Total rainfall volume per m^2 is .025 m^3/hour. This is approximately 500,000 randrops/hour or about 14 drops/second. Each drop has 1/2 * m * V^2 = 25 mJ of energy.
So putting it all together, this is generating 25 mJ/drop * 14 drops/second = .35 W/m^2, and that's only when its raining. (Edit: and this is assuming 100% conversion efficiency, which....no. Don't know anything about this technology, but probably cut that number in half again).
Sounds a lot like Solar Freakin Roadways.
Edit: Just a sidenote; back in college the best course I took was billed as a "Renewable Energy" but was really just a weekly set of unit conversion problems like this that proved how absolutely stupid most energy proposals are.
We did focus a fair amount on real technologies like Wind and Solar (and analyzing the shortcomings like storage, which haven gotten better since ~2009). The professor took a lot of joy in shooting down ideas like this though.
I wasn’t sure about the droplet analysis so took your same numbers (25mm/h, 10m/s) and just worked out aggregate mass: 25mm over 1m^2 = 0.025m^3 = 25kg
0.5mv^2 => 1250J/h… so looks like we agree.
And to add a simple economic analysis of why this is such a dead-end idea:
Mawsynram, in India, is apparently the rainiest city in the world with roughly 10,000mm of annual rainfall - 10x the global average.
A given rain energy harvesting panel, deployed there, would generate 500,000J/yr… or 0.138kWh. That’s significantly less than what a typical rooftop 1m2 solar panel would generate in an hour on a sunny day. 0.138kwh is worth around 1.3cents at 10c/kWh.
A big roof might get you $1-$2/year. You couldn’t pay to clean your roof for that. You couldn’t even pay someone to answer an email enquiry about the install costs for your system for that. This solution would have to be VASTLY cheaper than paint to stand a chance of being viable.
There is a reason our existing systems to collect power from rainfall rely on vast existing landscapes and aggregation mechanisms (rivers) to concentrate the rainfall for us.
It is - in my view - a dead idea.
Clearly storm systems can accumulate a large charge differential with the ground (i.e. lightning), but I don't know if that's the principle behind rain drop charge harvesting. Cursory googling[1] tells me electrostatic charge may be the source?
1 - https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.2023015...
Rain drops have been calculated to charge up to about 1/50 of an esu (electrostatic charge unit): https://agupubs.onlinelibrary.wiley.com/doi/10.1029/TE040i00...
Their paper is reporting 70V, so Joule/drop is on the order of 3.7E-11 J https://www.wolframalpha.com/input?i=%282.65E%E2%88%9211%29%...
(many, many orders of magnitude below the kinetic energy per drop)
It's important to note most of the research they've published is not using real rain, but laboratory generated droplets that is intended to push the harvester to the maximum.
While we're at it, can we extract any energy from lightning? Not sure if the blocker is being able to store energy delivered over a short period of time, or if it's the unlikeliness of a purpose-built structure being hit by lightning. Probably both.
> A tourist arrives in Vancouver on a rainy day. He gets up the next morning and it's still raining. In fact, it's still raining three days later. He goes out to supper and spies a young kid. Out of despair, he asks, "Hey kid, does it ever stop raining around here?!" The kid says, "How should I know, mister? I'm only six."
That seems like an utterly fanciful figure for kinetic harvesting, and AFAIK the droplet charge also wouldn’t be enough? What am I missing here?
“The peak power output of the bridge array generators is nearly 5 times higher than that of the conventional large-area raindrop energy with the same size, reaching 200 watts per square meter,” Li explained, “which fully shows its advantages in large-area raindrop energy harvesting.”
.35 W/m^2 vs 250 W/m^2 (after conversion losses) for solar, 2 solid orders of magnitude more energy. Even in cloudy conditions, solar is still going to deliver 15 W/m^2.
You'd be better off harnessing the power of wildlife to turn hamster wheels than spend money for the occasions to harvest a few joules when it rains. I imagine the operational costs and idle losses swamp any gains this system could ever hope to realize.
"Solar Roadways: When failure is worth 30 MILLION Dollars!"
"EEVblog 1534 - Solar Freakin' RAILways!"
There are apparently something like 30M lightning strikes per year (about 1 per second!), and random sources I found suggest the average strike has an energy north of 1GJ.
1GW is somewhat respectable. Good luck collecting it.
Worse, power-generating sidewalks.[1]
"Our kinetic energy solutions are inspiring brands all over the world to create a meaningful and lasting connection with stakeholders around sustainability and ESG practices. Our award-winning kinetic technology uniquely uses the renewable energy generated by a footstep, with the excitement of highly engaging experiences, to educate and inspire stakeholders."
Virtue signalling monetization as a business. They've been at this for 13 years now. "Crowdfunding soon".
> and analyzing the shortcomings like storage, which haven gotten better since ~2009
But then realised: it makes no sense. You’ve written “haven” — and I can’t tell if you meant to type “have” or “haven’t” … I mean “haven” falls right in between the two, your meaning is completely lost.
Overall the lecturer who relishes demolishing an idea… I grew up with that kind of intellectual attitude and had to do a lot more growing up before I realised that it’s a terrible habit in an educator. Taking actual joy in the debunking alone - that’s not the kind of joyful curiosity that a great educator would pass on, and it’s an abuse of young minds to act as if it is.
Looks like they ended up getting over $6m in funding. I can’t tell how alive they are but they received some FCC approval for the wireless connectivity in Jan 2022.
“So you are saying there’s a chance?”
A turbine on a down pipe isnt really worth it, so there's no way this will be.
The only plausible thing I can think of is some extremely low power rain sensor.
I'm sure that's a huge BUT for an engineer currently, I'm not trying to say it's easy or even doable, but that's the only reasonable use I could see, if it's just a touch extra energy for a solar panel, I could see it as a value-add, if the added cost is low enough.
Which means something that's engineered is made better by successive improvements from previous work.
2nd this is failing to consider different environment conditions and applications may make gathering energy from the environment in creative ways practical and useful.
Not saying this particular technology will eventually be practical from a commercial standpoint, only wishing to state it's more than just 'will this technology easily solve global energy demands'.
I agree we should keep an open mind regarding creative ways of collecting energy from the environment. But we should also abandon those which are quickly demonstrated to have no meaningful potential even if we were to perfect them.
The first iteration produces 0.5 units. The next produces 0.75. The third produces 0.875, then 0.9375 and so on. Each iteration improves on the previous, but no engineering will surpass the 1.0 limit set by fundamental physics.
> may make gathering energy from the environment in creative ways practical and useful.
There's no end of crappy ways to produce electricity. Get electricity from walking across the floor? Yep [1]. Get electricity from exercise bikes at the gym? Yep. Get electricity from plants? Yep [2]. "Solar Freakin Roadways"? Yep. [3]
[1] https://en.wikipedia.org/wiki/Pavegen#Criticism
[2] https://www.sciencedaily.com/releases/2018/12/181212093308.h...
[3] https://en.wikipedia.org/wiki/Smart_highway#Solar_road_panel...
> it's more than just 'will this technology easily solve global energy demands'.
You know how I know it's not a "potentially game-changing breakthrough in energy harvesting"? The best place to use this is not in extracting power from rainfall but in ultra-low-head hydropower energy production. Let the water fall on the panel as drops, and extract power.
The energy involved corresponds to a head of about 3m, so it needs to be more efficient than other ULH methods, which already exist.
Yet there's no mention of that application in the article. It's not like I'm an expert on the topic, so I conclude this is a solution in search of a problem.
Gatekeepers of domain knowledge usually do.
If an idea doesn't work on the back of a napkin, it's done. Think about other approaches to the problem and get another napkin. I think that's what he's getting at. Analysis from first principles is hardly gatekeeping.
You can—and it's usually safe to, in the translation from actual researchers to PR departments—ceertainly doubt the viability of this claim, but the whole article is devoted to at least explicitly claiming that they're aware of the obvious problems, and can overcome them:
> … the efforts to collect energy from falling raindrops have faced a technical hurdle that has made the concept inefficient and impractical. … as one might expect, the amount of power per raindrop is incredibly small. …
> Now, a team of researchers says they have found a design and configuration that greatly reduces the coupling capacitance issue and one they claim could make energy-harvesting rain panels a practical reality.
One key seems to be that, although you're obviously meant to think of rooftop panels, it seems more to be about large-scale installations:
> “The peak power output of the bridge array generators is nearly 5 times higher than that of the conventional large-area raindrop energy with the same size, reaching 200 watts per square meter,” Li explained, “which fully shows its advantages in large-area raindrop energy harvesting.”
(I do notice on re-reading that it says "… may lead to the development of rooftop, power-generating rain panels", but (1) one can freely claim that anything may lead to something else, and (2) one can claim that anything may lead to anything else, so I prefer to go by quotes from the researchers themselves.)
So your hypothetical 200 watts per square meter at 100% efficiency assuming spherical cow types of ideal conditions could possibly provide that power for 50 minutes per year under ideal circumstances or average 0.02w/m2 over a year. And average roughly 0.002w/m2 in extremely rainy though not world record setting locations.
Now, you ignore the kinetic energy in rainfall and try and harvest the potential energy when it lands at a high altitude, allowing a large collection area and large differences in altitude. But we call those things dams.
And if it wasn't free or near-free it's competing with putting one more panel or adding one more battery to the system. Or... just big, cheap funnel that is feeding water generator Sure you lose some of the velocity but it is so much simpler system
Like, generating energy from tiny movements have its niches (like sensors powered by piezoelectrics from vibrations of machine they monitor), but using it for raindrops just feels like a waste of time, most areas don't even get enough rain for that to matter.
Now if they figured out to do it to airflow with few moving parts, that would be more interesting.
"Technology improvements" can't get around the fundamental limit in the amount of energy available to harvest.
Edit: Oh, that's horribly misleading. They seem to be reporting the theoretical maximum output of this harvesting technology - e.g., how much energy could be recovered if they started blasting the panel with a firehose. The amount available in rain is on the order of .2-.4 W/m^2. At that rate you'd never recover the amount of energy as was used to produce the panel.
Congrats, you've invented the hydroelectric dam.
I pay about $30/month for local utility water. It's metered, but I rarely exceed the minimum billing amount.
If the generator cost $100 to buy and install, it would take nearly a century to pay back its install cost even in one of the rainiest locations in America.
I saw the construction of this thing by chance the other day. Is it so obvious it's a "shenanigan" if they're still researching it?
Do you want to know why you don't see power dams below a certain size and height? Because gravitational potential energy is not very much. And I say this as someone who gets more excited by the Niagara falls power station than by the actual waterfall.
I also question their math claiming 200w per sq meter.
https://www.insidescience.org/news/how-much-power-can-we-get...
In this case, the input energy available is ~4 orders of magnitude lower than that.
2. Would this work any better than a downspout turbine? I suppose that could also be wired in series.
https://en.wikipedia.org/wiki/Kelvin_water_dropper
The actual device is: https://arxiv.org/pdf/1309.2866.pdf
insert why not both? meme here
https://www.theguardian.com/environment/2018/mar/13/rain-or-...
Could a piezo collect current like this, on a solar panel sized sheet? I'd imagine it's not an insignificant amount of power during a downpour.
Can't do this homework at the moment...
Also make them harvest energy from the wind at the same time somehow.
“Come Rain or Come Shine”™
So the optimal strategy is solar facing the sun and rain on roof that is not.
Not describing the actual amount of "rain" they sprayed on the device also makes it unclear whether their "5 times higher" number is an apples to apples comparison, esp since 200W is a _lot_.
> When the area of the raindrop energy harvesting device is 15 × 15 cm2, the peak power output of BAGs is nearly 5 times higher than that of the conventional large-area raindrop energy with the same size, reaching 200 W/m2
I think perhaps the most important sentence on the posted article is (emphasis mine):
> Christopher Plain is a _Science Fiction and Fantasy novelist_ and Head Science Writer at The Debrief.
[1] https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=101...
But it is easy to say "this would be big if true" and difficult to prove "these cannot make anything close to enough electricity to be useful". So, we have a comments section filled with blind hype and hope, and little actual knowledge.
