Welcome. It is good to have you here. As usual, I like to begin with a little bit of background. The last seven days have seen me writing about Temperature, Heating and Air-Conditioning and How Stirling Engines Utilise Temperature Differentials to Drive Pistons which do Mechanical Work. It was not my intention to discuss both of these topics but I in the process of researching other issues, I stumbled upon them. I was particularly impressed by how Stirling engines can work even with as little heat as the heat from a cup of hot coffee. Then when I read that they find unique uses in Cogeneration or Combined Heat and Power systems, my curiosity was piqued. This post was meant to be about cogeneration and how they are used in distributed generation systems or stand-alone facilities to power buildings, provide heat and, in the case of Trigeneration, provide cooling. It was in the process of putting together a post on this aspect of cogeneration that the concept of Absorption Refrigeration sneaked up on me.

^{Pixabay CC0: Food Items in a Refrigerator}

Absorption Refrigeration

Absorption refrigerators differ from compression refrigerators and find uses in recreational vehicles. The principle of absorption refrigeration is also applicable to building air-conditioning by using waste heat from gas turbine generators and other heat sources.

Absorption refrigeration is a cooling process that uses a heat source to provide the energy for cooling. In practice, the heat source could be solar energy, fossil-fuel, combined heat and power (CHP) or cogeneration plants and waste heat from factories. It would seem counter-intuitive to expect that heat could be used to power a cooling process so how does absorption refrigeration achieve this purpose? Before examining the process of absorption refrigeration, I would like to discuss some physical concepts that are exploited in the process.

Partial and Vapour Pressure

To explore the principle of operation of an absorption refrigerator, let us consider a Single-Pressure Absorption Refrigerator. This type of refrigerator exploits the fact that the boiling point of a substance depends on the partial pressure of the vapour above the liquid. This dependence of boiling point on the partial pressure is such that the boiling point goes down with lower partial pressure. Even though the refrigerator has the same total pressure throughout the system, it maintains a low partial pressure for the refrigerant and as a result, a low boiling point in the part of the system that extracts heat from the interior of the refrigerator. However, in the part of the system that ejects heat to the outside of the refrigerator, the refrigerator maintains the refrigerant at high partial pressure and therefore, high boiling point.

Partial Pressure

A general property of gases is that the total pressure of an ideal gas mixture is the sum of the partial pressures of the gases in the mixture. In other words, if a container has a mixture of gases, each gas would exert a partial pressure equal to the hypothetical pressure that the gas would exert on the container if the whole volume of the container is occupied by only that gas at the same temperature. Even though this property is defined for an ideal gas, real gases behave similarly.

Mathematically, this property of gases is shown with the isotherm relation:

V_a/V_tot = p_a/p_tot = n_a/n_tot
V_a is the partial volume of any individual gas component a
V_tot is the total volume of the gas mixture
p_a is the partial pressure of gas a
p_tot is the total pressure of the gas mixture
n_a is the amount of substance of gas a
n_tot is the total amount of substance in gas mixture

Vapour Pressure

Liquids boil at temperatures where its vapour pressure equals the atmospheric pressure, and this is known as the liquid's ** normal boiling point**.Vapour pressure is the pressure of a vapour which is in equilibrium with its non-vapour phases (in the case of absorption refrigerator, water). Vapour pressure is also seen as the tendency of a liquid to evaporate or the tendency of a liquid or solid molecule to escape from it. It should become clear, from the ongoing, that the boiling point of a liquid would depend on the vapour pressure and also the prevailing atmospheric pressure since the atmospheric pressure boiling point corresponds to the temperature at its vapour pressure is equal to the surrounding atmospheric pressure.

At any given temperature, the higher the vapour pressure of a liquid, the lower the normal boiling point of the liquid.

How the Absorption Refrigerator Works

The cooling cycle starts at the evaporator with liquid ammonia at room temperature. The evaporator has greater volume than the volume of the ammonia liquid, with the remaining space containing a mixture of ammonia and hydrogen, the ammonia would have a lowered partial pressure due to the presence of hydrogen, and this helps to achieve a boiling point that is below the temperature inside the refrigerator.

The more common type of refrigerator is the compression type. The operation of the compression-type refrigerator and the absorption-type refrigerator are similar because both types use liquid refrigerants which have very low boiling points. Essentially, when the refrigerant is heated, it boils and evaporates, carrying the heat away from the chamber that needs cooling, then it condenses back to liquid. This change of phase from liquid to gas provides the cooling effect. This is by the kinetic energy definition of temperature discussed here. The difference between the operation of the compression-type refrigerator and the absorption-type refrigerator is the way in which the liquid refrigerant is condensed back to liquid after the cooling effect is achieved in a single cycle so that the cycle can be repeated. While the compression type uses a compressor to increase the pressure of the gas and return it to liquid, the absorption type applies only heat and works with no moving parts.

The absorption refrigerator cooling cycle is made up of three phases:

1. The liquid refrigerant is contained in a low-pressure vessel and readily evaporates when it picks up heat from the interior of the refrigerator due to the low pressure.

2. Secondly, the now-gaseous refrigerant is absorbed by the ammonia salt solution. Then the refrigerant-saturated ammonia liquid is heated, causing the refrigerant gas to evaporate from the solution.

3. The hot refrigerant gas passes through a heat exchanger, thereby transferring its heat to the air outside the refrigerator. When the gas loses heat through this process, it condenses back to liquid, and the evaporation phase is repeated from the first step as long as heat is applied to the ammonia salt solution.

Einstein's Refrigerator

Albert Einstein and his former student Leó Szilárd, patented an absorption refrigerator which has no moving parts, operates at constant pressure, and requires only a heat source to operate in 1930^[2]

The two scientists were motivated by a story of a Berlin family that was exposed to harmful substances and killed when the seal on their refrigerator broke. However, the technology did not quite find widespread applications until recently.In 2016, William Broadway of the Loughborough University Design School applied the technology to design a vaccine cooler called the Isobar.

William Broadway was inspired to create this design because of his experience travelling through Cambodia, Vietnam, Hong Kong, China and experiencing first-hand, the difficulty that aid workers have in the area of preserving vaccines long enough to reach the parts of the world that needs it. The design won him the prestigious James Dyson award which came with a cash prize of two thousand dollars. He plans to continue producing more prototypes.

According to him, the Isobar has two chambers which contain two sets of chemicals at the bottom. When heat is applied to the cooler, the substances separate and when flipped over, the chemicals recombine, giving off a cooling effect. Ultimately, the Isobar is a more efficient way of transporting vaccines to ensure their potency. The canister can also find application in organ donation and transplant when the specifics are adjusted. An image of the design can be found here.

I would like to thank you for reading.

---

References

1. RV Repair Club | How Absorption Refrigeration Works
2. Wikipedia | Einstein's Refrigerator
3. The Guardian | Einstein Inspired Isobar Vaccine Cooling System
4. Wikipedia | Cogeneration

---

^{If you write STEM (Science, Technology, Engineering, and Mathematics) related posts, consider joining #steemSTEM on steemit chat or discord here. If you are from Nigeria, you may want to include the #stemng tag in your post. You can visit this blog by @stemng for more details. You can also check this blog post by @steemstem here and this guidelines here for help on how to be a member of @steemstem. Please also check this blog post from @steemstem on proper use of images devoid of copyright issues here.}

---