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The insulation in the solar box cookers plays a vital role in performance. The current insulation technique involves using wood shavings in the gaps in the wall and in the bottom of the box cooker. This is done to reduce the convective heat transfer in the walls and floor of the oven. While this is the currently practiced technique for insulation, it is a known fact that the box ovens perform better during their first year of operation than in the years after. One of the theories associated with the diminished performance of the ovens over time is that the constant heat flow through the walls causes the insulation to settle to the bottom, and at that point the insulation would no longer be preventing convective heat transfer in the upper regions of the wall. And with an expected life of 10-12 years, diminished operation after the first year is something that needs to be analyzed. The motivation for this experiment is to essentially prove or disprove theories on the insulation in the box cookers and to improve the design in the most effective and economical way possible. A subteam consisting of Harrison Ko, Drew Hart, Abby Sterle, and Leif Paulson was formed following our trip to Nicaragua in order to begin work on this problem.
Experimental Setup
Figure 5 - Concept Drawing
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The idea of the insulation test is to isolate heat transfer through a single wall of a solar oven. Figure 5 depicts an insulated box with some sort of heat source on the inside. The heat inside the box will travel the path of least resistance, and with the other five faces of the box insulated it can be assumed that the majority of heat transfer is going through the experimental wall.
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Initially, the goal of this experimental idea was to be able to obtain an R-value for the experimental wall. After further research however, it was determined that obtaining an official R-value was rather arbitrary, and that as long as the data can be characterized in a logical and scientifically sound manner it can still be applied practically.
Construction
Figure
Figure 6 - Current Assembly
Our goal in construction is to make a box apparatus which has five highly insulated sides and a sixth side which is easy to change and manipulate so that a variety of insulation types can be tested. Ideally, the apparatus would mimic the performance of one of the real box ovens in terms of the relevant mechanisms for heat transfer out of the box, so we are using one of the old 30in x 30in box cookers as an internal frame and template. We have available sheets of mineral wool insulation, which is heat resistant and has an R-value around 3.1which is heat resistant and has an R-value around 3.1ft2 ℉ hr/Btu. Using three inches of mineral wool on each side gives an R-value contribution of approximately 10. Due to the rough and comparative nature of our calculations, this value need not and cannot be very precise.
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