by Chris Woodford. Last updated: July 1, 2020.
What's the best way to cool down your kitchen on a hot summer's
day? If your immediate answer is "Open the refrigerator door," you're way off target.
Every bit of heat a refrigerator sucks in
through its cool box is pumped straight out of
the metal fins at the back. If anything, because of the sheer
inefficiency of the machine, you'll make the room even hotter.
But using a refrigerator to cool a home isn't such a mad idea as
it might seem: with a few slight modifications, it's almost exactly
how an air conditioner works. Let's take a closer look!
Photo: A typical air conditioner unit outside a restaurant. This is the fan that
blows away the hot air. There's another fan you can't see, circulating cool air inside the building. Most
air conditioners are permanently fixed in one place, but you can get small portable air-conditioning units too.
How not to cool your kitchen
Artwork: Physics tells us you can't cool your kitchen by leaving the refrigerator door open,
because the heat energy "sucked" into the chiller cabinet is simply pumped out again through the cooling fins at the back.
A basic law of physics called the conservation of energy
says you can't make or destroy energy:
if you have some energy you don't want (such as heat in your kitchen),
you can't get rid of it completely. All you can do is change it into another form or move it to another place. If you
open your refrigerator door in the hope that you'll cool the kitchen,
all the heat that gets drawn in has to go somewhere else. The only
place it can go is out of the back of the machine. You may have
noticed that the grid of fins on the back of a refrigerator gets
pretty hot—and that's why: they're giving off all the heat that
would normally be inside. You can find out more in our article on
how refrigerators work.
How to build an air conditioner
But all's not lost! Instead of letting the power of science defeat us,
we just have to use it the right way.
Suppose you take a refrigerator and build your house around it, so
half the machine (the chiller cabinet) is inside your home and the
other half (the grid of hot fins at the back) is outside. Now if you
leave the door open, what you have in effect is a fully fledged air
conditioner. It draws in heat from inside your home and belches it
out again outside, gradually cooling your home in the process.
The simplest air conditioner units work in almost exactly this way, except they have
fans on both sides to circulate air more rapidly. They also have a
heating element in them so they can warm the air in a room on cold
days as well as cool it down on warm days. Machines like this are
sometimes called HVACs (heating and ventilation air
conditioning units). More elaborate air conditioners use long ducts to pipe
the warmed or cooled air throughout an entire building, but they
still work in essentially the same way.
Air conditioners in cars
Car air conditioners work much the same way as home and office ones, only they're a lot smaller. The chiller
part (which incorporates an expansion valve and an evaporator) is mounted behind the car's dashboard, the heat dissipater (incorporating a compressor unit and a condenser) is fitted near the car's radiator grille (where air blows past as you drive along), and the two things are connected by a circuit of pipes through which coolant flows when the air conditioning is switched on. Unlike with a static unit in a building, which is completely powered by electricity, the compressor unit in a car is powered by the crankshaft (driven by the engine, in other words). Usually there's a heater (so the temperature of the passenger compartment can be adjusted) and a dehumidifer (sometimes called a receiver/dryer unit) as well. Just like in normal air conditioning, the coolant cycles between gas and liquid, high and low pressure, and high and low temperature.
Artwork: How car aircon works (simplified).
1) The evaporator absorbs heat from the passenger compartment, which makes the coolant inside it boil and turn from a low-pressure liquid into a low-pressure gas.
2) The coolant flows out of the compartment into the compressor taking the heat with it. It enters the compressor as a low-pressure, relatively low-temperature gas.
3) The compressor squeezes the coolant so it becomes a high-pressure, high-temperature gas.
4) The coolant flows into the condenser and (surprise, surprise) condenses: it gives up its heat to the atmosphere, so it turns back into a high-pressure, low-temperature liquid and flows (via the receiver/dryer, not shown) into the expansion valve.
5) The expansion valve allows the coolant to expand into a low-pressure liquid.
6) The low-pressure liquid coolant enters the evaporator and the whole cycle repeats. Over time, heat is sucked from inside the passenger compartment to the air outside, cooling the car down.
How air conditioners can harm the environment
You probably love the feel of freshly chilled air on a hot day, but don't forget
that law called the conservation of energy! There's always a price to pay for getting something good in our universe. In this case, the price is the energy you have to use to run the air conditioning unit; using energy means there's an impact on your pocket and on the planet too in the shape of environmental problems like global warming. Environmentalists say we should use less air-conditioning, which sounds easier than it is in a really hot climate. It's important to remember that air-conditioning isn't just about luxury or comfort: an air-conditioned room can make you much more productive at work and it can have important health benefits too; some
public-health doctors have suggested that the greater use of air conditioning in the United States is one reason why there are fewer heat-related deaths there than in Europe, where air conditioning is used less. It's sometimes argued that if people don't have air conditioning, they're more likely to use things like electric fans, which work very inefficiently (rearranging hot air instead of removing it and generating heat with their own electric motors). But the biggest electric desk fans (typically rated 25–50 watts) use a fraction as much electricity as the smallest air conditioners (typically rated at 750–1000 watts); you could use about 20–30 fans and consume the same or less power than a compact AC unit!
Photo: The ozone hole over Antarctica that was caused by CFC pollution, mostly from air conditioners,
refrigerators, and aerosols. Picture courtesy of NASA on the Commons.
So what's the environmental damage? Let's consider the energy first. Every time you switch on the air conditioner in your car, you add an extra 10–20 percent to your fuel consumption (and an extra 10–20 percent to the price you pay at the gas station). At low speeds, opening a window instead is often a better option, though at higher speeds you create air resistance (drag) and waste more energy than you save. At home, using the air conditioner will add plenty to your electricity bill; when
physicist Tom Murphy tested his air conditioning scientifically, he found he used "more electrical energy in two days than we normally expend in a month." You could try other strategies like opening your windows all night but shutting them tight first thing in the morning and throughout the daytime to keep hot air out of your home. In really hot climates, you might find you simply cannot do without the AC; even so, you can dramatically reduce how much it's costing you (and how much energy you're using) simply by turning the thermostat to a slightly higher setting.
Air conditioning units used to have another very harmful effect on the environment as well. Until the late 20th century, most used coolant chemicals known as chlorofluorocarbons (CFCs) (so called because they are made from
the chemicals chlorine, fluorine, and carbon), which were also used widely in refrigerators.
When old air conditioners and refrigerators were broken apart for scrap at the end of their lives, the coolant chemicals
escaped into the atmosphere. Floating up into the stratosphere (the upper atmosphere), they rapidly damaged Earth's ozone layer: the natural sunscreen that helps to protect us from the Sun's harmful ultra-violet rays. Most modern air conditioners avoid CFCs (now banned in many countries under a global agreement called the Montreal Protocol) and use alternative coolant chemicals instead (typically halogenated chlorofluorocarbons or HCFCs). If you look closely at our top photo, you can see that the fan has a green "Ozone friendly" label on it, which means there are no CFC coolants inside.
Here to stay?
Love it or loathe it, we won't be getting rid of our air conditioners anytime soon;
in the United States, for example, all the trends are pointing the other way.
According to a 2009 survey by the US Energy Information Administration (EIA), 87 percent of US households now have air conditioning, with a dramatic increase in every region of the country since 1980. Changing expectations have helped to drive that trend:
around 90 percent of new homes are now fitted with AC. Affluent homes are more likely to use centralized air conditioning systems that cool the entire building; poorer homes rely on smaller, room-based air conditioning units fitted to windows or
walls. Although centralized systems are overwhelmingly the most popular in the South, Midwest, and West of the country, room-based units are still significantly more popular in the colder Northeast. Not surprisingly, a
2017 study by the EIA
found that residential air conditioning consumes 18 percent of US household electricity and causes a dramatic increase in
consumption during the summer months.
Who invented air conditioners?
Photo: One of Willis Carrier's air conditioner designs. This diagram is part of Carrier's US patent #675,144, filed in 1933 and reissued in 1941, which you'll find among the references below. Picture courtesy of US Patent and Trademark Office.
If you couldn't live without your air-con, thank Willis Carrier (1876–1950). He was the man who pioneered this "cool stuff" in the early decades of the 20th century. Here's one of his early designs—and note how closely it resembles my quick sketch up above. How does it work? Warm air is pulled in from a room (1), mixed with fresh air (2), conditioned, and blown back into the room by a fan (3). Heat is removed by the refrigerator chiller pipes in the center of the duct (4), which are fed and controlled by a system of pumps, compressors, valves, and thermostats (5).
Evaporative air coolers
If you're turned off air conditioning by the thought of expensive
electric bills and harming the planet, air coolers that work by
evaporation are another option to consider.
Evaporative cooling might sound complex, but it's pretty familiar
to all of us. Dogs keep cool without air conditioners just by
sticking their tongues out and panting; hot runners use a similar
trick, sweating profusely to shed heat from their bodies. When liquid
water evaporates and turns to water vapor, it absorbs heat, known as
the latent heat of evaporation, which it has to remove from something
nearby (a panting dog or a sweating athlete, perhaps). Putting this
science to practical use, we can use evaporation to remove the heat
from a room providing we have a handy supply of water nearby.
Portable air coolers (sometimes known as evaporative air coolers
or "swamp coolers") look a bit like air conditioners on wheels
but work in a very different way. Where an air conditioner works
like a fridge, expanding and then compressing a coolant chemical to
shift heat from inside a building to outside, an air cooler sucks in
hot air, passes it through or near to water to cool it down, and then
blows it back out into the room. There are two subtly different types
of air cooler:
- One of them passes the air through a kind of
water-filled "sponge," evaporating droplets of water into the
air, cooling it down, and making it more humid at the same time. This
is called direct evaporation because the air and water
meet—they exchange heat by coming into direct contact.
- In a slightly different setup, the incoming air blows through a heat
exchanger with cool water circulating in the opposite direction.
Here, the water and air don't come into contact, which is why this is
called indirect evaporation. Indirect evaporation involves
moving two streams of fluid instead of one, which needs an extra fan/pump,
so it tends to use more energy (electricity).
Artwork: How an evaporative cooler works. 1) Hot, dry air is sucked in through a grille.
2) The air passes through a pad soaked with cool water. 3) Some of the water evaporates, cooling the air and making
it more humid. A fan blows the cooled, humid air back into the room. 4) More water is added to the pad by an internal tank
that periodically needs refilling.
Air coolers can be much cheaper to run than air conditioners, but
they don't cool as dramatically. On the plus side, they are much more
portable; on the minus, they have internal tanks that need
periodically refilling with water (and ice, if you wish, to improve
performance) or permanently hooking up to a water supply with a
length of garden hose. They generally work best in hot, dry climates
where the humidity is fairly low (less than about 60 percent),
because lower humidity means more effective evaporation and cooling.
Unlike with air conditioners, which work best when you keep the doors
and windows closed, air coolers need to be placed in a good air flow
near an open window (where dry fresh air comes in) and with an open door
(for moist exhaust air to flow out). That makes sense if you think about it:
the water you're adding to the airflow is "soaking" up heat from the room, and if you constantly expel
moist air while allowing dry air to enter in its place, you're continually
removing heat. Air coolers that work by direct evaporation (adding water) can also be used as humidifiers though,
in that case, the doors and windows do need to be kept closed to allow the humidity to increase.