Air conditioning
Removes heat from enclosed spaces for comfort and health.
Air conditioning, known as A/C in the US and air con in the UK, is the practice of pulling heat out of a closed space to make the indoor temperature more comfortable, and sometimes also to manage humidity levels. This can be done with a mechanical unit or through methods like passive cooling and ventilative cooling. It falls under the broader category of HVAC systems, which handle heating, ventilation, and air conditioning. Heat pumps work much like air conditioners but include a reversing valve that lets them provide both heating and cooling. During hot weather, air conditioning helps prevent heat stroke, dehydration from heavy sweating, electrolyte imbalances, kidney failure, and other problems caused by overheating. Roughly 190,000 heat-related deaths are avoided each year because of it. In hot climates, air conditioners boost productivity, and historians consider them a major force behind postwar urban growth, alongside highways, cars, shopping centers, and suburban homes. As of 2022, air conditioning accounted for about 7% of the world’s electricity use and produced 3% of greenhouse gas emissions. Most air conditioners rely on vapor-compression refrigeration, and they come in sizes ranging from small units for vehicles or single rooms to huge systems that cool entire buildings. Air source heat pumps, which can both heat and cool, are becoming more popular in cooler regions. According to the International Energy Agency, 1.6 billion air conditioning units were in use worldwide in 2016. The United Nations has urged that the technology be made more sustainable to fight climate change, and that alternatives like passive cooling, evaporative cooling, selective shading, windcatchers, and thermal insulation be adopted.
**History** Air conditioning has ancient roots. In the prehistoric city of Hamoukar, in modern Syria, people built double-walled living spaces with a gap between the walls to encourage airflow. Ancient Egyptians also used a range of passive cooling techniques. These methods spread from the Iberian Peninsula across North Africa, the Middle East, and into Northern India. Passive cooling stayed common until the 20th century, when it fell out of favor and was replaced by powered air conditioning. Today, based on engineering studies of traditional buildings, these passive techniques are being revived and adapted for 21st-century architecture. Air conditioners let indoor environments stay fairly constant, regardless of outside weather or internal heat, and they made deep-plan buildings possible, allowing people to live comfortably in hotter parts of the world.
**Development** *Preceding discoveries* In 1558, Giambattista della Porta wrote in his science book *Natural Magic* about chilling ice below its freezing point by mixing it with potassium nitrate (then called "nitre"). In 1620, Cornelis Drebbel demonstrated "Turning Summer into Winter" for King James I of England, cooling part of Westminster Abbey’s Great Hall using a system of troughs and vats. Francis Bacon, a contemporary of Drebbel, described it later that year as an "experiment of artificial freezing," noting that nitre or salt added to snow or ice makes it colder. In 1758, Benjamin Franklin and Cambridge chemistry professor John Hadley experimented with evaporation to rapidly cool an object. Using a mercury thermometer bulb, they applied alcohol and ether, speeding evaporation with a bellows, and dropped the bulb’s temperature to −14 °C (7 °F) while the room was 18 °C (64 °F). Franklin observed a thin ice film form on the bulb at freezing point, and the ice grew to about 6 mm thick by the experiment’s end. He concluded that it might be possible to freeze a person to death on a warm summer day. The 19th century saw advances in compression technology. In 1820, Michael Faraday discovered that compressing and liquefying ammonia could chill air when the liquid ammonia evaporated. In 1842, Florida doctor John Gorrie used a compressor to make ice, then used that ice to cool air for his hospital patients in Apalachicola, Florida. He hoped to build a machine that could regulate building temperatures and even envisioned centralized air conditioning for entire cities. Gorrie received a patent in 1851, but after his main backer died, he couldn’t bring his invention to market. In 1851, James Harrison built the first mechanical ice-making machine in Geelong, Australia, and got a patent in 1855 for an ether vapor-compression system that produced three tons of ice daily. By 1860, he had started a second ice company and later argued against American advantages in selling ice-refrigerated beef to the United Kingdom.
*First devices* While early cooling systems typically...
Lore & Background
Air conditioning dates back to prehistory. Double-walled living quarters with a gap between the two walls to encourage air flow were found in the Bronze Age city of Hamoukar, in modern Syria, though their identification as passive cooling is not definitive. Ancient Egyptian buildings also used a wide variety of passive air-conditioning techniques, which became widespread from the Iberian Peninsula through North Africa, the Middle East, and Northern India. Passive techniques remained widespread until the 20th century when they fell out of fashion and were replaced by powered air conditioning. Using information from engineering studies of traditional buildings, passive techniques are being revived and modified for 21st-century architectural designs.
Reader's Guide
Air conditioning has profoundly shaped modern life. By enabling comfortable indoor environments regardless of external weather, it allowed deep plan buildings and made hotter parts of the world more livable. Historians rank it alongside highways, automobiles, shopping malls, and suburban housing as a key factor in postwar metropolitan growth. However, the technology carries environmental costs. As of 2022, air conditioning used about 7% of global electricity and emitted 3% of greenhouse gas. The United Nations has called for more sustainable technology, including passive cooling, evaporative cooling, selective shading, windcatchers, and thermal insulation. The International Energy Agency reported 1.6 billion air conditioning units were used globally in 2016. The development from passive techniques in ancient cities to powered systems in the 20th century, and now a revival of passive methods for 21st-century architecture, illustrates a continuous evolution. The work of early inventors like Willis Carrier, who formed The Carrier Air Conditioning Company of America, established the industry that today employs tens of thousands and is valued in the billions. The legacy of air conditioning is a balance between human comfort and health on one hand, and environmental sustainability on the other.
From Ammonia to Solar Thermal – The Long Arc of Absorption Cooling
In the late 1800s, engineers tackling the challenge of mechanical cooling relied on a mixture of ammonia and water as the working fluid in absorption systems. The basic principle was elegant: heat one end of a network of expansion and condensation pipes, and the opposite end would chill enough to form ice. At that time, natural gas served as the thermal driver. Over the decades, the chemistry evolved—lithium bromide paired with water became another widely adopted combination—while the heat source shifted as well. Propane now powers the absorption chillers found in recreational vehicles, and hot-water solar thermal collectors have emerged as what some describe as a free-energy heat input for modern installations. This trajectory, from fossil-fueled heating to sun-driven thermal input, represents a quiet but significant reorientation of how absorption cooling sources its energy.
The Grid-Tied Dilemma and the Rise of Direct Solar Cooling
For many years, the simplest way to run an air conditioner on solar was to install whole-building photovoltaic panels, invert the direct current to alternating current, and let the standard AC unit draw power as if nothing were different. The appeal was straightforward: no special electronics were needed on the cooling equipment itself. Yet the arrangement carried hidden costs. Most of these units topped out around a SEER of 14, the DC-to-AC conversion introduced efficiency losses before the electricity even reached the compressor, and the entire setup became dependent on grid availability. When a storm knocked out utility power, net-metered homes could not feed electricity back into a dead grid, leaving the air conditioner dark. Off-grid battery banks solved the outage problem but reintroduced conversion losses. The arrival of appliances that natively accept DC power made the old invert-then-rectify loop look increasingly wasteful, pushing the industry toward units that consume solar electricity in its native form.
DC-Powered Mini Splits and the Non-Compressor Frontier
A new generation of photovoltaic air-conditioning heat pumps now runs entirely on direct current, eliminating the conversion steps that plagued earlier designs. One mini-split variant taps a 48-volt DC bus and a small battery array—typically four 12-volt cells in series—dedicated solely to the cooling unit, allowing it to keep running at night or under cloud cover. Both employ variable refrigerant flow technology with high-efficiency variable-speed DC motors and compressors, demanding very little run power while also delivering heating. For larger commercial buildings, a third VRF-based configuration adds grid and battery backup plus optional net metering, and can simultaneously heat one zone and cool another from a single outdoor unit. Beyond compressors entirely, phase-change indirect evaporative coolers now achieve SEER ratings above 20 and up to 40 in dry climates, needing only a circulation fan and a water supply, as demonstrated at facilities like McCarran Airport in Las Vegas.
Policy Catalysts and the Economics of Sun-Powered Cooling
Government action has played a formative role in advancing solar cooling. The U.S. Earlier, a 1976 NASA-sponsored survey catalogued solar applications for cooling, covering absorption cycles, Rankine-cycle heat engines, and heat-pump approaches, and compiled an extensive bibliography. On the economics side, state-level photovoltaic subsidies in the range of $2.50 to $5.00 per watt can push the amortized cost of solar-generated electricity below $0.15 per kilowatt-hour, making the investment competitive in regions where utility rates have already crossed that threshold. Pairing photovoltaics with geothermal heat pumps further shrinks the required solar array: a high-quality geothermal installation can reach a SEER near 20, meaning a 29-kilowatt cooling load would draw less than 5 kilowatts while operating, dramatically reducing the panel capacity needed.
Frequently Asked Questions
Who is Air conditioning?
Air conditioning (commonly shortened to A/C or air con) is the HVAC process of extracting heat from a sealed indoor space so the interior stays at a comfortable temperature, and in many setups it also regulates humidity levels.
What are Air conditioning's powers and role?
Its core job is to pull thermal energy out of an enclosed area, whether through a mechanical compressor unit, passive cooling, or ventilative cooling. It sits squarely within the broader heating, ventilation, and air conditioning (HVAC) family of climate-control systems.
How does Air conditioning differ from a heat pump?
The two systems share much of the same refrigerant-cycle hardware, but a heat pump adds a reversing valve that lets it flip between heating and cooling modes, whereas a standard air conditioner is dedicated to removing heat from the space.
Why is Air conditioning important?
Beyond everyday comfort, it serves a genuine health function in extreme heat by lowering the risk of heat stroke and dehydration for people inside the conditioned space.
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