Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum Run of the River

The technological approach being implemented for the Tacana project and other prospective projects is known as “run of the river” hydroelectricity. In this type of hydroelectric generation, the power itself is created by the natural course of the river current – in short, the natural flow and elevation drop are used to generate electricity. Power stations of this type can only be employed effectively on rivers with a consistent and ample flow. Run of the river projects are dramatically different in design and appearance from conventional dams, and their overall environmental impact is substantially reduced. Ordinary hydroelectric projects constructed on rivers with significant seasonal fluctuations require a large reservoir in order to be productive during the dry season. This usually means that large tracts of land need to be impounded and flooded to enable continuous operation. In stark contrast, run of the river projects simply divert a small amount of water into a penstock pipe, which channels the water downhill to the power station turbines.
This system uses the basic laws of physics to ensure a steady power supply even in the absence of a huge reservoir. Because of the difference in relief as water travels down the penstock, potential energy from the water up river is transformed into kinetic energy during its descent. This gives the water flow ample speed to spin the turbines in the powerhouse, which in turn transforms the kinetic energy into electrical energy. This method also leaves downstream flows intact, since all diverted water is returned to the stream below the powerhouse.
Most run of the river power plants consist of a dam across the full width of the river to provide the head needed for running the turbines. The water that is not needed for generating electricity spills over the dam at a spillway. Such installations have a reservoir behind the dam, but flooding is minimal and the reservoir is not used to store water for later generation. Flooding the upper part of the river is not required as it doesn’t need a large reservoir. As a result, people living at or near the river don’t need to be relocated, and natural habitats are preserved. This reduces the environmental impact and is a friendlier alternative to the large-scale reservoirs that are a staple of major dams without run of the river design

Sounds cool, but it would have the disadvantage of creating a long stretch of river with reduced flow rate where it's being bypassed through the penstock. I bet environmentalists would hate it for killing off fish and what not!

I see they have found a way to make this much more complicated and of course less effective when power generation is the goal.

you can do this without a river.

but thats where this started.

Hi Kallog



I suppose you are talking about the speed of the water that
exits the system after generating the power.

if so , do you think that this water that exits the system is no longer affected by gravity?

because it is gravity that gave the water its speed in the first place.

all the water is falling at 9.8 m/s/s
it just travels horizontaly because it cant travel verticaly.

so almost as soon as the water is released from the system back into the river the water will accelerate back to its original or close to its original speed , because of gravity.

they may have already designed this into the system by allowing the discharge pipe to have a angle that would re-accelerate the water before it enters the river.

if they didnt design this into the system then the designers are probably getting kick backs from some energy interest.

because you could have back pressures on the downstream side of the turbine that could reduce power production.



if environmentalist hate it for killing off fish , we can just point them to the gulf oil spills and see if they still think that way.

Well, actually it isn't falling at 9.8 m/s/s, it is accelerating at that rate. And the actual flow rate at the bottom of the run won't match that acceleration, because of friction with the sides of the stream.



Actually it will come out of the bottom of the penstock at a significantly lower speed than it has when it enters the turbine. The turbine extracts a great deal of the energy from the flow and thus slows it down. I'm not sure of the details because I don't know exactly what the flow equations would be. But it is possible that the flow from the penstock would actually have a lower speed than the flow in the original stream bed. That would depend on a number of factors, including the percentage of the stream that was passed through the penstock.

We should also keep in mind that the flow rate in the original bed would be lower than just the percentage that is drawn off to feed the generator. That is if 50% of the water is drawn off to the generator then the flow will be 50%, but the stream beds area will not be cut in half. So the resistance to flow will not be cut in half. So the stream may not gain as much energy as it would at full flow.

I keep looking at that and thinking I should add some more to it, but I think that should be enough to think about for right now.

Bill Gill

well actually the water is falling at 9.8 m/s/s
just like everything else on earth that is falling at 9.8 m/s/s it just cant move because the other stuff is in its way.



I have not witnessed the successful stretching of water yet
so I believe that the speed of the water from the point where it enters the penstock to the turbine will be apx the same given that the penstock area does not change.



yes energy is extracted from the flow of water , but the water will not compress or expand a great amount durring the process and since there is no heat or cooling applied during the process any change in water volume would be minute.

therefore the water cannot be traveling faster at point (B)
than it is traveling at point (A)

it is the flow rate through the penstock and turbine and the discharge that determines the speed that the water travels through the system.



well if the system is designed just right the water that leaves the turbine can actually boost the performance of the turbine.

You got me there. Actually what I should have said is that the flow through the penstock will be considerably lower than it would be if the turbine wasn't there. The turbine will produce a large resistance to the flow of the water, so it will significantly reduce the flow through the penstock. Have you ever seen the demonstration of a generator operation? The one I am thinking of consists of a small generator with a hand crank on it. It is connected through a switch to a light bulb. When the switch is off the crank can be turned freely. But when the switch is closed the crank gets very hard to turn, so the person cranking it slows way down. The same thing happens when you run water through a turbine to run a generator. So the flow of water also slows way down. And the flow of water just below the penstock is also much slower. So the water coming out of the penstock may be slower than the water in the river.

Now as to improving the performance of the turbine. I guess if you could design the junction of the 2 flows as a hydraulic ram it might be possible to reduce the water pressure at the penstock output and increase the drive to the turbine. Of course if I understand how a hydraulic ram works correctly the flow from the main stream would need to be considerably larger than the flow from the penstock.

Bill Gill

a hydraulic ram was'nt what I had in mind.

but certainly one could be designed to out perform any turbine.

after all your dealing with water in a long pipe and if that water is moving then your dealing with a much larger force than a turbine could handle.

for instance if the penstock is 6 ft diameter and 1000 ft long then there will be a volume/mass of water in the penstock
of:
28 cubic feet of water for every foot in lenght of the penstock.

thats 1,763 pounds each foot and 1.7 million pounds in the 1000 ft lenght.

if the water is moving at 1 ft per second then the force required to stop the water in 1 second is 1.7 million pounds per second
per second.

or 1.7 million fps

if you stop the water in 1 second.

and yes there would be a need to allow time for the water to regain its speed after you stop the water.

but whats to keep you from having more than 1 1000 ft long pipe.

if it takes 3 seconds for the water to regain its speed then have 3 pipes.

that way you could get 1.7 million fps from the flow of the river.

if you live close to a river you could experiment with this by using 5" pipes attached together and a flapper valve.

put together 10 10 ft lenghts and attach a flapper or foot valve on the downstream end.

submerge the pipe into the river and allow the water to flow through it by opening the flapper with a stick or something.

dont use your fingers cause the flapper could cut your finger as it slams shut.

now let the valve close.

you will be surprised at the force the pipe will present as you try to hold on to it.

it should present at least 70 lbs or more if the water is moving at 1 ft per second.

if the river is moving faster than 1 ft per second then you should not try this with a 5" pipe , because you could get injured by the 100 ft of pipe comming at you.

I just thought I might need to add that bit.

In that case how do existing hydroelectric power systems work? They handle massive amounts of water dropping a long distance and don't seem to have any problem.



The turbine doesn't stop the water, it is turned by the force of the water, so this objection is not valid.

Your penstock length may be a little short also. I just checked and it appears that the average drop of the Mississippi River is approximately 1/2 foot per mile. You may need to find a steeper river than that to actually get much power out of it.

Bill Gill

buy they are not stopping the water , you did mention a hydraulic ram.

and heres a problem they seem to have.

the power of a hydraulic ram

the red armature thing in the top picture was in the hole in the bottom picture.




the water flowing had an enormous force behind it , it was'nt highly pressurized , no where near the 3000+ psi stress of the concrete , but the force of the water
as it suddenly stopped and all the water behind it is the reason the above occured.

I dont know why this occured yet , but if the turbine shaft somehow locked up this would have greatly decreased the flow resulting in a partial water hammer.


----------------------
but certainly one could be designed to out perform any turbine.

after all your dealing with water in a long pipe and if that water is moving then your dealing with a much larger force than a turbine could handle.

I found this about the above
this happened in 2009 it seems and it was a load cut that
caused the turbine to overstress .

http://depletedcranium.com/deadly-catastrophic-failure-at-russian-hydroelectric-dam/

Paul, that doesn't really have much to do with water power. We were talking about run-of-the-river power generation, not malfunctions. I never said that water couldn't do great damage.

On the other hand, I think I may have been limiting my discussion too much. We started off talking about penstocks, but thinking a bit more about it I don't think you would want to have a penstock for run-of-the-river power generation. The thing about this way of doing it is that you can only get as much power out as is generated from water flowing from the upper end of the penstock to the outflow from the generator. This amounts to a pretty small energy differential, compared to what they use at most hydroelectric plants, where they use a hundred foot or more fall.

I think that rather than using standard water turbines where the water is conducted to the turbine we should consider using a paddle wheel device in the river itself. I am picturing a paddle wheel on its side so that one half sticks out into the flow of the river. Or, thinking again, maybe a series of fans, shaped like ships propellers that are immersed in the river. The advantage of this type of design is that you catch the full flow of the river. Using a penstock limits the amount of water you can get into the turbine, and with the limited fall available in the normal run of a river you really can't get much power out of it. That's why most hydroelectric power plants are built with dams or at waterfalls. You can get a decent head on something like that. Anyway if you immerse your power capturing device, turbine or wheel, or what have you, in the river you can catch the full flow. That way you basically have the head from your station to the headwaters of the river as a power source.

There are of course some problems with this scheme. One is that on a river that really has much flow you will also have commerce. You will have to be able to get river traffic past your station, which will act somewhat as a dam on the river.

Bill Gill

unless the water is in a pipe and the pipe is elevated there would be no head pressures that could build up in the water.

--------------------


if you use a penstock you can extract the entire force of all the water in the penstock x its velocity , or the waters momentum.

and the longer the penstock the more energy you can extract.

and if you need more energy in the future you can simply add more penstocks or penstock lenght.

a paddle wheel only extracts energy from the blades that are in contact with the water.

and the water can simply slip around the blades when a load is placed on them.

but a hydraulic cylinder can extract all the force of the moving water.
because the water is trapped inside the penstock and cant flow around the cylinders.

using a penstock and piston cylinder system you could easily allow for traffic on the river.

you just sink the pipe in the river.

------------

they are already using propeller type turbines in rivers

some like in new york use tidal currents
http://www.popularmechanics.com/science/environment/4213223

the pipe that supplies water to new york city has turbines in it.

but these systems do not capture the full force of the water because water can simply bypass the blades and take the path of least resistance.

The amount of energy that you can extract from a column of water depends on the height of the column, not the length. Using the figure of .5 feet per mile that I gave above for the Mississippi slope you can only get a .5 foot head (height) from a 1 mile long penstock. The amount of energy you can get out of that is the same whether you use a penstock or the bed of the river. There is no magic way to increase the amount of energy you can get from the water. So it is more economical to build turbines in the river, as you say than to install a long penstock with no significant increase in power. At large hydroelectric stations such as Niagara Falls they use penstocks to deliver the water where they want it, not to increase the power available. It is just like the pipes in your home, which are there to deliver water where you need it.

I think that is about all I will have to say on this subject.

Bill Gill

so the water pressure from the head would only be .216 psi

so you could stand in front of the pipe and not get hurt.

thats the kind of thinking that causes damms to explode.

but you have the right to think as you like.

as in :

quick stop the flow we have a minor malfunction...

boooom



ok

some people are dead wrong , and alot of people are dead because other people are wrong.



if you put a 1 mile long pipe or penstock in the mississippi river and it is only 1 ft in diameter

it would have a cross sectional area of 113.09 sq inches
113.09 x 12 inch lenght = 1357.08 cu inches of water
the water in the 1 ft long section of pipe has a weight or mass of 1357.08 cu in * .0361 lbs = 48.99 pounds

if that water is moving at only 1 foot per second it has a
momentum force of 48.99 ft-lb/s

but if you have a 1 mile long pipe or penstock then the force is multiplied by 5280 times because there are 5280 ft in a mile.

48.99 * 5280 = 258670.30464 ft-lb/s

that is the force of momentum that is in the moving water
even if it only has a head pressure of 0.21 psi.

and if you tried to close a valve very quickly then that
would be the force that the end of that 12 inch diameter pipe would feel.

so each sq inch of the 113.09 sq in area of the end of the pipe would feel a force of
258670.30464 / 113.09 = 2287.296 ft-lb/s

just because you dont understand water flows and the dangers associated with flows of water doesnt mean that you can simply claim that the amount of energy that can be extracted from a flow of water depends on the height of the
column of the water because that is dead wrong.

I really don't see what safety has to do with this at all. Every kind of power plant is dangerous. Imagine if we never used turbines because somebody said "Oh, what if a blade gets a fatigue crack and brakes off, then the whole thing goes out of balance and breaks free of the bearings and goes running down the street." Sure disasters happen, but there are always ways to protect against them. In the case of having to absorb the momentum of a lot of fast moving water, just have a special weak part of the pipe that can break open and dump the same flow rate into the air as what would have gone through the turbine.

Anyway, check out this, it adds a whole new complication, and is also pretty amazing! It's related to the article on the main page of scienceagogo
Vortex hydro power plant in operation

yes , thats a nice system and it also shows how a high head pressure is not needed to use water as a power source.

notice in the video how it is the sheer mass of spinning water that drives the blades in the vortex , and not the pressure of the water.

I would like to see the thing with no water in it , to get a clearer idea of it.

I wouldn't mind calculating the rate of gravitational potential energy loss by the water moving through that small head difference, and comparing it to the power output of these plants. I suspect they're really just doing it in a way that's cheaper to build with high efficiency at low heads, not actually getting energy from a new source. But this one below does say it takes it from the rotation of the Earth!!!

Another vortex turbine

kallog

That sounds wonderful. But, I would really like to see a discussion of this by a third party, one who is not trying to sell the concept. This may be real, but I think before I would buy into it I would like to have a skeptical engineer review the whole thing. To me it sounds just a little too much like magic.

And no, I am not going to defend my point. To do it properly I would have to do a whole lot of research to see just what is happening.

Bill Gill

Yea me neither. Too much hard work :P I don't really doubt these things have efficiency advantages tho. Conventional power generation of all sorts typically falls way below theoretical limits of efficiency.