Raindrops To Energy: The Droplet-Based Electricity Generator

By Prof. Zuankai Wang 17 April, 2020

How can one drop of rain light up 100 LED bulbs? CityU's Prof. Wang shares with us their latest tech breakthrough

Water contains huge amounts of energy, yet harvesting through the triboelectric effect (two objects contact & separate) results in less charges generated & inefficient release of those charges
The new tech, droplet-based electricity generator, uses dielectric film to store charges when water droplets hit the surface & eliminates the surface effect, which dramatically improves efficiency
Despite efficiency still only at 5% (much less than hydro), it has the potential to harvest wave energy if scaled up & to even harvest the heart's mechanical motions if scaled down
Prof. Zuankai Wang
Author: Prof. Zuankai Wang
Prof. Zuankai Wang is currently a full professor in the Department of Mechanical Engineering and associate dean in the College of Engineering at the City University of Hong Kong. He was awarded the Changjiang Chair Professor by Ministry of Education of China in 2016. Professor Wang was awarded the Changjiang Chair Professor by the Ministry of Education of China, Fellow of International Society of Bionic Engineering and founding member of Hong Kong Young Academy of Sciences. Professor Wang was the winner of 2020 President’s Lectureship at the City University of Hong Kong, 2018 World Culture Council Special Recognition Award, 2018 President’s Lectureship at the City University of Hong Kong, 2017 Outstanding Research Award and President’s Award at the City University of Hong Kong, Outstanding Youth Award conferred by the International Society of Bionic Engineering (2016), OSA Young Scientist Award (2016), Chinese Government Outstanding Self-Financed Students Abroad Award (2007), and Materials Research Society (MRS) Graduate Student Silver Award (2007 Fall Meeting, Boston).
Read more from Prof. Zuankai Wang →

Electrical power generation consumes a lot of water. The good news is that a research team from City University of Hong Kong (CityU) has recently developed a new form of droplet-based electricity generator (DEG) – which could light up 100 small LED bulbs from one drop of rain. The research, published in Nature, was led by Professor Zuankai Wang from CityU’s Department of Mechanical Engineering, Professor Xiaocheng Zeng from the University of Nebraska-Lincoln, and Professor Zhonglin Wang, Founding Director and Chief Scientist at the Beijing Institute of Nanoenergy and Nanosystems of Chinese Academy of Sciences.

CWR sat down with Prof. Zuankai Wang, one of the lead researchers, to ask him more about this – how can this help advance scientific research into water energy generation and tackle the energy crisis?

CWR: Congratulations on your groundbreaking achievement! Could you start us off by explaining how a water droplet can be used to generate power for LED bulbs?

Prof. Zuankai Wang (WZK): Our work is about the energy harvesting from water droplet. As you know, energy is fundamental to us. We cannot imagine how the world and our tomorrow will be if we do not have energy. But on the other hand, energy is conservative, and the electric energy must be converted from other formats such as wind energy, solar energy, and water energy. Water covers more than half of the earth and contains huge amount of energy.  The motion of water has long been effectively harnessed through electromagnetic generators. But they are bulky, costly. Moreover, they become inefficient when water supplies are low.

The motion of water has long been effectively harnessed through electromagnetic generators…

…but they are bulky, costly & become inefficient when water supplies are low

Recent studies have come up with a new method through the use of the triboelectric effect. That is, when two objects contact and separate each other, we have opposite charge generated on both sides, and then we can have electricity generation. So far, however, this technique has proven to be highly inefficient. In our work, we develop a new technique which can dramatically enhance the energy conversion through a device with a simple configuration, which consists of a dielectric film (similar to the Teflon) and two electrodes.

CWR: Where did you get this inspiration from? What drove you towards developing this amazing technology?

WZK: When we started to work on this project, we realised that there are two bottlenecks in the conventional method. The first one is the amount of charge sources generated is less because this is a surface effect and the second one is how to release the generated charges efficiently.

When we struggle to overcome these limitations, one inspiration suddenly flows into my mind. That is field effect transistor, for short, FET. FET is the basic unit of integrated circuit. It is so important that its invention won the Nobel prize in 1956. Inspired by this, we designed the similar configuration similar to FET.

CWR: How could this be applied to harvest more energy? Could we go beyond powering LED bulbs?

WZK: When droplets continuously hit the surface, lots of charges are stored in the dielectric film. And at the same time, these charges can be effectively released owing to the formation of a closed-loop circuit (bulk effect), eliminating the bottleneck (surface effect) encountered in the conventional design. As a result, the energy harvesting efficiency is dramatically improved.

The efficiency is at 5%, much less than large-scale hydro

Still, the efficiency is only 5%, which is not comparable to large-scale hydropower. If we have adequate energy input, then it is possible to power beyond LED Bulbs.


CWR: What were the main challenges you encountered over the course of this research?

WZK: The challenges really lie in the development of a proper dielectric material which gives the best output as well as good durability and stability.

CWR: How do you see the future of this technology progressing? Can it replace the traditional thermal power generation soon?

WZK: Our design is general, which can be used to harvest any kind of water energy as long as there is the motion of water.

It is not limited to droplet, it is not limited to droplet impact. It will have a wide range of applications. However, lots of works remain to be done to further improve the energy conversion efficiency.

CWR: What are the next steps for your research? Are there any other innovations in the pipeline?

WZK: Scientifically we have provided a paradigm for water-based energy harvesting, which will find lots of applications.

In the large scale, this tech can harvest wave energy in the ocean…

…in the small scale, it can harvest the motion of our heart

In the future, we will seek to translate our basic research into practical products. For example, in the large scale, it can harvest wave energy in the ocean and in the small scale, it can harvest the mechanical motion of our heart.

However, lots of works remain to be done to further improve the energy conversion efficiency.

Further Reading

  • Race For Water – Fighting Plastic Pollution In Our Oceans – What does the world’s largest solar-powered catamaran have to do with ocean plastic pollution? The Race for Water Foundations’ Lee explores this sustainable solution for ocean conservation with us
  • Race For Water – Converting Plastic Waste To Energy – With most plastic waste fragmented into tiny particles, ocean clean-up is increasingly difficult. Enter Race For Water Foundation’s circular tech to transform 5-12t of plastic waste per day to energy. Their Rollin expands
  • T Park: Waste-to-Energy In Hong Kong -Hong Kong’s increasing waste load by 2030 will put tremendous pressure on its management capability.  Veolia’s Nina Cambadelis introduces T PARK, a state-of-the-art sludge treatment facility that turns waste into energy while achieving ‘zero wastewater discharge’
  • Renewable Energy: Bigger Than You Think -Renewables surge as coal wanes but the bulk of the renewable energy boom is yet to come. CWR’s Thieriot on why this aggressive surge won’t be enough to solve the climate-energy nexus

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