How Air conditioning works

Air Conditioning

We’ve just had air conditioning installed. Not too good for the environment, but with the heat waves hitting the Netherlands the past few years, well, I need to be able to sleep as well. I did some research into the matter, so let me share something about how air conditioning works.

The second law of thermodynamics

One of the most important properties of the universe, is that of entropy. Basically, it boils down to ‘things do not organize themselves’. This means, molecules don’t group themselves into a structure, atoms don’t align by themselves, and heat does not flow from a colder to a warmer area by itself. This last bit is crucial. Let me repeat it: heat does not flow from a colder to a warmer area by itself. This is the second law of thermodynamics.

At this point, you might shrug and say ‘fine, so what?’ The thing is, it is easy to forget about the effects in practice. Do you have a fan to blow cool air into your face? Well, the second law of thermodynamics says that the fan is actually just blowing air around while adding heat which is created by the fan motor. The reason it feels cold is because the air flow evaporates seat on your skin — your own personal air conditioning.

The same applies to refrigerators and portable air cooler. You can open up a refrigerator in a room, or place a portable air conditioner, but the second law of thermodynamics says it will only be pumping heat around, not making things cooler. No matter how efficient, they will also create heat, because of that same law.

Of course, there is a way around this: move the heat to some place else.

How a heat pump works

A heat exchanger is a device which exchanges heat with its environment. A heat pump is a device which uses two of these. One which absorbs heat, and one which sheds it. Connect the two, and you can absorb heat in one place, transport it to another, shed it there, then rinse and repeat. And that is how a heat pump essentially works.

So a heat pump is a device which can absorb heat in one place and expel it in another. Of course, that transportation is not 100% efficient. Extra heat will be generated by the system, courtesy of the second law of thermodynamics.

Still, it works perfectly well. A refrigerator transports heat from inside the refrigerator to the outside — the back, usually. The inside is cold, and keeps your beers and pizzas cold or frozen. A proper air condition transports heat from inside your house to the outside. That is why a proper portable air conditioning has a tube that has to be hung out of a window. It has to transport the heat somewhere for it to work.

Conditioning air the other way around

Modern air conditioners can pull heat out of your house pretty efficiently. However, no system is perfect. Moving heat around takes power, and that has an environmental impact. There really is no way around that, given it’s a law of nature.

That said, a little of the power drain is offset by what you can do in winter: reverse the flow. My shiny new air conditioning can pull heat from inside the house out, but also pull heat from outside the house in. Meaning, in winter, we can do very localized heating in a pretty efficient manner. Contrast that to central heating, which is another power drain.

Pulling heat from outside and depositing it inside is actually more environmentally friendly than using centralized heating. Centralized heating is just adding heat, not transporting it. This means that heating based on transporting heat can achieve a higher power to heat ratio. Efficient centralized heating can theoretically never convert more than 100% of its energy to heat, it can only add 1 watt of heat for 1 watt of power. By moving existing heat, a heat pump can achieve a higher ratio, for example add 2 watts of heat to your house with only 1 watt of power.

The environment

The above notwithstanding, anybody telling you air conditioning is good for the environment is a liar. I am a bit ashamed of having it installed, in fact, but not enough not to have it done. Heat waves are nothing to be laughed at, and the Earth is boiling.

In spite of what some might say, the second law of thermodynamics applies to our planet as well. All that fossil fuel we’re burning, that heat is here to stay. Top that with greenhouse gasses making more heat from the sun stick around, and well, we’re in pretty hot water as a planet.

Switching to renewable energy sources helps, because then we use the power of the sun — and wind, which is actually also powered by the sun — to move energy around, instead of expending energy stored in oil and wood.


The second law of thermodynamics. Bringing cool drinks and air conditioning to a home near you.

Martin Stellinga Written by:

I'm a science fiction and fantasy author/blogger from the Netherlands