Power shares

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Many of those sedate English rivers had open water wheels generating, maybe DC, but I'm sure we can better harness what we've got.
I'd appreciate if Jantar, or other people who have expertise in the field, could make some suggestions.

[Jantar]

Further refinement, Archimedes inside the intake, pelton useless there, but instead on trailing arms to accommodate different flows, at the discharge,
we've all seen photos of the plumes or rooster tails some dams give at that point, unharnessed.
If not a pelton, whatever is the appropriate type of water wheel for that application.

The amount of power available to a turbine is P=GQHe. Where
G is gravity, the driving force. 9.81 m/s^2
Q is flow of water in m^3/s
H is the head of water in m
e is the turbine efficiency

It doesn't matter what anyone does, that amount of power from the water source can not be exceeded. By using some power from the flow in the penstock, as you suggest, just reduces the head of water available to the turbine.

That "plume of water" that you mention is spill, not water going through the turbines. The amount of time that water is spilled is very small. So that would be millions of dollars spent for possibly a few days use each year.

[dibble]

The amount of power available to a turbine is P=GQHe. Where
G is gravity, the driving force. 9.81 m/s^2
Q is flow of water in m^3/s
H is the head of water in m
e is the turbine efficiency

It doesn't matter what anyone does, that amount of power from the water source can not be exceeded. By using some power from the flow in the penstock, as you suggest, just reduces the head of water available to the turbine.

That "plume of water" that you mention is spill, not water going through the turbines. The amount of time that water is spilled is very small. So that would be millions of dollars spent for possibly a few days use each year.

You seem to have become the goto expert..is it fair to presume water has some sort of terminal velocity so H must have a maximum before it becomes ineffective? Thanks.

[Snow Leopard]

time to talk Reynolds number ?

[Snoopy]

Is it fair to presume water has some sort of terminal velocity so H must have a maximum before it becomes ineffective? Thanks.

'Terminal Velocity' is a concept that is usually thought of when a solid object falls under the action of gravity. In theory a solid object pushed out of an aeroplane will keep on accelerating under gravity travelling faster and faster until it hits the ground. In practice most objects that fall out of aeroplanes have air resistance. This is a counter force that acts opposite to the direction of gravity. At some point the force of more and more air molecules hitting the falling object will reach a level that exactly balances the gravitational force. Thus the object that is falling will stop accelerating and keep falling at a constant velocity once gravity and the counter air resistance force reach balance. This maximum constant velocity that is reached is termed the 'terminal velocity'. Different objects will have different terminal velocities. All that friction from the air will heat up the object that is falling. If an object falls from high enough in the earths atmosphere this heat build up can cause a solid object to vapourise. The same can happen to a liquid object too, although more easily (it takes less energy to go from liquid to gas than from solid to liquid to gas).

If rain can fall from thousands of feet in the air and still come down to earth as droplets you would have to have a fairly high dam for the water to vapourise into steam as it came down over-energised at 'terminal velocity' don't you think? I don't foresee too much hydraulic operational risk from building a dam that is 'too high'.

SNOOPY

[Jantar]

You seem to have become the goto expert..is it fair to presume water has some sort of terminal velocity so H must have a maximum before it becomes ineffective? Thanks.

In free fall through air it would have a terminal velocity. That is not what is happening inside a turbine. It is the pressure due to gravity that is being used, not its velocity. A column of water 1 sq m and 1 meter high weighs exactly 1 tonne, so if a turbine has a column of water 60 m high then there are 60 tonnes of water pushing that turbine around. Now Onslow is going to have a column of water 600 m high, so 600 tonnes of water pushing on that turbine. That is where the power of the water comes from, not its speed.

K14 is also an expert on hydro power. I helped train him in the distant past.

[gains]

In free fall through air it would have a terminal velocity. That is not what is happening inside a turbine. It is the pressure due to gravity that is being used, not its velocity. A column of water 1 sq m and 1 meter high weighs exactly 1 tonne, so if a turbine has a column of water 60 m high then there are 60 tonnes of water pushing that turbine around. Now Onslow is going to have a column of water 600 m high, so 600 tonnes of water pushing on that turbine. That is where the power of the water comes from, not its speed.

To add to this, I think what you're really asking dibble is whether adding more head to a hydro-scheme will decrease efficiency?

If that is the case, in short, yes but a greater power output.
Comparatively to the pressure of water on the turbine, the efficiency would drop below what the system was designed to operate at. However, the power output could increase as a greater load could be applied to the turbines and therefore the generators. The turbines are designed and built for a particular load range and operating outside of this range risks the loss of efficiency and overloading the generators.

[Snoopy]

'Terminal Velocity' is a concept that is usually thought of when a solid object falls under the action of gravity. In theory a solid object pushed out of an aeroplane will keep on accelerating under gravity travelling faster and faster until it hits the ground. In practice most objects that fall out of aeroplanes have air resistance. This is a counter force that acts opposite to the direction of gravity. At some point the force of more and more air molecules hitting the falling object will reach a level that exactly balances the gravitational force. Thus the object that is falling will stop accelerating and keep falling at a constant velocity once gravity and the counter air resistance force reach balance. This maximum constant velocity that is reached is termed the 'terminal velocity'. Different objects will have different terminal velocities. All that friction from the air will heat up the object that is falling. If an object falls from high enough in the earths atmosphere this heat build up can cause a solid object to vapourise. The same can happen to a liquid object too, although more easily (it takes less energy to go from liquid to gas than from solid to liquid to gas).

If rain can fall from thousands of feet in the air and still come down to earth as droplets you would have to have a fairly high dam for the water to vapourise into steam as it came down over-energised at 'terminal velocity' don't you think? I don't foresee too much hydraulic operational risk from building a dam that is 'too high'.

I think I wrote my post too late at night.

The first paragraph is O.K. It is not impossible for a rain droplet to vapourise on the way down, even if reaching terminal velocity is not the only explanation for this. I was writing on the premise that if a droplet falls at a certain speed then the likes of a waterfall would fall at a slower speed because the turbulent interaction between water molecules would slow each neighbouring molecule down as it fell. While there might be some truth in that, the pattern of air resistance provided by the atmosphere is likely very different. So it is not correct to say that you can extrapolate the behaviour of a large body of water falling from what happens to a droplet.

I found this interesting hypothetical experiment in a 'What if' book.

https://what-if.xkcd.com/12/

They reckon that if a rainstorm was bundled up into one droplet it would fall faster and faster until it hit the ground at 450mph. That I think means terminal velocity for such a large body of water is greater than 450mph. So 'Terminal Velocity' wouldn't come into the operation of a turbine, where the water would fall from a far lesser distance than sky level. In summary, I told you the right answer but my reasoning was wrong.

SNOOPY

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My recollection of most of the old NZED, or what some referred to as state hydro sites in the North island, is they seemed to use the same diameter penstocks, whereas the length to the power house, and fall could vary considerably.
To adhere to Jantars formula, does this mean the normal DEPTH within each pipe can vary at each site, eg 40% of capacity at one site, And say 90% at another?

Also curious why MereMere was only intended to last 20yrs, was it because the coal was expected to run out?

[Snow Leopard]

I am getting bored with this diversion into hydro efficiency as we have yet to correlate the height of the dam and water temperature to the share price .

But if you want to talk about long distance gas transmission systems and especially line-pack I will join in again.