We’re all familiar with electrical heating, but cooling by electricity? Here’s a primer to get you up to speed.
The Thermoelectric Effect
Most engineers are familiar with the thermocouple. A pair of dissimilar electrical conductors are arranged in parallel and joined at each end. When the temperature of one junction is raised relative to the other, a voltage is produced.
What’s less well-known is that the effect is reversible. Applying a DC voltage can cool one junction and warm the other. Interestingly, if the voltage is reversed so the current flows in the opposite direction, the heat flows the other way.
This phenomenon was discovered in 1834 by retired French watchmaker Jean Peltier. It is known today as the Peltier Effect.
Peltier Cooling in the Semi-Conductor Age
The basic principle of thermoelectricity is that a temperature gradient makes charge carriers move through the conducting material. Conversely, when an external voltage moves the charge carriers, they carry heat with them and create a temperature gradient.
This effect can be magnified by using semiconductor materials that have been N or P doped, (similar to the doping in transistors,) to encourage electrons to move in a particular direction. In a thermoelectric cooler, pairs of N and P doped Bismuth Telluride pellets are arranged to be electrically in series but thermally parallel.
A thermoelectric cooler module looks like a metallic square less than 1/4” thick but 2” or more along the sides. (The size is determined by how much cooling is needed.) Inside, the Bismuth Telluride pellets are arranged in a ceramic matrix, so when current is flowing, one large flat face cools down while the opposite face warms up.
Heat always flows from hot to cold, so to lower the temperature in a transit case the cold face of the thermoelectric cooler is placed on the inside, the hot face then being outside. The thermal energy of the warm air in the case then flows into the cold face and is pumped across to the other face, where it is removed by convection.
To improve the performance of the system, heat sinks are mounted to both the hot and cold faces. Typically, fans are arranged to blow air over these sinks, keeping the whole assembly compact.
And that is how electricity is used for cooling. To learn more, be sure to read Part 1: Thermoelectric Cooling for Transit Cases.