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Love you for this! I had exactly the same “solar freaking roadways” thought, although at least that idea qualified by basic theoretical analysis of available energy and area for harvesting and conversion efficiency. It was an obviously terrible idea for other reasons :-) yet it still got a prototype…

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.

If the raindrops were caught with a funnel, so the actual surface area of the device is very small, but the funnel is large, would that improve the economics? Maybe add in a water tank + hydro power to capture more gravitational potential energy from the water,

Catch them with a patch of ground and use a river as the funnel and you’re really on to something.

Maybe you could like block the river just a bit to build up like a reservoir.

It wouldn’t help with kinetic energy harvesting from the raindrops as that would go into the funnel as heat.

It might provide a way to harvest the remaining gravitational potential energy of the rain (possible funnel being your roof and guttering) but the only upside is that you could concentrate the energy with something that’s already there (and hence harvest over a smaller area). The amount of energy (and hence value) available would be even lower - unless you had a really high roof.

This is also the reason I abandoned my high school scheme of hydro turbines at the bottom of downpipes.

As the comments below say - you need to be working at the scale of a few major geographic features as a funnel before it starts to get really interesting.

Given the explanation above, it would need to be a funnel that is cheaper than paint. If the analysis above Is accurate (I can’t vouch for it either way) then it’s hard to imagine a material strong enough at such a low price point.

It’s possible too that the proposed mechanism is related to electrostatic charge in which case funnelling would probably interfere.

I'm wondering if the charge generated by a rain drop could be from its static charge, rather than kinetic energy conversion?

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...

I'm getting well outside of my depth here, but some starting points:

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.

I feel like if we want static charges there must be some sort of potential difference between the top of a large building and the bottom during a thunderstorm. Probably not, though.

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.

> While we're at it, can we extract any energy from lightning?

Wikipedia has you covered.

https://en.wikipedia.org/wiki/Harvesting_lightning_energy

If we could harvest and store the anti-electrons they would be very efficient at generating heat.

Ah, excellent!

Yes, the TENG that they refer to in the paper is TriboElectric NanoGenerators https://en.m.wikipedia.org/wiki/Triboelectric_effect

And way more intermittent than solar. Seems like a really dumb idea.

Not in the PNW!

> 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."

This was true 20 years ago. Nowadays, we joke about Juneuary and this year we had Maygust. Our summers are hotter and longer, winters bringing weeks of freezing and snow where I grew up expecting a single day of snow. The calm weeks-long drizzles of my childhood are gone, replaced with days-long "atmospheric rivers" called "monsoon" in warmer climes.

Is Juneuary June in January or the other way around?

June in January, and August in May. As a kid, we wouldn't expect to even see the sun until late May.

noooo i'm sure it's the opposite here. January in June. (very accurate in 2023)

An amusing joke, but while rain is frequent in the winter in the PNW (Pacific Northwest USA) the rain is often light. Seattle residents tend not to use umbrellas unless the rain is particularly heavy. It might not be a very good power source.

All it took was a year and a half in Bellevue and I've never needed protection from the rain ever since.

Now that I'm back in California I get a lot of weird looks in the office when I go out into the rain on purpose. It's a lot easier to appreciate the rain here when it's not raining nonstop for nine months.

Come spend some time in the Gulf Coast! Rain here actually physically hurts, sometimes. Big droplets (though probably not as big as the ones in the Alaskan Panhandle[0]), falling at a very rapid pace, looks and feels like a curtain of water. It's not unusual to have an inch an hour.

[0]: https://en.wikipedia.org/wiki/Ketchikan,_Alaska#/media/File:...

I get the same puzzled bemusement here in Idaho. Originally from the Oregon Coast I now relish a good rain and love a long walk through it.

Seattle actually gets less rainfall per year than any major city east of the Mississippi. It's just overcast and misty/drizzly a lot in the winter. Summers are bone dry.

Yes and: Foggy. When I was a kid, typical day was foggy til noon.

As a fun data point, the capital of Sierra Leone gets more rainfall in a single month than the PNW gets in an entire year.

I was there in the rainy season, it rained harder than I ever imagined possible for ten hours a day, every day.

You sent me down a rabbit hole. Holy crap! The AVERAGE rainfall in Sierra Leone in August is over 600mm... 23 inches!

According to Wikipedia, the average August rainfall in the capital, Freetown, is over a meter (~42")[0]. Tropical wet-and-dry climates, they're wild!

[0]: https://en.wikipedia.org/wiki/Freetown#Climate

Freetown gets more rain in one August week than my town (Scottsdale) gets in a full YEAR

It was seriously, seriously impressive to live in that environment.

I wound up finding mould on my clothes, on my fridge, inside my Jeep, etc. etc. It was extremely hard, so after a month I drove north into Mali where the rainy season was basically over :)

I saw rain staggeringly heavy more times than I can count on the West Coast of Africa - it was truly incredible. Now that I think about it, I'm not sure I ever got it on film.

This same kid also walks around in Bergen, Norway

No, the kid i Bergen is twelve.

He didn't see sunny skies until he was already a man, and by then it was nothing to him but blinding!

Funny, but also very inaccurate for large parts of the PNW? Maybe it is always raining in Vancouver, but summer is remarkably dry in large parts of the area.

Edit: Granted, the time of the year that solar is likely too spotty, we do have rain.

I was visiting a friend in the PNW and we went for a walk. Shortly after we started, the drizzle started. Honestly, it felt great, the drizzle on the face. My clothes didn't really seem to be getting wet, either, that's how slight it was. Then, he mentioned "the rain" and I looked at him. Dude, you think this is rain? He said, yeah, it rains harder, but they consider this rain. I questioned him a bunch and it seems much of the time the rain was at drizzle level.

Now, this was in the 90s. I don't know if the rain is harder now, due to, you know, the changin' weather.

I think this is still very accurate. I tell folks from the south that ask if it rains a lot here, that they likely wouldn't call what we get rain. It definitely looks wet all the time in the winter. As if it always recently rained.

A drizzle is pleasant when it's warm but part of grim and cold weather when the temperature dips down (below 15-10°C?)

The other fun thing of the PNW is that it really doesn't get too cold that often. :D

Average daily high stays under 15ºC (~60ºF) from about November to April. Definitely cold enough that being damp is unpleasant, especially with any kind of breeze.

Depends on the rain. We don't have the cold rain I was used to from the south. Such that I'm perfectly fine biking in all year and basically all weather.

So you aren't wrong that it can certainly be unpleasant. But it isn't as bad as rain from where I grew up. By a long shot.

I lived in Vancouver for a number of years. I sure don’t miss the winter. Otherwise it’s a lovely part of the world. But the clouds and the rain really got to me.

Seattle rain festival; January 1st to December 31st.

100% it's always raining here it's especially bad in the summer.

Seattle has 156 rainy days per year, Vancouver has 168.

Honestly doesn't sound like that much to me. >150 is typical for most cities here in Germany. Munic has 193 rainy days per year.

So it just seems like the people living in the PNW are exaggerating a bit.

How are they claiming 200w/ meter^2?

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.”

Placing the rain power panel under a waterfall might generate a useful amount of energy, eventually. The key feature is some sort of vertical conductive channel that gets the charges away from each other quickly. Maybe the idea is that if they can produce power in a worst case, they can improve solar panel efficiency in low light condition. Anyway, it sounds like useful research that got into the hands of the marketing department. Lost in several layers of translation ?

It may be on some level a culture issue. Grand announcements in unreproducible results are more common in societies with a profound respect for learning. Think Dr. Pangloss …

Perhaps an unrealistic test environment?

Blasting the panel with a fire hose? Lol

I was just thinking the same thing. xD This is a totally useless idea (outside of Florida). If you wanted to harvest some energy reliably, there's plenty in wind and tides and ocean currents.

.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!"

https://youtu.be/ff-3MhQ7ri8

"EEVblog 1534 - Solar Freakin' RAILways!"

https://youtu.be/7vItnxhWRqw

Speaking of Florida…

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.

At least some research and experimentation has gone into the idea[1]. Doesn't seem like much has come from it to date, for good reason(s). But who knows, maybe somebody clever will come up with a mechanism for this that works, somewhere down the line.

[1]: https://en.wikipedia.org/wiki/Harvesting_lightning_energy

> Sounds a lot like Solar Freakin Roadways.

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".

[1] https://www.pavegen.com/

I was really thrown by this parenthetical

> 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.

Wow, I totally forgot about solar fricken roadways. That video was 2014.

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?”

https://solarroadways.com/faq-funding/

I'm surprised you even bothered to do the maths.

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.

Thank you. Collecting rainwater in a barrel is useful for gardening if you want to save a bit of water. This nonsense? Total bunk!

Heck it feels like the majority of my whole engineering degree was unit conversions

Yes it's just very, very inefficient solar power.

I mean this is true of nearly every single form of power available to us, maybe excepting nuclear.

It's still stellar power, but not from sol.

Yeah true. Geothermal I guess is another one.

Geothermal is nuclear with extra steps.

No this is more like each raindrop shimmying down the sky carpet and then poking the panel to deposit the collected static charge.

But if you could combine it with a solar panel, using the same footprint somehow, then you at least get some output while the sun isn't shining, which would be better than now.

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.

First engineering is an iterative process.

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'.

Correct, but what the parent here presents is a theoretical upper bound. A working product wouldn’t even get close. When the theoretical upper bound shows that something could never aspire to more than a vastly inferior alternative to existing proven technologies, the correct approach is to abandon it rather than invest in iterative improvements.

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.

> successive improvements from previous work

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.

"The professor took a lot of joy in shooting down ideas like this though."

Gatekeepers of domain knowledge usually do.

Ah yes. Physics, the ultimate gatekeeper.

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.

If you think a system not passing the sniff test because basic physics shows it to be mediocre, your problem is with reality, not a physics professor.