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| Fig. 1: Mechanical air conditioners on an apartment wall. (Source: Wikimedia Commons) |
Air conditioning refers to the process of controlling the temperature and humidity of the air. Mechanical and passive methods can be used to condition air; however, since the invention of the first modern cooling unit by Willis Carrier in 1902, the current gold standard for air conditioning uses mechanical technologies. Currently, mechanical air conditioners are widely used across developed countries, with individual houses and apartments often having a dedicated unit, as shown in Fig./ 1. Using external work to fight the Second Law of Thermodynamics, mechanical air conditioning moves air from low to high temperatures. Although it varies slightly, typical mechanical air conditioners follow four processes known as the reversed Carnot cycle that utilize phase changes to drive cooling: [1]
Evaporation: An evaporator with cool, low-pressure, liquid refrigerant flowing through it takes heat from the surrounding air. The refrigerant leaves as a saturated, low- temperature, low-pressure vapor.
Compression: A compressor adds external work to the system and turns the low-pressure vapor into high-pressure vapor, increasing its temperature as expected with the ideal gas law, which defines the relation between pressure, volume, and temperature. The refrigerant leaves as a saturated, high-temperature, high-pressure vapor.
Condensation: The high-temperature vapor runs through a condenser and releases heat to the surroundings to return a high-temperature, high-pressure liquid.
Expansion: The resulting high-temperature, high-pressure liquid passes through an expansion valve, which decreases its pressure, and thereby decreases its temperature. At this point, the refrigerant is once again a cool, low-pressure liquid, and it restarts its cycle.
A refrigerant is a liquid that can undergo phase changes to absorb and release heat at working temperatures and pressures. An ideal refrigerant is a liquid that has some of the following properties: a high latent heat of vaporization, chemical stability, low cost, non-toxic, non-flammable, non-corrosive, and has a low environmental impact, among others. [1] It is unlikely to find a refrigerant with all desired properties, so refrigerants are used on a case-by-case basis. Some refrigerants are now known to destroy the ozone layer. Some sustainable synthetic refrigerants have arisen as competitors to these ozone-depleting refrigerants.
The performance of a mechanical air conditioner is quantified through the Coefficient of Performance (COP). As the reversed Carnot Cycle requires work to move heat, the COP can be thought of as the simple ratio between the amount of heat able to be moved per unit of input work. [1] Currently, air conditioners' typical COP values range from 2-5. However, the COP value for an air conditioner will vary depending on the operating conditions; therefore, engineers in the United States of America looks towards the seasonal energy efficiency ratio (SEER). The SEER is a weighted average of a value similar to the COP under different operating conditions. The minimum SEER value is currently regulated at 13-14, and the maximum ranges from 25-30 due to technical limitations. The SEER does have units of BTU/Watt, and, just similar to the COP, represents how much energy (BTU) can be moved per energy input (Watt).
Although mechanical air conditioning dominates in first-world countries today, that wasn't always the case. Other cooling strategies still exist and may provide energy and cost-saving benefits. Some alternative active cooling systems include fans and evaporative coolers. Some passive cooling methods include shading systems, phase change materials (PCM), ventilation systems, radiation-cooled textiles, and the selection of thermally advantageous building materials. [2-4] Many of the mechanical air conditioning alternatives have drawbacks that revolve around effective working conditions. Nevertheless, some places may not have the infrastructure or income to sustain mechanical air conditioners, resulting in these alternatives being practical heat mitigation strategies.
An increasing demand for air conditioning has been one of the largest drivers for energy demand worldwide. Quantifying the energy demand of mechanical air conditioners' is a function of many variables, including model, weather, inhabitants' behavior, and building size. [5] Using global cooling reports published by the International Energy Agency (IEA), we can get a scale of the total energy usage for cooling. There has been a large increase in space cooling, contributing to 13% of the global electricity demand increases between 1990 and 2016, and is 10% of the total global electric demand, or equivalently 2000 TWh. [6] As the climate warms and the standard of living increases, the number of mechanical air conditioners increases. In places where the standard of living is already high, such as the US, 70% of the residential energy load can come from cooling alone. [6] In cities, the demand for cooling results in a cyclic effect. Mechanical air conditioners expel heat outdoors, intensifying the urban heat island effect (UHIE). One study reported city temperature increases of up to 1°C from air conditioning use. [6] Therefore, using air conditioning to be thermally comfortable results in driving further air conditioning energy consumption.
The Intergovernmental Panel on Climate Change (IPCC) projects that the global temperature will likely exceed pre-industrial levels by 1.5°C between 2030 and 2052. [7] To prevent further increases in the average global temperature, the IPCC posits that rapid changes at unprecedented scales to current infrastructure are necessary. [7] As the 1.5°C is just an average, local temperatures may see a significantly larger increase. In the face of these temperature increases, air conditioning use is projected to increase, with a concomitant increase in CO2 emissions. [6,8] By 2050, it is projected that 25000 GW of total residential AC cooling capacity will be installed, a threefold increase. [6] By 2050, emissions from air conditioners are expected to almost double from 1.135 Gt of CO2 to 2.070 Gt of CO2. [6]
© Erik Schreiner. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] G. F. Hundy, A. R. Tratt, and T. C. Welch, Refrigeration, Air Conditioning and Heat Pumps, 5th ed. (Butterworth-Heinemann, 2016).
[2] S. Liu, W. Ge, and X. Meng, "Influence of the Shading Nets on Indoor Thermal Environment and Air-Conditioning Energy Consumption in Lightweight Buildings," Energy Rep. 11, 4515 (2024).
[3] I. Oropeza-Perez and P. A. Østergaard, "Active and Passive Cooling Methods For Dwellings: A Review," Renew. Sustain. Energy Rev. 82, 531 (2018).
[4] X. Liu et al., "Radiation Cooling Textiles Countering Urban Heat Islands," Sci. Bull. 69, 3318 (2024).
[5] W. Tang et al., "Prediction and Evaluation of Air Conditioner Energy Consumption of Residential Buildings in the Yangtze River Basin," J. Build. Eng. 65, 105714 (2023).
[6] "The Future of Cooling," International Energy Agency (2018).
[7] H. Lee and J. Romero, eds. "Climate Change Synthesis Report," Intergovernmental Panel on Climate Change, 2023.
[8] E. De Cian et al., "The Impact of Air Conditioning on Residential Electricity Consumption Across World Countries," J. Environ. Econ. Manag. 131, 103122 (2025).
