Energy Harvesting From Falling Raindrops

The IoT explosion is well on its way and it’s here to stay, whether we like it or not. Much has been said already about the major hurdles that stand in the way of a seamlessly connected world of everyday things. Possibly the biggest of these is the power bottleneck that is inevitable from using battery-based solutions. On the other hand, there is burgeoning interest, all over the world, in coming up with ingenious energy-harvesting solutions to the power woes that have, so far, plagued IoT growth. 

At ONiO, we don’t think batteries are going to continue playing a significant role in the long term future of the Internet of Things. The IoT landscape is evolving at a breakneck pace and the key stakeholders involved are frantically trying to stay ahead of the curve. Energy harvesting has emerged as our best bet to secure a sustainable IoT future, where we won’t have to rely on our fast diminishing mineral reserves or decades-old power technologies that are practically obsolete. 

We’ve already all but shouted from the rooftops that we don’t think the gargantuan number of IoT devices that we are heading towards can realistically be powered by batteries - no matter what kind. And again, we’ve made no secret of the fact that we strongly believe energy harvesting to be the future when it comes to powering IoT devices. 

We’re not going to labour the point and be repetitive - Long story short, battery-based power solutions for IoT devices suffer from all sorts of shortcomings - To find out more about why batteries don’t represent an elegant power solution for IoT devices, check out our posts here or here. 

Background

Everyone and their grandmothers are savvy with solar energy today. Globally, we’ve come a long way towards embracing solar energy - be it through large-scale installations of panels or our beloved battery-powered cars. 

However, it doesn’t take a rocket scientist to understand that solar panels are all but useless when the heavens decide to open up. In a seemingly cruel twist of irony, some of the wealthiest countries in the world, which seem to be the most ideally positioned towards implementing solar-based energy solutions on a large enough scale, don’t get a whole lot of sunshine for big chunks of the year. 

But, for many years now, there has been widespread speculation about the possibility of harvesting practically usable amounts of energy from the falling rain. 

The principle is fairly straightforward - raindrops fall onto the ground with a significant amount of force in the form of mechanical energy. When these falling raindrops hit the ground, the impact causes tiny perturbations, which can then potentially be harvested and converted into usable electrical energy. 

But, How much energy could we possibly get from this? Not a lot right? It seems hard to comprehend that the impact of a few raindrops here and there could amount to anything significant. 

For the better part of two decades, this was also the broad scientific consensus on the matter. It was believed that looking into harvesting rain energy was far from realistic and that it wouldn’t translate into an economically viable, real-world solution. 

Today however, the landscape looks entirely different from even back in 2008, when some of the first serious studies on this topic were undertaken. Granted, this may not be a viable solution to our large-scale energy needs. But, recent efforts have highlighted just how much potential there could be when in powering autonomous microsystems. The massive strides we have made in semiconductor technology seem to warrant another serious look into the previously untapped potential of rain energy. 

“Water droplet energy is indeed small, so that people did not realize such energy can be harvested previously. But with the rapid development of ‘Internet of Things,’ many smart devices raise demand for the distributed sensors and energy sources.” 

What do the numbers look like?

It is important to bear in mind that any estimation of the kind of potential there is in rain-based energy harvesting will be relative. New technologies and modalities are cropping up constantly and are bound to alter the figures. Moreover, there is the obvious caveat that the numbers will vary from place to place and will also change temporally. 

This section is aimed more at giving a ballpark estimate of the figures - to show just how much potential there is in this. 

In their 2013 analysis, Lin et al, suggested that in France, the total available rain power could amount to about 1Wh per square meter per year. For comparison, in a country like Bangladesh, which typically has a significantly more wet climate, the potential energy output from rain could amount to a few KWhs per year, just from the monsoon months (June to September).

These may not be significant enough to power industries and keep lights on, but the numbers suggest that there is enough potential here for IoT systems and sensors. Even in temperate countries, energy harvested from falling raindrops is more than sufficient to power sensors, RF transmitters, and control units. And going by the progression of these technologies, in the near future (albeit in tropical countries), we might be able realistically power small electronic/electrical devices using raindrops. 

“Water droplet energy is indeed small, so that people did not realize such energy can be harvested previously. But with the rapid development of ‘Internet of Things,’ many smart devices raise demand for the distributed sensors and energy sources,” says Hao Wu, who published a paper on the subject. 

How do raindrop harvesters work?

Without getting too much into the dreary details, here’s the gist of how raindrop harvesters work.

The basic principle is that raindrops, when they hit the ground, cause an impact due to the kinetic force of their movement; This energy, when it hits the ground (or the harvester surface, in this case) causes small vibrations which can be harvested and converted into small amounts of electricity. 

When the raindrops fall on a dry surface, they tend to cause splashes and spread around, which results in a waste of energy. The presence of a “wet layer” on the harvester changes the scenario quite a bit. The weight of the water that is present on the surface adds to the total mass of the setup and therefore, reduces the frequency of the system. This means that when water is collected on the harvester, it vibrates along with the system, causing a damping effect. 

And when enough water is collected on the harvester surface, the impact caused by raindrops generates ripples which cause a further increase in gravity loads on the surface. Most raindrop energy harvesters are designed around this basic principle. Cantilever designs tend to be the most popular for such energy harvesters. 

Check out this journal article if you understand how piezoelectric raindrop harvesters function in more detail.

Powering 100 LEDs from raindrops?

A team from the City University of Hong Kong has recently, led by biomedical engineer Wang Zuankai has recently come up with a method to generate a voltage upwards of 140V (enough to power about 100 small LED bulbs), from 100ml of water dropped from 15cm!

This might sound like witchcraft or worse, clickbait! But, the scientists had to use a bunch of canny science tricks to make it happen. 

“The significance of this technology is the much enhanced electric power per falling rain droplet, which makes the device much more efficient to convert energy from a falling droplet to electricity” 

- Xiao Cheng Zeng, University of Nebraska-Lincoln.

One of the unique features of the team’s droplet-based electricity generator (DEG) was that they made use of a chemical called polytetrafluoroethylene (PTFE). That might sound like a mouthful, but believe it or not, it’s a chemical we’re all familiar with. Known more popularly as “Teflon”, this is the chemical that goes into the making of our household non-stick cookware. 

The team’s DEG contains a PTFE film which is able to progressively accrue surface charge as it continuously gets hit by water droplets until it reaches a point of saturation. The team found, much to their amazement, that as the droplets impact the surface and spread out, the drops function as a “bridge” of sorts that connects two electrodes - al aluminum electrode and an indium tin oxide (ITO) electrode (with Teflon).

This bridge acts as a closed-loop circuit. And ingeniously, the droplets and the surface coating perform the roles of resistor and capacitor respectively! The device also features a structure that resembles a field-effect transistor (FET), that allows for high energy-conversion efficiency. This results in the device’s power density jumping up thousands of times when compared to those without a FET structure. 
 
The significance of this technology is the much enhanced electric power per falling rain droplet, which makes the device much more efficient to convert energy from a falling droplet to electricity, says Xiao Cheng Zeng, from the University of Nebraska-Lincoln, about the breakthrough project. 
 
Click here to find out more about the project from CityU’s website. 

What this means and what lies ahead?

The human industrial enterprise has reached a tipping point of sorts. The rapid economic growth of the past few centuries has resulted in most of us enjoying unimaginably high standards of living. But the sorry fact still remains that this has come at a grave cost to the planet. 

The energy crisis that we are facing today requires all the brainpower that can be thrown at it. Innovations such as this DEG might not become commercially available for quite a few years yet. Nevertheless, these breakthroughs are crucial for us to feel optimistic about our future as a race on this planet. 

In our race to find more sustainable energy sources, we’ll take every drop we can find!