h a l f b a k e r yI think this would be a great thing to not do.
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I had the idea we could tap into the potential energy of pressurized city water, and in researching it, I came across the Baked idea "Emergency Inline Plumbing Microhydroelectric Generator" (EIPMG) by [hankthoreau]. It spurred an interesting discussion, and I thought I would revisit it with a variation
on the idea that might make it more feasible.
There is great potential energy in city water supplies, especially the kind of systems where water is pumped into a raised tank and piped to your home. The pressure in the system is stored energy, released when you open the tap, but to what advantage? You don't need high pressure to feed your dishwasher, or fill a tub, or even fill a toilet. High pressure is great in a shower, but not a necessity everywhere else - in fact we build devices to restrict the flow (faucets), but could potentially use it better.
My idea is to put a turbine in the main water delivery pipe to the home that would generate electicity from the delivered water, whenever the tap was turned on. The difference between my idea and the EIPMG is that the water is first delivered to a roof mounted reservoir. The reservoir is where you would draw water for your home from. When the reservoir gets depleted from use, the city water trickles in (slowed by the turbine converting some of that potential energy into electricity). The reason for the reservoir is to maintain a minimal amount of pressure in your home, and to even out demand.
If you were normally using water, you would open and close faucets and your water demand would spike and drop with that use. With a trickle system, you are utilizing the pressure of the city system to supply electricity and deliver a prolonged but reduced flow of water. Might also help with peak use in cities with water shortages.
I know you can't get something from nothing, but wouldn't removing all or the majority of water demands from a high-pressure system be in effect using less energy? You could of course to opt to put some devices on city water directly, avoiding the loss of pressure (but losing the energy gain).
Think of it this way. If you put a little paddle-wheel AFTER the water left the faucet, it would slow the water down a bit, generate electricity, but not change your peak demand a bit in most circumstances. It might take a little longer to fill up the dishwasher, or washing machine, or pool, but that might not matter to everyone.
Looking forward to comments.
Emergency Inline Plumbing Microhydroelectric Generator
Emergency_20Inline_...lectric_20Generator
Baked Idea [trekbody, May 25 2005]
[link]
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A quick end-of-a-backvelope
calculation suggests that for an average
height house using a generous amount
of water, you'd harvest about enough
energy in a day to run a 100W light bulb
for about 1.7 minutes (10,000 J). |
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If right, that kind of sucks - thanks basepair. But wouldn't the energy harvested be the difference between lifting all the used water to the height of the building and lifting it to the height of the water tower. That seems pretty signficant to me, but my lack of physics is showing. |
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Why have a water tower in the first place then. Just pump to the house reservoirs and forget pumping to the top of the tower. That's the best way to "harvest" the energy between the two heights. |
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Whenever you're looking at a potential energy source you need to consider the amount of power your getting. For instance, you could hook a generator up to your hamster's running wheel, but would it be worth it? See my new posting for hamster based power generation conversion tables. |
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Because the water tower allows for lossless distribution of energy (if there are no leaks). A pump would be much more complicated, and would have to turn on and off with demand. This is the reason they use water towers and pumps - to help level pressure demand and to make distribution easy. |
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Oh yeah, I see what you are saying. Well, then, I guess we better get some of that energy back, eh? [+] |
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//But wouldn't the energy harvested be
the difference between lifting all the
used water to the height of the building
and lifting it to the height of the water
tower. That seems pretty signficant to
me//
OK, so I'm not sure how
much higher the water tower is than the
building, but let's guess 30m (which is a
lot), and let's also guess that the
average household uses 1 cubic metre
of water per day (which will be in the
right ball-park at least). This water
weighs 1000kg, so the energy available
is at most 30 x 1000 = 30,000 Joules.
One Watt is an energy
consumption of 1 Joule per second, so
you've got 30,000 Watt-seconds of
power available over the course of the
day, or enough to run a 100 Watt
lightbulb for 300 seconds (5min). This
is higher than my original calculation,
which assumed only a 10metre head of
pressure. But still not a huge amount. |
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Thanks for info Basepair! |
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Very thorough, [Basepair]. Rough and dirty, but I can't find a flaw... Unless, of course, you use 100 or so cubic meters of water a day. And who does that? |
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//Rough and dirty, but I can't find a
flaw// I love it when people say things
like that. |
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Interesting discussion and I agree with daseva. only thought to add is that the water towers in my area are well over 30m from ground level AND sitting on the highest hill around. Vertical difference between the top of that tower and the roof of my house is considerably more than 30m. My area is biology so this a bit far afield for me but wouldn't the difference in elevations be key? Maybe could get 5 minutes of light? |
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Oh - and the other reason for the towers is failsafe water presuure for fire fighting in the event of power failure. |
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That 300,000J is the energy you'd have if the water was being dropped vertically from the water tower into your sink. Even with the taps on full, the water doesn't come close to that level of kinetic energy. Frictional losses in the system are more or less the reason that 30m of pressure head is needed in the first place. |
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The bottom line is that, even if all practical issues were worked out, and you got the best case scenario (50 minutes at 100 watts, or approx. 4 bags of microwave popcorn) it wouldn't make sense financially to do this. Refrigerators would be slightly more efficient if the pressure reduction from condenser to evaporator were achieved with a turbine feeding energy into the motor of a instead of a throttle (the isentropic expansion in place of an isenthopic one would be closer to an ideal carnot cycle), but a couple of Watt hours just isn't worth spending thousands on. |
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Hang on. In Britain, we're told that the power consummed by everyones 'Standby' lights on their TVs requires a full power station for everyone in the country - surely 10,000 J per household multiplies up to a considerable amount - worth considering further I think. |
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