How to build a thermoelectric system
Though putting together a thermoelectric device shares some of the same semiconductor processes used in the IT industry, there are in fact far fewer steps involved. Nonetheless, it is not a straightforward task, and the key stages are summarised below.
Step One: Base Materials
The thermoelectric effect was first discovered using base metals, but all current thermoelectric devices use compounds, with the most common being bismuth telluride, lead telluride and silicon germanium. The component elements of these compounds come in a range of forms such as powders, rods, strips, balls, and lumps, all of which may come at varying levels of purity.
Step Two: Ingot
As a compound is required, the elements need to be mixed together to form a single substance. This is most commonly done by using a ball mill and “flat pressing” the material, or by using a crystal growth method similar to those used in silicon production, such as the Bridgeman technique which involves heating the material above its melting point and slowly cooling it.
Step Three: Chips
Once a solid alloy has been formed, the material then needs to be sliced into wafers and cut into the basic units to be used in the thermoelectric device – at MicroPower we call them chips, though others refer to them as dice, legs, diodes, or blocks. Two types of semiconductor are required for thermoelectric devices: p-type (positively charged) and n-type (negatively charged).
Step Four: Module
To produce usable electricity, p and n-type chips need to be electrically connected in series via metal conductors while being configured thermally in parallel, all sandwiched between a non-electrically conducting hot and cold plates. This type of circuit is known as a thermoelectric module, the efficiency of which is significantly affected by the geometric design.
Step Five: System
For a thermoelectric system, you may need one, or many modules, as well as electrical connections and converters to deliver the electricity produced in the required form, and heat exchangers, both to deliver the available heat to the hot side of the modules, and move heat away from the cold side – this is usually done by ensuring cold air or water passes over this part of the system.