To implement photovoltaic cookers in Third World Countries, the availability of the burners with the optimal resistance must be widely available to us. However, Third World Countries do not have the technology to make the modern electric stove burners available to us, and we do not have the funds to buy electric stove burners. Also, it is an inconvenience to hack the burners to the optimal resistance for our PV panels. Therefore, we developed a method to make inexpensive burners that can be modified to the optimal resistance of the PV panels.
Our first prototype was pouring a cement mixture containing 1 part mortar clay, 2 part cement, and 5 part sand into cardboard box wrapped with plastic. A 80 resistance-26 gauge nichrome wire was then placed inside the cement mixture. Two nichrome burners were made using this process. One nichrome wire was measured to be 2.9 and the second nichrome wire was measured to be 3.5 ohms. The graph in Figure 1 was used to determine the length of wire needed to obtain a specified resistance. The nichrome cement burners can be seen in Figure 2.
|Figure 1. Nichrome wire 80 26AWG resistance versus length resulting in 2.571 ohms per foot.|
|Figure 2. Custom made cement nichrome burner 2.9 ohms.|
This nichrome burner was used on the second prototype (the straw bale with a cement layer in the inner wall of the insulation) and the third prototype (cooker with hay stuffed into a plastic drum). The 2.9 ohm burner was proven to heat at a faster rate than the 3.5 ohm burner. Hence, the optimal burner is close to 2.9 ohms, more test will be done. There were some shortcoming when using this burners. When placing the nichrome wire into the cement mixture, it was difficult to judge if the nichrome wire was not protruding out of the cement. As a result, the nichrome wire of the 2.9 ohm resistor was visible in a couple of places shown in Figure 3, which in turn may cause a short. Therefore, a different method was used to make these cement burners.
|Figure 3. Defect on Nichrome Cement Burner|
The second fixture was made out of a cardboard and then shape into a box with no top. The cardboard was then wrapped in duck tape in order for easy removal of the cement burner once it harden. Loops of fish line were attached to the sides of the box in order to keep the nichrome wire afloat. In other words the nichrome wire will be about a quarter inch from the bottom and a quarter an inch from the top. Figure 4 displays the final results of the fixture with the nichrome wire threaded through the loops. Two nichrome burners were made using this method as shown in Figure 5: a 2.7 ohm resistor and a 2.9 ohm resistor.
|Figure 4. Cement burner fixture.|
Figure 5. The top and bottom of both nichrome burners.
Using this method, the weight of the burners was cut in half, from 0.95 lbs to 0.45 lbs; also, the surface area remained the same. Therefore, the heat transfer rate should remain about the same for the new burner, while the thermal storage in the new cement burner is cut in half.
Figure 6. A) old burner 0.95 lbs B) new burner 0.45 lbs
|Figure 7. Power meter.|
Afterwards, a power meter was attached to the individual burners to gather data on voltage, current, and power. The Figure above shows the power meter used for gathering the data. The table below displays the max power, max voltage, and max current that the burners acquired using two different PV panels. We used one panel that was available to us at the student experimental farm which was quite dirty, and another PV panel provide by Ryan Wang and Ryan Perry which was new.
The Table below reveals the results from our test. The test was performed in a sunny, clear day at 11:30 am. Therefore, the PV panel was in direct sunlight so it should be providing maximum power. Panel 1 is the panel provided by Pete Schwartz, and Panel 2 is the panel provided by Ryan Wang and Ryan Perry. Figure 9 show the display of the PV panels specifications.
|Table 1. Power, voltage and current of cement burners.|
The calculated resistance for PV panel 1 and panel 2 were 3.25 ohms, 3.1684 ohms. Our results reveal that a 2.9 ohm resistor provides the most power. Therefore, we will be making more nichrome cement burners ranging from 3.3 to 3.0 ohms to acquire the optimal resistance.