
How much power can we expect from rainfall? - avoidboringppl
https://eighteenthelephant.com/2020/02/12/can-the-teardrops-that-fall-after-reading-bad-science-writing-generate-renewable-electricity-yes-they-can/
======
Someone
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)

~~~
fnord123
>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.

~~~
gherkinnn
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.

~~~
bathtub365
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...](https://curiocity.com/vancouver/wp-
content/uploads/2019/08/powerhouse.jpg)

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

~~~
bathtub365
What is the culture of reddit? I don't read or post there.

~~~
jshevek
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.

------
nstart
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.

~~~
thereisnospork
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?

~~~
saagarjha
> 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?)

~~~
jmalicki
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.

------
Zenst
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.

~~~
bryanrasmussen
>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...](https://www.cleanproguttercleaning.com/are-gutter-guards-worth-it/)
(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.

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

~~~
bryanrasmussen
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.

~~~
DanBC
Here's a nice video.

[https://www.youtube.com/watch?v=S6oNxckjEiE](https://www.youtube.com/watch?v=S6oNxckjEiE)

~~~
bryanrasmussen
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.

------
jemski
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...](https://www.improbable.com/category/the-weekly-improbable-research-
podcast/)

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

EDIT: formatting.

------
londons_explore
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.

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

~~~
throwaway_pdp09
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

------
goldenkey
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!

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

~~~
dalemyers
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.

------
Animats
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/](https://pavegen.com/)

~~~
saagarjha
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!

~~~
leghifla
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.

------
jerzyt
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.

~~~
gen220
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.

~~~
qchris
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?

~~~
gen220
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...](https://energy.economictimes.indiatimes.com/news/renewable/india-
shelves-gujarat-west-bengal-tidal-power-projects/74547357)

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.

------
marcus_holmes
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?

~~~
zdragnar
> "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.

~~~
marcus_holmes
Interesting. What is the point of journalism then?

~~~
zdragnar
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.

~~~
marcus_holmes
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.

------
stevage
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?

~~~
r_parthasarathy
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...](https://www.merckvetmanual.com/management-and-
nutrition/nutrition-exotic-and-zoo-animals/nutrition-in-primates)), converting
kcal/day into Watts. I don't know what their peak power is. Anyway, you'd need
a few monkeys, but not hundreds!

------
murftown
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.

------
ctdonath
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.

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

~~~
ctdonath
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.

~~~
smaddox
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.

------
jzer0cool
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](https://www.youtube.com/watch?v=fhj2BN-cbvg)

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

------
acqq
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.

------
alpb
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.

~~~
mohn
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.)

------
m463
Tears are a higher form of power than raindrops. They can indicate joy, or
solve intractable problems or eliminate traffic tickets.

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

~~~
djrogers
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!)”

------
hirundo
"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.

~~~
acidburnNSA
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.)

~~~
aetherspawn
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.

~~~
saagarjha
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.

