Generator Redesign Tries to Catch a Good Wave

While many ocean energy projects think big, laying large buoys that convert waves into electricity through mechanical means, there is another approach to developing wave energy: going small. For more than a decade, materials scientists have been tweaking materials on tiny scales to harvest electric charge from moving water. Now, by making a fundamental change to one such device’s design, a group at Central South University in Changsha, China, has managed to more than double its current collection capabilities.

To be precise, the Changsha group’s device is called the triboelectric nanogenerator (TENG). True to their name, TENGs rely on the triboelectric effect: If one material rubs across another, electric charge can transfer from one to the other. Given the correct materials, a TENG can then harvest that moving charge as electricity. The “nano” part comes in the material: Creating an effective TENG requires shaping the material with very small structures in order to tweak the electric exchange into drawing more current.

New ideas in wave energy are needed. The field has had plenty of false starts, and less than a gigawatt of capacity has been installed so far. But one optimistic scenario posits that the world might host 100 gigawatts of wave power by 2050. Wave power is particularly attractive for island nations and remote island communities, where land for power plants can be scarce, but waves are plentiful.

Nanogenerators Go Triboelectric

The TENG is a relatively recent invention. Materials scientists from the University of Science and Technology Beijing and Georgia Tech first created one in 2012 while attempting to construct a magnetic-field sensor. Researchers quickly realized that the device could work as an energy harvester. A year later, the same Georgia Tech group unveiled a TENG that relied on water. As waves of water washed across an array of nanoscale silicone pyramids, the apparatus generated a very small current.

Part of why researchers are so keen on these kinds of liquid-solid combination TENGs is that the water itself is one of the partners in the electric exchange. The device therefore should suffer much less wear and tear from friction. What’s more, it doesn’t need springs, rotors, turbines, or any other mechanical components. However, liquid-solid TENGs have a long way to go before they can find use as a reliable power source. They don’t produce enough current to be useful, and they don’t generate current with the reliability that is required.

The Changsha group’s device generated 3.5 times the voltage and 2.3 times the current of earlier designs.

So even the gadget’s basic design is still under scrutiny in an effort to improve its performance. A standard liquid-solid TENG might look like a sealed tube, partly filled with water, and with an electrode placed in its middle. As the tube rocks back and forth in the waves, water sloshes around inside and crosses the electrode to generate current.

The Changsha group decided to move the electrode to one end of the tube. Ideally, this would expose the electrode to a greater volume of water, more electric charge, and generate a larger current. After demonstrating the benefits of their design through computational modeling, the researchers crafted one in its polymer flesh, filled it with deionized water, planted it on a tilting horizontal platform and let it seesaw back and forth.

They did, in fact, observe a dramatic change. Compared to a standard liquid-solid TENG, the Changsha group’s device generated 3.5 times the voltage, 2.3 times the current, and 3.5 times the charge. Moreover, using water sloshing around in a 25-centimeter-long tube, the researchers were able to light up an array of 35 LEDs, blinking them off and on.

As a bonus, the researchers showed that their new and improved TENG can generate a small radio signal. They linked one up to an electrode submerged in a separate water tank. A receiving electrode picked up the signal from 40 cm away. The researchers dream of planting TENG-powered devices like these on the seafloor, where they might be able to act as nodes in some kind of underwater communications network.

These are small steps, and TENGs like these probably will not make a big splash in the power grid any time soon. But the Changsha group’s experiments suggest that this device could one day serve as a useful power source for small gadgets in water, far from a shore-bound power line.

“It could be utilized for environmental monitoring, underwater exploration, and underwater data transmission in various industries including marine research, offshore energy, and underwater robotics,” says Guozhang Dai, an applied physicist at Central South University.

The group published their work on 3 April in the journal ACS Energy Letters.