Naked Science Forum

General Science => General Science => Topic started by: skaraman on 22/10/2019 08:08:35

Title: Can you use unlimited mini hydro generators on the end of a faucet?
Post by: skaraman on 22/10/2019 08:08:35
I imagine this is a fairly ignorant question, but having looking up a number of resources, I cannot find a specific enough answer.

Someone I know wants to attach mini hydro generators to faucets throughout their house to save power. A question they posed was "since these generators can be attached to the end of the faucet where the water comes out, and are powered by the motion of the water, what is stopping me from attaching a huge number of mini hydro generators to the end of the faucet, essentially creating an endless supply of electricity?"

My knowledge of science is fairly limited, and I found the question to be interesting, so I pose the same question. Why can't my friend attach a potentially unlimited amount of hydro generators to the end of a faucet and get free energy, so long as the faucet's water is flowing downward?
Title: Re: Can you use unlimited mini hydro generators on the end of a faucet?
Post by: syhprum on 22/10/2019 08:48:33
If you connect a hydro generator to a faucet you are not only using the water supplied by the water company you are using the power that they used to pump it.
you will of coursed be charged for both by the companies !
in the early part of the ninetieth century before electric power transmission got going power was transmitted by water pressure that was used to work cranes and lifts and customers who wanted a small amount of electricity could use small hydro generators which was convenient but expensive compared to electricity supplied by cable that soon made it obsolete.
Title: Re: Can you use unlimited mini hydro generators on the end of a faucet?
Post by: evan_au on 22/10/2019 10:30:03
Quote from: OP
Why can't my friend attach a potentially unlimited amount of hydro generators to the end of a faucet and get free energy, so long as the faucet's water is flowing downward?
It's a matter of pressure - the water in your water taps has a finite pressure.
A hypdropower generator works on pressure - the turbine blades are driven by the difference in pressure between the inlet pipe and the outlet pipe.

You get the maximum amount of power if the outlet pipe has "zero" pressure - the water just goes down the drain, and the inlet pipe is at full water pressure.

However, if you connect another hydrogenerator to the output of the first one:
- The first turbine outlet pressure is no longer zero, so it doesn't generate as much electricity
- The second turbine's inlet pressure is now much lower than the water supply pressure, so it doesn't generate as much electricity, either.

So there is a finite amount of power you can extract from a water pipe of a given diameter and pressure.
- Adding an infinite number of turbines does not make the power infinite.

There are cases where you might put several devices in series - it was common in steamship engines, for example:
- One small diameter cylinder uses high-pressure steam direct from the boiler.
- The output of that cylinder is at a lower pressure, and this goes to a second, larger-diameter cylinder
- Which feeds a third cylinder at very low pressure discharges into the atmosphere
- But each additional cylinder produces less power and adds more complexity, so in practice they commonly used triple-expansion marine steam engines.
See: https://en.wikipedia.org/wiki/Compound_steam_engine#Multiple-expansion
Title: Re: Can you use unlimited mini hydro generators on the end of a faucet?
Post by: alancalverd on 22/10/2019 10:48:19
Hydroelectricity is indeed free of charge as long as you collect the water in your own reservoir at the top of your own mountain and move it through your own pipes. It may be worthwhile comparing your domestic piped water charges with your electricity bill.

UK domestic water costs about 4 per tonne (you pay more for the sewerage to take it away than you do for delivering the clean stuff!). Assume you have a 5 m pressure head from a 1000 liter brake tank in the loft. 1 tonne falling through 5 m will deliver

m g h = 1000 x 9.81 x 5 =  49,050 joules = 13.625 Wh for 4, about 30,000 times the cost of the same amount of domestic electricity.

If you drive your turbine direct from the water main, at say 5 bar pressure, you will reduce the cost to about 3,000 times the cost of mains electricity.
Title: Re: Can you use unlimited mini hydro generators on the end of a faucet?
Post by: vhfpmr on 23/10/2019 22:15:14
Hydroelectricity is indeed free of charge as long as you collect the water in your own reservoir at the top of your own mountain and move it through your own pipes. It may be worthwhile comparing your domestic piped water charges with your electricity bill.

UK domestic water costs about 4 per tonne (you pay more for the sewerage to take it away than you do for delivering the clean stuff!). Assume you have a 5 m pressure head from a 1000 liter brake tank in the loft. 1 tonne falling through 5 m will deliver

m g h = 1000 x 9.81 x 5 =  49,050 joules = 13.625 Wh for 4, about 30,000 times the cost of the same amount of domestic electricity.

If you drive your turbine direct from the water main, at say 5 bar pressure, you will reduce the cost to about 3,000 times the cost of mains electricity.

Some people in the UK can use as much water as they like for a flat annual charge. Many years ago I pondered the idea of using it to generate electricity as a provocative way of demonstrating to the water board what a daft tariff system it is. Unfortunately, I had already switched to a water meter by then.