Although raindrops, evaporating water, or even air moisture are all potential sources of decentralized clean electricity generation, most of the technologies that take advantage of these widespread sources of energy remain at the lab-bench stage. To provide a comprehensive map of these developments and their possible future, a recent study published in the journal Nano Research Energy has surveyed the opportunities and challenges this young field currently faces.
Huge hydroelectric dams, large tidal barrages, or wave-energy converters on the sea surface are some of the first things one thinks of when considering sustainable energy generation. However, these options depend upon heavy, bulky, and above all centralized forms of harvesting energy from water. According to the scientists leading this new study though, there are myriad of other technological pathways that could take advantage of the ubiquitous presence of water on our planet.
“Water is everywhere. It is ambiently available like no other entity. So all this clean energy is just sitting there, unused and waiting for us to take advantage of it,” said study senior author Zuankai Wang, a mechanical engineer at the City University of Hong Kong. “It makes sense for us to tap into this vast reservoir of energy not just for bulk electricity production, but for a range of applications such as sensors and wearable devices where a micro-scale of energy harvesting is much more appropriate to the use it is being put to.”
Unfortunately, the development of such distributed water-energy technologies remains very much in its infancy, with many of the current concepts for water-energy harvesting techniques suffering from poor durability, scalability, and low energy conversion. “And yet the rest of nature has figured out thousands of different ways to do exactly this. Evolution has basically perfected the process of extracting energy from ambient hydrologic processes in ways that are extremely efficient,” Wang explained.
For instance, the lotus leaf allows droplets of water to roll across its surface with extremely low resistance, the Araucaria leaves cause liquids with varying surface tensions to flow in different directions, while some carnivorous plants (the so-called “nepenthes” or pitcher plants) are able to direct liquid through their surface structure. These natural phenomena inspired scientists to develop superhydrophobic surfaces or “slippery liquid-infused porous surfaces” that can repel liquid extremely efficiently, allowing for constant electrical output from droplets in harsh environments with high humidity, high salt concentrations, and low temperatures.
Besides plants, animals such as electric eels inspired researchers to develop artificial electric organs making use of hydrogel arrays (highly absorbent polymers which don’t dissolve in water) that function like eel membrane components.
Unfortunately, much of the design of such bio-inspired water-driven electricity generators remains at a lab-bench stage, and the devices currently constructed have a very short lifespan. Further theoretical and experimental research is needed to better understand the underlying structures of water-driven electricity generation in order to manage to efficiently construct durable and scalable bionics that could mimic nature’s capacities.
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