Thermal harvesting

Thermal energy can be converted into an electric current for any device exhibiting heat flow. Thermal energy can be converted into electrical energy by two basic processes:

• Thermoelectric: Direct conversion of thermal energy into electrical energy through the Seebeck effect.
• Thermionic: Also known as thermotunneling. Electrons are ejected from an electrode that is heated, and into an electrode that is cool.  

The thermoelectric effect (Seebeck effect) is produced when a gradient of temperature exists in conducting materials. The flow of carriers from a hot to a cold region between two dissimilar electrical conductors creates a voltage differential. A thermocouple, or thermoelectric generator (TEG), could effectively produce voltage simply based on the temperature difference of a human, based on their core body temperature and outside temperature. A temperature difference of 5 degrees Celsius could generate 40 uW at 3V. As heat flows through the conduction material, a hot-side electrode induces electron flow to a cold-side electrode-producing current. Modern thermoelectric devices use n or p-type bismuth telluride in series. One side is exposed to the source of heat (called the thermocouple), and the other is isolated. The energy harvested by the thermopile is proportional to the square of the voltage and equivalent to the temperature difference between the electrodes. One can model the energy harvested by a thermocouple by the following equation:

Here S₁ and S₂ represent the different Seebeck coefficients for each of the two materials (n and p-type) in the thermopile when there is a temperature differential, T_H -T_L. Since the Seebeck coefficients are functions of temperature and there exists a temperature difference, the result is a voltage difference. This voltage is generally very small, so many thermocouples are used in series to form a thermopile.

One substantial problem with current thermocouples is the poor efficiency of the energy conversion (less than 10%); however, their advantages are notable, including their small size and ease of manufacturing, resulting in fairly low costs. They also have a very long lifetime of over 100,000 hours. The main problem, of course, is finding a relatively constant source of thermal variance. Using such a device in an environment throughout multiple seasons and temperatures is challenging. For IoT devices, thermoelectric generation typically resides in the 50 mW range.

Thermionic generation is based on the ejection of electrons from a hot electrode to a cold electrode over a potential barrier. The barrier is the work function of the material, and is best used when there is a significant source of heat energy. While its efficiencies are better than thermoelectric systems, the energy required to jump the potential barrier makes it generally unsuitable for IoT sensor devices. Alternative schemes such as quantum tunneling could be considered, but it currently remains a research activity.