Capturing the energy of rain

When it rains, solar panels are useless. This could change with the miracle of “triboelectric nanogenerators”. This technology invented in 2012 generates energy from movements that are too light for current turbines.

Sometimes you get an electric shock when you touch a handle. Or when you manage to hold a light balloon on a wall after rubbing it on your hair. Until 2012, the miracle of static electricity had almost no useful applications. That changed with an invention by Zhong Lin Wang, a materials engineer at the Georgia Institute of Technology (Georgia Tech) in Atlanta.

“I myself am working on using this discovery to power marine drones or ocean sensor networks,” emphasizes mechanical engineer Zhiqun Deng.

Earlier this year, Wang and collaborators published two studies of solar panels that can generate (static) electricity when it rains, by exploiting the friction of raindrops on the panels. For the moment, the electricity produced is equivalent to the solar energy produced by the panels on overcast days. This is tens of milliwatts, or 10,000 times less than the production of solar panels in good weather.

The ABCs of triboelectricity

Static electricity was the subject of research early on, from the 17th century^e century. In 1831, British physicist Michael Faraday invented the electromagnetic generator, which produces electricity from the movement of a magnet. It is the basis for electricity generation by bicycle dynamos and hydroelectric power plant turbines, among others.

“Electromagnetism works at high frequencies, but not at low frequencies, such as waves, raindrops and everyday shocks, such as walking on floors,” says Deng. On the contrary, “triboelectricity (static electricity) uses these movements and frictions at very low frequencies. The energy collected is modest, but triboelectricity uses very, very widespread movements.”

The vast majority of publications on triboelectricity involve Chinese researchers. “There is possibly more funding in China than in North America for triboelectricity,” said Ramakrishna Podila, a physicist at Clemson University in South Carolina. “And prestigious researchers like Wang [qui dirige l’Institut de nanoénergie et de nanosystèmes de Pékin] have a lot of influence within the Chinese Academy of Sciences to direct research funding toward triboelectricity. »

In the United States and Europe, public authorities are demanding more fundamental research before funding new technologies, according to Mr. Podila. “However, there are many unknowns in the fundamental science of triboelectric nanogenerators. This is the area of ​​study of my research group. We develop dynamic quantum mechanical models and validate them with atomically thin materials. »

Three hypotheses have been put forward to explain the operation of triboelectric nanogenerators, according to Mr. Podila. These could be charged particles like electrons that pass from one material to another during friction. Or they are ions, larger charged particles, that make the jump from one material to another. Or, tiny pieces of one of the two materials come into contact and could be responsible for generating energy, by breaking apart.

Sampada Bodkhe, a mechanical engineer from Polytechnique Montréal, confirms that there is an enigma – we do not know how triboelectric electricity is produced on a molecular level – at the basis of Mr. Wang’s invention.

Shoes and filters

Mr. Wang has founded a company in China, Nairteng, which is preparing to market air filters powered by the friction of feet on the floor of a building. Shoes that store this energy, which can then be used to charge a phone, are also part of Nairteng’s plans.

Other researchers are working on textiles capable of generating triboelectric energy thanks to the movements of those who wear them. Mr. Wang also developed a sphere inside which another sphere moves with the waves when the whole is placed in the sea, thus generating triboelectric energy. In lectures, the Georgia Tech researcher has argued that a network of less than a square kilometer of these spheres could power a small city.

Mr. Deng, for his part, tested sensors in interior reservoirs whose instruments and telecommunications antenna are powered by the movement of waves.

Will we be able to power electrical devices that need more energy, for example household appliances? Mr. Deng believes that it will be enough to increase the efficiency of triboelectric nanogenerators by a factor of 100. “It’s a young technology, so anything is possible in the next decade. »

Mr. Podila, for his part, believes that triboelectric nanogenerators will be able, at most, to power LED lamps and electronic devices such as telephones and perhaps small computers. “I’m skeptical about the ability to proportionately scale up this technology,” says the South Carolina researcher. They are good at the microwatt or milliwatt scale. And there seems to be a lot of variation in effectiveness with temperature and humidity. »

Blood pressure and structural integrity

M^me Bodkhe, meanwhile, is working on blood pressure sensors powered by piezoelectric materials. These materials generate current when they are compressed or stretched.

Piezoelectric energy works better than triboelectricity if the movement frequencies are constant, according to the Polytechnique researcher.

“We could also examine the integrity of structures with piezoelectric sensors. We send waves into the structures; they power the sensors, which then analyze how the waves travel within the structures. »