Today, the high temperature for Baltimore was 37 Celsius, which is body temperature. Much of the US is experiencing a heat wave with many places in the south and southwest experiencing highs over 40 C. It's pretty clear that without air conditioning (which is a room refrigerator), life in the summer would be unbearable. I think the refrigerator is one of the greatest inventions in history. Now of course things like writing, the wheel, the printing press, electric lights, etc. may have had more impact on the world but I think pumping heat (which is what a refrigerator does) is the greatest because it required an extremely deep understanding of physics and math before it could happen. It would be very unlikely to accidentally discover how to pump heat.
Given the importance of refrigeration and air conditioning to modern life, I think it is imperative that everyone knows how they actually work. People probably know that it involves some sort of refrigerant that is pumped around. They may even vaguely remember learning about the Carnot cycle in their high school or college physics classes. However, my guess is that many people don't really get intuitively how a refrigerator works, unlike say a furnace, which they do get (i.e. it burns a fuel that makes hot air that is blown around the house, or it burns gas to heat water that is pumped around the house).
To make a refrigerator, there are a few things you need to know. The first is that there is this thing called energy that is conserved. This is known as the first law of thermodynamics. What is energy? Physicists and philosophers could debate that into the wee hours of the night but suffice to say it is a thing that can take many forms but cannot be destroyed. More importantly, energy can take the form of work (i.e. the ability to move something) or heat, which is a form of internal energy measured by temperature. The second thing you need to know is that heat flows from a higher temperature to a lower temperature. This is the second law of thermodynamics and has to do with entropy always increasing, which is a topic I will visit in the future.
The second law is less obvious than it seems. If you boil some water and leave it on the counter, it will cool down to room temperature. Heat flows out of the hot water and into the room. However, if you take a cup of cold water out of the fridge and leave it on the counter then it will warm up to room temperature. So, if you were not careful you would might believe things just flow to room temperature. The question is why did the cup get warmer but the room not get colder if heat flowed from the room to the cup. Likewise in the first example, the hot cup got colder but the room didn't seem to change either. Why? Well it is because the room is really big compared to the cup and how much the temperature changes when you add or subtract heat depends on how much stuff (called heat capacity) you have. Removing a cup's worth of heat from the room only changed the temperature by an imperceptible amount because the room has a lot of heat capacity. The excess energy was basically spread out over a large volume. Refrigeration is hard because you need to make heat flow from a cold area to a hotter area. The laws of thermodynamics, which are absolutely not obvious and one of the greatest human achievements, also explain why perpetual motion machines are not possible (another post perhaps).
The first two laws of thermodynamics still don't tell you how to make a heat pump. They only tell you that it requires work (energy). But what work do you need to do? Most heat pumps use some form of compression and expansion cycle because when you compress a gas/fluid (i.e. do work on it) the temperature goes up and conversely when you let a gas expand against something like a piston (the gas does work) and the temperature will go down. Because energy is conserved, when you do work on a gas that energy is converted into heat or "internal" energy, which is manifested by an increase in temperature. When the gas expands against something it must use some internal energy and the temperature will decrease. Finally, most gases will cool down just by moving from high pressure to lower pressure through a small opening (called a throttle). This is called the Joule-Thompson effect and occurs because the gas molecules have an attractive force for each other (Van der Waals forces) and thus expansion requires work to pull them further apart.
Now, we almost have all we need to build a refrigerator. As you can anticipate, we will use expanding gas to cool our fridge. One possible design is to have a container of compressed gas with a throttle attached so the gas cools as it is released and expands. Pump this cool gas into the fridge where it can absorb heat from the inside of the fridge (heat flows from the warmer interior of fridge into the colder gas and heats that gas). Pumping out the gas and compressing it makes it a full cycle. Energy is used to pump the gas around and to compress it. This is called a reverse gas or Brayton cycle. It uses energy to move heat from a cold place to a warm place. In the forward direction, the Brayton cycle is a heat engine (in fact it is a jet turbine) where you convert heat into work. The problem with the Brayton cycle is that it is very inefficient (or impossible) to use as a heat pump at room temperatures or lower. This is because you can only get so much cooling when you throttle a gas at low temperatures (modern freezers can go down to -40 C).
Thus, most modern fridges and air conditioners use a vapour-compression cycle, where a liquid/gas combination refrigerant is used. The principle is similar, you get cooling by expansion but the added twist is you don't just expand a gas but you let a liquid "boil" into gas when expanded. This will result in a much larger drop in temperature but requires finding a magical substance that is liquid at room temperature when under compression and then turns into gas when sent through a throttle. Freon, which is such a refrigerant, was invented in the mid-twentieth century and made refrigerators practical and affordable to the masses. However, Freon (and related CFCs) also destroyed the ozone layer that protects the earth from harmful UV rays and was internationally banned in 1994 in the Montreal Protocol. We should note that this beneficial agreement would not have happened so quickly if chemical companies hadn't already found replacements (and could thus enforce new patent monopolies since the old ones had expired). Regulation can happen when the interests of capitalists are aligned. Something to note when dealing with climate change.
There is one final trick that makes the modern fridge/AC more efficient and that is when you compress the gas back into liquid, it raises the temperature to much higher than room temperature and even higher than the temperature in Texas during a heat wave (although barely). This hot liquid is then cooled by releasing heat into the less hot environment and giving it a head start before it passes through the throttle. A commercial heat pump is an air conditioner that can move heat in both directions. When it is in air conditioning mode the refrigerant expands and cools when inside the house and is compressed outside of the house and when acting as a heater it does the opposite.
Finally, it is actually possible to build a refrigerator with no moving parts using what is called the thermoelectric effect (Peltier effect) where passing electricity across a junction of two different materials can cause one side to cool down and the other heat up due to a quantum mechanical effect, which I may explain later). Now that you know the laws of thermodynamics and how a refrigerator works, explain why you can't cool a room by leaving the fridge door open.
