17 January 2014

Motoring up nano magnets

Free energy inventions, perpetual motion machines have been mooted for millennia. Humanity has always had a hankering for the something-for-nothing. Of course, we’ve known since at least the early nineteenth century that thermodynamically there is no free lunch. That’s not to say, however, that we cannot make power, if not with greater than 100% efficiency, then at least by exploiting natural phenomena that require no more input from us than to build and maintain the devices from which we might tap that power – wind, solar, tidal, nuclear.

Even the most efficient power station is never going to give us perpetual power indefinitely without additional, ongoing input of resources. So, it is always with interest when researchers suggest an alternative power supply that ostensibly breaks not the first nor the second laws of thermodynamics, but seems to offer an interminable source of power with apparently no ongoing demands. Josef Polácek and Petr Alexa of the Institute of Physics and Institute of Clean Technologies, at the VŠB – Technical University Ostrava, in the Czech Republic, however, are not working on the tidal nor the wind farm scale but at the level of Brownian motion.

To quickly recap, Brownian motion is the random movement of particles due to thermal fluctuations and was named for botanist Robert Brown whose 1827 work identified the motion of pollen grains suspended in water. Brownian motion was characterized theoretically in detail by Einstein in his annus physicus, 1905, in which the constant buffeting of the pollen grains by water molecules provided the explanation for the motion. This work was the foundations on which Jean Perrin’s experiments of 1908 to provide evidence of atoms and molecules was built.

The advent of microelectromechanical systems (MEMS), lab-on-a-chip devices and the micro and nanotechnology has led to the recognition that Brownian motion can be a problem as well as a provider. It can interfere with the movement of fluids in small-scale structures but might also enable a “Brownian motor” that could propel those fluids. Much work is being done in this area. However, Polácek and Alexa have now suggested that Brownian motion might be exploited to generate electrical power on this scale.

They have considered a system, a low-density gas, containing myriad magnetic nanoparticles the movements of which might be used to induce random voltage pulses in a microscopic electric circuit. Such a circuit would contain a conducting coil with a very large number of turns and a diode rectifier that would allow current to flow in the coil in one direction only and thus produce a direct current. Of course, on such a scale, the amplitude of the voltage pulse is, they estimate, going to be around 30 billionths of a volt, a coil with at least a million turns would be required – perhaps based on self-coiling or bio-templated nanowires – conducting to allow a current to reach the rectifier and be put to use. Nevertheless, the team is enthusiastic that such a system, driven only by thermal agitation, would produce an adequate output to drive microscopic devices. One might imagine arrays of such nano-generators powering long-term remote sensors in deep, hot caves or perhaps devices onboard spacecraft heading for the stars.

Such blue-skies thinking is invaluable in fundamental science, but I do wonder whether such a system would ever have practical application. Major advances will be needed in nanotechnology to construct the requisite million-turn coils and microscopic rectifiers. The production of uniform magnetic nanoparticles en masse will be required. This will have to be sufficiently small and not too strong magnetically that they the constant Brownian buffeting of the gas molecules breaks apart any aggregates that form otherwise, the particles will simply precipitate out of the gas. But, smaller, weaker particles would mean a smaller induction effect.

“We are well aware that the practical application is a question for the future”, Alexa says. “In our paper we tried to test whether the Brownian motion of magnetic nanoparticles could be theoretically exploited to generate electric current. For this purpose we constructed a ‘toy model’. This model provides an estimate of basic conditions that have to be fulfilled to reach the rectifying region and to enable the system to work,” he adds.

However, we might assume that technical such issues could ultimately be circumvented as nanotechnology advances, so are there any inherent limitations? Concerning its utility let us ignore the details of how these generators might work, and instead focus on their practicality from the thermodynamic perspective. Ultimately, extracting power from Brownian motion means converting thermal energy to electrical energy. If the process is successful at generating electrical energy, then conservation laws dictate that the process depletes the supply of thermal energy. So that thermal energy must then be replenished somehow. Heat from the sun, geothermal heat or some other source might be exploited, for instance. But, if such a heat supply is essential to operation then we might think of this Brownian generator not as a new, untapped energy source, but simply a power conversion device just like any other electrical generator.

If the device is merely a converter, albeit on a much smaller scale than a nuclear power plant, then whether or not it is practical becomes a question of efficiency. If one must tap sunlight or geothermal energy to drive it, then there might be far more efficient and effective ways to utilize that energy even on the very small scale of MEMS and other devices without having to construct arrays of million-turn coils and such like in order to do so.

Conventional rotating generators can achieve 95% efficiency, losses arise mainly from the recovery and transmission processes, leading to 40% end-to-end efficiency. Solar are expensive and achieve anything from 2 to 30% efficiency, while Peltier effect thermoelectric generators manage just 5. All of these are likely to outperform a Brownian generator, but admittedly they operate on an entirely different scale.

Alexa adds that, of course, “thermodynamically there is no free lunch.” He points out that the efficiency of microscopic mechanical systems has been discussed and calculated before and there are perhaps inherent limitations to nano machines. Regardless, it is a fascinating idea at the cutting edge of nanoscience. I just wonder whether a magnetic Brownian motion generator idea will stick.

Polácek, J. and Alexa, P. (2013) ‘Brownian motion of magnetic nanoparticles as a source of energy?’, Int. J. Nanotechnol., Vol. 10, No. 12, pp.1109–1114.

Motoring up nano magnets is a post from: David Bradley's Science Spot

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