Thermoelectric devices are seen very often in various appliances. Small refrigerators, semiconductor chip coolers, medical chillers. Thermoelectric effect works in both directions – it can generate temperature difference when current flows or it can generate current when temperature difference is applied.
About thermoelectric effect
It is known for more than 100 years. There are several scientists who discovered this effect in one or another way. Probably you’ve heard Peltier effect. Jean Charles Athanase Peltier discovered that if you apply electrical current to junction of two materials it gets cold or hot (depending on current direction).
The reverse effect when electricity is generated due to temperature difference was discovered by Thomas Johann Seebeck. So this effect is Honored by his name. But for now lets focus on Peltier effect and thermoelectric devices.
The most common element is ceramic coated device with two wires popping out. They are made of semiconductor materials. One is N type and another is P. Instead of single Junction there are hundreds of them – simply speaking they are connected in series, but thermally they are parallel. Multiple joints increases the surface are and so the efficiency. If you look inside the Peltier element you will see a sandwich made of small silicon bricks electrically connected in series. The ceramic plate is used as carrier and heat transferee.
Each individual pair works same way, the heat is absorbed from one side of plate and transferred to another. If we go deeper in to working physics we can see that when current flows electrons in N-element are pushed against the current while holes in P element same direction as current. Both are mowing from one side of element absorbing heat. The heat is carried to another junction. Hope this is enough for understanding what happens there.
And again. If we apply different temperatures to both sides of plates we get electricity (Seebeck effect). It is not common usage as it’s efficiency is very low. Some space programs have been using those as alternative energy source.
So why thermoelectric coolers aren’t used widespread? It seems that they are beneficial in many ways including solidness (no moving parts), failure proof if used correctly and are easy to control. It appears that they aren’t as efficient we would like it to be. Compressor based systems are several times more efficient which keeps them in refrigerator business. Peltier elements can reach temperature difference between surface to about 70ºC. The bigger difference is the worse efficiency becomes. So it is used in compact coolers, in environmental friendly applications where efficiency isn’t the key. You’ve probably seen an USB powered cooler for soda cans – it wouldn’t be possible without thermoelectric coolers.
Peltier elements are pretty standardized devices and you can tell some features from its ID. For instance I have couple of TEC1-12705 elements what can be decoded in to;
TE – thermoelectric;
C – size. C – standard; S – small;
1 – number of stages. Normally 1.
127 – the number of P-N couples inside the plate;
05 – current rating. In this case 5A.
As all semiconductors it is sensitive to overheating. It is advised not to power Thermoelectric element without heat-sink on hot side. Probably you can connect it for a short time to determine which side cools and which heats.
Typical cooler application
If you are going to build a cooler, your typical setup should be as follows:
It is mandatory to put a heat-sink on a hot side of Peltier element which would take a transferred heat. For better transfer put thermal paste. It is good idea to have temperature sensor on heat-sink in order to avoid overheating. On cold side of plate you can put what ever you designed – a cold plate, another heat-sink with blower to get cool air or what ever you need. Here also you can put a temperature sensor to keep cold point. If hot heat-sink gets too hot maybe you need to put a fan on it. It depends on your application and other circumstances. Next time I hope to do more physical testing of thermoelectric cooler.