Description of the Problem

Recently, the Nicaraguan women have requested a larger oven so that they can cook in larger pots and make larger quantities of food.  This request, in essence, is a desire for greater cooking efficiency, an issue that has been around in one form or another from the inception of the solar oven project.  Accordingly, the solar oven team decided to test different means of cooking with the box oven. We investigated cooking in parallel (multiple pots simultaneously) as well as in series (pots in sequence). The team also chose to investigate the issue of heat leakage through the door, a problem the solar oven team noticed during its testing this semester. While the women of Nicaragua have not explicitly mentioned this issue, it directly relates to cooking efficiency and thus merits investigation. The solar oven team hopes that our findings will help empower the women of Nicaragua to cook more food in a shorter amount of time.

Methods

Testing Pots in Parallel

Black pots and their caps were weighed on a scale and the weights were recorded to four decimal places. The cap of the pot was removed and the scale was then tared. Three pounds of water were poured into the pot. The weight of the water in the pot was recorded to four decimal places. The pots were covered and placed in solar oven, with their placement in the solar oven recorded.

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Figure 1 - Pot Configuration and Lighting for Parallel Cooking Tests

Testing Pots in Series

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.

Figure 2 - Pot Configuration and Lighting for Series Cooking Tests

Comparing the Cook Time of 3 lbs of Water with New Door

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

Results

Heating in Parallel

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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Figure 7. Visual depiction of weaker lighting zone

Series Testing

The series testing did not prove to be more efficient than the parallel testing. Heating 2 pots of 3 lbs of water to 95⁰C in series took 32% longer (86 minutes) than heating 2 pots of 3 lbs of water simultaneously.  Similarly, heating 4 pots of 3 lbs of water to 95⁰C in series took 40% longer (112 minutes) than heating the 4 pots simultaneously.  Therefore, based upon observations with up to 12 lbs of water, cooking in series is much more time consuming than cooking in parallel.  The Solar Oven Group does not recommend that the women of Nicaragua cook in series if cooking in parallel is an option. 

New Door versus Old Door

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.

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Figure 9 - Comparison of times required for solar oven to reach 95⁰C before and after the addition of the wood door seal.

Next Step for Solar Oven Project

While the Solar Oven Group determined that cooking in parallel was the most efficient method of cooking, the group did not explore the maximum capacity that the oven could handle without being "overloaded".  This could be a noteworthy topic to investigate in coming semesters.