Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

How much power can we expect from rainfall?

eighteenthelephant.com

Post reply on HN
Print view View original

61–70 of 110 posts

Re: How much power can we expect from rainfall?

#61

Post by CraigJPerry »

Zenst wrote: post #4

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…

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

Re: How much power can we expect from rainfall?

#62

Post by Retric »

londons_explore wrote: post #42

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…

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.

Re: How much power can we expect from rainfall?

#63

Post by mikro2nd »

pjc50 wrote: post #39
Zenst wrote: post #4

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…

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.

Re: How much power can we expect from rainfall?

#64

Post by 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)

Re: How much power can we expect from rainfall?

#65

Post by leghifla »

saagarjha wrote: post #53
Animats wrote: post #25

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.

Re: How much power can we expect from rainfall?

#66

Post by bryanrasmussen »

DanBC wrote: post #60
bryanrasmussen wrote: post #50

Earlier quoted context omitted.

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.

Here's a nice video. https://www.youtube.com/watch?v=S6oNxckjEiE

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.

Re: How much power can we expect from rainfall?

#67

Post by jmalicki »

saagarjha wrote: post #52
thereisnospork wrote: post #38

Earlier quoted context omitted.

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 '…

> 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 l…

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.

Re: How much power can we expect from rainfall?

#68

Post by londons_explore »

throwaway_pdp09 wrote: post #58
londons_explore wrote: post #42

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…

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

Re: How much power can we expect from rainfall?

#69

Post by throwaway_pdp09 »

londons_explore wrote: post #68
throwaway_pdp09 wrote: post #58

Earlier quoted context omitted.

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 tur…

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.

Re: How much power can we expect from rainfall?

#70

Post by jshprentz »

bryanrasmussen wrote: post #29
Zenst wrote: post #4

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…

>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 ro…

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

Post reply on HN