Welcome to the Refrigerator Blog! This site will evaluate and provide information on the household refrigerator through the Thermodynamic Principle, Best Efficiency Based on Thermodynamics, Engineering Details, Environmental Impacts, and our team’s Personal Viewpoints. We hope you enjoy and learn something new. This blog site was put together by Team B in Dr. Chen’s Thermodynamics & Kinetics (CHEM 320) class.
The 2nd Law of Thermodynamics states that heat flows from hotter bodies to colder bodies spontaneously, and for heat to flow in the reverse, work must be applied. This is what is meant by the Clausius Statement: heat cannot be transported from a cold reservoir to a hot reservoir spontaneously.
A refrigerator is an example of a Carnot cycle in reverse. Instead of heat flowing from the hot reservoir to the cold reservoir and producing work, work is applied and heat flows from the cold reservoir to the hot reservoir.
Below are the results of the Carnot cycle (heat engine) and Carnot cycle in reverse (refrigerator).
Best Efficiency Based on Thermodynamics
The coefficient of performance, nr, of a reversible Carnot refrigerator is defined as the ratio of the heat withdrawn from the cold reservoir to the work supplied to the device:
nr = qcold/w = qcold/ (qhot – qcold) = Tcold/ (Thot – Tcold)
If Thot were to decrease from 0.9 to 0.1, nr would decrease from 9 to 0.1. This demonstrates that if the refrigerator is required to provide a lower temperature, more work is required. Additional work is required to extract a given amount of heat.
A household refrigerator typically operates at 255 K (~0 °F) in the freezing compartment, and 277 K (~39 °F) in the refrigerating section. Using the lower of these temperatures for the cold reservoir, and room temperature of 294 K as the hot reservoir, the maximum nr value is 6.5. For every joule of work applied to the system, 6.5 J of heat can be extracted from the refrigerator. This is the maximum coefficient of performance, and is only applicable to a reversible Carnot cycle with no dissipative losses. When losses are taken into account, it is difficult to have an nr value greater than 1.5. This shows a significant loss of efficiency in an irreversible dissipative cycle.
A refrigerator uses 5 major components: An expansion device, evaporator and condenser coils, a compressor and a refrigerant. The refrigerant is a liquid that enters in the expansion device, as it passes through the sudden drop of pressure makes it expand, cool, and turn into a gas. As the refrigerant flows around the evaporator coil it absorbs and removes heat from the food inside. The compressor squeezes the refrigerant, raising its temperature in pressure. It is now a hot, high pressure gas. The refrigerant then flows through condenser coils on the back of the fridge, radiating its heat to the atmosphere, then cooling back into a liquid as it does so. The refrigerant then re-enters the expansion device and the cycle repeats itself. So basically, heat is constantly picked up from the inside of the refrigerator and taken outside of it.
When the refrigerator was invented the design was rather basic. It wasn’t until the 1950’s that major enhancements began. The freezer increased in size and the futuristic look was created to match a home’s decorating scheme. In the 1990’s, a refrigerator was aimed to be sleek and modern. The French style doors and stainless steel were introduced. Today, the refrigerator market has a broad range of choices to fit one’s needs. Although the design evolution of the refrigerator has drastically changed, the future may hold many more changes.
The environmental impacts of the refrigerator include depleting ozone levels due to the chlorofluorocarbon (CFC) fluid. The CFC’s were used as a working fluid in the refrigerator, the material that the compressor squeezed down into a liquid that produced heat. Then the liquid expanded back into a gas, absorbing heat that the refrigerator removed from its interior. When the CFC is exposed to our atmosphere it can undergo reactions resulting in ozone depletion.
The chart below illustrates the declining levels of ozone over time, largely due to the refrigerant.
The ozone layer helps block much of the UVB radiation before passing through the stratosphere. Since ozone is decreasing, UVB radiation can make its way into the biosphere which allows the high energy radiation to disrupt the cells of the organisms. This causes damage to plant life and causes skin cancer in humans.
The chemistry behind CFC’s:
Today, refrigerators are a necessity in our everyday lives. The cool temperature decreases the natural reaction and biological rates that take place in our food, therefore keeping it fresh for an extended period of time. Keeping food fresh means less pollution due to food waste! Although refrigerators and refrigerants cause pollution, scientists continue to create more sustainable and energy efficient refrigerators as technology advances.
Meet Team B:
Thank you for visiting the Refrigerator Blog! Maybe you now feel like a refrigerator expert. We hope you learned something new and continue to be curious about the science behind your everyday life. Thank you to Dr. Chen for teaching us all about Thermodynamics, Kinetics, Carnot Cycles, efficiency, and everything else that makes up the common refrigerator. Feel free to leave a comment if you have any questions, comments, concerns, or additional details about refrigerators.
Boutin, Chad. “For Refrigeration Problems, a Magnetically Attractive Solution.” NIST, 8 Jan. 2018, www.nist.gov/news-events/news/2009/01/refrigeration-problems-magnetically-attractive-solution.
Chen, F., PhD. “Atmospheric Chemistry of the Ozone Layer.” Thermodynamics & Kinetics Lecture. Thermodynamics & Kinetics Lecture, Nov. 2018, UW Green Bay.
Engel, Thomas, and Philip J. Reid. 2010. Thermodynamics, Statistical Thermodynamics, & Kinetics. 2nd ed., Pearson / Prentice Hall.
ITC. “THE SECOND LAW OF THERMODYNAMICS For Mechanical and Industrial Engine…” LinkedIn SlideShare, 21 July 2016, www.slideshare.net/eurekacifto29/the-second-law-of-thermodynamics-for-mechanical-and-industrial-engineerig.
Khemani, Haresh. “Applications of Second Law of Thermodynamics: Part-2: Refrigerators.” Brighthub Engineering, 12 Nov. 2018, www.brighthubengineering.com/thermodynamics/4352-applications-of-second-law-of-thermodynamics-with-refrigerators/.
“NIST Chemistry WebBook, SRD 69.” Octadecanoic Acid, National Institute of Standards and Technology, webbook.nist.gov/.
Wald, Matthew L. “When Refrigerators Warm the Planet.” The New York Times, The New York Times, 26 Apr. 2011, green.blogs.nytimes.com/2011/04/26/when-refrigerators-warm-the-planet/.