1-D Scaleup Testing

Five tests were run to characterize the new, larger oven. Tests 1 and 2 were run using the two older 30"x30" box cookers, and Tests 3, 4, and 5 were run using the new 1-D scale up of the original ovens. The tests involved heating a measured amount of water in the solar ovens and observing the time it took to heat from 40°C to 95°C. The pots used to contain the water had a top diameter of 8 inches and were 5 inches tall. With an upper lip thickness measured to be ¼ inches, the inner diameter of the pots was calculated to be 7.5". The pots' exterior and their lids were painted black, and can be seen in the picture below.

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Figure 1. Black pots used for heating water in the solar oven tests. 

The temperatures of interest were measured using seven thermocouples. To determine if calibration was needed for the thermocouples, all seven were placed in a single pot of water and their outputs were tested. The mean temperature reading was calculated after readings had stabilized, and showed that there was a 2.4°C difference between the minimum and maximum mean temperatures with a mean standard deviation of 0.38°C. The thermocouples were determined to be accurate enough for our purposes, though calibration would not affect the results because the data was analyzed for the time it took to heat from 40°C to 95°C. A temperature reading too high or too low would not affect this measured time, though it would explain why water in some pots was measured to be over 100°C towards the end of test runs.

The testing was done using the lighting system, where the insulation from the halogen lamps give approximately the same amount of energy as the sun does on a bright, clear day in Nicaragua. A pyranometer was used to measure the insulation during the five tests. During Test 1 and 2, the pyranometer was placed at the center of the lighting system, straddling the tops of the two older ovens. For Tests 3, 4, and 5, the pyranometer was moved to the back left corner of the new oven, where insulation was weaker. However, the amount of insulation recorded was similar for each position, and probably did not change between testing of the older and new ovens. Figure 2 shows the pyranometer reading for each test, though the data was interpreted by the program reading results from the thermocouples, and the units of the pyranometer output, which were converted from the pyranometer voltage output to °C, do not make sense. However, this graph shows that the insulation was relatively constant when the pyranometer was placed in the center for the first two tests, and that it was relatively constant when the pyrometer was placed in the corner of the light bed during the last three tests.

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Figure 2. Insolation during the five tests in °C units converted from voltage output.

Images of the test set up can be seen in Figure 3 to 6.

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Figure 3. A view of the oven interior from the door opening.

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Figure 4. New oven during Test 4.

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Figure 5. New oven being tested using the light system.

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Figure 6. Computer used to log temperature data with new oven being tested in the background.

Test 1 involved the two older ovens, each with approximately 2.7 kg of water in a single pot. Test 2 also involved the two older ovens, but each with approximately 4 kg of water divided into two pots. Test 3 involved the newly constructed oven with approximately 8 kg of water divided into four pots. Test 4 and 5 involved the new oven with approximately 4 kg of water divided into two and four pots, respectively. These results are illustrated in Figures 7 to 11, and a consolidated table of test details and results are in tables following the figures.

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Figure 7. Test 1 temperature results. Note that probes T2 and T3, which read the temperature of the black plate in each oven, became detached at different points in the run.

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Figure 8. Test 2 temperature results.

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Figure 9. Test 3 temperature results.

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Figure 10. Test 4 temperature results.

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Figure 11. Test 5 temperature results. 

Table 1. Consolidated list of test details and results. In the columns listed as "Time" is the time measured for each pot of water to heat from 40°C to 95°C. In the last column labeled "Min/kg" is the maximum time from the Time (min) column divided by the total mass of water heated in each test.

These results indicate that the time required to heat all the water from 40°C to 95°C to be mainly a function of the total mass of water being heated, regardless of whether the pots are in two smaller ovens or one larger one. Test 2 and Test 3 both involved heating approximately 8 kg of water, though in Test 2 each older oven had 2 pots containing 2 kg of water in each pot, and in Test 3 the newer oven contained 4 pots containing 2 kg of water in each pot. It only took 1.77 minutes more for all the water in Test 2 to heat from 40°C to 95°C than it did in Test 3, and this minor increase in time could be attributed to the 179 g difference in the total mass of water.

Additionally, increasing number of pots and decreasing the amount of water in each pot did not have a significant effect on the time it took to heat the water in Tests 4 and 5. Test 4 only took 0.87 minutes, or 52 seconds, longer than Test 5 to boil 4 kg of water, even though Test 4 had 2 pots each containing 2 kg of water and Test 5 had 4 pots each containing 1 kg of water.

Even though the amount of time required to heat from 40°C to 95°C increases as the mass of water heated increases, the difference in time required decreases. This suggests that there is a fixed amount of energy used to heat the oven, since the relationship is not linear, as can be seen in Figure 12 below.

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Figure 12. Time required to heat water from 40°C to 95°C as a function of the total mass of water.

This is further evidenced by the decrease in per kilogram heating time as the total mass of water being heated, as illustrated in Figure 13.

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Figure 13. Per kilogram heating time as a function of the total mass of water.

While this data suggests that the time required to heat all the water from 40°C to 95°C is heavily dependant of the total mass of water being heated, more testing is needed to fully understand the affect of other variables, such as the surface area of the water touching metal painted black. However, this data does indicate that the surface area of the pots in contact with the black plate on the bottom of the oven does not have much influence on the heating time. To get a complete understanding of the best way to heat large amounts water, which will help give an understanding of the best way to cook large amounts of food, testing needs to be done using pots of different sizes, which were not available at the time of testing.

The data has shown that making the oven wider has negligible effects on heating time. However, testing should also be done with a 3-D scale up of the oven when built, since the bigger pots that the women in Nicaragua would like to use tend to be taller, to see how oven height affects heating time. Contrary to results obtained by previous tests done with the solar ovens, where heating time decreased when more pots with smaller amounts of water were used, the results obtained in this series of testing indicate that using larger pots in wider ovens do not lengthen heating time.