Rain Power: Harvesting Energy from the Sky

January 22, 2008 By Lisa Zyga, Phys.org feature
Rain Power: Harvesting Energy from the Sky
The testing apparatus controlled drop size, frequency, and height, as drops of water fell and impacted a piezoelectric material at the base. Credit: Romain Guigon, et al.

Researchers who study energy harvesting see energy all around us – we just need to find a way to capture that energy. One of the latest energy harvesting techniques is converting the mechanical energy from falling raindrops into electricity that can be used to power sensors and other electronics devices.

Scientists from CEA/Leti-Minatec, an R&D institute in Grenoble, France, specializing in microelectronics, have recently developed a system that recovers the vibration energy from a piezoelectric structure impacted by a falling raindrop. The system works with raindrops ranging in diameter from 1 to 5 mm, and simulations show that it’s possible to recover up to 12 milliwatts from one of the larger “downpour” drops.

“Our work could be considered as a good alternative to power systems in raining outdoor environments where solar energy is difficult to exploit,” Thomas Jager told PhysOrg.com. He explained that the system could be used for both mobile outdoor devices as well as indoor power. “For example, we intend to develop remote sensor nodes in cooling towers, but abandoned sensor networks are also one of the foreseen applications for this type of system.”

As Jager and coauthors Romain Guigon, Jean-Jacques Chaillout, and Ghislain Despesse explain in a recent issue of Smart Materials and Structures, the physics of how a raindrop impacts a surface is not fully understood. However, to build a rain energy harvesting system, the important part is to estimate the recoverable energy during the impact.

When a raindrop impacts a surface, it produces a perfectly inelastic shock. The amount of energy generated by the impact can then be estimated using a mechanical-electric model.

To capture the raindrops’ mechanical energy, the scientists used a PVDF (polyvinylidene fluoride) polymer, a piezoelectric material that converts mechanical energy into electrical energy. When a raindrop impacts the 25-micrometer-thick PVDF, the polymer starts to vibrate. Electrodes embedded in the PVDF are used to recover the electrical charges generated by the vibrations.

The group experimented with raindrops of different sizes, falling heights, and speeds. They found that slow falling raindrops generate the most energy because raindrops falling at high speeds often lose some energy due to splash. By using a micropump to generate and test the properties of raindrops, the researchers demonstrated that, for low drop heights, the electrical energy is proportional to the square of the drop’s mechanical energy, while voltage and mechanical energy are directly proportional.

The largest raindrops caused the largest vibrations on the PVDF, and therefore generated the greatest amount of electrical energy. The researchers demonstrated that their system could generate 1 microwatt of continuous power as a worst-case scenario, while simulations showed that a single large raindrop might generate up to 12 milliwatts of power.

“The recoverable energy depends directly on the size of the piezoelectric membrane, the size of raindrops, and their frequency,” Jager explained. “The available energy per drop varies between 2 µJ from 1 mJ depending on its size.

“The corresponding instantaneous converted power starts from a few µW up to 10 mW for a converter area of a several square centimeters. An interesting figure to keep in mind could also be the available rain power per year in common France regions with a continental climate: almost 1 Wh per square meter per year.”

In the future, the scientists plan to develop a method to store the electrical power to provide a steady current for practical use.

More information: Guigon, Romain, Chaillout, Jean-Jacques, Jager, Thomas, and Despesse Ghislain. “Harvesting raindrop energy: theory” and “Harvesting raindrop energy: experimental study.” Smart Mater. Struct. 17 (2008) 015038-9.

Copyright 2008 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

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2.7 / 5 (3) Jan 22, 2008
Quite clever, but honestly now... Just how practical could this be?
3.8 / 5 (5) Jan 22, 2008
"1 Wh per square meter per year"

A common rechargeable AA battery can have a capacity of 1600 Ah, or 2.4 Wh of energy. It will take almost two and a half years to recharge with a 1m^2 rain collector.

This is unbelievably insignificant. Why would they even bother with it?
4.2 / 5 (6) Jan 22, 2008
For reference - average annual sunlight power density is more than 100 watts per square meter even in Antarctica.
3.3 / 5 (3) Jan 22, 2008
Renewable Energy for Weather Sensors and stuff, wind/solar would be far more practical. This doesn't seem to be useful.
4.1 / 5 (7) Jan 22, 2008
Why not implant piezoelectrics continuosly under the freeways and roadways surface.

As cars and trucks pass over, the weight of the vehicles would compress the piezoelectric generating alot more electricy per minute.

Not patentable now as this idea has been disclosed on this comment screen.

2.3 / 5 (3) Jan 22, 2008
They'd be better off making a rain bucket engine....
1.8 / 5 (5) Jan 22, 2008
That's so insignificant it's ridiculous...
2 / 5 (5) Jan 23, 2008
BIG DEAL! my daughter built a tabletop device like this in 1980 for her sixth grade science fair project. no government grants were used LOL.
3.3 / 5 (7) Jan 23, 2008
You people who mock this are incredibly shallow, and closed minded. And I hope that you are here in search of a change of thinking and not just visiting to hear yourself speak. Otherwise, I can't imagine why you would come to a site like this. This is a quite brilliant technology. This idea will inevitably lead to something bigger. We need positive thinkers in the world today, and not those who only look for a way to complain or mock everything.
3.3 / 5 (7) Jan 23, 2008
Solar power seemed pretty insignificant in its infancy. Look where it is now? What if our roof tiles were made of a material which converted rain drops into energy? But, think five or ten years into the future when this technology is more advanced. Or what it turns into...

Twenty years ago, we were not able to power exterior lights with 3 inch solar panels. Be we can do it now.
3.3 / 5 (4) Jan 23, 2008
True this technology may have room to grow, but it's going to have to grow a LOT to become useful. At this point, the energy used to produce it is probably much more than the energy that it can give over it's ENTIRE lifetime.
3 / 5 (2) Jan 23, 2008
Most of the technology "what ifs" discussed previously have all been researched and could be in every building and home today,only if we stop the use of nonrenewable fuels for our power needs.We will have to deal with it someday or at least our grandchildren will have to.
4 / 5 (1) Jan 23, 2008
Hey visual (01/22/08 11:41) - where can I pick up a couple of those 1600Ah AA batteries? They would really change the world as we know it! ;-) and nice work CEA/Leti-Minatec... I like the idea of a hydro-electric power plant on my roof, without the hassles of all that turbine maintenance!
1.7 / 5 (3) Jan 23, 2008
Why not implant piezoelectrics continuosly under the freeways and roadways surface.

As cars and trucks pass over, the weight of the vehicles would compress the piezoelectric generating alot more electricy per minute.

Not patentable now as this idea has been disclosed on this comment screen.

if the polymer was put under a solar panal, there could be power with/without sunlight and if used to cover buildings with a lot sound from machines runing/ music playing industry could be increased to allow an increase of industry that is big enough to let the U.S. outcompete china and take over the world with eco-friendly economic weapons of mass production.
4.3 / 5 (3) Jan 24, 2008
Unfortunately sound energy is very small scale smarterthanu. I seem to remember reading that at current conversion rates it would take a human screaming for thousands of years to produce enough energy to heat a single cup of coffee. Or something like that.
3.5 / 5 (4) Jan 24, 2008
Perhaps I'm a pedant, but a single large raindrop ought to be quoted in terms mJoules of energy - not 12mWatts of power (or if so with a statement of pulse duration). Perhaps they mean that large raindrops falling at the rate of 1 drop per second produces 12milliWatts. Not indicative of good science.

1.8 / 5 (5) Jan 26, 2008
Hey guys check out my new patent for piezoelectrics under roadways.
3 / 5 (7) Jan 26, 2008
Just because you have a limited imagination and can only think in terms of how this technology would help you, by powering your high-energy-demand consumer appliances, it doesn't mean there aren't other applications for this technology. Powering remote sensors springs to mind.
1 / 5 (1) Jan 26, 2008
Because a compact universal energy device would just be too "limited."
1 / 5 (1) Jan 29, 2008
First rain drops, how about the energy available in micro wind? If they can get a material to harvest raindrops how about the slightest movement of wind on a seemingly flat surface?
4 / 5 (1) Feb 10, 2008
Why not just put a turbine in the downspout of rain gutters? Wouldn't that be a simpler system since it could be all in one (or at most a few) places rather than dispersed over a broad area of a roof?
1 / 5 (1) Mar 19, 2008
Hey visual (01/22/08 11:41) - where can I pick up a couple of those 1600Ah AA batteries? They would really change the world as we know it! ;-)

Right, good catch - it is mAh of course.
I did use the correct figure in the conversion to watt hours - the result is still 2.4

So my post is still valid. But it seems it flew right over most people's heads.

1Wh per year - that is 0.000114079553 watts
Compared to sunlight's over 100 watts in even the poorest conditions.
... very useful, yeah.

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