Chilly chemical properties

Because it's the middle of winter here in Canada, I think today is a good day to talk about refrigeration.

Just kidding. Actually it's because the ISS had to replace a piece of its refrigeration system last week, and I thought that was a good excuse to talk about refrigeration.

Most modern refrigeration involves the chemical property \(\Delta H_{vap}\), or enthalpy (heat) of vaporization. Every substance has a heat of vaporization, and the amount of heat energy required to vaporize a substance is independent of what temperature the substance boils at. To choose a rather extreme contrast, water boils at 100C while lead boils at 1750C, but water requires 539cal/g to convert from liquid to gas while lead only needs 208cal/g, less than half that required by water. This amount of heat does not account for how much is required to get to the boiling point, and if you remember your high school chemistry, the temperature does not change with the additional heat input while it changes from liquid to gas.

The basic principle in use here is that when a substance evaporates, it draws heat energy from its surroundings (or the more familiar form: when you add heat to a substance, it will evaporate), and when a substance condenses, it releases heat energy back to its surroundings. Put an insulated barrier between these two sides of the process, and you have refrigerators, freezers, and air conditioners which get colder "inside" and warmer "outside".

Easily the most common is the refrigeration cycle powered by an electric compressor. This is in your refrigerator and also in the ISS. (Note that there are more components in the system, such as controls and safety systems. I'm only discussing principles, not construction.)

The compressor causes the refrigerant to condense by raising the pressure, then the condensed liquid is given a chance to shed its heat. Remember how old fridges had their entire back side covered with tubes that went back and forth, and the cupboard above the fridge was constantly warm? The ISS has a similar construct that allows the warm ammonia to shed heat outside of the station. Because the ISS is in a near-vacuum, it can't rely on convection to carry the heat away (to the cupboard above the fridge) but can only radiate heat, which is a much slower process. The cooled, condensed liquid is then released back into the cool side where it can evaporate again, and while doing so draw more heat from the cool side.

Both ammonia refrigeration (which is being used in the ISS, as well as many other large industrial scale refrigerators) and freon refrigeration (household refrigerators, freezers, and air conditioners) work the same way. Ammonia refrigeration is far more efficient due to its larger \(\Delta H_{vap}\), meaning one kg of ammonia soaks up a lot more heat while evaporating compared to one kg of freon, but ammonia is extremely irritating to the eyes and throat if there's a small leak, and a large leak can be quite dangerous. The various halocarbons known as Freon are less efficient, but don't have the acute toxicity to people nearby if there's a leak. (They are, however, being phased out due to long-term environmental effects which could not be measured when they were first developed.)

Somewhat less common but still using \(\Delta H_{vap}\) to gather heat is the "propane fridge", which cools the inside of the fridge by running a propane burner on the outside. This one confuses a lot of people because you're adding heat to the system via the propane burner, in order to cause the inside of the refrigerator to cool down. It's often used in situations where a propane tank is easier to come by than electric power: camping, boats, remote cabins, and other off-grid situations.

Instead of varying the pressure with a compressor to control whether the refrigerant (ammonia) is liquid or gas, the entire system is pressurized, and a few other interesting physical properties are used.

Cooled liquid ammonia from the condenser flows into the evaporator, as with conventional refrigeration, but instead of lowering the pressure to evaporate the ammonia, the gas side of the evaporator is connected to an absorber outside of the refrigerator, where ammonia gas from the evaporator is absorbed into the water. Hydrogen, there to balance the system pressure, is not absorbed and returns to the evaporator. This runs constantly, because as ammonia is absorbed into the water, there is a lower partial pressure of ammonia in the absorber; gases tend to equalize pressure in connected spaces, so more ammonia gas flows toward the absorber from the evaporator, and more ammonia evaporates to replace what flowed out. The hydrogen runs in a circle. (Green arrows.) No fans or pumps are required here, because the ammonia partial pressure difference between evaporator and absorber is what causes the gas circulation. If the water were to become saturated with ammonia, then the ammonia partial pressure would equalize, ammonia would stop evaporating, and the inside of the refrigerator would stop being cooled.

To make sure this saturation and equalization doesn't happen, the ammonia-rich water from the condenser flows to the heater, where ammonia (and some water vapour) is boiled off. The water and ammonia are separated in the separator, and the water with very little ammonia is returned to the absorber. (Blue arrows.) This keeps the concentration of ammonia in the absorber water below saturation, so the hydrogen loop, and thus the cooling, keeps running as long as heat is applied.

Dry, warm ammonia gas is then sent to the condenser, where it sheds its heat and condenses to liquid before flowing back to the evaporator.

The third type of refrigeration I'm going to talk about is why I wrote at the top of the post that most refrigeration involves \(\Delta H_{vap}\): this one is purely electrical, and uses the Peltier effect.

This type of refrigeration is substantially less efficient than either of the types described above, but it has advantages in some applications—a big advantage is that Peltier coolers can be made very small.

Peltier effect coolers (and heaters!) are basically the opposite of a thermocouple, much as an electric generator and an electric motor are opposites. In this case, instead of turning motion to electricity or electricity to motion, a thermocouple or a thermoelectric generator turns heat into electricity and a Peltier effect cooler turns electricity into heat or cold. (This is distinct from the heat generated by running electricity through any wire: that's not reversible.) Both of them depend on the point where two different metals come into contact. If that junction is heated, a current is developed in one direction; in thermocouples it's a single junction and a tiny bit of electricity used for measurement, and in thermoelectric generators it is many junctions and enough to run entire spacecraft.

A Peltier effect cooler is when electricity is applied to a junction between two different metals. Depending on which way the electrons are flowing, the junction will get either hot or cold; the second junction where the circuit is completed will go the opposite way. If used in refrigeration, there would be insulation between the hot end and the cold end.

Yes, this is one case where Star Trek's famous "reverse the polarity!" pseudoscience actually does something useful: for example, a camp cooler with a Peltier effect cooler built in can actually be used as a "keep warm" container by reversing the polarity of the power source—which is to say, putting the battery in backwards.