Thermogenerators or thermoelectric generators enable the direct conversion of heat into electrical energy. They make use of the Seebeck effect, a thermoelectric effect in which voltage is generated between two metals under temperature difference. These generators offer an efficiency of approximately four to eight percent. They are therefore primarily used where electricity is to be generated from residual heat. For example, from heat flows from waste incineration plants and various industrial and IT processes.
Generally, the required heat flow through the generator is approximately 10w/cm². The maximum yield is guaranteed when the load resistance corresponds to the internal resistance of the module. Furthermore, the voltage generated depends on the temperature and the number of leg pairs. Efficiency can be achieved by using materials with excellent thermoelectric properties. Good examples are alloys of bismuth telluride (up to 300 degrees Celsius) or bismuth lead (up to 360 degrees Celsius). A homogeneous mixture of lead telluride is suitable for higher requirements. This is stable up to a temperature of 500 degrees Celsius.
Generators made of half-Heusler alloys can even withstand a temperature of 600 degrees Celsius, but these are currently still in development. The application is already possible on a limited scale. The optimum integration into the thermal environment must also be taken into account. Unnecessary temperature losses impair power generation and should be avoided to a large extent.
Furthermore, thermogenerators are used in Micro Energy Harvesting (MEH) and replace batteries in some areas. With a generated voltage of several hundred millivolts, they can, for example, take over the self-sufficient power supply of sensors in the Internet of Things (IoT). Thermogenerators are also increasingly being used in places where there is no grid connection and a battery change involves a great deal of effort.
Mounting material / TIM
As with any Peltier element, the thermoelectric generator should be installed with the utmost care. The filigree semiconductor elements can break even with the smallest deformation or too high contact pressure. The generator is clamped between the cold and warm sides and fixed there with a screw connection. The use of heat conducting media between the connecting surfaces is advisable. The thinner the coating, the better the heat transfer. Since high-frequency electromagnetic radiation should be prevented in some applications, the use of ceramic insulating discs is recommended. These have a high electrical insulating capacity and prevent oscillation caused by crosstalk in HF transistors or in LF power semiconductors.
Using a CFD tool (Computational Fluid Dynamics), we are able to test all parameters of the system prior to prototype construction and avoid any complications that might occur in advance. Numerical fluid mechanics can be used to test thermal systems for weak points in a simulated test environment. Thermogenerators can thus be optimally integrated into the project and adapted to the requirements down to the smallest detail. This results in a significant shortening of the development time, which would be required for conventional prototype construction. In addition, development costs are reduced to a minimum since no material is required for the construction of virtual models.
Heatpipe heat sink
The constant running of a thermogenerator requires the permanent maintenance of a temperature gradient. This makes the correct installation of a sufficiently dimensioned heat sink and the appropriate heat pipe even more important. The heat sink itself is usually made of copper, aluminium or, in rare cases, silver. Aluminium heat sinks take advantage of the low weight, but do not offer optimal heat conduction. Copper, for its part, is heavier and conducts heat much more effectively. However, the high weight can be problematic in some applications, which is why copper heat sinks are usually smaller and equipped with a fan for optimum performance. Silver elements offer very good thermal conductivity, but are significantly more expensive to purchase.