Bio-inspired water-driven electricity generators:From fundamental mechanisms to practical applications

Harvesting water energy in various forms of water motion,such as evaporation,raindrops,river flows,ocean waves,and other,is promising to relieve the global energy crisis and reach the aim of carbon neutrality.However,this highly decentralized and distributed water energy poses a challenge on conventional electromagnetic hydropower technologies that feature centralization and scalization.Recently,this problem has been gradually addressed by the emergence of a myriad of electricity generators that take inspiration from natural living organisms,which have the capability to efficiently process and manage water and energy for survival in the natural competition.Imitating the liquid-solid behaviors manifested in ubiquitous biological processes,these generators allow for the efficient energy conversion from water-solid interaction into the charge transfer or electrical output under natural driving,such as gravity and solar power.However,in spite of the rapid development of the field,a fundamental understanding of these generators and their ability to bridge the gap between the fundamentals and the practical applications remains elusive.In this review,we first introduce the latest progress in the fundamental understanding in bio-inspired electricity generators that allow for efficient harvesting water energy in various forms,ranging from water evaporation,droplet to wave or flow,and then summarize the development of the engineering design of the various bio-inspired electricity generator in the practical applications,including self-powered sensor and wearable electronics.Finally,the prospects and urgent problems,such as how to achieve large-scale electricity generation,are presented.

Rotating Surfaces Promote the Shedding of Droplets

Achieving rapid shedding of droplets from solid surfaces has received substantial attention because of its diverse applications.Previous studies have focused on minimizing contact times of liquid droplets interacting with stationary surfaces,yet little consideration has been given to that of moving surfaces.Here,we report a different scenario:A water droplet rapidly detaches from micro/nanotextured rotating surfaces in an intriguing doughnut shape,contributing to about 40%contact time reduction compared with that on stationary surfaces.The doughnut-shaped bouncing droplet fragments into satellites and spontaneously scatters,thus avoiding further collision with the substrate.In particular,the contact time is highly dependent on impact velocities of droplets,beyond previous descriptions of classical inertialcapillary scaling law.Our results not only deepen the fundamental understanding of droplet dynamics on moving surfaces but also suggest a synergistic mechanism to actively regulate the contact time by coupling the kinematics of droplet impingement and surface rotation.

In situ Reduction of Silver Nanoparticles on Chitosan Hybrid Copper Phosphate Nanoflowers for Highly Efficient Plasmonic Solar-driven Interfacial Water Evaporation

The development of water purification device using solar energy has received tremendous attention.Despite extensive progress,traditional photothermal conversion usually has a high cost and high environmental impact.To overcome this problem,we develop a low cost,durable and environmentally friendly solar evaporator.This bilayered evaporator is constructed with a thermal insulating polyvinylidene fluoride(PVDF)membrane as a bottom supporting layer and plasmonic silver nanoparticles decorated miero-sized hybrid flower(Ag/MF)as a top light-to-heat conversion layer.Compared with the sample with a flat silver film,the two-tier Ag/MF has a plasmonic enrichment property and high efficiency in converting the solar light to hcat as cach flower can gencrate a microscale hotspot by enriching the absorbed solar light.On the other hand,the PVDF membrane on the bottom with porous structure not only improves the mechanicalstability of the entire structure,but also maintains a stable water supply from the bulk water to the evaporation interface by capillarity and minimizes the thermal conduction.The combination of excellent water evaporation ability simple operation,and low cost of the production process imparts this type of plasmonic enhanced solar-driven interfacial water evaporator with promising prospects for potable water purification for point-of-use applications.

Hydrodynamic constraints on the energy efficiency of droplet electricity generators

Electric energy generation from falling droplets has seen a hundred-fold rise in efficiency over the past few years.However,even these newest devices can only extract a small portion of the droplet energy.In this paper,we theoretically investigate the contributions of hydrodynamic and electric losses in limiting the efficiency of droplet electricity generators(DEG).We restrict our analysis to cases where the droplet contacts the electrode at maximum spread,which was observed to maximize the DEG efficiency.Herein,the electro-mechanical energy conversion occurs during the recoil that immediately follows droplet impact.We then identify three limits on existing droplet electric generators:(i)the impingement velocity is limited in order to maintain the droplet integrity;(ii)much of droplet mechanical energy is squandered in overcoming viscous shear force with the substrate;(iii)insufficient electrical charge of the substrate.Of all these effects,we found that up to 83%of the total energy available was lost by viscous dissipation during spreading.Minimizing this loss by using cascaded DEG devices to reduce the droplet kinetic energy may increase future devices efficiency beyond 10%.

Achieving ultra-stable and superior electricity generation by integrating transistor-like design with lubricant armor

Extensive work have been done to harvest untapped water energy in formats of raindrops,flows,waves,and others.However,attaining stable and efficient electricity generation from these low-frequency water kinetic energies at both individual device and large-scale system level remains challenging,partially owing to the difficulty in designing a unit that possesses stable liquid and charge transfer properties,and also can be seamlessly integrated to achieve preferential collective performances without the introduction of tortuous wiring and redundant node connection with external circuit.Here,we report the design of water electricity generators featuring the combination of lubricant layer and transistor-like electrode architecture that endows enhanced electrical performances in different working environments.Such a design is scalable in manufacturing and suitable for facile integration,characterized by significant reduction in the numbers of wiring and nodes and elimination of complex interfacing problems,and represents a significant step toward large-scale,real-life applications.

A droplet‐based electricity generator with ultrahigh instantaneous output and short charging time

The past several years have witnessed the rapid development in effectively transforming randomly distributed water kinetic energy into electrical energy,especially triggered by the emergence of droplet‐based electricity generators(DEG).Despite this,it still suffers from relatively low average power density,which is also achieved at the cost of long charging time,the time to reach stable and saturated surface charge density either through continuous droplet impingement or precharging.Although the harvested energy per droplet in DEG remains as the dominant metric,ultrahigh instantaneous output and short charging time are equally important in some specialized applications such as instantaneous luminescence.Here,we conduct systematical modeling and optimization to build the link between the hydrodynamic and electrical systems,which enables us to determine ultrahigh instantaneous output and short charging time by tailoring parameters such as dielectric layer thickness,droplet ion concentration,and external load.We envision that this strategy in achieving ultrahigh instantaneous output as well as shortening charging time would provide insights and design routes for water energy harvesting.