There are several problems with TE modules used in waste heat recovery applications. First, the TE modules themselves are not very efficient; although some heat energy gets converted into electricity, still much of it is wasted.
The next problems have to do with construction of the module itself and durability in its desired environment. Inside a TE module, semiconductors are soldered in series to copper pads to conduct the electricity. They are limited by temperature; in many situations where waste heat recovery would be the most desirable, higher temperatures exist than the melting point of the solder used to hold the modules together. The modules would literally melt and fall apart.
Another problem has to do with the coefficient of thermal expansion (CTE). When the module is heated, brittle intermetallics form due to the diffusion of Ni (the coating) into the substrate (Bi2TE3). The CTE of the newly formed Ni2Te3 is drastically different from the CTE of the surrounding materials. It is easy to see how the shear stresses at the interface generated by high or cyclic temperatures would have a tendency to crack the brittle intermetallic that forms in the modules.
A thermo-electric module is a semi conductor based electronic component that, up until recently, has been used as a small heat pump. By applying a low DC voltage to the module, one side will become hot while the other side becomes cold. If the polarity is reversed, the sides of the module will also be reversed: the formerly hot side becoming cool and vice-versa. The effects are somewhat proportional; the more voltage is applied, the more temperature differential there is.
Applying a voltage differential to a TE module in exchange for a heat differential is taking advantage of something called the Peltier effect, so named for the French Physicist, Jean-Charles Peltier. Thermo electric modules successfully utilize the Peltier effect for all kinds of machinery ranging from temperature controllers for sensitive electronic equipment to water heater/coolers to electric travel cooler chest/food warmers that plug into an automotive cigarette lighter.
When one applies a voltage differential to a TE module, a temperature differential is produced, but the opposite is also true. When a temperature differential is applied to the module, the leads produce a DC voltage differential. This is called the Seebeck effect, named after the German physicist, Thomas Johann Seebeck. One can see how a TE module utilizing the Seebeck effect would be useful for waste heat recovery. Wasted heat energy can be converted into useful electric current.
BMW is one company taking advantage of the Seebeck effect. Engineers are putting TE modules on the exhaust system of BMW automobiles; energy that would otherwise be wasted as hot exhaust gasses are vented from the car is used to help charge the car’s electrical system.
What the TE team is doing is attempting to make the TE modules more durable and more efficient by increasing their cyclic and high temperature durability using three main unconventional processes for joining the semiconductors in the modules to the copper pads. First, we are researching methods for using an ultrasonic welder to join the copper pads directly to the semiconductors without the need for low melting temperature solders, resulting in much higher potential operating temperatures.
Secondly, we are exploring the possibility of using a microwave welder to provide the heating in both the semiconductor and the coating material more evenly relative to each other during joining in an attempt to decrease the shear forces at the interface during heating and prevent intermetallic formation during joining. (The TE team is designing and building their own microwave welder, as no commercial models exist.)
We are also researching ways to use a ultrasonic welder to weld the leads directly to the copper pads, increasing electrical conductivity and again getting rid of the need for a low melting temperature solder.
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