Benefiting from hot air (Image: Lehtikuva OY/Rex Features )

The hot gases passing through a vehicle’s exhaust could be tapped to generate power, using “cuffs” made from a new carbon-nanotube-based material. The “thermocell” produces electricity at a similar cost per watt as commercial solar cells.

All around us there are opportunities to soak up wasted heat and convert it into electricity, says Ray Baughman, who works on thermocells with colleagues at the University of Texas at Dallas. Car exhaust pipes and power stations are just two forms of technology that waste a lot of heat and could be improved by building thermocells into their designs to recover lost energy.

However, to date the most effective thermocells have been based around expensive platinum electrodes, making them impractical. Baughman and colleagues have now shown that cheaper carbon nanotubes can be used instead, because the nanotubes pack a huge surface area into a tiny volume, and electrons transfer quickly between the electrolyte and nanotube electrodes. They have made thermocells three times as efficient as any before.


Energy flow

The basic design is simple. Each thermocell contains two electrodes, positioned at either end of a temperature gradient: for example, one right next to a hot pipe and the other closer to the surrounding cooler air.

In between is a chemical mix, in which the heat encourages chemical reactions that push electrons around an external circuit. Ions in the mix shed electrons at the hotter electrode and pick up electrons at the cooler one to complete the circuit.

One of the team’s thermocell designs is intended to be wrapped around a hot pipe, inspired by the fact that heat leaks out from such structures in many situations, such as chemical factories and power plants. “You could harvest energy from the tailpipe of a car,” adds Baughman.

The “hot” electrode wrapped around the pipe is surrounded by a heat-resistant layer, which is itself encased in a ‘cold’ electrode. An aqueous solution can move through pores in the heat-resistant layer, allowing ions to circulate between the reactions at the two electrodes.

In tests, a prototype thermocell functioned well for 90 days. With an electrode temperature difference of 60 °C it produced energy for $5.14 per watt based on materials costs for the prototype – comparable with that of mass-produced silicon solar cells.

Journal reference: Nano Letters, DOI: 10.1021/nl903267n