Technology Opportunity: Nanowire-Based Piezoelectric Renewable Power Generation

A thin piezoelectric film on the windowpane, absorbing the noise of the street and converting it into energy to charge the phone. Pedestrians on sidewalks, metro escalators, which charge through piezo converters batteries of autonomous lighting. Dense streams of cars on busy roads, generating megawatts of electricity, which is enough for entire cities and towns.

Fantastic? Unfortunately, so far, yes, and such can remain. There is a high probability that soon there will be an agiotage around sensational reports about the wonderful prospects of energy generators on piezoelements. And we will again dream of a safe, renewable and, to put it mildly, cheap electric energy, obtained with the involvement of other phenomena. After all, the list of physical effects is remarkably great.

The phenomenon of piezoelectricity was discovered by the brothers Jackson and Pierre Curie in 1880 and has since become widespread in radio engineering and measurement technology. It consists in the fact that the force applied to the sample of the piezoelectric material leads to the appearance of a potential difference on the electrodes. The effect is reversible, i.e. the opposite phenomenon is also observed: applying a voltage to the electrodes, the sample is deformed.

Depending on the direction of conversion of energy, piezoelectrics are divided into generators (direct conversion) and motors (reverse). The term "piezo-generators" does not characterize the conversion efficiency, but only the direction of energy conversion.

It was the first phenomenon connected with the generation of electricity under mechanical influence that engineers and inventors became interested in in recent years. As from the cornucopia, reports of the possibilities of obtaining electric energy were scattered, utilizing street noise, the movement of waves and wind, loads from the movement of people and machines.

Today several examples of the practical use of such energy are known. The metro station "Marunuchi" in Tokyo installed piezo-generators in the hall for the purchase of tickets. Accumulation of passengers is enough to control the turnstiles.

In London, in an elite disco, piezo generators feed a few lamps that stimulate the dancing and ... the sale of soft drinks. Piezoelectric lighters have become commonplace. Now, any smoker has his own "power station" in his pocket.

More recently, the world public blew up a message about testing systems for generating energy from moving vehicles. Israeli scientists from a small firm Innowattech estimated that 1 kilometer of the highway can generate electric power up to 5 MW. They not only carried out the calculations, but also uncovered several tens of meters of the liner of the freeway and mounted below it their piezo-generators. It seemed that finally there was a breakthrough in the field of alternative energy. But this raises serious doubts.

Let us consider in more detail the physics of the processes taking place in the piezoelectric. To get acquainted with the principles of energy generation by piezoelectric materials, it is sufficient to understand several basic mechanisms. When the piezoelectric element is mechanically applied, the atoms move in the asymmetric crystal lattice of the material. This displacement leads to the appearance of an electric field, which induces (induces) charges on the electrodes of the piezoelectric cell.

Unlike a conventional capacitor, the lining of which can retain charges for a long time, the induced charges of the piezoelectric element are retained only as long as the mechanical load is acting. It is at this time that you can get energy from the element. After removing the load, the induced charges disappear. In fact, the piezoelectric cell is a source of a negligible current, with a very high internal resistance.

Since the specialists of Innowattech did not consider it necessary to share the results of their experiment with the general public, let us try to make a rough numerical estimate of the efficiency of piezoelectric work as an energy source. As an object for calculations, let's take the usual household piezo lighter - the only product that has now become widely used.

Of the abundance of technical characteristics of piezomaterials, we need only a few. This is the value of the piezoelectric module, which for piezoelectrics, which is common (and so far the industry does not produce), is 200 to 500 picoculons (10 to minus 12 degrees) per newton, and characterizes the efficiency of charge generation under the influence of force.

This characteristic does not depend on the size of the piezoelectric element, but is completely determined by the properties of the material. Therefore, trying to make more powerful converters by increasing the geometric dimensions is meaningless. The capacity of the lining of the piezoelectric elements of the lighters is known and amounts to about 40 picofarads.

The lever system of force transfer to the piezoelectric element creates a load of approximately 1000 Newtons. The gap in which a spark jumps is 5 mm. The dielectric strength of air is taken to be 1 kV / mm. With such initial data, the lighter generates sparks with a power of 0.9 to 2.2 megawatts!

But do not be afraid. The discharge duration is only 0.08 nanoseconds, hence the huge power values. The calculation of the total energy generated by the lighter gives a value of only 600 micro-joules. At the same time, the efficiency of the lighter, taking into account that the mechanical force through the lever system is completely transferred to the piezoelectric, is only ... 0.12%.

The energy extraction schemes proposed in different projects are close to those of lighters. The individual piezoelements generate a high voltage, which breaks through the discharge gap, and the current flows to the rectifier, and then to the storage device, for example, an ionistor. Further transformation of energy is standard and does not represent interest.

From lighters to the task of obtaining energy on an industrial scale. Let the most efficient elements generating 10 milliwatts per element be used. Collected into clusters (groups) of 100-200 elements, they are placed under the roadway. Then, in order to obtain the declared power value of about 1 MW per kilometer of the road, it will take only ... 100 million separate elements with individual energy-removal schemes. There remains the task of its summation, transformation and transfer to the consumer. In this case, the currents of the elements, taking into account the varying load on the roadway, will lie in the range of nano or even picoampere.

Getting acquainted with similar projects for obtaining energy from the piezoelectric effect, an analogy with the hydroelectric power plant, in which the turbines operate from the moisture of the morning dew gently collected from the surrounding fields, involuntarily suggests.

But what about the experiment of the Israeli company? The report on the results of the "wrecking" on the liner of the motorway did not appear. But in fact ahead of the fulfillment of the contract on receiving energy from the motorway Venice - Trieste, which was concluded by Innowattech.

In this regard, there is one version: it is a company of the "start-up" type, i.e. with a high risk of investment capital. Having received more than modest preliminary results of the researchers, its founders decided to justify the investors' money spent and turned a magnificent marketing move - conducted a spectacular test involving the press. And the whole world started talking about a small company. And in this noise the main question was lost: where are the megawatts of cheap energy?

Summing up, we can draw only one conclusion: piezoelements will never become alternative sources of electricity on an industrial scale. The range of their applications will be limited to low-power (micro-power) power supplies and sensors. And sorry, such a beautiful idea!

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References for Text and Images:

1. https://newatlas.com/finger-movements-could-power-negt-gen/47135/
2. http://thecoolgadgets.com/piezoelectric-nanogenerators-convert-motion-for-greener-energy-source/
3. https://electronics.howstuffworks.com/nanogenerator.htm
4. https://www.sciencedirect.com/science/article/pii/S2211285511000085
5. https://www.nasa.gov/ames-partnerships/technology/technology-opportunity-nanowire-based-piezoelectric-power-generation
6. https://edition.cnn.com/2011/10/13/tech/innovation/pavegen-kinetic-pavements/index.html
7. https://techcrunch.com/2011/08/25/nanopower-shoes-could-generate-20w-of-electricity-during-walking/
8. https://www.theguardian.com/sustainable-business/power-generating-tiles-renewable-energy
9. https://www.engineering.com/ElectronicsDesign/ElectronicsDesignArticles/ArticleID/5879/Power-Walking-with-Energy-Floors.aspx

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