Turning Waste Heat Into Energy

If we were somehow able to harvest waste heat, our energy problems would be over. A lot of consumer electronics and technology produce excess heat, that if we could somehow harness, we could use as an energy source.

Attempting to harvest this heat poses a difficult problem: its temperature is too close to that of the environment. One feasible option for converting waste heat into energy is the thermoelectric effect; some metals and semiconductors can create a current along a heat gradient. Currently, the efficiency of thermoelectric materials isn’t cost-efficient enough to produce on a large scale, but two new papers released this week may bring us closer to that goal and demonstrate possible applicable thermoelectric devices.

The efficiency of thermoelectric materials is measured by “a figure of merit.” These units are measured in zTs. To make economical sense, the thermoelectric materials would need a zT higher than two. The best materials available right now have a zT of about one, meaning we’re close to where we want to be.

Thermoelectric Efficiency

The efficiency of thermoelectric materials can be increased in a few different ways. One of the latest papers published makes use of a method called “valley degeneracy.” The process involves increasing the effective mobility of electrons present in the materials. This would enable the materials to produce more current with less of a temperature difference.

Two Different Perspectives

One of the papers’ authors produced a model that allowed them to measure how valley degeneracy will work with thermoelectric materials. Unfortunately, the high temperature that is produced when working with the materials will limit their use to strictly industrial buildings that can handle those extreme temperatures.

An alternative paper focuses on a method that doesn’t require such high temperatures or the use of toxic materials. This study notes that the best semiconductors would have a zT of about one. The authors also turned to an organic conductor: poly(3,4-ethylenedioxythiophene) or PEDOT instead of using a traditional metal semiconductor.

Ordinarily, conducting polymers wouldn’t work well for thermoelectric purposes. The reason that they’re so desirable in this experiment is that they conduct heat poorly, making them ideal candidates because they’re able to keep the device from reaching thermal equilibrium. In addition, these conductors are cheap and flexible. The study had to utilize oxidation methods in order to get the conductors to optimal thermoelectric efficiency. After experimenting, they were finally able to get the conductors at a zT of 0.25 at room temperature.

In order to further demonstrate the effectiveness of their organic conductor, the team fabricated several devices using an inkjet printer. They were able to produce more than 40 nanowatts/cm² at a temperature difference of 10^oC.

While both papers present interesting theories, neither one of the projects proposed are ready for commercial use yet. With a bit of nudging these daring projects may be able to push the boundaries. More importantly, maybe both teams will be able to find a way to utilize their devices at varying temperatures so that the commercial uses of the devices will expand beyond industrial and power facilities.

ars technica Photo by : Wikipedia