After putting a HOBO in my fridge, something unexpected happened…

Ambient conditions:
The refrigerator is kept along a wall which lies across from a large 4’x4’ single pane window. Throughout the day, the refrigerator receives a large amount of reflected luminance. The kitchen receives some patches of direct light; however, the houses adjacency to a neighboring building greatly reduces the daylight factor within the kitchen, and a majority of the light that enters the room is reflected.

Initial Hypothesis:
The internal temperature of the fridge will experience 3 distinct instances of temperature swings near peak periods of use when the door is ajar, in which the internal air is exposed. During these periods there will be an increase in the mean max temperatures within the 24-hour period. Additionally, the rise in external temperatures within the kitchen as a result of increased exposure to solar gains will result in a higher mean temperature within the afternoon, and will result in a lower mean temperature within the evening when external temperatures drop. Five people share the refrigerator, so activity should be evident during times of food preparation during meals.
Estimating temperature swings, we sketched our predicted interior temperatures over the course of a twenty-four hour period (shown in red), including the appropriate temperature range and fluctuations. We then recorded actual data (shown in blue) and compared the data on a single graph (below).

Conclusions:
I had expected to see three peaks in temperature, varying with the use of the fridge nearing meal times, as well as with the different external temperature fluctuations from varying levels of solar gain during daytime hours. In my initial hypothesized temperature curve I had assumed that the insulation of the refrigerator would create an appropriate thermal break between the interior and exterior air temperature. I also expected the temperature to reach a constant base temperature once given time to level out, with longer cooling periods when there was a higher external temperatures.
The actual temperature data recorded revealed an oscillating pattern throughout the course of the day, without an average base level temperature when left untouched. The fluctuation of temperature from high to low appears to repeat about every 20 to 30 minutes. The original assumption being that temperature differentiation would be the result of the door being left ajar would appear to be false, for it would mean that the refrigerator was being opened every 20 to 30 minutes. However, an examination of how a refrigerator works, reveals an explanation for the pattern of temperature range from low to high that repeats throughout the day.

How a refrigerator works:

·  The compressor compresses the refrigerant gas. This raises the refrigerant’s pressure and temperature (orange), so the heat-exchanging coils outside the refrigerator allow the refrigerant to dissipate the heat of pressurization.

·  As it cools, the refrigerant condenses into liquid form (purple) and flows through the expansion valve.

·  When it flows through the expansion valve, the liquid refrigerant is allowed to move from a high-pressure zone to a low-pressure zone, so it expands and evaporates (light blue). In evaporating, it absorbs heat, making it cold.

·  The coils inside the refrigerator allow the refrigerant to absorb heat, making the inside of the refrigerator cold. The cycle then repeats.

The data indicates a high degree of fluctuation in the internal temperature of the fridge. There appears to be a regular schedule of temperature swings that on average range from 32 to 37 degrees Fahrenheit, with a change of about 5 degrees on average. Doing some quick research on the processes of a fridge, I found an internal compressor regulates the temperature by compressing the refrigerant gas. The frequent oscillation of the temperature reveals the inefficiency of the insulation as a thermal break between internal and external temperatures. As the fridge is clearly not opening on regular intervals every 20 minutes, thermal loss through the envelope of the fridge due to the greater external temperatures influences the internal temperatures within, requiring the internal compressor to regulate the increase in temperature. The swings occur as the temperature within rises causing the internal compressor to automatically switch on and lower the temperature again.
Since I was younger, I have been programmed to quickly open and shut the door of a fridge, and scolded by my elders for wasting energy when it wasn’t rapid enough. However, it appears that their energy should have been directed elsewhere, possibly more appropriately at the manufacturers who do not properly insulate the fridge. I would conclude that it is not the opening of the refrigerator during use that causes temperature fluctuation within the internal temperatures, but instead the inefficiency of the envelope’s thermal resistance and role as a thermal break. Going further with this experiment, I would be interested in calculating the appropriate resistance value required to allow for minimal energy use by the compressor to regulate the internal temperature. Following such calculations it would be interesting to see current advancements in the means of insulating refrigerators, and other means of reducing the energy used within the device. Considering the inefficiencies of thermal insulation within refrigerators, I am anticipating that there is a large amount of energy consumed by outdated refrigerators that have less efficient compressors that account for larger energy loads.

This entry was posted on Wednesday, September 15th, 2010 at 6:59 am and is filed under Refrigerator Data. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.