All kinds of surprising things can generate an electrical charge, even rainwater. Probably not many of us think of getting electricity from rainwater, but scientists are looking at it.

Students in Mexico even devised a working prototype for one idea.

In this post, we look at several ideas under study. Some will require considerable time and refinement before they come to a rooftop near you. Some of them may never be practical. But it’s always interesting to watch experts poking around in odd corners to see what they might find.

Principles of hydropower

Hydropower, or waterpower, uses water falling on a turbine from a suitable height. The force of the water turns the turbine. A dynamo connected to the turbine converts that mechanical energy into electricity. Usually, a dam over a river supplies the height and volume of water to run the turbine.

But what about water falling from an even greater height? Like rainwater.

The average annual rainfall in the US is about 77 cm. For easier arithmetic, though, let’s use 100 cm. The standard roof on a house is about 200 square meters.

That amount of water on that size roof amounts to about 20 kilograms of water per year. It falls from clouds at an elevation of about 2000 meters. During the time it falls that distance, it accelerates.

If used for hydropower, it could produce about 111 kilowatt hours. That’s not much, but add together enough roofs to make up a square kilometer. That amount of energy turned to electricity would power about a thousand houses.

A small scale project of getting electricity from rainwater

There’s no need for hydropower to work on a massive scale. In 2014, students at the Technological University of Mexico designed a system that makes electricity from the rainwater that falls on a single roof. They called it Pluvia.

In this case, electricity is a byproduct of purifying the rainwater to make it safe to drink. Poor neighborhoods in Mexico City, and in much of the rest of the world, lack clean water. That’s in part because it requires energy to purify and distribute water, and these same neighborhoods also lack dependable electricity.

Here’s how Pluvia works:

• Rainwater falls on a roof that may or may not have a gutter and downspout. If it doesn’t, the system adds sheeting to the roof to create a slope to channel the water to a specific outlet.
• The water passes through a filter to remove all the pollutants the water has absorbed on the roof. From there it goes into a tank.
• Water flows from the tank over a microturbine. It’s ten inches high and two inches wide. The spinner fits in a half-inch pipe.
• The turbine charges a 12-volt battery.
• After flowing past the turbine, the water passes through a charcoal filter for more purification.
• The purified water meets or exceeds the standards of Mexico City’s network water supply.

As first implemented, the turbine didn’t operate the pump, but it did supply some light in the house. I haven’t found a more current description. It has probably been refined. Like all experimental technology, it will take long effort and refinement to make it practical for widespread use. Meanwhile, whatever good it does is more than those homes had before.

The promise of graphene

Numerous academic research teams are exploring a revolutionary new material called graphene. It is a single layer of carbon atoms arranged in a hexagonal lattice structure.

It’s much thinner than a sheet of paper, yet stronger than steel. Among its other features, it holds energy. Therefore, it has valuable potential for renewable energy.

It can serve as an anti-reflective coating for solar cells. And here’s where it gets interesting. Researchers at the Ocean University of China found that it can generate electricity from rainwater.

Pure water exists only in the form of distilled water, not in nature. The minerals in rainwater contain both positive and negative ions. Somehow the graphene can convert these unbalanced charges into electricity.

Solar panels produce less electricity in rainy conditions than in sunny conditions. In principle, a graphene coating can have the added benefit of boosting the panels’ capacity to generate electricity in the rain.

The material is too new for scientists to know what all it can potentially do. Many applications, including electricity, require much more research before they can become both practical and commercially viable.

So far, I have described some ways of generating electricity from rainwater that appear to be useful in the foreseeable future. Now, let’s look at a couple of pieces of basic research.

The triboelectric effect and the droplet electricity generator.

Researchers from the University of Nebraska at Lincoln and the City University of Hong Kong have been working on a “droplet electricity generator” (DEG) for the past two years.  They recently announced that they produced 140 volts of electricity from a single raindrop.

They coated the generator with tetrafluoroethylene, a hydrophobic polymer. Hydrophobic means that it avoids water. It doesn’t get wet. It also has a unique property scientists call a quasi-permanent electric charge.

When a drop of water falls on this coating, it bridges the gap between two electrodes. It releases whatever electric charges exist on the polymer in what is called the triboelectric effect. That is, some materials get a charge when they come in contact with another material and then come apart.

Static electricity is an example of the triboelectric effect. It’s not very strong or very predictable, but the three-terminal field-effect transistors in the DEG greatly increased the energy by thousands of times over previous attempts to get electricity from a raindrop.

The piezoelectric effect

Rainwater generally doesn’t have enough volume to run a very large turbine, but raindrops can fall on piezoelectric materials. Those materials can generate electricity.

“Piezo” is a Greek word that means to squeeze or press. Piezoelectric materials under stress become deformed, which upsets the balance of electrical charges in them. Quartz is a common example. It has long been used in clocks, and more recently in electronic devices, to keep time.

So rain falling on a piezoelectric substance bends it out of shape and makes electricity. Just not very much.

But raindrops are free. With a little imagination and a lot of hard work, it may be possible to generate usable energy from a good hard rain and do it economically.

This idea appears to be the least practical of the ones in this post. On the other hand, plenty of breakthroughs have happened from research that, at first, doesn’t look very practical. So check again in about fifty years to see if anything came of using piezoelectric materials to make electricity from rainwater.

Sources:

Can we generate electricity from rainfall? / Rajat Lunawatt, ScienceABC. June 28, 2019 (Updated January 15, 2020)
Little turbine filters rainwater and makes electricity / Janet Fang,, ZDNet. April 4, 2014
The next renewable energy source could be rain / Irina Slav, Oilprice. February 16, 2020
Solar cells that could generate energy from rain / Robin Whitlock, Interesting Engineering. April 13, 2016

Photo credits:
Raindrops on window. Image by Wolfgang Mennel from Pixabay
Hydroelectric power diagram. Public domain from Wikimedia Commons
Rain on solar panel. Some rights reserved by h080
Cat demonstrating static cling. Public domain from Wikimedia Commons