The conversion of thermal energy into electrical energy dates back to physicist Thomas Seebeck in the 1820's, who created an electric current by forming a metallic loop joined at two places, of different temperature and separated by a junction. Since Seebeck's original experiment, this phenomenon has since been explained as the thermoelectric effect.
The guiding principles of the thermoelectric effect suggest that the voltage produced by a temperature gradient is defined by the material properties of the metals and the temperature difference. However, a recently discovered phenomenon, named thermopower waves by its discoverers in the US and Korea, appears to have produced powers that far exceed classical thermoelectric calculations.
The key to this new technique of generating energy is the usage of the carbon nanotube as a thermoelectric device. These microscopic structures have recently started to increase in popularity as components of thermoelectric systems due to their high electrical conductivity (10,000 S/cm) and Seebeck coefficient (12 μV/°K). [1] A thermal gradient is created along the tube by coating them with a layer of chemical fuel such acetonitrile and TNA, then ignited at one end using a precision laser pulse or high voltage spark. As the fuel combusts, a high speed wave of heat with a temperature of 3,000°K speeds along the tube. The resulting heat wave was shown to also produce an electrical current along the carbon nanotube guide and a peak power of 7 kW/kg, a number far exceeding the output power predicted by thermoelectric calculations. [2] Despite the short duration of the output, its max power density is competitive even against modern lithium ion batteries. By introducing multiple coats of the chemical fuel, there is also possibility of creating a nano-sized fuel cell. The exact cause for this for this high power output is still undergoing research, but the discoverers have attributed it to the thermal wave attaching to electrical charges and carrying them down the tubes at speeds 10,000 times faster than the speed of the fuel combusting. [1] The carbon nanotubes and their alignment have also proved to be a crucial part of the of the thermopower waves. Due to the nearly one-dimensional confinement within the nanowire wave guides, the electron-phonon coupling appears to be physically enhanced. When experiments were attempted with other materials, it was shown that much more initiation energy was required, and in the case of unaligned nanotubes, the reaction was initiated but did not fully propagate. [3]
Although the science behind this technology is still in development, there is already a lot of speculation on this new energy generation technique can be used. One of the most intriguing possibilities is in nanoelectronics. While the size of electronics has been constantly decreasing, the size of their power sources has not always had the same reduction. The tiny size of the carbon tubes is perfect for usage in optoeletronic devices, microchips for medical drug delivery, and even airborne sensors. A second possible usage utilizes the fact that unlike most energy cells, there is no self discharge rate for these fuel encoated carbon tubes, and energy from these cells can be preserved forever. A recent experiment has even shown that it is possible to obtain an alternating voltage output with opposing polarities, in addition to the indefinite shelf-life of the cell, can be combined to justify usage in high dependability industrial systems. [2] When cascaded in large arrays, these systems can support very power intensive devices. While current implementations of this system have a very low efficiency, advancements are being made at a very quick pace. Researchers have experimented with the structure of the carbon nanotubes, including coating t hem with polyaniline and doping the carbon from p-type to n-type. [1] As the efficiency of this system increases, so does the possibility of the thermopower wave becoming a realistic new energy source.
© Jerry Zhou. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] W. Choi et al., "Chemically Driven Carbon-Nanotube-Guided Thermopower Waves", Nature Materials 9, 423 (2010)
[2] S. Walia et al., "Sb2Te3 and Bi2Te3 Based Therompower Wave Sources", Energy Environ. Sci. 4, 3558 (2011).
[3] S. Shimizu et al., "New Concepts in Molecular and Energy Transport Within Carbon Nanotubes: Thermopower Waves and Stochastically Resonant Ion Channel", Phys. Status Solidi 248, 2445 (2011).