Last week’s article on cooling got away from me a little bit. I had actually intended to discuss a very interesting, lesser known cooling method but it was necessary to lay the groundwork of how temperature works. That’ll make this article a bit shorter and to the point.
If you want to cool a large area, like a server room, a conventional air conditioner will always be your best bet. For the job, they are the most efficient. For the purposes of this article, we’re going to refer to air conditioners by a more technically accurate name: heat pumps. This is what air conditioners are actually doing. They are pumping heat from one area to another. Equipment in our server room generates heat, and the heat pump effectively absorbs that heat and pumps it outside. The heat in the server room heats up the cool decompressed gas and then this warmed, decompressed gas is directed outside, where its acquired heat will be dumped by a heat sink. Heat pumps can also work in reverse to warm an area. If our server room was an iguana habitat, a heat pump would take the small amount of heat present in cool, outside air, and compress the gas causing it to heat up, and then vent this heat, via a heat sink, into our iguana habitat.
Generally speaking, any cooling mechanism has a heating component to it. This type of heating/cooling all works by moving heat energy around. Other types of heating and cooling are called exothermic and endothermic chemical reactions, respectively. An exothermic reaction converts potential chemical energy into heat energy. An endothermic reaction requires more energy input (in the form of ambient heat) so the result is the reactants become cool because they’re absorbing the heat. The applications for this type of heating and cooling are limited, but specific. For example, chemical hand warmers that heat up when you break them, or ice packs that cool when you break them.
What if you had a small area, or even a small device that you needed to precisely cool, to within fractions of a degree? Chemical reactions and compressed gasses cannot be controlled precisely enough for this task. Enter the solid state Peltier effect. Any time you have two different types of metals joined together, heat energy can be generated or removed at the junction when electric current is passed through it. This will work with any two metals, but for high performance, useable cooling, Peltier devices normally use P N semiconductor junctions, and there are often dozens of them all packed into single wafer. When current is applied, one side of the wafer will become hot, and the other side will become cool. If left this way, the device can eventually self-consume. However, if you place heat sinks on both sides, the heat being “pumped” to one side of the wafer can be dumped. A heat sink on the cool side will allow for better cooling by providing more surface area for heat energy to pass through the device, to the hot side. Remember, heat energy is everywhere. What we call “cool” is just less heat. Unless “cool” is absolute zero, there’s still heat energy there.
Because these junctions can be manufactured at virtually any size, and current through them precisely controlled, they can create extremely precise temperatures. They have no moving parts, no refrigerant gas, compact and flexible shapes, can be used in almost any environment and have far, far longer service life than a refrigerant-based heat pump. Their disadvantages are that they are not very energy efficient in terms of cooling/heating performance vs a refrigerant-based system and they are unable to create large temperature differentials. A typical device will only be able to create a temperature difference of 70 degrees celsius between the hot and cool side.
Disadvantages aside, these Peltier coolers have many applications. Any small device (even USB-powered) that advertises cooling ability, will be a Peltier device. Small 12V coolers for vehicles often use Peltier elements. Spacecraft use them to reduce temperature differences on the spacecraft itself by pumping the heat generated by exposure to the sun to the shaded side of the craft. Some lasers use the coolers as part of the laser circuitry to maintain a precise temperature to stabilize the laser wavelength. Sometimes the coolers are even used as active (as opposed to passive) cooling devices for computer equipment.
I think this about wraps up most of the heating/cooling methods I’m aware of. If there are any that I’ve forgotten about, or anything else you’d like to know about, send me an email.