13-07-2023 | By Robin Mitchell
As researchers try to explore new power sources for a renewable future, some have turned their attention to hygroelectric systems, those that harness electricity in humid air. What exactly is hygroelectricity, what have researchers done regarding this field of study, and is it a viable power source for future electronic systems?
What exactly is hygroelectricity?
Upon reading the term hygroelectricity, you may think that this is a typo and is supposed to say hydroelectricity. Well, no matter how much my spell checker wants to change it to hydroelectricity, hygroelectricity is indeed a real thing. While hydroelectricity is concerned with extracting electrical energy from flowing water, hygroelectricity is the act of extracting electricity from humidity.
The best example of hygroelectricity in the natural environment is lightning. While the process is not fully understood, it is generally believed that as warm air rises, the motion of tiny water droplets results in a build of static charge, and when the charge difference between two extremes in a cloud becomes too great, dielectric breakdown results in an instantaneous release of electrical energy (i.e., a lightning bolt). Of course, the process is far more complex, with streamers looking for opposing charges and the direction of current, but for the sake of this article, moving water droplets carry charges as they rub against objects such as dust particles and each other.
While humans have known about lightning for as long as humanity has existed, the idea that moving moisture generates static charges called the Armstrong Effect, was only recognised in the 19th century, when a train operator named Patterson noticed that he received electric shocks after touching steam emanating from the train. After telling others about the phenomenon, it would be William Armstrong who, with a team of others, would go on to investigate the phenomenon and understand its mechanism.
Researchers exploring hygroelectricity as a source of power
Despite hygroelectricity having been discovered well over a hundred years ago, it is only recently that some researchers have turned their attention to hygroelectricity to see if it can be used as a viable power source.
In the case of one research team, their journey to harness electricity from humidity came by accident. Initially, the researchers from the University of Massachusetts Amherst were developing a humidity sensor, but during testing, the student operating the device forgot to turn the power on. Despite having no external power, the sensor was still producing a voltage, indicating that it was generating electricity from humidity.
After realising their mistake, it sparked intense research into materials and design structures to find out how humidity can generate electricity and what an ideal generator would look like. Interestingly, it was discovered that the shape and structure of the material is far more important than the material itself, and this is generally believed to be because the motion of the water droplets generates power and not the interaction between the water droplet and the material. Of course, an exchange of charge is required, so not all materials are ideal for a hygroelectric generator, but using a more efficient charge-carrying material is not as important as its structure.
Moreover, the research team from the University of Massachusetts Amherst has made significant strides in the field of hygroelectricity. Their accidental discovery of a humidity sensor generating electricity without external power led to the development of a new type of hygroelectric generator. This generator, which uses a unique structure of carbon nanotubes, has shown promising results. The high surface area and excellent conductivity of the carbon nanotubes make them an ideal material for hygroelectric generators. Their research has opened up new possibilities for the practical application of hygroelectricity1.
Understanding Charge Separation in Hygroelectric Generators
In another study published in Nature Communications, researchers found that the charge separation in hygroelectric generators is primarily due to the difference in humidity between the surface and the interior of the material. This difference in humidity leads to a difference in the amount of water absorbed, which in turn leads to a difference in the amount of charge that can be transferred. This finding has significant implications for the design and efficiency of future hygroelectric generators2.
In another interesting development, Yao and his colleagues published a scientific paper in 2020 that described how tiny protein nanowires, produced by a bacterium, could harvest electricity from the air. The exact mechanism is still under discussion, but the material's tiny pores appeared to be able to trap floating water molecules. As they rub against the material, the water molecules also appear to lend it a charge. Yao explains that, in such a system, most molecules stay near the surface and deposit lots of electrical charge while a few others penetrate more deeply. This creates a difference in charge between the upper and lower parts of the material layer. This insight was shared by Chris Baraniuk in a BBC article.
Furthermore, a European Union-funded project called Catcher is also exploring ways to extract electrical energy from humid environments. They have developed small discs made from zirconium oxide that can trap water molecules and force them down tiny microchannels, stripping the water molecules of any static charge. Although their work has not yet been peer-reviewed, it represents another exciting development in the field of hygroelectricity1.
In a recent study published in Advanced Materials, researchers developed a new type of hygroelectric generator that uses a unique structure of carbon nanotubes. This structure allows for a high surface area and excellent conductivity, making it an ideal material for hygroelectric generators1.
The researchers also noted that some of the water droplets penetrate deep into the material while others remain at the surface. Those at the surface are able to transfer more charge, while those inside the material cannot. As such, this charge separation at different layers can be exploited to generate an electric current.
A study in Nature Communications further supports this observation. The researchers found that the charge separation in hygroelectric generators is primarily due to the difference in humidity between the surface and the interior of the material. This difference in humidity leads to a difference in the amount of water absorbed, which in turn leads to a difference in the amount of charge that can be transferred2.
In the research team’s most recent publication, devices thinner than a human hair have been able to generate a fraction of a volt (as high as 600mV), but the combination of these generators could result in significantly greater output voltages. While not enough to power a large computer, it could be sufficient for powering ultra-low energy processors used in IoT devices if scaled up. Furthermore, the researchers have managed to show energy generation at humidity levels as low as 20%.
Demonstration of water molecules creating charge separation – Courtesy University of Massachusetts Amherst
Another team of researchers working on a European Union-funded project called Catcher is also looking to find methods of extracting electrical energy from humid environments. In their case, small discs made from zirconium oxide can trap water molecules and force them down tiny microchannels that trip water molecules of any static charge.
Again, like the researchers from the US, the disc developed by the European researchers is only able to power small devices such as LEDs. However, as their work has not been peer-reviewed, their claims must be taken with a pinch of salt.
Are hygroelectric systems a viable energy source?
While hygroelectric systems developed by researchers have proven to work, they are far from being ideal energy sources, and it is highly likely that they won’t be used in the near future. The tiny amount of energy they provide combined with the need for a humid environment restricts where they can be used, and even though researchers have noted that their devices generate electricity starting at 20% humidity, this doesn’t mean that the electricity is useful immediately.
However, a recent development in the field of hygroelectricity might change this perspective. A study published in Energy & Environmental Science presented a flexible, low-cost, and scalable hygroelectric generator made out of Kraft paper coated with exfoliated and reassembled graphite (ERG). This generator can deliver 250 nA of electric current through a 2 MΩ resistor for days, and its voltage and current outputs can be scaled up by connecting multiple generators in series or parallel. This "green" alternative for producing electricity could pave the way for more practical applications of hygroelectric systems3.
The same can be said for triboelectric generators. Even though they are able to turn the movement of an ant into electricity, the quantity produced is so small that it has no practical application. As such, it might be that a sensor powered by a humidity hygroelectric generator will only operate in high-humidity environments.
Furthermore, the tiny amount of energy available in humid air means that larger systems, such as houses and cars, will never be powered by hygroelectric systems. Instead, it is likely that hygroelectric devices will be IoT, smart systems, and tracking devices utilising low-energy technologies.
This isn’t to say that what the researchers have developed isn’t exciting. But when it comes to energy harvesters, it is essential that we manage our expectations.