How does an air conditioner actually work? - Anna Rothschild
Typically, with any piece of technology,
you get out what you put in.
Pump one unit of energy
into an electric toaster
and you get about one
out in the form of heat.
That’s just the first law
of thermodynamics:
energy has to be conserved.
But there's a piece of technology
called a heat pump,
where for every bit of energy you put in,
you get 3 to 5 times as much heat out.
What wizardry is this?
Heat pumps have been hailed
as a climate-friendly heating solution
to traditional heaters,
most of which operate
by burning fossil fuels.
So much so that in 2021,
heating buildings was responsible
for about 10%
of global energy-related CO2 emissions.
Heat pumps reduce emissions in two ways—
first, they run on electricity,
meaning less fossil fuel use as grids
make the switch to renewable energy.
And second, they're more efficient
than their counterparts,
using less energy to produce
the same amount of heat.
Where a typical oil or gas boiler is,
at best, about 90% efficient,
some heat pumps can achieve
500% efficiency.
Heat pumps rely on the same technology
as air conditioners.
And in fact, they often double
as air conditioners,
heating your home in the winter
and cooling it in the summer.
How? Air conditioners take heat
from your home and move it outside.
To do so, they harness the second law
of thermodynamics.
That’s the one that says that heat will
always move from a hotter object
to a colder one.
When you turn on your A/C,
a fan blows the hot air from your home
over coils
containing a substance
called a refrigerant.
A refrigerant’s molecules turn to gas
at relatively low temperatures,
so as it collects thermal energy
from the hot air in your home, it boils.
Then, it passes into a compressor,
which pushes the gas molecules
closer together,
heating them up even more.
Now that gas is hot—
way hotter than the outside air.
So when a fan blows over the refrigerant,
thermal energy transfers to the
comparatively cold air outside.
As the refrigerant releases heat,
it starts to liquefy.
It goes through an expansion valve,
which decreases the pressure,
causing it to get even colder.
Now, it’s ready to pick up more heat
from your house and start the cycle again.
In winter, heat pumps work
exactly the same way.
But this time they pick up heat
from outside and move it into your home.
Of course, it’s sometimes freezing outside
when you want to use your heater.
But the air doesn’t need to be warm—
it just needs to be warmer than the
refrigerant to transfer its heat.
All this sounds great, but for now there
are some drawbacks to this technology.
First, refrigerants can be
potent greenhouse gases.
Hydrofluorocarbons are some
of the most popular refrigerants.
But a single hydrofluorocarbon molecule
can have 2,000 times
the global warming impact of CO2.
While in use, the refrigerant stays
contained in a closed loop.
But when heat pumps, A/Cs, and
refrigerators are improperly installed
or thrown into landfills,
the refrigerant can leak out.
So scientists are trying to create
new refrigerants
that are better for the environment.
Also, the colder it is outside,
the less efficient an air-to-air
heat pump will be.
Nevertheless, over half the buildings
in icy Norway use heat pumps.
Some people there have opted for pumps
that draw heat from under the ground,
which stays more consistently warm,
rather than heat from the air.
Finally, there's the cost.
In the US, installing a small heat pump
usually costs several thousand dollars,
though some people need
more powerful systems,
depending on the size of their home
or the temperature in winter.
Often they’re only a little more expensive
than installing a new A/C system,
and the heat pump can save money
on utilities in the long run.
But replacing a working system requires
an upfront investment
that a lot of people just don’t have.
Still, as the risks
of climate change loom,
many countries are offering subsidies
to help with the costs.
And some cities are creatively harnessing
seas, sewage, and data centers
as heat sources,
using heat pumps in manufacturing,
and even creating giant heat pumps
for entire districts.
So, are heat pumps actually breaking
the first law of thermodynamics?
Of course not.
They’re just not using
their electricity to make heat.
They're using it to power the compressor
and spin the fans.
They get the extra energy for free—
from heat in the air or underground.
Which is how, by putting in 1 unit
of energy,
you get 3 to 5 units of heat out.
Seems like magic, but it's just physics.
