In this blog post, we’ll examine how an air conditioner transfers heat from indoors to outdoors and explore the operating principle of the heat pump that makes it possible.
Imagine a sweltering summer day: you’re riding a crowded subway, walking past the scorching asphalt, and finally stepping into an office or classroom where the air conditioner is blasting. The moment you’re met by refreshing, cool air that cools your sweat-soaked body, the air conditioner becomes a truly welcome presence. A little while later, once you’ve cooled off sufficiently and have a moment to think, let’s take a closer look at the air conditioner that gave us that refreshing air. How exactly does an air conditioner keep providing cool air? Where did all the heat from my body and the office or classroom go? Understanding the principle of a heat pump makes the answer clear.
The principle of a heat pump is easier to understand when compared to that of a water pump, which we are all familiar with. First, everyone knows that water generally flows from a higher place to a lower place. However, we can use a water pump to draw water from below up to a higher level. Similarly, heat generally tends to flow from areas of higher temperature to areas of lower temperature. However, by using a heat pump, we can reverse this natural direction of heat flow and transfer heat from a lower-temperature area to a higher-temperature area. Air conditioners utilize this very principle of heat pumps to maintain a cool indoor temperature by continuously extracting heat from the relatively cooler indoor environment and transferring it to the warmer outdoor environment.
An air conditioner can be viewed as a large heat pump; it is fundamentally composed of the main components of a heat pump: a compressor, a condenser, an expansion valve, and an evaporator. Among these components, the condenser and evaporator function as heat exchangers in the outdoor and indoor units, respectively, while the remaining components—including the compressor and expansion valve—regulate the circulation of the refrigerant. All components are connected by pipes, through which the refrigerant continuously circulates. The refrigerant acts as a medium for transferring heat; while Freon-based refrigerants were widely used in the past, modern air conditioners and heat pumps utilize various refrigerants—such as R-32, R-410A, and R-454B—depending on the specific unit and its intended use. During the circulation process, the refrigerant’s chemical properties remain largely unchanged; it alternates between liquid and gas states depending on pressure and temperature, thereby transferring heat from indoors to outdoors.
Heat pumps utilize the refrigerant’s ability to change between gas and liquid states based on pressure and temperature to transfer heat from one location to another by releasing heat to the outside during the condensation process and absorbing heat during the evaporation process. This property of the refrigerant—vaporizing or liquefying depending on pressure—can be easily understood by imagining a middle or high school classroom. If we think of the students in the classroom as the refrigerant, when the “pressure” of the teacher is present, the students cannot move freely and cannot stray far from their seats. In other words, this resembles the liquid state, where the movement of particles is restricted and they are clustered closely together. On the other hand, when the “teacher”—representing pressure—leaves the classroom, that is, when the pressure decreases, the children move freely, leaving their seats and wandering around. This analogy helps illustrate the gaseous state, in which molecules move actively.
Now that we understand how the refrigerant liquefies or vaporizes depending on pressure, let’s examine how this process occurs inside an air conditioner by following the refrigerant’s circulation. Since the refrigerant circulates continuously, the result is the same no matter which of the four components we start with; however, for convenience, let’s begin with the compressor. First, the refrigerant enters the compressor in a low-pressure gaseous state. The compressor then uses electrical energy to convert the low-pressure gaseous refrigerant into a high-pressure gaseous state. During this process, both the pressure and temperature of the refrigerant increase. The refrigerant, now in a high-temperature, high-pressure gaseous state, moves to the condenser, where it condenses into a liquid while releasing heat at a temperature higher than that of the outdoor air. During this process, the refrigerant releases heat to the outside; as a result, through the continuous processes of compression and condensation, it discharges to the outside the heat drawn from the indoor space and the heat corresponding to the energy supplied to the compressor.
After releasing heat through the process described above, the refrigerant becomes a high-pressure liquid. It then passes through an expansion valve, which reduces the refrigerant’s pressure—the opposite of what the compressor does. As the refrigerant passes through the expansion valve, its pressure and temperature drop, bringing it to a low-temperature, low-pressure state. A portion of it enters the evaporator in a mixture of partially vaporized liquid and gas. Subsequently, as the refrigerant evaporates in the evaporator located indoors, it absorbs heat from the room and enters the compressor again in a low-pressure gaseous state. Since the heat required for the refrigerant to evaporate is absorbed from the indoor air, heat is removed from the room, and the room cools down. The refrigerant, having absorbed heat from the indoor space through the processes of expansion and evaporation, returns to a low-pressure gaseous state and enters the compressor—the first stage of the cycle. To summarize, within the heat pump, the refrigerant releases heat to the outside through compression and condensation, and absorbs heat from the room through expansion and evaporation. By continuously repeating this cycle, the heat pump ultimately achieves its purpose of transferring heat from the room to the outside. In the case of an air conditioner, since this heat pump cycle is used to cool the room, it is generally referred to as the cooling cycle. Conversely, when the direction of refrigerant flow within the heat pump is reversed, the roles of the evaporator and condenser inside and outside the building are swapped. In this case, the heat pump absorbs heat from the outside and transfers it indoors; since the heat pump is heating the interior, this can be used as a heating cycle. In other words, by reversing the direction of refrigerant flow, a single heat pump can be used for both cooling and heating, and this principle can be observed in air conditioners and heat pump systems that combine cooling and heating functions.
