In this blog post, we’ll examine the principles, operating process, advantages, and limitations of absorption chillers, which use the heat generated by burning natural gas for cooling.
On September 15, 2011, a baseball game in progress at Mokdong Stadium was suddenly suspended due to lighting issues, and in residential areas, many people were startled and rushed out into the streets—causing significant disruption. This was all due to a power outage. The outage at that time resulted from a combination of factors, including a short-term surge in electricity demand caused by abnormally high temperatures, failures in forecasting electricity demand and assessing supply capacity, and inadequate management of reserve power. Actual electricity demand rose to 67.26 million kW, exceeding the initial forecast of 64 million kW. Following the September 15 blackout, public interest in electricity demand grew, and campaigns encouraging people to reduce electricity usage during summer peak hours continued. However, at the time, the rise in electricity demand was recognized as a major social issue, as news reports repeatedly highlighted that demand had reached an all-time high.
When examining the reasons for this increase in electricity demand, the amount of electricity used for air conditioning during the summer stands out as one of the primary causes. In fact, cooling demand has a significant impact on summer peak electricity consumption. The Korea Power Exchange continues to advise the public to reduce air conditioner use during summer peak hours and recommends minimizing the use of electric air conditioning in favor of district cooling or gas-powered air conditioning. However, even though many people find the electricity bills for air conditioning burdensome, they often end up turning their air conditioners back on due to the unbearable summer heat. So, during the summer when electricity demand spikes like this, what methods are available to stay cool while reducing the burden of electric cooling?
Have you ever heard of a cooling method that uses relatively little electricity? Furthermore, would you believe it if that method utilized the heat generated by burning city gas? Upon hearing this, most people would likely ask in return, “Isn’t city gas used for heating, not cooling?” However, there are indeed devices that use city gas for cooling. That is precisely the absorption chiller introduced here. Unlike conventional electric chillers, which operate primarily using a compressor, absorption chillers generate cooling by utilizing thermal energy; they can use not only city gas but also waste heat or other heat sources.
Absorption chillers work by taking advantage of the fact that water evaporates more easily at low temperatures when external pressure decreases. Consider the phenomenon that occurs when cooking rice in the mountains, where atmospheric pressure is low: water, which normally boils at 100°C, boils at a lower temperature there, making the rice prone to being undercooked. This is based on the same principle, so it’s easy to understand. Absorption chillers utilize this principle to maintain the pressure of the water at a very low level, causing it to evaporate at a low temperature. When water changes from a liquid to a gas, it absorbs heat from its surroundings and uses that heat for its phase change. Therefore, as water evaporates, it draws heat away from its surroundings, causing the ambient temperature to drop. This is the same principle that makes you feel cool when water on your body dries on a hot summer day, or when you feel a chill as sweat evaporates during exercise. In other words, an absorption chiller creates low pressure to facilitate water evaporation, thereby maximizing the property of water to absorb heat as it evaporates. When water evaporates smoothly, the process of removing heat from the object being cooled can occur continuously.
In an absorption chiller, the low-pressure space where water evaporates easily is called the evaporator. Pipes through which the water to be cooled flows pass through the evaporator, and the absorption chiller operates by spraying water—which acts as the refrigerant—onto the surface of these pipes to cause it to evaporate. As the water evaporates and draws heat away from the pipes, the water flowing inside the pipes cools down. This cooled water is then used to cool the indoor air, and when the cooled air is supplied into the room, cooling is achieved.
However, upon closer examination, it becomes clear that this process cannot continue indefinitely. If water continues to evaporate inside the sealed evaporator, water vapor accumulates inside, and as the amount of water vapor increases, the pressure rises. When the pressure rises, the water becomes less likely to evaporate, causing the cooling efficiency to gradually decline. This is where the lithium bromide solution comes into play: it removes the water vapor to maintain an environment conducive to continued evaporation. Lithium bromide solution has a strong ability to absorb water vapor. Connected to the space containing the evaporator, this solution absorbs the water vapor, thereby keeping the vapor pressure inside the evaporator low and allowing water to evaporate easily once again. You can easily understand this by imagining a kettle boiling in a small room alongside a moisture-absorbing dehumidifier. In actual absorption chillers, a system that uses water as the refrigerant and lithium bromide as the absorbent is widely employed.
So, does the lithium bromide solution absorb water vapor almost indefinitely? Of course not. For this device to continue operating, the lithium bromide solution—which has become diluted by absorbing water vapor—must be concentrated again. When we need to concentrate a solution, what method comes to mind first? That’s right. Just as when making sugar syrup or adjusting the seasoning of a bland spicy fish stew, the method of “simmering down” is also used to concentrate the lithium bromide solution. To supply the heat required for this evaporation, the heat generated by burning city gas is utilized. The diluted lithium bromide solution is transferred to another container, where it is heated and separated once again into concentrated lithium bromide solution and water vapor. The concentrated lithium bromide solution returns to the absorber, where it repeats the process of absorbing water vapor. In this way, an absorption chiller generates cooling by circulating the refrigerant and absorbent using thermal energy, rather than compressing the refrigerant with electricity.
However, the water vapor must not be allowed to return directly to the original evaporation chamber. Since it is in a vapor state, it cannot be reused as a refrigerant in the original evaporation chamber, and the heat generated during the condensation process must be dissipated. Therefore, the water vapor is cooled in a separate condenser until it condenses into water, after which it recirculates as a refrigerant. The heat generated during this process is discharged to the outside via cooling water and a cooling tower. A cooling tower can be thought of as performing the same role as the outdoor unit of a typical home air conditioner—discharging the heat generated during the refrigeration cycle to the outside. In an absorption chiller, the cooling tower is a critical component because it must simultaneously generate cooling in the evaporator and discharge the heat generated in the absorber and condenser to the outside.
As this process repeats, the indoor air eventually cools down. If you consider the process carefully, you’ll realize that a certain amount of electricity is required to operate the cooling tower and cooling water system, circulate the cooled water, and move the lithium bromide solution as it dilutes and then concentrates again. However, since it does not directly drive a large compressor with electricity like a typical electric chiller, utilizing thermal energy can significantly reduce the amount of electricity needed for cooling. In particular, energy efficiency can be improved by utilizing heat that is difficult to use for other purposes—such as waste heat from power plants or heat discarded by industrial facilities—for cooling. Therefore, it is more accurate to understand absorption chillers not as “cooling that uses no electricity at all,” but rather as “cooling that uses thermal energy as its primary power source instead of electricity.”
Although this article uses city gas as an example, the fuel used for heating does not necessarily have to be city gas. Current absorption chillers can utilize fuels such as natural gas directly, or use heat sources such as waste heat from generators and combined heat and power (CHP) plants, waste heat from industrial facilities, steam, and hot water; methods for utilizing thermal energy such as solar heat and geothermal energy are also being researched. In particular, absorption chillers are significant as a means of improving energy efficiency because they can utilize waste heat for cooling. On the one hand, absorption chillers have the advantage of reducing the burden on electric cooling during periods of peak electricity demand; however, they require cooling towers and cooling water systems, and the equipment can be large. Additionally, the conventional lithium bromide-water system uses water as a refrigerant, making it difficult to achieve a low chilled water temperature of 5°C or below. Therefore, while they are suitable for general building cooling, other methods are required for low-temperature refrigeration. Recently, technologies for utilizing waste heat, improving efficiency, and leveraging various heat sources have been continuously developed. In South Korea as well, district cooling and gas-powered cooling systems are increasingly being utilized to alleviate peak electricity loads. If more people come to understand and embrace the principles of absorption chillers, might we one day see a reduction in the burden placed on electric cooling during the height of summer—when electricity demand surges—and the ability to utilize a wider variety of energy sources?
