For centuries, man has had to deal with spoiled food and the money lost from purchasing said food. Enter the refrigerator, a device used to lower the temperature of food and beverages and to slow the formation of bacteria. Our blog will detail the thermodynamic principle associated with the refrigerator, as well as the working chemicals and environmental effects of the release of the aforementioned chemicals. Chemists and physicists have, for the sake of calculations, assumed the Sun was composed of an ideal gas, and we are going to validate or refute that claim. Welcome, ladies and gentlemen, to Team B’s Blog!
One of the fundamental concepts in thermodynamics is the concept of the heat engine. This engine involves the Carnot Cycle, in which a hot reservoir transfers heat to a cold reservoir, and during the process, work is done. However, if we reverse this cycle, and instead add work, usually in the form of electricity, we can transfer heat from the cold reservoir to the hot reservoir, and create a refrigerator, according to the following diagram:
Now, the way a refrigerator accomplishes this is by using a compressor that uses electricity (work, or W) to compress a refrigerant such as 1,1,1,2-tetrafluoroethane and creates a superheated vapor by transferring heat to the system (the refrigerant.) This vapor travels through coils on the back of the refrigerator, known collectively as the condenser and is cooled down by ambient air, and undergoes a phase change into a cooler liquid still under high pressure. Eventually, the gas enters what is known as the expansion valve, where about 1/2 of the liquid vaporizes and expands due to a lower external pressure, and performs expansion work on the surroundings causing the refrigerant to lose heat and decreases its temperature. A fan then blows across the tubes filled with this cooled gas, and transfers the heat from the cold reservoir (the inside of the refrigerator) to the air surrounding the refrigerator (the hot reservoir.)
The Carnot Cycle operates by using 4 reversible steps. Since these steps are reversible, these steps can be reversed and create the Carnot Refrigeration Cycle. The four reversed steps are: 1. Adiabatic Compression 2. Isothermal Compression 3. Adiabatic Expansion 4. Isothermal Expansion. The refrigerator undergoes this cycle by first compressing the refrigerant gas so that no heat escapes into the surroundings (adiabatic compression.) Then, the gas is compressed so that the final temperature is equal to the initial temperature (isothermal compression.) After that occurs, the gas is expanded so that no heat is lost or gained by the surroundings (adiabatic expansion.) Then, the remaining gas expands so that there is no temperature change in the gas (isothermal expansion.)
In thermodynamics, there exists a quantity called efficiency. This quantity defines how much heat you are able to remove from the cold sink based on a certain input of electrical energy. Efficiency for Carnot refrigeration is not actually called efficiency, but is instead given the name coefficient of performance, and it is defined by the following equation:
So, let us say that the cold reservoir doesn’t go any lower than freezing (273.15 K.) Let us also say that the hot reservoir that is receiving the heat is the ambient air at room temperature, or 298.15 K. The maximum coefficient, then, would be 273.15/(298.15-273.15) or 10.926 CoP units, meaning that 10.926 times more heat is removed than the electricity that is supplied.
There are two things that need to be known for refrigeration.
- A gas cools on expansion.
- When you have two things that are different temperatures that touch or are near each other, the hotter surface cools and the colder surface warms up. This is a law of physics called the Second Law of Thermodynamics.
A refrigerator uses a series of coils where a coolant is trapped inside. As the coolant travels through the coils it makes a circuit, changing back and forth from a liquid to a gas. To make this possible the refrigerator has five major components: compressor, heat-exchange pipes, expansion valve, heat-exchanging pipes, and refrigerant (liquid that evaporates inside the refrigerator to create the cold temperatures).
- The compressor compresses a refrigerant gas which could be chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), hydrofluorocarbon (HFC), perfluorocarbon (PFC), or blends made from ammonia and carbon dioxide. The compressed gas heats up as it is pressurized (orange). For the above diagram lets use ammonia.
- The coils on the back of the refrigerator let the hot ammonia gas dissipate its heat. The ammonia gas condenses into ammonia liquid (dark blue) at high pressure.
- The high-pressure ammonia liquid flows through the expansion valve. You can think of the expansion valve as a small hole. On one side of the hole is high-pressure ammonia liquid. On the other side of the hole is a low-pressure area (because the compressor is sucking gas out of that side).
- The liquid ammonia immediately boils and vaporizes (light blue), its temperature dropping to -26.6 degrees C (the average temperature of a modern day refrigerator. This makes the inside of the refrigerator cold.
- The cold ammonia gas is sucked up by the compressor, and the cycle repeats.
Refrigerants have a direct impact on the environment as a result of leakage of gases into the atmosphere which leads to ozone depletion. The highest ozone depleting refrigerants historically have been Chlorofluorocarbons or CFCs and Hydro chlorofluorocarbons or HCFCs which have mostly been phased out due to the Montreal Protocol. New legislation and consumer awareness of these substances has created a focus on reducing the global warming potential (GWP) and the ozone depletion potential (ODP) of refrigeration systems; as well as an emphasis on leak detection and effective recovery, reclamation and destruction of refrigerants. ODP is characterized as the potential for a single molecule of refrigerant to destroy the ozone layer; it uses R11 (single CFC) which has been given an ODP of 1.0 as a reference. GWP is a measurement of the effect a given refrigerant molecule will have on global warming in relation to Carbon Dioxide where CO2 has been given a GWP value of 1 as well. As stated before R11 has an ODP of 1 and a GWP of 4000. The next worse refrigerant for the environment is R22 (HCFC) which is currently being phased out in Europe; it has an ODP=.05 and a GWP of 1700.
The carbon-chlorine bond in these compounds is broken in the presence of light. The Cl atom catalyzes the conversion of ozone into O2. Ozone absorbs UV-B radiation, so its depletion allows more of this high energy radiation to reach the Earth’s surface.
Cl + O3 → ClO + O2 (chlorine atom changes an ozone molecule to ordinary oxygen)
ClO + O3 → Cl + 2O2 (ClO from the previous reaction destroys a second ozone molecule and recreates the original chlorine atom, which can repeat the first reaction and continue to destroy ozone)
The use of safer refrigerants has increased however; these refrigerants include:
R134A (HFC) ODP=0, GWP=1300.
R407C (HFC) made up of a mixture of various refrigerants. ODP=0, GWP=1610.
R410A (HFC) ODP=0, GWP=1890.
R290 (propane) which is a naturally occurring hydrocarbon and refrigerant has an ODP of zero and a GWP of only 3. Although it is arguably the best refrigerant for the environment it is also highly flammable, something that needs to be considered in its implementation. Ammonia is very similar to propone in that it’s safe for the environment but very toxic to handle.
We believe that there’s still a lot more progress to be made to lessen the impact that refrigerants have on the environment from recovering refrigerants, making products more energy efficient, reducing the amount of refrigerant needed for refills, and better products to prevent refrigerant leakage.
Overall, our group was quite impressed with the engineering and thermodynamic feat that is a refrigerator. However, the refrigerants that are used are often ozone depleting, global warming agents, or both. While propane and amonia offer a low global warming potential and a low ozone depletion potential, we feel that the toxicity of ammonia and the flammability of propane make them both unsafe for handling. We propose 1,1,1,2-tetrafluoroethane, a refrigerant that has 0 ozone depleting potential, but a high global warming potential. However, we feel that if this gas is contained, it should have a minimal effect on global climate change.
It’s not what’s on the outside that counts, but what’s on the inside.
A simple equation that is used by chemists working with gases that are considered “ideal” use what is known as the ideal gas law, displayed here:
This equation assumes two things, that the particles are not attracted to each other, and they are point particles, meaning they take up no volume of their own. If we assume these two things are not true, it behaves as a real gas, which behaves according to this equation:
where a = the amount of interaction between the particles in terms of attraction or repulsion, and b = the excluded volume, or amount of volume the particles occupy. Since the sun is composed mostly of helium and hydrogen, we can analyze the a and b values for these two gases to determine whether they can be considered ideal enough to use the ideal gas law equation. Since helium and hydrogen are the smallest atoms known, they have very few protons and electrons and therefore very small inter-molecular forces, so the a term is negligible, and these gases can be considered ideal from the first assumption standpoint.
Since the first part of the ideal gas law equation assumptions have been satisfied, we must now undergo a process to prove that the excluded volume of the hydrogen and helium particles is negligible compared to the total volume. To do this, a worksheet has been prepared, shown here:
From this worksheet, we have proven that the excluded volume of the sun’s particles is negligible, and so the b term assumption is confirmed. Since the a and b terms have now been proven to be accurate assumptions of the sun’s ideal gas behavior, we have concluded that it is justified for scientists to use the ideal gas law for the sun.