Electricity from falling water droplets

| Hans Wolkers

Scientists from the University of Twente and the South China Normal University developed a device that can generate electricity from falling water droplets. The technology is simple and elegant, but it will not easily solve the energy problems. ‘It just doesn’t rain that often in the Netherlands.’

‘Although the technology is efficient, there are serious complications to generate substantial amounts of energy using the method,’ says PhD scientist Niels Mendel. But with some adaptations, the design is also suitable to generate electricity from other sources of mechanical energy than rain drops. This allows the application in small wearable devices, like watches.

Very slowly a droplet of water drips out of the tip of a syringe and falls on a small black, glossy plate, measuring about two times three centimeters. Once it hits the plate, the droplet flattens out before sliding down the surface. A thin, silver-colored wire, attached just above the plate’s surface, is connected to an oscilloscope, and catches the sliding drop. Immediately, a yellow spike shows on the small screen of the oscilloscope. Electricity!

‘As you can see, the set-up of our electricity generating system is quite simple,’ PhD scientist Niels Mendel says almost apologetic. ‘But its simplicity is elegant at the same time and the mechanism of electricity generation is invisible to the eye.’ The heart of the system is the special design of the tiny plate: it is build-up out of several layers. Mendel: ‘This is where the magic happens and where the mechanical energy of falling droplets is converted into electricity.’

Bigger current

The plate acts like a permanently charged capacitor, where a redistribution of charge occurs when droplets hit the plate. This results in an electrical current. It consists of three layers, each with different properties. In between a Teflon top layer and a conductive doped bottom layer, a middle layer of non-conductive silicon oxide is sandwiched. To get the plate working as a capacitor, an extra electrical, negative charge is beforehand injected into the Teflon top layer. This negative charge attracts a positive charge in the bottom layer, but because of the insulating middle layer, the negative and positive charges remain separated and are lined up against each other, almost like soldiers in opposing trenches. A thin conducting wire runs from the positively charged bottom layer to just above the top of the negatively charged Teflon top layer.

‘When a water droplet runs down the Teflon top layer of the plate and hits the wire, an electrical connection between top and bottom layer is established,’ Mendel explains. ‘Due to the excess negative charge in the top layer, the positive charge from the bottom layer partly flows through the wire into the water droplet: an electrical current is generated.’ The bigger the contact area between water droplet and Teflon layer, the more charge flows from the bottom to the top, resulting in a bigger current. While the water droplet slowly flows from the plate, the charge flows back to the bottom layer, resulting in a second current in the opposite direction.

Optimize efficiency

The new method is rather efficient. ‘With the current set-up, about twelve percent of the mechanical energy of the droplet is converted into electricity,’ Mendel explains. ‘That is roughly seventy percent of the efficiency of solar panels.’ But the efficiency can dramatically be improved. For example, by maximizing the contact surface of the droplet with the plate, resulting in a bigger charge transfer. This can be achieved by optimizing the ‘spreading’ of the droplet, by varying the height from which the droplet falls, as well as the angle of the plate: both have an influence on how the droplet spreads over the plate and thus how large the contact between droplet and plate is.

However, the most effective way to optimize the efficiency is by increasing the applied charge in the top layer. ‘Doubling the charge results in a four times efficiency increase, roughly to forty to fifty percent, which is more than double the efficiency of solar panels,’ says Mendel. Although the technology is simple and elegant, it will not easily solve the energy problems. Mendel: ‘The most important limiting factor to widely apply the new technology and generate electricity at a large scale is that it just doesn’t rain that often in the Netherlands. The total energy in drops that fall on a surface per year is simply not so is big.’ Even when placed in a rainforest, the approximately five times gain in energy generation would not be sufficient to compete with solar panels in The Netherlands.

Considerable optimization

However, for large-scale applications, instead of falling rain drops, other sources of fluid motion can also be used, for example the energy in waves at sea. According to Mendel, the efficiency and amount of energy extracted of such a system, depends very much on its design. For example, it is more efficient to have multiple smaller, connected systems, than a single large one. In addition, the system will have to be really robust to withstand the forces of the waves. Mendel: ‘It will take considerable optimization efforts to make this work efficiently. And then the question remains how much energy can eventually be generated.’

‘It will take considerable optimization efforts to make this work efficiently'  

Realistic applications

Because of the challenges to make the system work for large-scale generation of electricity, the scientist sees more realistic applications in small devices though. As a kind of side application, the method can be applied as a very accurate rain sensor: each raindrop results in an electrical current, which size depends on the drop size. The total electricity generated is a measure for the amount of rain.

For a wider spectrum of applications on a smaller scale, the system can be redesigned to generate energy from movement. If the water droplets are replaced by a conducting plate that can move to and from the charged top layer, the kinetic energy resulting from movement, can be converted into electricity in a similar way as falling raindrops. Mendel: ‘Such a device can more easily be applied in small, portable equipment, like a watch and can be applied in the near future. The arm movement of the person carrying it can thus be used to move the plate and generate energy for the watch.’

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