An energy breakthrough from the City University of Hong Kong finds power in a single drop of water – up to 140 volts, in fact.

The droplet-based electricity generator, or DEG, supports renewable energy possibilities, such as creating electricity from falling rain.

"The power generated can light up 100 small LED lights," said lead researcher Professor Wang Zuankai  from CityU's Department of Mechanical Engineering.

Droplet-based energy generators have largely relied on the triboelectric effect, which creates a charge when dissimilar materials come in contact and separate. Triboelectric generators, however, have traditionally produced low amounts of energy.

The City University of Hong Kong system's power density reaches up to 50.1 W/m. The increased power density, thousands of times higher than similar devices, offers short bursts of instantaneous power.

The CityU-developed DEG employs a structure based on the basic unit of an integrated circuit: the field-effect transistor. The "FET" has three terminals – gate, source, and drain – to control the current flowing through a device.

Falling water, in effect, helps to complete an FET-like circuit.

The Hong Kong team's design features a material with a quasi-permanent electric charge. The material combines an aluminum electrode with an indium tin oxide (ITO) electrode layered with a synthetic known as PTFE.

When a drop hits the PTFE/tin surface, the water bridges the two electrodes and creates a closed-loop circuit. The researchers discovered that a continuous barrage of water droplets increases and saturates the surface charge, overcoming the inefficient, low-charge density encountered in previous work.

After the spreading water connects the two electrodes, all the stored charges on the PTFE can be fully released for the generation of electric current. As a result, both the instantaneous power density and energy conversion efficiency increase significantly.

Fig a) is the schematic diagram of DEG: an ITO glass slide is coated with a thin film of PTFE and an aluminium electrode is put on top of it. Drops of water act as the gate of the transistor and complete the circuit when they hit the surface of the glass. Fig b) is the optical image showing four parallel DEG devices fabricated on the glass substrate. (Image Credit: CityU)

“Our research shows that a drop of 100 microliters of water released from a height of 15 centimeters can generate a voltage of over 140 volts,” said Professor Wang.

In an edited interview with Tech Briefs below, Prof. Wang reveals the range of renewable possibilities with droplet-based energy.

Tech Briefs: What do you consider to be "groundbreaking" about this achievement?

Prof. Wang Zuankai: We cannot imagine how our tomorrow will be if we do not have energy, especially sustainable energy, such as wind energy, solar energy, and water energy.

Water covers up more than half of the Earth and contains huge amounts of energy. The motion of water has long been effectively harnessed through electromagnetic generators, but they are bulky, costly, and inefficient when water supplies are low.

Recent studies have come up with a new method based on the triboelectric effect or friction effect. That is, when we contact and separate two objects, we will have positive and negative charges on each side. By connecting to an external circuit, we can transform the flow of charge into power.

This technique, however, suffers from very low energy conversion efficiency because, by nature, this process is a surface effect. In our work, we develop a new technique that solves this problem.

We show that we can light up 100 LEDs using a single droplet. From this point of view, our work is groundbreaking.

Tech Briefs: How much power can be created, and how do you envision this kind of system being used? What applications do you envision with this kind of system, both in the near future and beyond?

Prof. Wang Zuankai: The peak power density is 50W/ m2. Our design is general, It is not limited to the droplet; it is not limited to droplet impact. It can be used to harvest any kind of water energy as long as there is the motion of water. The system will have a wide range of applications.

In the large scale, the DEG can harvest wave energy in the ocean and in the small scale, it can harvest the mechanical motion of our heart.

Tech Briefs: What are the limitations of traditional droplet energy generators?

Prof. Wang Zuankai: We are a new player in the energy area. When we start to worked on this project, we realized that there are two fundamental bottlenecks in the conventional method.

The first one is the amount of charge sources generated is small. The second is that even when you have an increase in the stored charges, there is no way to release them.

Tech Briefs: What inspired this work, to create power from water?

Prof. Wang Zuankai: As we struggled to overcome these limitations, one inspiration suddenly flew into my mind: the field effect transistor, or FET for short. FET is the basic unit of an integrated circuit. It is so important that its invention won the Nobel prize in 1956.

An FET has three terminals: one is the source, which contains lots of electrons. Another is the drain, which draws the electrons from the source. In normal conditions, however, the electrons cannot flow into the drain due to as barrier below the source and drain.

That is why there is the third terminal, the gate, above and between source and drain. When we apply a voltage in the gate, all the electrons in the source can flow to the drain.

Inspired by this, we designed a configuration similar to an FET. We have a special material called PTFE and ITO which behave as sources. storing high density of charges; another electrode is the drain. With the flow of water, there is electricity generation. When a droplet hits the surface, the source and drain are connected; all the charges are transported to drain.

Tech Briefs: What is most exciting to you personally about this achievement, and what is possible?

Prof. Wang Zuankai: The energy we harvested is from nature and the process does not involve any chemical reaction. It is a totally green process.

The research was led by Professor Wang Zuankai from CityU's Department of Mechanical Engineering; Professor Zeng Xiaocheng from the University of Nebraska-Lincoln, US; and Professor Wang Zhonglin, Founding Director and Chief Scientist at the Beijing Institute of Nanoenergy and Nanosystems of Chinese Academy of Sciences.

The findings were published in the latest issue of Nature .

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