Timeline for answer to What is the utilitarian purpose of artificial waterfalls? by Joe Bloggs
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| Dec 18, 2018 at 14:29 | comment | added | pluckedkiwi | @Tim yes cheap because supply is plentiful relative to demand and expensive because supply is short relative to demand - such semantic quibbles are meaningless as the vernacular is perfectly clear. Intermittent differences between sources and overall load can lead to times when additional load is needed to bleed off excess generation to maintain grid stability (the price literally turns negative). Utility-scale water pumps are good for that if the infrastructure is already in place and they can be highly responsive, but when you run out of storage for more water, then waterfalls. | |
| Dec 17, 2018 at 22:40 | comment | added | Tim | @pluckedkiwi your terminology is probably what is leading to your confusion. Electricity is never “plentiful” or “short”. Electricity is generated exactly as it is demanded, or the grid would turn off. Electricity does, however, vary in price. It gets cheaper when there is lots that could be generated than is being generated (night time mostly). This is when a company will pump water up (because they pay less than they get when they release it in the daytime). | |
| Dec 17, 2018 at 22:15 | comment | added | pluckedkiwi | @Tim I was thinking more along the lines of the waterfalls representing reservoir spillover from overfilling. My brain cannot really wrap around a concept that when power is short you would spend it pumping water up and out into the air, where as I regularly see hydroelectric dams with spillover from excess water flow and notice the lack thereof during drought (when power generation is reduced due to insufficient water behind the dam). | |
| Dec 17, 2018 at 11:48 | comment | added | Falco | @nobody good concept - it can also be a good practice to keep the water moving and the pumps going on a smaller rate when the reservoirs are full - so the water doesn't get algae and the pumps don't clog up. So the waterfalls are just the small portion of spill-over extra water, when the reservoirs are full. | |
| Dec 17, 2018 at 7:59 | comment | added | nobody | Core idea is great, the second part seems flipped tho: Power generation happens via pipes, you don't see that. (No wasting energy when it's scarce.) With surplus, lakes/reservoirs are filled. When they are full, they spill & create waterfalls. (Why? With some methods of power generation (e.g. solar), you have to do something with the output, in the simplest case by intentionally short-circuiting. That may not be viable for your method of generation (heat problems?) and there's no other sink available, so you keep the pumps running… Hence, waterfalls.) | |
| Dec 11, 2018 at 20:55 | comment | added | Joe Bloggs | @CactusCake: very cool. | |
| Dec 11, 2018 at 19:40 | comment | added | CactusCake | See en.wikipedia.org/wiki/Dinorwig_Power_Station for the (piped) real-life version. | |
| Dec 11, 2018 at 15:57 | comment | added | Harper - Reinstate Monica | just as electrical power is volts (pressure) x amps (flow), hydraulic power is also pressure x flow. Pressure is caused by the depth of water above the tap point. At the top of a waterfall you have flow alone, no pressure to speak of. Ditto at the top of a pipe. Water below the turbine can't contribute to pressure through the turbine, except via syphon effect, and that is limited to 30'/10m/1 atm. | |
| Dec 11, 2018 at 7:19 | comment | added | Joe Bloggs | @Harper the proof being in the flow rate of the water. If the water is being forced through the pipe/turbine system at its maximum velocity then no matter where you put the turbine the water coming out the other end of the pipe will be at that velocity. As energy can’t be lost elsewhere (assuming frictionless, straight pipes, naturally), the power extracted from any pipe/turbine arrangement is equal, no? | |
| Dec 11, 2018 at 7:12 | comment | added | Joe Bloggs | @Harper :I’m just talking pure classical mechanics. I will admit I assumed laminar flow in the pipeline (IE no air in the pipe), but it’s trivial to show that energy in = energy out no matter where you put the turbine there. A waterfall is less efficient than an enclosed system, but i literally said that in my answer. | |
| Dec 11, 2018 at 7:09 | comment | added | Joe Bloggs | @immibis: See the sentence where I wrote that exact thing. I’m just trying to give a semi-plausible reason here (or contribute to the stack of potential reasons). | |
| Dec 11, 2018 at 2:24 | comment | added | Stack Exchange Broke The Law | @JoeBloggs However (AIUI) you only get energy from the length of the pipe. Once the water escapes into free air, you don't get to extract energy from the rest of the drop since it doesn't generate suction. So it's better to have the pipe go all the way down to where the waterfall would end, instead of having the pipe go halfway down and then having a waterfall. | |
| Dec 11, 2018 at 2:20 | comment | added | jpmc26 | Real life example of an energy storage lake, although a waterfall isn't used. @pluckedkiwi It's actually the reverse. The stored water would be going back down to release the stored energy. When that's happening, energy is not being generated using the normal mechanism, whereas when the water is being pumped back up, excess energy to store is being generated. | |
| Dec 10, 2018 at 23:21 | comment | added | Harper - Reinstate Monica | You're talking about the impact energy of free water landing, which is a diffent thing. It's hard to do. It's also not what Joe said that I challenged. | |
| Dec 10, 2018 at 21:57 | comment | added | Vaelus | @Harper the water will gain kinetic energy as it falls, untill it reaches terminal velocity. The kinetic energy of the water can be converted to kinetic energy of a turbine at the bottom. The integral of force (weight) times distance (height) remains the same, so the same amount of work is done on the water. Of course, there is some loss to air resistance, non-elastic collision, and lost water, but it's not true that all of the energy of free falling water is lost. | |
| Dec 10, 2018 at 21:15 | comment | added | Harper - Reinstate Monica | @JoeBloggs that is flat wrong. | |
| Dec 10, 2018 at 20:54 | comment | added | Joe Bloggs | @Harper: A turbine at the top of a pipe will pull out exactly the same energy as a turbine at the bottom of a pipe. The only energy wasted is that which is lost to air friction and dispersion. If it’s a problem then have a set of stepped reservoirs for even more architectural joy. | |
| Dec 10, 2018 at 20:41 | comment | added | Tim | @pluckedkiwi that’s the wrong way round; the water is pumped up during “plentiful” electricity consumption, and released when they need more electricity. And plentiful is a poor descriptor; electricity is produced precisely as needed. | |
| Dec 10, 2018 at 19:19 | comment | added | Harper - Reinstate Monica | Except the waterfalls are fairly useless for that, the visibly falling water is wasted height/energy. You can only capture energy from the water that falls into a turbine. | |
| Dec 10, 2018 at 14:04 | history | edited | Joe Bloggs | CC BY-SA 4.0 |
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| Dec 10, 2018 at 14:03 | comment | added | Joe Bloggs | @pluckedkiwi :Excellent point!! | |
| Dec 10, 2018 at 13:50 | comment | added | pluckedkiwi | As a bonus you have a great visual and auditory signal to all residents that it is a time of plentiful energy, so they should conduct any energy-intensive activities while the waterfalls are going. If the waterfalls are not apparent, city is on restricted power consumption and everyone should be attempting to conserve power. | |
| Dec 10, 2018 at 13:11 | history | answered | Joe Bloggs | CC BY-SA 4.0 |