Harvesting rain energy

By Rushan Ziatdinov

Rainwater has been collected for centuries to be used to irrigate the land. Nowadays, the use of rainwater reduces the depreciation costs of pumping and purification equipment and lowers personal expenses. But can rain be used to generate electricity like the sun or the wind, for example?

This is a good question. Scientists and engineers have already found answers. As of now, "rain energy" and "energy of rain" phrases are found in the titles, abstracts or keywords of 81 Scopus indexed documents, 28 of which were published in the last five years.

Rain has potential as an energy source and an alternative way of generating electricity. Harvesting the energy of rain provides a lot of advantages. First and foremost, rainwater energy harvesting helps lessen the wide and ever-pressing need for water. Additionally, it lessens the demand that withstands imported water. It is also an efficient way to develop both the quantity and quality of groundwater.

As a sustainable energy source, rain energy harvesting can be useful in providing an alternate way of generating electricity. It is beneficial, in that it acts as an alternative energy source, particularly in preventing electricity shortages in communities and cities, especially in places where the rainy season is long.

In a 2016 article written on the Engadget multilingual technology blog, rain ― which is known to be a solar energy cell's sworn enemy ― was cast as an alternative and advantage. Engadget reports that Chinese scientists have proposed a solar cell with a graphene layer that is as thick as an atom. This solar cell is specifically designed to collect energy from raindrops, allowing it to be used in misty or stormy conditions. The graphene allows for the captivity of water, which then translates it into a natural capacitor.

The performance of a piezoelectric energy harvester was tested on actual rain in a 2017 study published in Elsevier's Energy journal. In the study, three real rain events were used as the basis for energy harvesting. Throughout the study, rain parameters including the rain rate, rainfall depth and drop size distribution were observed.

Among the three rain events, 0.34 was the largest record for added mass coefficient. Using this figure as a basis, it can be said that the modal mass of the water layer is 34 percent lower than the piezoelectric beam's modal mass. The results of the research suggest instances of low energy because of low rain activity. The researchers suggest coming up with an efficient energy storage system for the rain impact energy harvester.

Engadget released an article in 2020 that showed the effective translation of rainwater into energy, after several different potentials by different groups. This article demonstrates the design combination of an aluminum electrode and an indium tin oxide electrode along with polytetrafluoroethylene, which has a "quasi-permanent" electric charge.

Each raindrop that falls on the combined surface acts as a bridge for two electrodes, resulting in a closed-loop circuit, which is necessary for letting go of charges that are contained. Engadget also mentions that this particular technology helps keep collected rainfall. A continuous raindrop allows for charge accumulation, which then results in a saturation point.

In a 2020 article on Discovery Channel's official website, one particular method makes use of polytetrafluoroethylene film with an indium tin oxide electrode and an aluminum electrode, which then act as a bridge when the raindrops hit the device, fabricating an electrical system that is a closed-loop.

This method is capable of garnering twice as much energy and twice as much efficient work with gigantic amounts of water, compared to traditional ones. Through this method, raindrops generate kinetic energy that is then converted to electrical energy. The power generated from the given method can give energy to about 100 small LED light bulbs, as described by Wang Zhong Lin, the chief scientist and director of the Beijing Institute of Nanoenergy and Nanosystems at the Chinese Academy of Sciences.

In a 2021 research work published in Mechanical Systems and Signal Processing journal, a piezoelectric collection-based rain energy harvester was created and proposed by Wang. This particular rain energy harvester uses a structure that is funnel-shaped in collecting rainwater, which is then put in a tank that is self-releasing because of its huge surface.

The differences from the typical rain energy harvesters can be witnessed through the capacity of a self-release tank to let go of rainwater on its own even with the absence of external monitoring and active control during the times that the tank is filled with rainwater.

Rain energy harvesting is promising, but consideration for the actual amount of raindrop energy has to be given. Water droplet energy is indeed small, therefore people weren't aware that it might be harvested before, reports an article by Wanghuai Xu et al. that was published in the Nature journal in 2020. But as the Internet of Things expands quickly, the demand for distributed sensors and energy sources grows as a result of the rise of smart devices.

According to Statista, there was a nationwide average of 1,591.2 millimeters of precipitation in South Korea in 2020. What if some of this rain was turned into energy? Wouldn't that be possible for a technologically advanced country like South Korea? Rain energy harvesting devices, although still not commonly known and used, can be an alternative and can open the door to a renewable energy source that is not only innovative but sustainable.

Rushan Ziatdinov (ziatdinov.rushan@gmail.com) is a professor in the Department of Industrial Engineering at Keimyung University, Daegu. In 2010-2011, he was a postdoc in the College of Engineering at Seoul National University.