Refrigerator Project

What Does This Project Look Like?

Refrigerators are common household items that we use every day. These appliances have evolved from primitive clay pots in the ground to modern day stainless-steal units that allow us to keep food and drinks cold and last longer. The refrigerator project will teach us the basic concepts of thermodynamics and cooling. Our goal is to build a working mini-fridge to cool a bottle of water as cold as we can.

Project Goals

The goals of our project are simple; make a bottle of water as cold as we can. We are trying to learn the fundamentals of thermodynamics and heat transfer within a system. Using 1 1/2 inch rigid insulation we are to construct a box that can hold a mini water bottle and will serve as the fridge. To cool the inside of the refrigerator, we are using small temperature affecting device called a Peltier. Using electricity, the Peltier device gets hot on one side and cold on the other. Our goal is to use these materials to construct the most efficient refrigerator possible.

Version One

To begin, my partner Laney and I cut four sides, one top, and one bottom out of the insulation. Our design uses 45 degree angles to create a better seal between the sides. Instead of having a straight sides line up against each other,our design forms a perfect box with more efficient seals. Other groups used questionable box cutters for their fabrication which resulted in rough, imprecise lines. To avoid this, we made us of the chop saw which has a handy angle feature to cut things -- in this case it was insulation -- at anywhere from 0 to 45 degrees. This gave us very clean, precise pieces to build with. We used hot glue to keep attach the sides to one another, then used duct tape to seal the inside and outside seams. The duct tape prevents any cold air from escaping and hot air from entering through the corners. To create an effective lid on the top of the fridge, we added anextra piece of insulation to the inside face of the lid. The extra piece was the same size as the interior, and we secured it to the lid using hot glue and duct tape. This extra piece of insulation allows us to remove the lid without needing to seal it every time we put it back on by forming a seal on the inside of the fridge when it is in place.

The Peltier Device

The Peltier device is a flat ceramic square that is often used for heating and cooling. The Peltier runs on an electric current which passes through two different substances within the device. When the electricity goes through these substances, it creates a heat differential; one side becomes hot and the other becomes cold. There is a ceramic plate on both the top and bottom to distribute this heat or lack there of. Unfortunately, the hot side become so hot that they burn out and fry the device within seconds of turning it on. To prevent this, we apply thermal paste to the ceramic surface, then a heat sink and a CPU fan. The heat sink transfers the energy from the excited atoms on the hot side of the Peltier into its metal tongs. The CPU fan circulates air over the heat sink to remove the that energy (heat) from the metal and transfer it into the air. The thermal paste allows more of the heat energy to be transferred from the Peltier to the heat sink.

We cut a hole centered in the upper part of one of our sides for the Peltier device. Although the insulation is an inch and a half thick, the Peltier devices must be put just inside the wall so that it is even with the exterior. This allows air to be pulled into the heat sink's tongs and removed by the fan. Without this air circulation, the heat sink and CPU fan would be useless and the Peltier would burn out. We could not glue the fan to the heat sink due to the high temperatures it reaches, therefore we had to tape the fan down. The hole for the Peltier must be exactly the same size as the device if not a little smaller. This prevents air and heat from traveling between the interior and exterior. At first, our hole was too big. To solve this, I cut slivers of the insulation, about 1/8th of an inch think, and duct taped them to the inside of the holes. This made the holes the perfect size for our Peltier.

The Results

After we had finished assembling our refrigerator, we drilled two holes in the lid to insert temperature probes. One went through the lid and into a hole we cut in the top of the water bottle; this recorded the temperature of the water. The second probe was in the fridge to record the ambient air temperature. We also had a probe in the room recording ambient temperature to act as a normal.

The data is disappointing. The temperature in the refrigerator rose, along with the temperature in the water and the ambient temperature outside the fridge. Our Peltier system failed to cool the water bottle even one degree. This graph shows the steady increase in temperature over an hour and a half which prompts the question, "was the device even on?" The answer is yes and unfortunately this graph was a common one throughout the class. After a demonstration from our teacher Mr. Varvil, we came to the conclusion that we were not evacuating enough heat from the system. The heat energy created by the Peltier was exceeding the amount of energy being conducted and removed by the heat sink and CPU fan. Thus, the more heat energy we remove, the cooler the Peltier can become.

Version 2

After our version 1 refrigerator failed to meet our project goal of cooling the water bottle, we set out to improve our fridge design. Laney and I changed three aspects of our design to improve efficiency. First, we added a block of insulation to the bottom of the interior of the fridge to reduce the volume of air the the Peltier device had to cool. Second, because we speculated that the more heat we remove, the cooler the Peltier will become, we removed the heat sink on the Peltier device and replaced it with a mega-heat sink. This mega version consisted of 9 heat sinks attached to a large piece of thin aluminum sheet metal. This was to help disperse more heat from the hot side of the device, thus allowing the interior to become colder. In addition to the larger heat sink, we upgraded our fan to an industrial size. This fan provided a greater amount of air that would flow through the heat sink and remove more of the heat energy. Lastly, we placed a regular heat sink and fan on the interior side of the Peltier device. We believed that the increased air flow over the cold device would remove more energy from inside the fridge and reduce the heat even further.

The results of the version two test revealed that the Peltier devices can cool the refrigerator if enough heat is removed from the hot side. In the beginning 100 seconds of this test the temperature inside of the fridge drops steeply to 19.5 degrees, roughly a 2 degree drop. The water temperature followed this trend, however not as dramatically. After the first 100 seconds, the fridge temperature reversed and began to rise at a similar rate to how it fell. Once it reached the same temperature it started at, the line begins to level out and slowly increases about one degree over the rest of the test. It's important to note that at the same time the ambient temperature outside the fridge was rising as well, about one and a half degrees over the course of the test. This is most likely because we started the test in the morning and the outside temp rose as the day went on. I suspect that the Peltier device reversed it's cooling trend because we were not evacuating enough heat from the hot side. Once the heat produced from the hot side overcame the amount of heat energy we were removing, it spread the excess heat throughout the device.

This graph shows the comparison between the version one and the version two data. In version one, we saw zero proof of cooling from the Peltier and all three probes rose in temperature. In version two, we saw a noticeable initial drop in the temperature inside the fridge and in the water. However this did not last long as the temperatures quickly reversed momentum and began increasing. Although the version two fridge was not successful, we did learn that the potential for our fridge to work is there, we just have to evacuate more heat from the Peltier to maintain a cooing trend.

Conclusion

The goal of this project was to design and build a refrigerator that would cool a water bottle. Unfortunately, Laney and I failed to cool our bottle at all. This failure was not uncommon. Only one group was able to use the Peltier devices and cool their water bottle. However, I believe that given the assignment to build a version three, we could effectively build a fridge that reached met this goal. Although we did not actually succeed, we did learn the fundamentals of thermodynamics and the definition of heat. We also learned scores about Peltier devices and the necessary evacuation of heat needed to successfully cool something. Overall, I think this project, although a bust, was fun and informative. In the future, I think a more effective path for the project would include more emphasis on the design of cooling systems for the Peltiers.

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