In this blog post, we’ll take a detailed look at the scientific principles and structure behind how air conditioners cool indoor spaces, as well as their advantages and limitations.
Recently, due to the effects of climate change, summer heat waves and tropical nights have become increasingly frequent. Consequently, air conditioners have evolved beyond mere convenience appliances to become essential household appliances that play a crucial role in maintaining health and quality of life. During summers marked by persistent heat waves, sales of air conditioners and demand for replacements are steadily increasing. So, what is the structure of an air conditioner, and what principles allow it to keep indoor spaces cool even during the hottest summer days?
The most fundamental principle of air conditioning is cooling through the use of latent heat of vaporization. The states of matter can be broadly categorized into three types: solid, liquid, and gas. When heat or pressure is applied to a substance, it changes from its current state to another; this is called a phase change. Generally, to induce a phase change, the temperature of the substance is raised or lowered. Since thermal energy weakens the intermolecular forces, heat must be absorbed when a solid turns into a liquid or a liquid turns into a gas. Conversely, when a gas turns into a liquid or a liquid turns into a solid, heat is released.
Air conditioners utilize pressure to induce these phase changes. Since a gas occupies a much larger volume than a liquid, applying high pressure to a gas causes its volume to decrease, causing it to condense into a liquid. At this point, the refrigerant releases thermal energy into the surroundings. Conversely, lowering the pressure of a liquid causes it to vaporize easily, absorbing heat from the surroundings. The thermal energy absorbed during the process of a liquid turning into a gas is called the heat of vaporization. Air conditioners utilize this heat of vaporization to absorb heat from the indoor air and produce cool air.
Air conditioners are primarily composed of a compressor, a condenser, an expansion valve, and an evaporator, and cooling occurs as the refrigerant circulates through these components.
First, the gaseous refrigerant enters the compressor located in the outdoor unit. The compressor strongly compresses the refrigerant, transforming it into a high-temperature, high-pressure gas. The compressed refrigerant then moves to the condenser. In the condenser, the refrigerant condenses into a liquid as it exchanges heat with the outside air. Since heat of condensation is released to the outside during this phase change from gas to liquid, hot air is expelled from the outdoor unit.
As the refrigerant becomes a liquid, its pressure drops sharply as it passes through the expansion valve. While previous articles have explained this solely using Bernoulli’s principle, in reality, the expansion valve is a device that significantly reduces pressure by regulating the refrigerant’s flow rate; during this process, the refrigerant enters a low-temperature, low-pressure state where it can easily evaporate.
The refrigerant, now at a lower pressure, moves to the evaporator in the indoor unit. In the evaporator, the refrigerant evaporates into a gas while absorbing thermal energy from the surrounding air. As heat from the indoor air transfers to the refrigerant, the temperature around the evaporator drops, and the fan circulates this cooled air into the room, providing cooling. The gaseous refrigerant then returns to the compressor to repeat the same process. As this cycle continues, the air conditioner continuously transfers heat from the interior to the outside.
Many people think of air conditioners as machines that generate cold air from scratch, but in reality, they are devices that utilize the principle of a heat pump to transfer heat from indoors to outdoors. In other words, rather than generating cold air, they cool the interior by expelling heat from the room to the outside; conversely, hot air is discharged from the outdoor unit.
This refrigeration cycle is not significantly different from the basic principle of the modern electric air conditioner developed by American inventor Willis Carrier in 1902. Of course, while compressor performance, control technology, refrigerant types, and energy efficiency have advanced significantly today, the basic principle of transferring heat by compressing and condensing the refrigerant, then repeatedly expanding and evaporating it, is still used today.
From a scientific perspective, this is a prime example of the highly effective application of the fundamental principles of phase changes in matter and heat transfer.
Of course, air conditioners operating on this principle are not without their drawbacks. In the past, CFCs and HCFCs were widely used as refrigerants, but since these were found to deplete the ozone layer, their use has largely been discontinued or is being phased out. Currently, HFC-based refrigerants that do not deplete the ozone layer and eco-friendly refrigerants with a low global warming potential (GWP) are gradually being adopted, and technological development to reduce environmental impact continues.
Furthermore, since air conditioners are home appliances that consume a significant amount of electricity, prolonged use can lead to high electricity bills. However, South Korea’s electricity rate structure has become less progressive than in the past, and with the widespread adoption of high-efficiency air conditioners equipped with inverter compressors, it is now possible to achieve the same cooling effect using significantly less power. Recently, energy efficiency has been further enhanced through the application of AI-based automatic power-saving features and smart cooling technologies that autonomously analyze indoor environments.
As such, air conditioners are a prime example of scientific and technological innovation that delivers exceptional cooling performance while utilizing relatively simple scientific principles—namely, changes in the state of matter and the transfer of heat. As eco-friendly refrigerants, high-efficiency compressors, and smart control technologies continue to advance, air conditioners will evolve to provide even more comfortable indoor environments while reducing their environmental impact. Air conditioners are expected to continue playing a vital role in helping people spend hot summers more safely and comfortably.
