Physics buffs: drain pipe turbines

Johnny Girth

Johnny Girth

Johnny Girth

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I had an idea the other day that maybe the power of rain that fell on roof-tops could be harnassed and turned into energy.

However, after my calculations show that on an average size 3 bedroom house, assuming you had a 100% efficient turbine you would only generate about 1.4M joules per year, a saving of about 5p.

So not worth it. Anyone else want to have a stab in case I've gone completely wrong (A level mechanics was so far back...)
 
Johnny Girth said:
I had an idea the other day that maybe the power of rain that fell on roof-tops could be harnassed and turned into energy.

However, after my calculations show that on an average size 3 bedroom house, assuming you had a 100% efficient turbine you would only generate about 1.4M joules per year, a saving of about 5p.

So not worth it. Anyone else want to have a stab in case I've gone completely wrong (A level mechanics was so far back...)
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Well...the rainiest parts of the UK get about 2000mm of rain per year, or 2m.

If a house can be considered to have a capture area of 20m by 20m then you'd collect 20 * 20 * 2 cubic m of water per year, or 800 cubic metres, weighing 800,000 kg. If this were to drop 5m, you'd get energy of 800000*10*5 Joules (roughly), or 40MJ.

Ive been quite generous with my calculations though (taking the wettest part of the country, assuming a big collection area and big drop), so they're not inconsistent with your answer.
 
Yeah that does seem quite generous :)

I measured this house (roughly) and got 12 x 4 metres (4 bedroom student house) so I think 20m x 20m must be a bit of a palace! Also assumed house was about 10m tall which is probably a bit much. Took rainfall as that in london which is about 0.6m per year.

So probably still not worth it even if you live in a mansion in a wet area. Ah well.
 
I have enough trouble keepig a water butt filled, let alone powering the house off it.

Really dont think you can capture enough rainfall for it to be even worth considering. You'd be much better off putting a propeller on the generator and using the wind.
 
Johnny Girth said:
Yeah that does seem quite generous :)

I measured this house (roughly) and got 12 x 4 metres (4 bedroom student house)
So probably still not worth it even if you live in a mansion in a wet area. Ah well.
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Thats tiny! My bedroom is is only half the size of that. I would say 10x10 minimum for a small 4 bedroom house.

Is the saving of 5p just the saving of electricity or is that the cost of a turbine as well?
 
To continue from visagaes example:

I unit of electricity at 8p unit = 1000*3600 =3.6MJ so 40MJ is worth 40/3.6*.08 = £0.88

The water itself is much more valuable, I pay approx £1 per m^3 so 800m^3 is worth a lot. One person uses about 100m^3 per year.
 
It's just the electricity saving, going on a 100% efficient turbine and 1kWh being 10p. 1kWh = 3.6MJ so unless you've got an enormous house that all runs through one drainpipe in a wet area you would probably never even pay for the turbine.

Ah well, if it's the thought that counts then the environment owes me one.
 
Just stick a net type thing over the hole, like they do now and the leaves won't reach the turbine. :)
 
Then put the wire net thing over all the open guttering. You can buy it from most garden stores. With its curved shape the leaves just blow away. ;)
 
Why not put something in the sewers though, loads of water and energy there as all water has to flow down out of houses
 
Simon said:
Why not put something in the sewers though, loads of water and energy there as all water has to flow down out of houses
Click to expand...


Still wouldn't save money it would cost to set up, plus it would clog the blades. waste water has solid bits in it.
 
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