There is an old saying that a refrigerator runs more efficiently when full than when empty.
Refrigerators are one of the most energy consuming household appliances, using an average of 150 kWh per month. Everybody has a refrigerator and, for those of us who rent, our refrigerators usually haven't been bought with conservation in mind. Furthermore, most of us use our refrigerators "as is", never endeavoring to increase their efficiency and decrease their operating costs. With energy costs rising and our refrigerators contributing a large portion to our monthly energy bills we quickly decided, for the health of our student loan payments, to investigate the matter further.
We discovered many suggestions on how to accomplish a reduction in energy use, one of which involved filling a refrigerator with jugs of water. This related directly to the aforementioned adage so we decided to conduct an in depth study.
Filling a refrigerator with water filled jugs is also a good simulation for a refrigerator full of food, which may be a more likely enacted scenario than adding and removing jugs of water. It raises the pertinent issue of our shopping habits. Can we shop like bachelors and keep only a bottle of champagne on hand? Or, for the sake of our pocketbooks, should we prepare for the last supper?
One of our teammates recently bought a used refrigerator from St. Vincent de Paul, so we conducted the experiment in her kitchen.
There are 2 phases in the refrigeration cycle: condensation and evaporation.
The refrigerator compressor is the most active and energy consuming component of a refrigerator. It receives the refrigerant in a gaseous phase and compresses it into a liquid. The Ideal Gas Law dictates when the pressure of a gas increases, so does its temperature. This is how heat is removed from the refrigerator. Heat is bled by the liquefied refrigerant through a network of small coils on the back of the refrigerator.
The evaporator then reduces the pressure of the refrigerant, returning it to a gas. The reverse of the Ideal Gas Law is also true, that if the pressure of a liquid/gas decreases, as does its temperature. As the colder refrigerant runs through the body of the refrigerator, it pulls heat from the compartment before returning to the compressor and repeating the refrigeration cycle.
See The Refrigerator Energy Use Story for a brief discussion or Refrigeration Basics for a comprehensive lesson.
The compressor only needs to run if its interior begins to warm. Room temperature, insolation, proximity to furnace vents and ovens are but a few of the numerous heat sources causing a refrigerator to warm. For a more complete list of heat sources, consult Tips for Using Refrigerator/Freezers.
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