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Could someone cleverer than me help me out?

I had a crazy thought going through my head the other day and I can't lay my mind to rest until I get an answer.

Q. How much energy could be produced by using mains water pressure to turn a generator? And would it be feasible to install a system to feed whatever is produced back to the grid? Assuming that the system would be installed in a building where a constant water supply is needed so the generator would be turning continuously, and a rough water pressure of around 3-4 bar.

Thanks in advance for any help

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  • $\begingroup$ Thanks for your comments, so is anyone clever enough to do the math (I am not) to figure out the potential energy that could be produced. I would estimate a conservative flow rate of 15 litres per minute at a pressure of 3 bar. $\endgroup$ – user42737 Mar 18 '14 at 0:05
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    $\begingroup$ potential energy = pressure X volume $\endgroup$ – DavePhD Mar 18 '14 at 0:48
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    $\begingroup$ 3bar= 300,000Pa 15L/min = 0.25L/s = .00025 cu.m./s so 75 watts $\endgroup$ – DavePhD Mar 18 '14 at 0:54
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    $\begingroup$ Some sump pumps are designed to work from mains water pressure in the event of loss of electrical power. For example, see libertypumps.com/Products/Category/SubCategory/Product/… $\endgroup$ – Peter Diehr Mar 13 '16 at 22:18
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A generator converts mechanical power to electrical power; pressure alone is insufficient.

Assuming the flow in = flow out and a constant flow, the power output of the generator would then be proportional to the pressure difference between the inlet and outlet.

Thus, subtract the minimum pressure required by the building from the mains pressure and multiply that by the flow to find the potential power available for conversion.

Since generators aren't 100% efficient, the actual electrical power generated will be less.

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  • $\begingroup$ Ahhhhhh right, so what determines the amount of electricity that can be produced is the flow rate? Any ideas on how efficient generators are? $\endgroup$ – user42737 Mar 17 '14 at 23:41
  • $\begingroup$ @user42737, the flow rate and the acceptable amount of pressure drop. When water is flowing through your turbine, the outlet pressure will be less than the inlet pressure. The power that you are able to generate will be proportional to the product of the flow rate and the pressure drop. Drop the pressure by too much, and you will have to redesign your bathroom fixtures to work with the lower pressure. $\endgroup$ – Solomon Slow Nov 13 '15 at 21:58
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We pay 3 dollars per cubic meter for water where I live. At 400 kPa (60 psi) that's 400 kJoules per cubic meter maximum theoretical power. But that's only about a tenth of a kilowatt-hour, which costs about a penny at 10 cents/kW-hr. So it's not that good a proposition.

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    $\begingroup$ Yes but surely if would be worthwhile setting the system up if the water was free/already paid for. $\endgroup$ – user42737 Mar 18 '14 at 0:27
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    $\begingroup$ @user42737 Let's put some more numbers to it. In the USA, new showers must use < 2.5 gallons of water per minute. So a 10 minute shower uses 25 gallons, which converts to roughly 0.1 cu. m. So maximum energy to extract is 40 kJ per 10 minutes assuming 100% efficiency in the generator. That's basically 66W of power generated during those 10 minutes. So you could power 1 lightbulb in the bathroom while you showered and that's about it. $\endgroup$ – tpg2114 Mar 18 '14 at 0:56
  • $\begingroup$ Not to mention that if you extracted all the energy from the flow, you wouldn't have any water pressure left with which to shower. You'd really just be standing under a slow drizzle. All the energy in the flow went into the generator. So actual power generated with inefficiencies in the generator and leaving some pressure in the water to use the shower correctly is much smaller. $\endgroup$ – tpg2114 Mar 18 '14 at 0:58
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You could certainly make electricity this way, it just wouldn't be cost effective. 3-4bar would be the same pressure as a 30-40 meter hydroelectric dam. The energy per time unit depends upon the flow rate (which depends upon the 4th power of pipe diameter).

potential energy = pressure X volume

I wouldn't want to see your water bill!

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    $\begingroup$ Thanks for your comments, lets assume the water was being used anyway and was paid for. What would be the mathematical equation for figuring this out and what information would I need to do this. Given enough time and a unlimited amount of caffeine I could probably figure out how much energy this could potentially produce but what sort of amount do you lose due to generator efficiencies? $\endgroup$ – user42737 Mar 17 '14 at 23:33
  • $\begingroup$ potential energy = pressure X volume $\endgroup$ – DavePhD Mar 18 '14 at 0:34
  • $\begingroup$ efficiency can be about 70-90% nrel.gov/docs/fy01osti/29065.pdf $\endgroup$ – DavePhD Mar 18 '14 at 0:44
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A domestic cold water tap full on could produce enough power to run a big TV assuming 100% efficiency. Currently working on the idea of using tap water energy to lift well water to a storage tank in the top of the house for use in supplying the loo and garden. Once most of the energy has been extracted from the tap water then there could be enough left to deliver the water for normal consumption.... best wishes Physics teacher

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  • $\begingroup$ Any calculations or experiments to prove the first sentence? $\endgroup$ – Kyle Kanos Mar 13 '16 at 22:23

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