Conductivity and Refrigerators

Thermal Conductivity:

So we have now established that refrigerators are able to use energy to produce cooling. However, simply reducing the temperature inside an enclosed space can be an extremely inefficient method of keeping food cool. The reason for this is the thermal conductivity of the insulation materials surrounding the refrigerator.

Thermal conductivity can be defined as "an exchange of kinetic energy between microscopic particles in which the less energetic particles gain energy in collisions with more energetic particles." Therefore, if a refrigerator is cool on this inside, and the temperature outside the fridge is around 70 degrees Fahrenheit, then the less energetic particles are going to want to collide with the more energetic particles outside of the fridge.


This picture describes the exchange of heat through a surface, such as that of a refrigerator wall (Courtsey of Physics for Scientists and Engineers).

In order to slow this exchange of energy, insulating materials may be used. An insulating material is simply something that does not allow for energy transfer readily through its surface. For instance, would a glass-enclosed refrigerator keep food cool efficiently? Probably not, as flat glass has a low R Value of .89 (ft^2 * F * h / btu). What is an R value you may ask? It is a term that compares the thermal conductivity (k) divided by the material thickness (l).

With these new values explained, here is the equation for calculating the thermal conductivity of several slabs (and thus the rate of energy transfer):

P = A (t2-t1) / Sum( R )

where P is equal to power, A is equal to area, t2 is the outer temperature and t1 is the inner temperature, and R is representative of the R values of all the materials summed (Serway 624). The equation allows for a person to examine all the materials in a refrigerator and then determine how much energy is going to be lost through thermal conduction. Foam typically has an R value of around 5, while state-of-the-art vacuum sealed panels have R values of around 25 (Glacier Bay Insulation Test Result Tables 1).


This picture describes the exchange of heat through multiple surfaces. A refrigerator has several different materials, which encapsulate its cold compartment (Courtsey of Physics for Scientists and Engineers).

With this is mind, it is ideal to have the best insulator as possible. While you can achieve the same level of insulation with foam, the required thickness of insulation becomes bulky and impractical. The vacuum-sealed tiles now used in refrigerators can yield energy savings of up to 20%-30% over traditional foam insulation for the same thickness (Glacier Bay Insulation Test Result Tables 1).

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