Piezoelectricity: The energy under our feet

Por: Rodrigo Angelo Albuquerque Mendes e Maria Luiza Viana Sampaio

There are numerous mechanical collisions on the ground every day: people walking in shopping malls, cars driving on asphalt, airplanes landing at airports. In 1880, the brothers Pierre and Jacques Curie discovered piezoelectric materials, substances that enable a way to take advantage of such pressure or mechanical tension to generate electrical energy, known as the piezoelectric effect, or vice versa, to transform electric potential difference into increase or retention volume, known as the reverse piezoelectric effect. Its first practical use was made by Paul Langevin, in the development of sonars, during the First World War. However, due to the difficulty of producing piezoelectric transducers, a device made of quartz that transforms one type of energy into another, started the production of synthetic materials, such as piezoelectric ceramics from Barium Titanate, developed by the USSR and Japan, and those from Chubo Titanium, produced by the USA.

Piezoelectric transducer Source

Once the context of the piezoelectric effect is understood, its concept is based on the property of some crystals to produce electric charge under mechanical stress, proportionally, called the direct effect. The reverse effect is characterized by generating mechanical stress resulting from an applied electric field. Among the examples of crystals, the most important natural is quartz, but ceramics are usually based on Barium Titanate or Lead Zirconate. The piezoelectricity effect, talking about molecular geometry, is not seen in substances with a central symmetry, however pure elements such as Selenium and Tellurium are exceptions. A very important feature is that when these crystals are subjected to vibrations above 20 kHz, they produce ultrasonic movements.

Main natural piezoelectric crystal: QuartzSource

The growing demand for energy can lead to the depletion of the main sources, like petroleum, coal, natural gas, and others, so it is essential to discover or desenvolve alternative sources that cause little or no damage to the environment, such as wind, solar, and piezoelectric, which is little explored. Among the various applications of piezoelectric ceramics, energy generation is included in:

Nightclubs: a dance floor can use human movement to provide lightingSource

Automobile tires: nanogenerators, tiny wires of piezoelectric materials, inserted inside them, being initially stored in a capacitor and, later, passed into the vehicle by a switch.

Roads and train tracks: consists of taking advantage of the impulse resulting from the weight and speed of the vehicle to send energy to a generator.

In addition to energy generation, they are also used in medicine, in ultrasound devices; like sound amplifiers and impact detonators, for example.

In view of this, it is possible to analyze the energetic potential of the various applications of piezoelectric materials, according to the following examples:

  • Israeli technology company Innowattech has installed generators on a road in the northern Israeli city of Haifa that convert the pressure exerted by vehicle tires into electrical energy. According to the company, a double strip of 1 km in length equipped with generators is already supplying Israelis with 0.5 MW per hour, enough to light 600 houses for a month.
  • Other initiatives, such as the Repsol project “Roads as an energy body”, are also based on the development of piezoelectric roads and, thanks to the devices integrated into the pavement, they can transform the pressure and vibration generated by vehicles into electrical energy at a rate of 1 kWh for 12 m of road.

Taking into account the continental proportions that Brazil has, with 1,720,700 km of national roads and highways, according to the NTC Yearbook 2018, National Transport Confederation (the fourth largest in the world), it would be possible, according to the data from the Repsol project, to generate approximately 143,391.6 MWh, which would be equivalent to 10 times the capacity of Usinas de Itaipú (largest hydroelectric plant in Brazil), making it possible to light more than 286 thousand houses during a month (taking into account the average consumption of the Brazilian population of 152.2 KWh per month, according to “Laboratório de Eficiência Energética em Edificações” of
federal university of santa catarina).

It is worth mentioning that the calculations consider the value found in the Repsol project of 2009 and do not consider that the efficiency of energy production per meter of road can be greater in Brazil, since 61.1% of all cargo handled in the Brazilian territory pass through these highways and that the piezoelectric plates are directly proportional to the load exerted on them.

Examples of piezoelectric ceramics installations in pavements:

Although it is clean energy, it is still questionable why so much energy potential still does not have the necessary investments to take advantage of this source that is “wasted” every day. From this questioning it is possible to focus on the points that limit the effectiveness of piezoelectricity on the roads.

In this context, one of the main factors that limit the implementation of this technology is its cost and production time, which are considerably high. Numerically, it is possible to mention the investment, made by the Israeli technology company Innowattech, of US $ 650 thousand for the aforementioned project. It is also worth mentioning the difficulty of storage for later use, because its immediate use is simple, as a form of self-sufficiency, however very expensive capacitors are needed to store it. Moreover, another factor is the fluctuation in efficiency that occurs due to extreme temperatures.

However, the researcher at the Universidade Estadual Paulista (Unesp), Elson Longo, already produces in the laboratory a material that promises to be cheaper than those currently available. “The piezoelectric ceramics that are made today take 30 to 48 hours to be ready – and this in very small quantities”, says Longo. “Using another process, which involves microwaves, we can do the same in just one hour,” he explains.

In addition, there are researches, led by professors Walter Sakamoto and Maria Aparecida Zaghete, also from Unesp, who have been positive in the search for the production of a material capable of capturing piezoelectric energy efficiently and at a lower cost than that found in the market. While the materials made today are a mixture of Lead, Titanium, Zirconium and ceramics, Brazilians replaced the last ingredient in the recipe with a polymer made with castor oil, similar to plastic. “We use a renewable and cheaper raw material,” says Sakamoto.

Therefore, even though studies and investments are still scarce, this source is very promising, given that the current technological scenario favors its development.


Bibliography:

SEMANA ACADÊMICA. Piezoeletricidade como fonte de energia alternativa. Disponível em: https://semanaacademica.org.br/system/files/artigos/robson.pdf. Acesso em: 10 mai. 2020.

NÚCLEO DO CONHECIMENTO. Conhecendo a piezoeletricidade uma nova forma de geração de energia elétrica. Disponível em: https://www.nucleodoconhecimento.com.br/engenharia-eletrica/geracao-de-energia-eletrica. Acesso em: 10 mai. 2020.

CVDENTUS. ULTRASSOM PIEZOELÉTRICO: DUAS TECNOLOGIAS A FAVOR DA ODONTOLOGIA. Disponível em: http://blog.cvdentus.com.br/ultrassom-piezoeletrico-duas-tecnologias-a-favor-da-odontologia/. Acesso em: 12 mai. 2020.

HBK COMPANY. como um transdutor de forca piezoelétrico funciona ?. Disponível em: https://www.hbm.com/pt/7318/como-um-transdutor-de-forca-piezoeletrico-funciona/?gclid=EAIaIQobChMIvPv7s-Ox6QIVhwaRCh0oaQAdEAAYASAAEgI5BfD_BwE. Acesso em: 11 mai. 2020.

TOMORROW.MAG. Recovering energy from traffic: positive energy roads. Disponível em: https://www.smartcitylab.com/blog/urban-environment/recovering-energy-from-traffic-positive-energy-roads/. Acesso em: 12 mai. 2020.

MUNDO EDUCAÇÃO. Fontes de energia do Brasil. Disponível em: https://mundoeducacao.uol.com.br/geografia/fontes-energia-brasil.htm. Acesso em: 13 mai. 2020.

QUORA. What is a piezoelectric road?. Disponível em: https://www.quora.com/What-is-a-piezoelectric-road. Acesso em: 12 mai. 2020.

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