since we are testing which prototype and burner is the best design. Therefore, there is no need to bring the water to a boil.