Insulation Subteam
Members : Sarah Clement, Harrison Ko and Julianne Schwartz
Based on the experiences of the women in the Nicaraguan communities, 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 wood shavings insulation to settle to the bottom, and at that point the insulation is no longer able to prevent convective heat transfer in the upper regions of the wall. With an expected life of 10-12 years, diminished operation after the first year is issue what must be analyzed.
In order to simplify the analysis of the experimental data, heat transfer was isolated to a single wall of a solar oven. As depicted in Figure 1, the experimental unit consisted of an insulated box with a heat source inside. In order to ensure that any significant amount of heat transfer only occurred through the experimental wall, five of the six sides of the experimental oven were designed to have a thermal resistivity roughly ten times greater than air. This ensured that the heat inside the box would then travel through the path of least resistance, the less insulated experimental wall. The experimental wall was designed to mimic the walls of a conventional solar oven and allowed for interchangeable insulation types for testing.
Figure 1: Concept Drawing
A measuring and control system that turns on and off the heating system of four 500W halogen light bulbs to maintain a user defined temperature. LabVIEW to display the temperature at different locations throughout the experimental unit, seven thermocouples were places throughout the system:
Figure 2: A Diagram of Thermocouple placement
The insulation materials chosen for testing were wood shavings, fiberglass, and rice hulls. Wood shavings were tested since they are the current insulation material in the oven. The women of Nicaragua and our faculty adviser suggested fiberglass and rice hulls.
Figure 3 shows the typical data file acquired from running an experimental trial.
Figure 3: An Example of Temperature vs. Time for a Rice Hulls Trial
This study focused on the temperature readings from thermocouples 4 and 6, representing the "insulation inside" and "insulation outside," respectively. These temperature readings were of particular interest due to the fact that these thermocouples flanked the insulation in the experimental wall. The difference between these thermocouples would in essence represent the thermal storage capability of the insulation.
Temperature Differential vs. Time for Thermocouples 4 and 6
There is a clear separation between the temperature differences of the insulations and the air. The air trials are all consistent and are lower than those of any of the other insulation types. This is to be expected since air is the control and is known to be a poor insulator due to the convective currents in the air during heating. The 2 fiberglass trials appear to have the greatest temperature difference between thermocouples 4 and 6 and that indicates that fiberglass has the greatest heat storage capacity and is therefore the best insulator. Wood shavings and rice hulls appear to be fairly comparable in their temperature differences, with wood shavings appearing to show a slight edge.