Thermoelectric (Seebeck) Power generation modules have a large thermal expansion (and contraction) through the range of allowable temperatures -60ºC to 300ºC. It is vitally important that the mounting system allow for this expansion and contraction while maintaining an even pressure on the module. Please see TEG Mounting.
Hot and Cold Side Identification:
Thermoelectric (Seebeck devices or TEGs) generators will only generate electricity if there is a temperature difference across the module. That means that the “cold” side must be at least colder than the “hot” side for there to be any power generation. The hot side is generally attached to a source of heat while the cold side is typically connected to a heat sink that is air or liquid cooled (water block).
NOTE: Keep in mind that the heat from the hot side must travel through the TEG to the cold side in order for there to be electricity generated. It is vitally important that the heat sink on the cold is able to quickly get rid of this heat in order to stay cool. If the cold side cannot get rid of this heat then it will not be the cold side for very long. Use of a Water Block or liquid heat exchanger will significantly increase heat dissipation thereby increasing power generation.
Examine the diagram below. The view is seen as if the module is placed on a table in front of you as shown with the wires pointing towards you.
The Cold Side of the TEG module can work continuously at a maximum temperature of 160ºC (320ºF). Do not allow the cold side temperature to exceed this limit or irreversible TEG damage and failure will occur.
The Hot Side of the TEG module can work continuously at a maximum temperature of 300ºC (572ºF). It is important that you only connect the hot side of the TEG module to the heat source. Accidently reversing the module and placing the cold side towards the heat source will cause irreversible TEG damage and failure.
A Microscopic Look at Surfaces
Even when you have two “flat and smooth” surfaces, they are far from truly flat or smooth. The diagram below shows what’s really going on at a microscopic scale.
As you can see, the two surfaces may look flat and smooth, but in reality, when examined under magnification, they consist of “hills”, “peaks”, and “valleys”. When these two surfaces are brought into contact with one another, only the peaks make contact. It has been calculated that the average amount of contact between any two smooth surfaces is in reality only 5%. The other 95% are voids!
The above image shows how the remaining valleys create voids through which heat energy can barely pass through, in effect creating an insulated area – not the ideal thermal interface.
Surface Finish and Preparation
As a minimum, any surface intended to be part of a thermal interface should be flat to +/- 0.001 inches over the entire interface surface and smooth to a surface finish of 32 micro inches or better.
If possible, a polished surface is better since it ultimately reduces the size and depth of the “peaks” and “valleys”. A polished surface finish of #6 to #8 is good, but the economics of polishing may determine a different stopping point for your product. If polishing is used, make sure to remove all polishing residues – see cleaning below.
The interface surface must be thoroughly cleaned, once all machining and polishing is completed. Ultrasonic cleaning first in a heated degreaser solution and then in a solvent such as acetone is highly recommended to remove all oils and debris. (Never place a TEG or Peltier device in an ultrasonic cleaner!) Do not touch the surface with bare hands (skin oils) or allow any contact with other materials. It is better to complete this final cleaning stage just before assembly so as to minimize any dust or contamination.
Thermal Interface Materials (TIM)
A Graphite Foil TIM has been pre-applied to both sides of your TEG. (It is dark grey in color) The purpose of this TIM is to fill the valleys and gaps with a compressible material that has a much higher thermal conductivity (ability to transfer heat) than the air gaps it replaces. This essentially makes the entire interface transfer heat instead of just where the peaks were contacting. The following image shows how the situation has been dramatically improved.
Notes about the Graphite Foil TIM
The graphite foil TIM has been pre-applied to both sides of your TEG. Handle the TEG by its edges and try NOT to handle it by touching the graphite foil surfaces. The graphite foil is actually fragile and can easily be damaged. Grooves, scratches, dents in the graphite foil will decrease its ability to conduct heat. DO NOT apply any other TIM (thermal grease or compounds) to the graphite foil. It is ready to use in your power generating assembly.
TEGs should be mounted using the compression method. That is, the TEG is compressed between a hot plate and a heat sink (or water block) that will be cooler. The compression or clamping should be created with stainless steel machine screws on either side of the TEG. See Exploded View and Section View images below.