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How much power can we expect from rainfall? (eighteenthelephant.com)
175 points by avoidboringppl 3 months ago | hide | past | favorite | 105 comments

Before it starts falling, a rain drop of mass m has potential energy of mgh, with g earth’s constant of gravity, and h the height of the drop above ground.

Let’s pick a drop height of 1 km (low for cloud cover). That would make the kinetic energy about 10⁴m (where m is the mass of the raindrop)

At time of impact, potential energy is gone, and the drop has a kinetic energy of ½mv².

Taking the (high) impact speed of the rain drop of 10m/s of the article, that’s 50m (where m again is the mass of the raindrop)

So, at least 99.95% of energy is lost, heating the air or accelerating air downwards.

⇒ We could build a huge vacuum tube and let rain fall into that :-)

in real life, the alternative way of not letting air take much energy from the falling water is easier to build: make larger “droplets”, and have them tailgate each other really closely (also called a waterfall)

>in real life, the alternative way of not letting air take much energy from the falling water is easier to build: make larger “droplets”, and have them tailgate each other really closely (also called a waterfall)

To make this "aqua electricity" viable in the real world you would need huge bodies of water penned up and ready to be released at the appropriate time. Like the size of a lake. Pretty ridiculous.

That body of water also needs to be placed high enough for it to release a waterfall. I can’t think of something. Some large geological structures, perhaps? I dunno.

Maybe Elon Musk has a wacky idea to inverse this problem and build tunnels below lakes to make this work.

Coquitlam Lake actually has a tunnel leading to Buntzen Lake for the purposes of supplying it with water for power generation. Pipes then carry the water down to a generating station on the ocean shore. https://curiocity.com/vancouver/wp-content/uploads/2019/08/p...

The culture of hackernews is being poisoned by the culture of reddit during this pandemic.

What is the culture of reddit? I don't read or post there.

There are many aspects of the culture of reddit which is poisoning the culture of hackernews during this pandemic, to save time I'll just address the portion of reddit culture that is evidenced here: the practice of pretending to invent something that already exists and naming it in a manner somewhat analogous to its existing name.

The is a special case of the culture of repetitive, unimaginative, formulaic humor threads in which a series of people pretend to engage in a conversation in earnest, but the entire thread is satire, or meta, or fake invention, etc.

Edit: It seems to me that there is a correlation between the users who participate in these kinds of threads, and the users who damage the culture of HN in other ways. I'm not speaking of the specific users in this particular humor thread, but what I've seen in the past of humor threads in general. Clicking on their profile and viewing past comments, I've found that humor-thread participants often also engage in ideological battle, rely heavily on emotionally manipulative rhetoric, make low quality fallacious arguments, etc.

Yes, the joke is that we are describing a hydroelectric dam.

I absolutely love the gist of the article: interpreting quantitative research is something most of us are fundamentally illiterate on.

That's definitely how I feel about it at least. Others might feel differently.

That said, how does one actual build skills in this area. It feels like the author is able to do this here because it's a research paper in an area they have knowledge in.

How does one build up a general ability to critically evaluate numbers in a case like this? Does one have to know how to go look up formulas?

I recall an article from some time ago where authors released a paper about chocolate and health benefits or something similar but they intentionally gamed the numbers to find something that correlated. The press ate it up before the authors revealed the truth.

How does one go about becoming more literate in evaluating those kinds of papers and conclusions?

I'm willing to put in the time to read carefully and calculate. I just wish I knew what skills to acquire.

To evaluate stuff like this, you just need basic physics knowledge (enough to do dimensional analysis with) and estimation skills. One fun read along these lines is "The Art of Insight in Science and Engineering".

For stuff like the bad social science you mentioned, often you can catch it if you know basic statistics along with the common tricks people use to p-hack. See, for example, "The garden of forking paths" by Gelman.

Even if the statistical analysis is not given explicitly, or the issue is more subtle, for social science that touches on everyday life, you can usually tell if it'll hold up just with common sense. Try yourself using this quiz: https://80000hours.org/psychology-replication-quiz/

Fermi questions[0] (oft maligned when asked in interviews) are a good practice for this sort of thinking.

Anything that is either out of range on scale, or that ducks the question, is worthy of further evaluation and at least some mild skepticism. Theranos promising hundreds of blood tests out of a thimble of blood would be the former. The article referring to Volts of electricity is like a car salesman replying to 'how many seats does it have?' with 'The seats are upholstered in Fine Corinthian Leather' the statement isn't incorrect, it is just useless and, in context, intentionally misleading. Salesmen do it to sell cars, scientists to sell research, journalists to sell papers, and conspiracy theorists for, whatever reasons, I guess.

[0] A few examples: How many piano tuners are in New York City? How many ping pong balls could fit in a school bus?

> Fermi questions[0] (oft maligned when asked in interviews)

I really don’t get when people bemoan these. Do they not get that it’s supposed to be an estimation, and it’s not actually a quiz of whether you’ve memorized a seemingly useless bit of trivia? Do they not see how it can be a useful skill to have, especially in an engineering field where you frequently deal with many orders of magnitude? (If acquiring this lock takes 100 ns, am I doing this often enough for this to cause a user-perceivable delay longer than doing it single-threaded?)

The problem is more when you're a software engineer, and you're asked something like "how many manholes are in Los Angeles" - it is so completely tangential (at best) to any of the estimation skills you'd use on the job.

As the author notes in the article, all this takes is basic knowledge of physics units, and the only math required is addition and multiplication. So it’s a matter of basic skills to some extent, but that’s not really the key.

The way you get better at this is by doing it, a lot. Every time you read such an article, or some one relays such information to you, run the math through your head as much as you can. Make notes of the things you don’t know that prevent you from pushing the thinking further, and methodically learn about those. Don’t think of yourself in terms of particular labels - eg “I’m not much of a physics person” or “I’m a software guy” - your job is to understand the world, solve problems, answer questions.

It’s a constant effort and it’s not easy. It helps to surround yourself with others with the same drive. It helps to teach, as it forces you to constantly reassess the fundamentals, and the common mistakes beginners make (see Feynman’s writing on that topic).

This is roughly High School level physics (basic, not AP or any honors or anything).

In this particular case, there's the knowledge of the first basic law of thermodynamics (You can't win, the limiting case on energy conversion) and Kinematics, typically taught as the first half/third of physics. (Broadly: Mechanics/Kinematics, E/M, Optics and Waves)

In most physics courses, you’ll hit conservation of energy even before it’s presented in thermodynamics packaging. This kind of calculation should be less than a week’s worth of casual effort for someone who is familiar with algebra.

A physics undergrad degree will teach you all of these skills. I imagine, you can memorize a fairly large set of skills needed to do this in a few months, but practicing these skills to high proficiency takes a few years - which is the length of an undergrad degree.

I looked at this area a few decades ago and two approaches stood out as potential, though not stand out.

1) Roofs already collect a large surface area and funnel into a single point via roof guttering - so tapping the flow from the gutter run off would enable a viable volume of water turn a motor in the drain pipe as it descended and generate power.

2) Piezo electric generating from the roof, though was looking at a form of artificial grass format that would capture the wind as well as the rain hitting it, generating the vibrations to the base which would be a piezo element.

The first idea, maybe has some traction, though the volume of water (yearly rainfall) with roof surface area and the napkin maths didn't show a huge gain and bit of a limited market due to rainfall you would need to generate anything noticeable.

Second idea, kinda got mooted as roofs solar panels did better with the real-estate, and whilst for non south facing or some area's it may well been useful. Then the area's were solar is not as good due to latitude, well, then you have snow. So I kinda self-killed that avenue off as well.

Figured that tapping the drainage flow of water from a whole area combined would work better and for that need a organised drainage system all connected and yes you do get those. Though they are also the sewage system and with that, the cleanish flow of rainwater would be less common once you factor in the rest of the mix.

But I'm all for micro based generation, like a wall of small wind turbines instead of one large one and more aesthetic and palatable to add to existing buildings and area's.

>Figured that tapping the drainage flow of water from a whole area combined would work better and for that need a organised drainage system all connected and yes you do get those.

If I'm not mistaken, the technical term for these broad-area drainage systems is "river"

I think you’re on to something there. What if we made on very large, and collected lots of water at a central point and could control how much of it we use for electricity? It’s a damn good idea.

It will never work! The collection structures will fail left and right, the surrounding soil will be saturated and cause mudslides, you'll have to evict a bunch of people and buy their land, and besides, what about the downstream farmers? They don't give a dam!

How will salmon get back up into the gutters to spawn?

Just unleash a horde of 1st level characters to go XP grinding on the salmon, and the salmon will simply respawn after a few minutes.

Salmon tubes!

>If I'm not mistaken, the technical term for these broad-area drainage systems is "river"

In some area's/countries - yes. What I was referring to would be your towns/cities and heck in the UK, we have most accomodation plumbed for water going in and out.

Some countries/area's have what they call storm drains.

Yes they all end up at the river, ideally for the sewage - after it is treated, though again mileage may vary from country to country.

Joking aside, the power of any water driven electricity generation is dependent on the difference in water levels on either side of the generator. Dams don't get built in deep canyons for convenience: the sites are chosen because they maximize the height of the water column.

The potential energy of a municipal sewage system, while not zero, since there is flow, is so small as to be useless, unless your city is bisected by a 400 foot cliff.

The first question one should ask is whether one can reasonably expect to make back the energy embodied in the generator. For example, strapping a generator to a stationary bicycle is unlikely to reach this point. Small scale wind turbines have a similar problem [1].

I really doubt that generators in individual houses drain pipes would be viable.

[1] http://theoildrum.com/node/6954

A roof is about 200 m^2 (more or less), so at 10 mw/m^2, you could generate a whopping 2 watts. While it's raining. California gets 563 millimeters of rain a year, so you might end up generating an entire 100 watts. For the year.

A watt is a unit of power, not energy —- it represents the rate at which energy is produced (joules per second), not the total amount of energy produced. I’m not sure how you came up with the number 100, but the units for that number should be energy (such as joules or watt-hours).

Meant to type watt-hours for that last sentence. Thanks for catching it.

Most of the potential energy of rain is wasted as it falls through the sky. What you really want is a huge funnel held up by enormous hydrogen balloons that catches the rain directly from the cloud, and miles-long hose coming out of the bottom. Then you'll get some real bang for your buck!

Or you could just pile a lot of rock on top of rock, as much as five miles high if you like. Make a wall of this stuff hundreds or even thousands of miles long.

Warm clouds full of evaporated water would hit it, be forced up into colder air, and release the water.

It would then run down the rockpile and freeze, but the ice would flow down to warmer air closer to the surface, where it would melt amd you could harness the energy of the aggregated raindrops as they flow back to the sea.

If “building the wall” is politically unfeasible due to liberal tears, you could find some drifting continent and get it to smash into another continent. Wait a while, and the continents smashing together will make the wall for you.

No need for balloons, just be patient.

>Roofs already collect a large surface area and funnel into a single point via roof guttering - so tapping the flow from the gutter run off would enable a viable volume of water turn a motor in the drain pipe as it descended and generate power.

the purpose of a gutter is to direct the water to areas where it can seep into the ground without damaging the house (while carrying away leaves and other things that would rot and damage the roof), at least where I live, a motor in the drain pipe would slow down the gutters in its function, which you don't want to slow down in some of the rain you get in Denmark, and finally would leave it open to more easily blocking the drain. Of course one could put a guard up on the gutter to keep large things from coming down but as I understand it gutter guards aren't really a good idea https://www.cleanproguttercleaning.com/are-gutter-guards-wor... (from a company trying to sell gutter cleaning services so maybe should be taken with grain of salt but the arguments sound pretty reasonable) and at any rate I believe would still have to let things through that would be problematic for the motor to have through.

So essentially having a motor in the gutter for generating energy would mean the gutter could no longer be able to perform its function as a gutter and a new way to handle that would need to be devised.

Quint B.U.I.L.D.s developed a system that (barely) charges his cell phone with downspout water flow. His four-part (so far) video series documents his engineering, trials, and improvements. He tries different turbine designs, builds his own alternator, manages the current and voltage, and buffers the water flow. He shows the engineering challenges of implementing a back-of-envelope design. Good fun.

1. How to Harvest Free Energy From Your Roof with a Hydro Electric Generator! https://youtu.be/S6oNxckjEiE

2. 3D Printed Alternator vs. DC Generator for More POWER From the Rain! https://youtu.be/YLb4enCgnP4

3. Fidgeting With Turbines! https://youtu.be/vify0k2sHlQ

4. Charge a Smartphone With The RAIN!!! https://youtu.be/amu5LJaDUPY

You put a big tank with an overflow outlet. No impediment to downflow, smooth bursts and etc

Do you have a link to the thinking behind it/how it would work, would this be good for a house as opposed to a bigger building? I'd like to see if there is a solution that would match my house, although I don't think there would be.

Ok, now I get it, the original comment implied the wheel would be inside the gutter which would lead to the problems I noted, I guess refuse coming down the pipe would come out the end, not especially higher chance of clogging the pipe.

Couldn't you just make the drainpipe longer to ensure the water is carried far enough from the house?

Concentration of rainfall into artificial lakes to generate power was already done centuries ago:


The mine drainage system was later gradually replaced with steam engines, however there is a ~megawatt hydropower electrical plant still functional.

> But I'm all for micro based generation, like a wall of small wind turbines instead of one large one and more aesthetic and palatable to add to existing buildings and area's.

The problem with these is terrible efficiency as you scale down, due to fluid dynamics and the added redundancy from having multiple small units rather than one large one. This is why generators are huge.

Wouldn't the water back up and then you'd have to somehow contain a ton of water on your roof?

Roof line guttering is only about 8 metres high on residential property. 8 meters doesn’t seem like much potential energy for something that weighs 1kg per litre “processed”, that energy source isn’t going to run anything interesting at all

The efficiency of wind turbines and their area for capturing wind goes up with size, so a single large one is better than lots of small ones. Which is why domestic ones have not become popular except off grid.

Not to mention that small wind turbines are quite noisy.

Has anyone seen piezoelectric shoes that have wires running up your pant legs to charge stuff? Could be good for the outdoors and off the grid lifestyle. Probably charges way faster than solar.

You could build this grand gutter power generator and have power that costs 100x more $/kWh than your grid hookup.

Funny no one has cited Randals What-If yet: https://what-if.xkcd.com/23/ (third question on the list)

Back of the envelop calculations would have break even after about 100 years...

If you're at all interested in useless research (and not necessarily anything to do with ecology or the environment) I strongly urge you to check out the podcast "Improbable Research"[1]. I'm a long time lurker, but this felt so perfectly relevant I had to make an account and chime in.

It was started awhile back, then stopped. Recently a dear friend of mine [2] became involved in re-editing the episodes and releasing them again. I know they'd appreciate the support from like-minded individuals. Hope to see more submissions like this one. It's a great thought experiment.

[1]: https://www.improbable.com/category/the-weekly-improbable-re...

[2]: https://sethgliksman.wordpress.com/

EDIT: formatting.

This raises a question in my mind...

Current hydroelectric systems rely on building a dam, say 50m high, and generating electricity from a large mass of water falling that 50m.

Might another approach be to build a tower as high as possible, say 500m, and catch rain up there. You might catch less rain, but you'd have a lot more height.

Obviously the challenge with this is getting a large catchment area very high up. I guess you'd need some kind of aerodynamic design like a kite to allow a massive square meter-age to stay aloft during storms.

Rivers collect water over thousands of square miles. Thus turning inches of rainfall into serious amounts of water. Going 10x as high, but collecting 1/1,000,000th the water is not going to provide much power as energy is linear with height. https://en.wikipedia.org/wiki/Drainage_basin

The Hover Dam for example collects water across 167,800 square miles.

One can argue that these dams are already catching rainfall indirectly, from a very large area with very high efficiency.

I suspect theoretical energy generation efficiency is in disproportion to the difference in potentials (IIRC the hotter something is, the more efficiently you can extract energy from it - not more energy, but more in proportion - but IANAexpert).

So while dams may catch rain efficiently, if they were 10x higher perhaps the extra potential energy usable is more than 10x

Dams are very efficient to construct, because you take advantage of natural valleys - erosion has done most of the heavy construction work for you. The catchment area isn't just the surface area of the reservoir, but huge areas of surrounding land. Major hydroelectric reservoirs are unimaginably vast.


Interesting idea. As jdietrich says, I presume the reason we don't see this is simply the question of scale and cost. If you want to catch kilotons of water, a reservoir is far cheaper than an enormous raised platform.

On the flipside, there's the question of how far down you can get the water to drop, before feeding it to your generator. This is something already taken advantage of - https://en.wikipedia.org/wiki/K%C3%B6lnbrein_Dam#Reisseck-Kr...

Some dams use this with their generators being situation in the valley below. I see little point to do this unless you are restricted by some other factor, for example if you are a flat country with no access to natural reservoirs or vertical drops.

Catching much rain very high up sounds stupid but at the same time that's something that needs considering. Stupid suggestions can be made un-stupid in my occasional experience. Could you elaborate please? Genuinely interested.

[1] probably explains the downsides best (which I had not considered).

I guess considering that, the only way to make it viable is if there isn't much rainfall, or if the terrain is very flat/porus, or if you can make the collection devices very cheap (for example a kite that flies into a cloud and can collect a few tens of liters per second 1km up could generate 100 kilowatts, which is a lot less than a big wind turbine, so needs to be a lot cheaper to compete)

[1]: https://news.ycombinator.com/item?id=22994225

Yeah, the effectiveness of wide area water collection is why your suggestion is 'stupid'. That's ok. Where it's possibly not stupid is in the comment you linked to "...as energy is linear with height"

Sure potential energy is linear with height, but energy conversion efficiency is I think non-linear. You can I think collect more than 10x useful energy from an object that is 10x as hot. Equally I suspect you can get more than 10x useful energy from a column of water 10 miles high than from a column of water 1 mile high.

I'm pretty sure your idea won't work but that doesn't mean it can't possibly work. It's more likely harvesting wind energy via kites (as has been proposed) is more realistic, but still, I don't like dismissing dumb ideas. Sometimes they can be made useful.

> Equally I suspect you can get more than 10x useful energy from a column of water 10 miles high than from a column of water 1 mile high.

For a given flow rate, no.

High pressure water can be fairly efficiently converted to electrical energy, which itself can fairly efficiently be converted between low voltage+high current to high voltage+low current.

All the conversions are linear and well understood - there is no magic squared here to make this idea magic.

OK, you sound like an expert (not sarcasm) so if I may (and please note I never suggested a squaring relationship),

> High pressure water can be fairly efficiently converted...

And lower pressure less efficiently converted? Therefore the higher the column, the higher the pressure and the better the efficiency of the conversion?

I think that even if it is a good concept, this reservoir would never fill.

I don't even have to read this article to know that the best way to harvest rainfall is already being done: rain flows into rivers and we already use hydroelectric dams!

There is an interesting series of videos on Youtube about harvesting energy from gutter flow: https://www.youtube.com/watch?v=S6oNxckjEiE

I've been watching this and I'm quite impressed at the lengths he has gone through to make this work. Designing and creating a custom alternator is definitely one of the more impressive things I've seen in engineering videos on YouTube.

ive been enjoying this video series, i especially liked the video where he built his own alternator from scratch, i found it extremely educational.

Remember the "pavement that generated electricity"? [1] Raised £2.6m by crowdfunding. Turns what they really power is "exciting brand activation". By this they mean something you step on which interrogates your cell phone and ships the identity data off for marketing purposes.

[1] https://pavegen.com/

But you can click on the demo on their website and generate 3 Ws (wouldn’t it be nice if there was a convenient unit for this?) of energy fit each triangle you interact with!

1 Ws is just 1 J (1W = 1J/s). Just for comparison with a more widely used unit, you need 3.6 million J to make one kWh.

I'm surprised that tidal energy is not yet a big player. The amount of potential energy is massive, and it's very reliable, unless the moon falls of its orbit.

I have a friend who was very interested in investing in this space, and did some research.

TLDR, even though the tech is probably mature enough, the economics just aren’t there for private industry yet. It’s require major state-level funding to get it off the ground at this point, IMO. Very similar to how solar used to be.

First, tidal turbines are very specialized pieces of hardware (compared to wind turbines), that have to survive corrosive brine for a decade to pay for themselves. There are not many competing manufacturers, and they are expensive. The wires connecting them to land are also expensive, both in installation but especially in maintenance. So the up front and ongoing capital expenditures are relatively high, and there isn’t an economy of scale like there is for wind turbines and solar panels.

Second, there aren’t as many places that are well situated for tidal as you think. You need a basin of relatively shallow ocean, close to metropolitan power grids. There are only a handful of places like this in the world (around the UK, India, Australia), and developers would require a lot of work to get approval to develop this space, compared to solar or wind, where permit-seeking has relatively well-trodden path.

That would be fine, and we’d still be in business, but third, when you compare it to the opportunity cost of investing in a more traditional (or even “renewable”) power plant of a similar output, the story really falls apart. The ROI for the builder just isn’t there, and it makes way more sense to build solar or wind farms.

Some forward thinking governments are trying to help the ROI angle by providing subsidies on the generated electricity, but as a developer this is sketchy, because you still aren’t breaking even until 5-10 years, even with subsidies, and subsidies themselves can be politically fickle. When you combine this with the uncertainty attached to the turbines being relatively bespoke technology (compared to the commodity of wind turbines and solar panels), it makes sense why we haven’t seen more development here.

This is a fantastic summary! In the United States, I believe that the Pacific Northwest and parts of the Maine coastline were also considered, but for many of the reasons above, I don't think any of those projects really achieved critical mass for wider adoption.

I'm not bullish on the future of tidal power--there may be some use case for it, but the reality of the engineering challenges associated with putting hardware in a marine tidal zone for years at a time presents serious obstacles with limited advantages over versus a wind turbine or solar array of the same capacity.

I'm curious, do you know if your friend ended up investing?

The friend did not.

He was looking specifically to invest in Gujarat, because of family ties. However, the government decided the project wasn’t worth it and binned it. IIRC they were more interested in developing solar. I tried googling around for articles related to it, this the best I could find: https://energy.economictimes.indiatimes.com/news/renewable/i...

Australian government wasn’t super interested in developing tidal resources because of the coal special interest groups, but also because solar resources are lower hanging fruit. The UK project was very legitimate and institutionally mature, but was more of a science project for proving the technology than an investment opportunity.

Thanks for sharing - great info. I still think that the potential energy (no pun intended) is huge, so if the engineering challenges are solved, it may not compete with solar, but should be competitive with wind.

Looks like the first tidal power plant was built in 1966 in Brittany: https://en.wikipedia.org/wiki/Rance_Tidal_Power_Station.

It was also the largest for 45 years until a bigger one opened in South Korea: https://en.wikipedia.org/wiki/Sihwa_Lake_Tidal_Power_Station.

Not sure if someone has an analysis about why those aren't more widespread ? It seems like the existing ones are pretty massive and in very specific spots, maybe smaller installations aren't cost-effective enough.

The point of science journalism is not to report on great science, or "speak truth to power" (the point of journalism). It is (like all other online publications) to get people to click on titles so they get shown adverts. It's not really got anything to do with how educated the readers are, it's just the business model.

I read this after reading the article about banning "persuading" advertising. In my head the two are related. If we remove "supported by advertising" as a viable business model for journalism, then it has to go back to being paid for by readers. We would get much better journalism if this happened. In fact, if we removed "supported by advertising" for everything, we'd get much better everything.

But, as TFA says, it's worrying that Nature published this crap. Nature is supported by a non-advertising business model, and although there are lots of problems with their business model, it should in theory produce good results. I wonder what happened for this article?

It looks like the tech they developed is at least scientifically interesting, or at least cool. Doesn't have to be practical to be publication worthy. Still weird that it made it through nature's editors that claim to want importance (which tends to invite p hackers and outrageous claims), but there is a role for fun clever research trying to solve silly problems, the tech developed could be useful elsewhere (under a waterfall on mars?).

> "speak truth to power" (the point of journalism)

I think that might be a bit of wishful thinking. An "investigative journalist" might take that on as a motto, but it certainly isn't true of all journalism as a general goal.

Interesting. What is the point of journalism then?

Obviously, that depends on where in the world you are. Assuming a press that is relatively free from government and adversarial editorial oversight, journalism can likely be summed up as "obtaining newsworthy information and publishing it to the public".

According to Wikipedia, at least, there are at least 242 ethics standards for journalism worldwide, with something of a general consensus on truthfulness, accuracy, impartiality and fairness.

"truth to power" may be a very narrow aspect (if, perhaps, an important one), but the simple fact is that there is plenty of newsworthy information that is fundamentally unrelated to struggles against powerful people, bureaucracies, political entities and companies. The "point" of journalism, then, is much simpler: gather information, attempt to filter it through a lens of accuracy and impartiality, and disseminate it. Not quite tautological, but there you have it.

Yeah. I guess most of that "information" aspect is now being done by t'intertubes.

Maybe I should rephrase my original to be "the important point of journalism is speaking truth to power".

Seeing what a farce the UK media is making of Assange's trial (sorry, extradition hearing), it seems we're nowhere near that at the moment.

I'm really skeptical about the claim that a monkey could generate 100W with a hand-crank. Serious human cyclists only produce 250-350W with their legs, and a monkey is tiny compared to that - and with only one or two hands?

The monkey number was a very rough order of magnitude comment, and shouldn't be taken too seriously. But: You’re technically correct — I didn’t actually look up the metabolic rate of monkeys. The average value for humans (2000 Calories / 24 hours) is approximately 100 Watts. Very quickly looking into it, rhesus monkeys are around 30-50 Watts average power (https://www.merckvetmanual.com/management-and-nutrition/nutr...), converting kcal/day into Watts. I don't know what their peak power is. Anyway, you'd need a few monkeys, but not hundreds!

I didn't read the article, and I know it's more about math than an actual plan, but as a tangent I wanted to say that there's a cost to preventing rain from reaching the earth - even if we eventually returned the water there and didn't pollute or reduce the water. I think by altering this age-old cycle, we would lose some of the impact on the soil, and plants, the way it rolls downwards into increasing rivers.

We've caused a lot of damage with pavement and structures. The benefit of the power generated would have to be weighed against this cost.

One sentence that should have been elaborate on a bit more to mirror/complete the article’s approach: “For a sense of scale, a solar panel gets around ten thousand times this, or 100 Watts per square meter.”

Solar power has the same problem of terminal optimism as the proposed raindrop energy harvesting: in short, the source 1300W/m^2, all factors considered, becomes a real world 13W/m^2. Better than raindrops falling on my head, but still way less than most people comprehend.

Do you have a source for 13W/m^2? That looks about an order of magnitude low to me.

Math. Once you account for night, impingement angle, weather, atmospheric absorption, panel efficiency, dirt, seasons, storage efficiency, etc it averages out around 13W/m^2 long term.

Experience. I run my “office” all summer on solar. 1m^2 panel gives 70W max; usable light about 8 hours, average over a day is 23W. Halve that for winter, overcast, etc.

Gotcha. That still seems a bit low for where I live, assuming 18% efficient panels. Roughly 5kW.hr/m^2 averaged over the year, so 5000/24*0.18 = 37.5 W/m^2.

And of course solar thermal in the desert can do quite a bit better than that.

I had come across these videos in past:

A Drop of Water can light up 100 LEDS LightBulbs. https://www.youtube.com/watch?v=fhj2BN-cbvg

Sparks from Falling Water: Kelvin's Thunderstorm https://www.youtube.com/watch?v=rv4MjaF_wow

I have also read one estimate about how fast the water would have to fall during 40 days and 40 nights of Noah flood to cover the "highest mountain" on Earth -- apparently that speed alone would kill everything on Earth and also destroy any ark made of wood.

It's complicated cheating physics.

I'm having trouble understanding what the article's title actually means right now. Can HN parse this down for me, please:

> Can the teardrops that fall after reading bad science writing generate renewable electricity? Yes, they can.

A text is written on a scientific topic and it is so poorly written that when people read it, they cry. Can we harvest the kinetic energy in those falling teardrops and turn it into electricity? Yes.

(But it is not very much energy.)

Tears are a higher form of power than raindrops. They can indicate joy, or solve intractable problems or eliminate traffic tickets.

Wrong. One can harvest power from raindrop. It's called an hydro-dam. Seems to be working pretty well from what I know.

Not sure why the snark is called for - FTA explicitly calls this out: “ (There is, by the way, a very good way to harness rain for renewable electricity: let geography concentrate the water into a river, then build a dam and a hydroelectric plant!)”

"Shockingly, and inexcusably in my opinion, the researchers only state the instantaneous power (50 W/m2) of their device..."

That's not entirely fair. If Nature demanded that articles be written defensively, anticipating possible misinterpretations from non specialists, they would be very different articles with lower bandwidth for the target audience, who already know the difference between average and instantaneous power. The journal would not be improved by writing for endgadget instead.

It would be nice to have an implications section that put the effects in scale, but hardly inexcusable not to.

I think it is pretty misleading if they just gave the peak device power density. As in the car analogy given in TFA, if you were to burn all 270 Calories of a single Snickers bar in a thousandth of a second, your device would have a peak power of about 1 gigawatt, comparable to a large nuclear power plant. Yet if your publication said you had made a 1 GW device instead of saying that your device could power a single average home for 17 minutes, you reasonably be accused of misleading. Surely a subtext of the research was that energy sources are important to humanity.

(Sidenote: Huh, a snickers bar can power a home for longer that I expected.)

I would be interested to see a food chart that described how long various things can power a home in minutes.

Can a salad power a home longer than a snickers bar? Or what about a potato.

While I don’t have numbers, your questions are easy to answer: foods with more calories in them can power your home longer. A typical 700 calorie meal can, if used 100% efficiently, can power a 10 W LED lamp for about three days.

Yeah, Dietary Calories are actually Kilocalories in engineering terms, so it's a pretty energy dense material.

Actually, having read the Engadget end of things, I can see exactly what the professor is saying in his blog post.

The potential energy capture from any rain-related technique is trivial, negligible is a better word. The article sounds completely clueless to anyone who understands the energy flow involved. The fact that it was written should be a professional embarrassment for that author.

The fact that the original paper was treated with any respect is yet another level of embarrassment. A high school science project sounds more believable.

None of the original ink / bits should have been wasted. But I suppose some sort of nanoscale power supply could be some potential use, so maybe, maybe, there's some redeeming value.

But not the way the paper or the article tried to spin it.

"Shockingly, and inexcusably in my opinion, the manufacturers only state the clock speed (3.2ghz) of their CPU..."

I suppose it depends on the intent. If the intent is to impress you with numbers by giving you a seemingly "good" number, then they're being intentionally deceptive for the sake of getting published in Nature. Does instantaneous power actually matter in context, even to knowledgeable experts?

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