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The pots were left in the solar oven until the water inside the pot reached a temperature of 100°C. The pots were removed from the solar oven and weighed on the scale. The total weight of the pot, cap, and water was recorded to see if there was any significant water loss. A total of four parallel cooking experiments were conducted: 1 pot, 2 pots, 3 pots, and 4 pots. See figure 1below for pot configurations and lighting arrangement.
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A similar experiment was conducted to collect data regarding cooking pots in series. The same procedure as testing pots in parallel was used. However, instead of cooking up to four pots at one time, pots were split into multiple cooking times. For example, to cook two pots of water, one pot was placed in the oven until it reached 100°C, removed, and replaced with second pot filled in three pounds of water. A total of two experiments were run in series: 1 pot followed by 1 pot & 2 pots followed by 2 pots. See figure 2 below for pot placement and light arrangement.
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To compare the cook time of 3 lbs of water with a new door installed on the solar oven, the same procedure as testing pots in parallel with 1 pot was used. For pot configuration and lighting arrangement see figure 3 below.
Figure 3 - Pot Configuration and Lighting for New Door Test
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We performed four different tests to evaluate the effects of water mass on cooking speed in the solar cooker. We varied the test by adding an additional 3-lb of water filled pot to each successive test. Figure 4 below shows the plots of temperature vs. time for the four parallel cooking tests. While the boiling time increased with water mass, the increase was not a direct relationship. The time to boil four pots of water (12 lbs) was far less than four times the amount of time to boil 1 pot of water (3 lbs).
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To further examine the difference in temperature verses time, figure 5 shows the different temperatures after 60 minutes of cooking in the form of a bar graph. The data shows that the difference between 1 and 2 pots of water is somewhat small---a difference of 3.3%. However, the drop in temperature increases somewhat drastically from 2 pots to 3 pots, after 60 minutes of cooking. It is important to note that this drop, nevertheless, is not a decrease in time per unit thermal mass of water. The data shows conclusively that the greater the thermal mass of water heated in parallel the greater the efficiency.
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Another comparison we implemented was the difference in time it took each pot to reach 95°Celsius (figure 6). Again, it is clear to see that the difference between 1 pot and 2 pots is relatively small, and then there is a jump between 2 and 3 pots. Also, the data collected for the 4 pot test, on average, never reached 95 degrees Celsius.
Figure 6 - The time (minutes) that it took the average temperature in the pots for each test to reach 95°C. \ [The average data collected for the 4 pot test never reached 95°C, hence it's absence in this bar graph.\] Wiki Markup
An important note is that the temperature in 3 out of 4 of the pots for the 4 pot test reached 95 degrees. After 80.6 minutes, 3 pots were at 95 degrees or hotter. However, there was one pot that remained below 75 degrees for the entire testing period. Using the data from this pot in the average for the 4 pot test drastically reduced the average temperature for the test. It is tempting to blame the low temperatures on a misreading of the thermocouple or a mistake in the set-up of the system. However, we determined that the thermocouple was not broken or msitaken but rather that the temperature in the pot must have actually been less than the temperature in the other 3.
This led us to re-examine the set up of the solar cooker. The pot in question was located at the front left-hand-side of the cooker as shown in figure 7 below. Consulting our lighting set up, we noted that there are several lights that are burnt out on the left side of the cooker. We also noted that this particular pot was near the door and the door eventually had to be fixed due to bowing of the wood. Both of these issues could have influenced the lower temperature of the water in the front, left pot.
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Approximately one month into testing, a gap between the oven door and the body of the oven was discovered and predicted to be a significant source of heat loss that need not be overlooked. After attaching a wood stiffener "seal" to the oven door, the 1 pot- 3lbs test was repeated to investigate the effect of the anticipated heat loss on the oven's performance.
As shown in figure 8, the addition of the wood seal to the solar oven door did not have a significant impact on the overall temperature performance of the oven.
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The time it took the oven to reach 95⁰C decreased from 61.1 minutes for the old door to 59.4 minutes for the new door for an overall difference of 2.6% (figure 9). After 60 minutes, the solar oven with the new door was 2.8% hotter compared to the old door.
The Solar Oven Group concluded that the performance improvement of the oven with the new door was small enough that the data collected for the 2 pot-3lbs, 3 pot-3lbs, and 4 pot-3lbs tests was accurate enough for the purposes of the group's analysis.
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