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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.
Experimental Setup
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Figure 1: Concept Drawing
The idea of the insulation test is to isolate heat transfer through a single wall of a solar oven. Figure 1 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.
The experimental wall will be built to mimic the walls of a conventional solar oven. The main difference would be the interchangeable interior of the wall. The typical solar oven wall consists of a pine frame with insulation to fill in the hollow space. The experimental wall of this setup will be built the exact same way with the only exception of being able to change the insulation for different tests. This is done to be able to get the most relevant data from the setup.
The insulated box is designed to have the five non-experimental faces have a thermal resistivity far greater than that of the experimental wall. In the case of this experiment, the thermal resistivity is assumed to be similar to that of air. The faces of the experimental box were decided to be approximately ten times the thermal resistivity, to ensure that heat will most likely travel through the experimental wall.The high thermal resistance of the insulated walls is very easy to achieve since thermal resistance in series is additive. This allows for the insulated walls to simply be extra thick to increase the thermal resistance.
The experiment will be conducted to obtain the most relevant information possible. The plan is to measure temperature of the inside air, the outside air, and on many locations on the wall. The idea of is to obtain simple data from the air temperature measurements, as well as to obtain a heat gradient from the measurements through the wall.
The two basic factors of how well the solar oven is insulated can be described by two characteristics: the time it takes the oven to heat up to cooking temperature and how well the oven retains the heat. Therefore, the plan is to heat up the inside air of the box until it reaches a typical cooking temperature for the solar box cookers (~300˚F) and then immediately shut off the heat source. At this point, the time it takes for the inside air to become equal with the outside air can also be measured. These measurements can provide practical characteristics of different types of wall designs and insulation materials.
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.
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