L’onda termica dei nanotubi

Un gruppo di ricercatori del Massachusetts Institute of Technology (MIT) ha scoperto un fenomeno fisico mai osservato fino ad ora. Attraverso l’utilizzo di nanotubi si può generare energia elettrica direttamente da energia chimica.

Abstract: Abbiamo studiato la possibilità di utilizzare nanotubi di carbonio come mezzi di conduzione energetica. Il particolare effetto può essere opportunamente sfruttato per la produzione di energia elettrica, in assenza di combustione e formazione di gas a effetto serra. I nanotubi di carbonio vengono ricoperti con uno strato di combustibile ad elevata reattività il quale, infiammato ad una delle estremità del nanotubo mediante un laser o una scintilla ad alto voltaggio, va ad innescare un’onda termica che viaggia lungo il nanotubo stesso. Il processo consente di generare energia elettrica (corrente alternata) direttamente da energia chimica, senza la necessità di parti meccaniche intermedie, passibili di usura. Eliminando la perdita di energia dovuta all’attrito delle parti meccaniche, la generazione di corrente alternata si avvicina ai suoi limiti termodinamici (70-80%). Nonostante i primi esperimenti abbiano utilizzato combustibili solidi, anche combustibili comuni quali la benzina possono fungere da sorgente di energia chimica, disponibile nell’ambiente circostante al veicolo sotto forma di emissione diffusa o perdita durante le manovre di rifornimento. I generatori di onde termiche possono quindi assorbire gli idrocarburi dall’ambiente grazie all’idrofobicità dei nano tubuli ed alla loro ampia superficie, incrementando considerevolmente l’efficienza delle macchine ibride e diminuendo l’inquinamento.

Portable energy generation and storage is an increasingly important part of global energy demand, as electronic devices continue to proliferate. On another scale, the transportation sector is seeking alternatives to fossil fuels that can store energy for long times at high density but release it quickly for high power applications such as vehicle acceleration. The self-discharge and power density limitations of batteries and capacitors make them less than ideal for these sectors, but thermopower wave generators may find applications in this space. Their energy is stored in chemical fuels, which are very stable compared to electrochemical energy storage in batteries. However, unlike today’s engines, the mechanism of thermopower waves involves a direct chemical to thermal, then electrical energy conversion, and does not require combustion or the formation of greenhouse gases for power generation. In principle they can convert energy from fuel more efficiently because they do not use mechanical parts (like engines), which lose energy due to friction. And the fuel does not have to be a fossil fuel but rather could be generated renewably like some biofuels. The following graph compares power densities of various energy storage devices and the size to which they have been miniaturized.
The power density of thermopower wave generators surpasses other technologies of similar size. The only technologies with similar or higher power density discharge in a matter of weeks to years, whereas thermopower wave generators use chemical fuels that are stable indefinitely. (Some volumes were estimated from published device descriptions).How do thermopower waves work? Thermopower waves are a new, unconventional, and potentially transformative way of generating electricity directly from chemical energy. When a high-energy chemical reaction is guided along tiny molecular wires called carbon nanotubes (CNTs), the CNTs rapidly conduct the heat created by the reaction ahead of the reaction zone to unreacted fuel (as shown in the left portion of the figure below). This reaction can be initiated by a laser or a hot wire. The heat delivered by the CNTs sparks new reactions, which then generate more heat, leading to a fast-moving reaction wave. CNTs conduct heat more quickly than any fuels, so the reaction wave can be accelerated by as much as a factor of 10,000. The rapidly moving thermal reaction wave also pushes along electrons, like leaves caught up on an ocean wave (right portion of figure). It produces a pulse of electricity that can be up to seven times more powerful, per unit mass, than that obtained from today’s lithium-ion batteries.
Thermopower waves are possible
The movement of heat (phonons) and the movement of electrons are more strongly linked in these materials
**The generation of these thermopower waves appears due to configuration of carbon nanotubes. Phonons are units of vibrational energy that arise from atoms oscillating together in a crystal. Structures like nanotubes and nanowires confine the movement of phonons leading to their increased ability to carry heat. This leads to acceleration of chemical reaction wave along the length which in turn leads to improved ability to carry electrons and electric current.

Possible applications
Thermopower waves with oscillating velocities have recently been demonstrated. Since the velocity of the wave is directly linked to the power it generates, these waves could generate AC (alternating current) power directly from chemical reactions without the need for any moving parts. AC power is commonly generated today using mechanical devices like turbines, dynamos, and alternators. Power plants, cars, and boats use these devices widely, but their many moving parts mean that friction limits their efficiency. Eliminating friction energy losses could push AC power generation efficiency closer to its thermodynamic limits, perhaps up to 70-80%, compared to today’s maximum of 60%. Furthermore, the unusual physics of thermopower waves mean that their moving temperature gradients produce more power than conventional thermoelectric systems where the temperature gradient is static. The motion of the wave gives the electrons an extra “kick.”
Although the first demonstrations of thermopower waves have used solid fuels, they could also use common fuels like ethanol and gasoline for their required source of chemical energy. These fuels are available in the environment around vehicles in the form of fugitive emissions that escape due to leaks or during filling. Thermopower wave generators could then adsorb hydrocarbon fuels from their surroundings, due to the high surface area (up to 1000 m2/g) and hydrophobicity of carbon nanotubes. For example, single-walled CNTs can absorb up to 46% of their own mass at room temperature. Capturing this energy and converting it to electricity could increase the efficiency of hybrid automobiles or aircraft or power on-board electronics or distributed networks of tiny sensors around the vehicles.
Likewise, another potential energy source is uncombusted fuel in engine exhaust. For example, a jet like an F-16 consumes enormous amounts of fuel (as high as 6000 kg/hr under full thrust). Even if the engine achieves 98% combustion, 120 kg/hr of fuel is lost, and this amount increases with the rate of fuel fed to the engine. More fuel is lost under high-performance conditions. By capturing such fugitive emissions from aircraft and automobiles, networks of thermopower wave generators would both increase overall vehicle efficiency and decrease pollution. To power such basic devices as micro-sensors or actuators, they would only need to capture a tiny fraction of the escaping fuel to operate, and they would be continuously re-supplied as long as the aircraft was in flight. Furthermore, their high power density would decrease vehicle weight, providing further energy savings.

(Left) A schematic of a rapidly moving reaction wave along a carbon nanotube (CNT). The nanotube, an excellent thermal conductor, is wrapped with fuel (here labeled as TNA). (Right) Illustration of the experimental setup used to measure thermopower waves. The sign of the voltage pulse created depends on the direction of the wave relative to the contacts. Adapted with permission from Reference 1.
The newly discovered phenomenon of thermopower waves can open many avenues for sustainable energy generation and storage. They have the potential to improve the efficiency of converting chemical energy in fuels to electricity, since they are not limited by mechanical energy friction losses. These fuels could also be derived from biological sources. Storing energy in chemical bonds can preserve it for longer times than electrical storage in batteries and supercapacitors. The high-power electrical pulses from thermopower waves also outstrip today’s batteries. Thermopower waves could have an important impact on transportation, portable energy storage, and future micro- and nano-devices.
1. Wonjoon Choi et.al. “Chemically driven carbon-nanotube-guided thermopower waves”. Nature Materials (2010), 9, 423-429
2. http://www.britannica.com/EBchecked/topic/457336/phonon

Michael Strano
Professor MIT (Massachussetts Institute of Technology) Department of Chemical Engineering
Sayalee Mahajan
PhD Candidate, Massachusetts Institute of Technology
Joel Abrahamson
PhD Candidate, Massachusetts Institute of Technology


Massimiliano Fanni Canelles

Massimiliano Fanni Canelles Head of CAD Nephrology and Dialysis, Health Department with University of Udine Adj. Professor in Alma Mater University in Bologna of International Cooperation Editor of SocialNews Magazine President of Auxilia Foundation Twitter. @fannicanelles Instagram @fannicanelles