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Fundamental Mechanisms of Bio-Inspired Water-Driven Electricity Generators

Raindrops, evaporating water, and even moisture in the air are all potentially sources of decentralized clean electricity generation, but many of the technologies that take advantage of this ambient and vast source of energy—many of which are inspired by the electricity harvesting techniques of plants and animals—remain at the lab-bench stage. A group of researchers and engineers have put together a survey of the opportunities and challenges this very young field face.

Their review paper was published in the journal Nano Research Energy on November 30, 2022.

Enormous hydroelectric dams are perhaps the first thing one thinks of when considering sustainable electricity generation, or possibly large tidal barrages. If one is very familiar with the state of play in clean energy development, one might also be aware of wave-energy converters on the sea surface or seabed that convert the energy from high-intensity waves into usable electricity.

All of these options depend upon heavy, bulky and above all centralized forms of harvesting of the energy contained in water. Yet there are a myriad of other potential technology pathways that can harvest electricity from water in much more decentralized fashion, taking advantage of water’s ubiquitous presence almost everywhere on the Earth. These would produce usable electricity from processes of evaporation, condensation, rainfall, moisture, and even minute flows of water at the scale of a droplet falling off a leaf, and the very tiniest of waves.

Proposed technologies along these lines take advantage of various physical phenomena, including the piezoelectric effect (whereby electric charge accumulates in response to the application of stress or pressure), triboelectricity (in which certain materials become electrically charged after they are separated from a different material with which they had been in contact), thermoelectricity (the conversion of heat to electricity and vice versa), and the hydrovoltaic effect (in which electricity is generated via interaction between water and nanomaterials).

“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 Zuankai Wang, paper author of the review and researcher with the Department of Mechanical Engineering 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.”

Much of the work in the development of such distributed water-energy technologies remains very much in its infancy however. Many of these lab-bench concepts for distributed water-energy harvesting techniques suffer from poor durability, poor scalability and, worst of all, low energy conversion. This latter problem means that for all the effort put into harvesting energy out of such processes, not much is squeezed out.

The development of generators that are driven by water vapor in the air for example uses materials that so far exhibit poor capacity for water adsorption (adhesion to the surface), resulting in incomplete interaction between the water and the material, producing low electrical output, and declining even more in the face of harsh environments.

“And yet the rest of nature has figured out thousands of different ways to do exactly this,” added Wang. “Evolution has basically perfected the process of extracting energy from ambient hydrologic processes in ways that are extremely efficient.”

The lotus leaf for example at the micro and nano scale enjoys an extreme hydrophobic structure that allows droplets of water to roll across its surface with extremely low resistance—essentially on a cushion of air.  This phenomenon has inspired engineers to study textured superhydrophobic surfaces. The asymmetric 3D ratchets of the Araucaria leaf causes liquids with varying surface tensions to flow in different directions. And the ability of nepenthes, the group of carnivorous plants also known as pitcher plants, to direct liquid through its surface structure, inspired the authors of the review paper to develop a ‘slippery liquid-infused porous surface’ (SLIPS) system that can repel liquid extremely efficiently. A water-energy generator with durable SLIPS allows for constant electrical output from droplets in harsh environments with high humidity, high concentrations of salt, and even ultralow temperature.

And it’s not just plants. As water-driven electricity generators are well suited for harvesting energy from human motion due to their deformability and compact size, another group of researchers inspired by electric eel membranes developed artificial electric organs making use of hydrogel arrays (highly absorbent polymers that do not dissolve in water) that work as analogues of the eel membrane components.

Despite the explosion in development of such bio-inspired engineering, or ‘bionics’, for water-energy harvesting, the current generation of water-driven electricity generators remains largely ad hoc. The researchers felt that a comprehensive review of the field was urgently needed to place it on a firmer theoretical foundation and identify research gaps in order to better guide design of systems and development of novel materials.

The review covers the main mechanisms of electricity production for bio-inspired water-driven generators. It also offers a tour d’horizon of the various bio-inspired devices that have been developed, specifically evaporation, moisture, rainwater, and wave and flow-driven generators, covering three use cases: sensors, wearable electricity generators, and self-powered electronics.

The researchers concluded that the underlying structures of water-driven electricity generation remains undertheorized, in particular that of charge transport and transfer, as well as of energy conversion. Most notably, there is no general theory of charge transfer at the interface of solid materials and water, and proposed mechanisms for this remain hotly debated.

In addition, liquid residues on solid surfaces can significantly reduce electrical output, and so how to avoid or reduce such residues is one of the most vital avenues of research for the field. Most efforts have focussed on textural microstructures in materials that produces a super-hydrophobic surface in order to achieve an incomplete contact between liquid and solid. While this produces the desired water residue reduction, it also inevitably limits the solid-liquid contact area, reducing charge induction and thus lowering electrical output, producing the same result as a residue.

In other areas, improving the ability to absorb water from the environment will be key to improving electricity generation. The researchers recommended that a greater focus be applied to the study of organisms that have evolved over a long period of time in extremely arid areas, such as deserts.

Finally, the authors noted that much of the design of bio-inspired water-driven electricity generators remains at the lab-bench stage, with such devices confronting only a fairly mild experimental setting rather than the rough and tumble of real-world conditions.

The life-span of these technologies even in the laboratory only survive a few days or at most a few months. This compares poorly to roughly 25-year life-span of a solar panel or the half-century or longer of a nuclear plant or hydro dam. There may be use cases, perhaps in medical applications, where a short lifespan poses few problems or is even desirable, but for wider adoption of the technology, such unsatisfactory lifespans will need to be overcome.


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