# Electrical generation from used water in my home

#### Avraham Bookatz

Hi everyone. I was just out in my back garden and was watching the water from someone's shower pour down the pipe and out into a drain. I was thinking if all of the used water from my house just goes down a drain then why not try make use of it.

If the water system continues after use and takes the water to let's say a turbine in the celler, and then another, and another for as long as it's feasible in the space given. Wouldn't I be able to provide myself with a decent amount of power.

I'm not talking about modification on an old system but to make such a system from scratch.

Any thoughts would be appreciated.

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#### AZFIREBALL

The problem is, that water contains very little potential energy. Unless it is moving very fast, falling from a great distance or under great pressure, it is not very useful as a source of power. The best use of so called 'grey water' is in watering your garden.

#### Avraham Bookatz

what if all of the 'grey water' was channeled together and run through a 'vortex turbine' before moving on to wherever it goes on to.

#### russ_watters

Mentor
It should not be difficult to calculate how much energy is available:

-How much water do you use?
-How far can you make it fall?
-Do you know the gravitational potential energy equation?

#### essenmein

Often wondered in high rise buildings if it would make sense to recoup some of the energy in the falling waste water to offset the fact that someone had to pump it up there.

#### russ_watters

Mentor
Often wondered in high rise buildings if it would make sense to recoup some of the energy in the falling waste water to offset the fact that someone had to pump it up there.
So calculate it!

Literally, I read the OP before I got out of bed and calculated the answer while taking a shower. There's no need to guess or wonder, guys, this is easy!

#### essenmein

So calculate it!

Literally, I read the OP before I got out of bed and calculated the answer while taking a shower. There's no need to guess or wonder, guys, this is easy!
The physics is easy, the amount of flow unknown, and that particular piece of idle wondering is not that high on my list of things I should actually calculate.

#### russ_watters

Mentor
The physics is easy, the amount of flow unknown, and that particular piece of idle wondering is not that high on my list of things I should actually calculate.
Fair enough. In my house and I would think The shower is the biggest water user. Perhaps the roof drains.

#### davenn

Gold Member
The physics is easy, the amount of flow unknown, and that particular piece of idle wondering is not that high on my list of things I should actually calculate.

just pick a reasonable figure and work with it to give you an idea

#### essenmein

just pick a reasonable figure and work with it to give you an idea
Or pick some nominal value to come up with a general idea, lol, 1kg over height of 1m on earth is 10J. So 1L/s water over 1m is 10W (assuming 100% energy conversion and that the water is starting with zero kinetic energy, ie just the grav potential energy). This is linear, so 1L/s falling from 30m is 300W.

1L/s is quite a lot of water and not generating that much power.

Then average household consumption of water in US/Can is ~140gal/day, so in grown up units that is about 530L/day, or average of 6.1e-3 L/s so its already obvious why we don't see power generating systems on drains.

#### Tom.G

Often wondered in high rise buildings if it would make sense to recoup some of the energy in the falling waste water to offset the fact that someone had to pump it up there.
It turns out that hi-rise buildings need a "jog", a short horizontal section, in the drains at each floor. That is to avoid the solids being in free-fall and going right thru the pipe when they hit bottom. Oops! Messy.

#### OmCheeto

Gold Member
I did a calculation on something similar about 12 years ago(posted 03-20-2007 08:28 AM) on another science forum.
It turned out, if I collected all the rainwater from my roof, it would generate about 0.73 kwh/year.
I just redid the calculation, and I came up with 0.62 kwh/year.
At my current electrical rates, and an estimated cost of installing such a system to capture that energy of $200, it would take me 5000 years to recoup my investment. (Estimation of system efficiency of 50% was used this time. I think I ignored that, 12 years ago.) As I said back then; "It's a good idea to do the math before building some of this stuff." #### OmCheeto Gold Member So calculate it! Literally, I read the OP before I got out of bed and calculated the answer while taking a shower. There's no need to guess or wonder, guys, this is easy! I tried that on the back porch. I can only do it if I round everything to 1 significant figure: 100 m^2 roof * 1 m rain = 100 m^3 water * 1000 kg/m^3 = 100,000 kg * 10 m/s^2 (gravity) * 3 m (∆elevation) = 3 million joules / (4 million joules/kwh) = 3/4 kwh hmmm.... I wonder if I can do that non-metrically? 1000 ft^2 * 3 ft rain = 3000 ft^3 water * ??? lbm/ft^3 Nope. #### Greg London 100 gallons of water per day, per person. Which is roughly 800 pounds. Say water drops 10 feet in a house from first floor to sewer. Thats 10,000 joules a day. 3.65 million joules per year. Or... Almost exactly one kilowatt hour. Worth about 12 cents. So...... probably not worth it. #### sophiecentaur Science Advisor Gold Member let's say a turbine in the celler, So 1L/s water over 1m is 10W It turned out, if I collected all the rainwater from my roof, it would generate about 0.73 kwh/year. Almost exactly one kilowatt hour. Worth about 12 cents. What counts is the Energy and not the Power. The Energy from such a scheme is virtually nothing - particularly when you think of the capital expenditure involved. Payback time would be decades and that doesn't include breakdowns - which more schemes tend to suffer from unless they are wildly over-engineered. This is yet another example of so called Energy Harvesting and only a very few EH schemes are worth their salt. Save money by turning down the heating and having fewer or shorter showers; that really can make a difference. #### Avraham Bookatz Did anyone actually look at the technology I mentioned. If this technology was applied to a central water pipe that all used water runs through (including rainfall from the roof) would that have an effect on the calculation. Bear in mind I was planning this in a particularly rainy city like Manchester, England. #### willem2 Did anyone actually look at the technology I mentioned. If this technology was applied to a central water pipe that all used water runs through (including rainfall from the roof) would that have an effect on the calculation. Bear in mind I was planning this in a particularly rainy city like Manchester, England. I doesn't really matter what technology you use, you can't get out more than 100% of the potential energy of the water. This is equal mgh. (m = mass, h is altitude difference, g = acceleration of gravity). To get useful energy out of the water you need a sufficient amount of water and enough altitude difference. I don't think manchester is hilly enough. You might want to have to use energy to pump away the water. Another thing you'll need is some kind of reservoir. If you can only produce energy when it's raining, that's not useful at all. You'll probably need to store at least 10% of the yearly rainfall. If you have 830mm of rain each year you would need a 100x100 meter lake, 8.3 meters deep, for every square kilometer. #### sophiecentaur Science Advisor Gold Member Did anyone actually look at the technology I mentioned. If this technology was applied to a central water pipe that all used water runs through (including rainfall from the roof) would that have an effect on the calculation. Bear in mind I was planning this in a particularly rainy city like Manchester, England. I know it's disappointing but PF has had dozens and dozens of threads about Energy harvesting and only a very few - where the sums actually add up - have any legs at all. If Hydroelectricity was a go-er in Manchester then it would be used on a big scale. Water Power went out of fashion as soon as alternatives came along and that wasn't just because it was not cool. Hydroelectric Power generation is only used in the UK where there are serious mountains (where dams can be built big enough) and a lot of rain (= in Scotland mainly). The money to be made in the home basically involves Energy Saving and there are hundreds of quid to be saved, even over the medium term. Energy Sources = Engineering = Cost effectiveness. You have to do the sums before ordering stuff off eBay. #### russ_watters Mentor Did anyone actually look at the technology I mentioned. People ignored it because the harvesting technology doesn't impact the answer, but it was probably worth pointing out that the idea looks like a perpetual motion machine, based on a misunderstanding of flow and conservation of energy. What you propose wouldn't extract more than the energy available If this technology was applied to a central water pipe that all used water runs through (including rainfall from the roof) would that have an effect on the calculation. Bear in mind I was planning this in a particularly rainy city like Manchester, England. Most modern cities have separate systems for rainwater and sewage. #### sophiecentaur Science Advisor Gold Member Most modern cities have separate systems for rainwater and sewage. . . . . also seperate 'Grey Water', which is from baths, showers and hand basins. That can be used for washing cars and watering the garden. It would also have some finite mgh. #### Greg London Did anyone actually look at the technology I mentioned. If this technology was applied to a central water pipe that all used water runs through (including rainfall from the roof) would that have an effect on the calculation. Bear in mind I was planning this in a particularly rainy city like Manchester, England. Lets try the Empire State Building. It is 87,000 square feet and 1250 feet tall. NYC gets 45 inches of rain a year, or 3.75 feet. This means that 87000*3.75 = 326,000 cubic feet of water falls on the Empire State Building a year. 1 cubic foot of water weighs 62.4 pounds. which means 20million pounds of water falls on Empire State Building every year. roughly 10million kg. which falls a height of 380 meters. 380 * 10mill kg * 10m/s2 = (rounding) 40billion joules. there are roughly 4million joules per kilowatt hour. 40bill jouls / 4mill joules/kwh = 10,000 kwh A kilowatt hour costs about 12 cents. So, all the rain that falls on the empire state building would generate about$1,200 worth of electricity per year.

Put another way, rainwater runoff would generate 10 mega-watt-hours of electricity per year.
the Empire State Building uses about 5,000 mega-watt-hours of electricity per year.
About 0.2% of its total consumption.

#### Greg London

Oh, one other caveat for the empire state building calculation: if you purchase only one rain water generator for the building, then you will need to put it on the first floor, feed it through a 1250 foot tall pipe to the roof funnel, AND you would need to STORE that water until you could run it through the generator while keeping the pipe full of water.

If you just feed the water through the generator as it falls as rain, you might only fill up a few feet of the pipe, and the generator wont be able to see that 1250 foot drop. It might see, say, a 12.5 foot column of water, at which point, it generates $12 dollars of electricity a year instead of$1200.

Your system will have to add some other level of complexity to actually recover the energy from that 1250 foot drop. Which adds to the cost. Which means it is likely never going to pay for itself.

#### Greg London

18 sq ft of solar panel generates 265 watts of electrical power. The empire state building fits 4800 panels, producing about 1.2 megawatt of power when its sunny. Say its sunny for only 3 hours a day im nyc. Thats 3.8 Mwh a day. Times 365 days. Thats 1.4 Gigawatt hours of electricity a year.

Solar panels pay for themselves after about 7 years, and they are usually good for about 30 years.

So per year, for empire state building we get:
Rainwater runoff: 10 mwh
Solar panels on roof: 1400 mwh
Total consumption: 5000 mwh

"Electrical generation from used water in my home"

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