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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
    Commented Mar 18, 2014 at 0:05
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    $\begingroup$ potential energy = pressure X volume $\endgroup$
    – DavePhD
    Commented Mar 18, 2014 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
    Commented Mar 18, 2014 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/… $\endgroup$ Commented Mar 13, 2016 at 22:18
  • $\begingroup$ @PeterDiehr Broken link $\endgroup$
    – user121330
    Commented Aug 19, 2022 at 15:55

5 Answers 5

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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
    Commented Mar 17, 2014 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. $\endgroup$ Commented Nov 13, 2015 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
    Commented Mar 18, 2014 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
    Commented Mar 18, 2014 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
    Commented Mar 18, 2014 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
    Commented Mar 17, 2014 at 23:33
  • $\begingroup$ potential energy = pressure X volume $\endgroup$
    – DavePhD
    Commented Mar 18, 2014 at 0:34
  • $\begingroup$ efficiency can be about 70-90% nrel.gov/docs/fy01osti/29065.pdf $\endgroup$
    – DavePhD
    Commented Mar 18, 2014 at 0:44
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According to https://www.watersafe.org.uk/advice/common_plumbing_questions1/pressure/what_is_the_minimum_water_pressure_that_a_water_supplier_must_supply/ , in the UK the domestic water supply should be expected to provide a minimum pressure equivalent to a 10m head, and a 9l/minute flow rate at the kitchen tap.

Then at https://www.renewablesfirst.co.uk/hydropower/hydropower-learning-centre/what-is-the-minimum-head-and-flow-i-need/ there is some information on micro-hydro schemes. That reckons a 380l/s flow rate with a 10m head should generate 25kW.

So assuming(!) the same efficiency could be achieved, the amount of power which might expected to be generated by the domestic supply's 0.15l/s flow is 25000*0.15/380 = ~10W.

Equivalent of a couple of old (pre-"rapid charging") USB3 ports (900mA * 5V = 4.5W).

Hard to imagine this being of any practical use, even for those with an unmetered water supply. 10W solar panels are dirt cheap and far less wasteful of a precious resource like treated, potable water.

Update: It's been bought to my attention that items such as this https://www.amazon.co.uk/Yosoo-Water-Turbine-Generator-Charging/dp/B00ZCBNNOC/ exist (Amazon has many other almost identical looking and probably made in the same factory items, but this one has the most interesting reviews).

I note it claims to produce 10W power - pleasingly in line with my 10W estimate above - however there seems to be no more detailed information given specifying power output in terms of flow and pressure.

  • One review claims to have used one to power some LED lighting at an "alpine hut".
  • One review claims to have used one in a working model of a hydro-electric plant!
  • One review reckons the claim of a 10W output is rather optimistic and might need a 50m head to achieve. (Which might indicate the assumption in my calculation that the efficiency of a "pro" 25kW generator could also be achieved by a 10W "toy" one is rather dubious).
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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
    Commented Mar 13, 2016 at 22:23
  • $\begingroup$ The latter part sounds like a use-case for a hydraulic ram pump ( en.wikipedia.org/wiki/Hydraulic_ram ). Those take input water at some pressure and flow rate, and output a proportion of it at higher pressure and lower flow rate so it can be elevated (the rest of the flow is effectively discarded at the pump's level). $\endgroup$
    – timday
    Commented Aug 19, 2022 at 17:43

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