The Power Crisis

[Bullitt]

One thing thats always confused me is the different capacity of different hydro plants. eg Benmore Avimore and Waitaki all flow exactly the same amount of water (more or less) but have vastly different outputs due to (presumably) the size of the station and when it was built.

Is it possible to knock down Waitaki dam and build a new one that puts out three times as much power while using the existing (albeit temporarily drained) lake and therefore not do any environmental damage?

[Mikkel]

One thing thats always confused me is the different capacity of different hydro plants. eg Benmore Avimore and Waitaki all flow exactly the same amount of water (more or less) but have vastly different outputs due to (presumably) the size of the station and when it was built.

Is it possible to knock down Waitaki dam and build a new one that puts out three times as much power while using the existing (albeit temporarily drained) lake and therefore not do any environmental damage?

It's the elevation change which is crucial. The bigger the difference the more energy you get out...

[Jantar]

One thing thats always confused me is the different capacity of different hydro plants. eg Benmore Avimore and Waitaki all flow exactly the same amount of water (more or less) but have vastly different outputs due to (presumably) the size of the station and when it was built.

Mikkel got it. The power produced (P) is a function of thre different values. The flow of water (Q), the height of the water in the dam (H), and the efficiency of the plant (e). Most hydro stations will have an efficiency of between 0.87 and 0.91, so there isn't a lot in it. There is also a constant that man can't change, ie Gravity (G).

P = GHQe

So on a river system like the Waitaki, the only real reason for the diffence in output is due to the height of the dams. This height is measured as the diffence between the water surface at the top of the dam minus the water surface at the downstream side of the dam minus any head loss in the penstocks. So if Waitaki was to be knocked down, and a new higher dam built, that would raise the downstream water level at Avimore, and hence decrease the available power at that site.

[Mikkel]

Mikkel got it. The power produced (P) is a function of thre different values. The flow of water (Q), the height of the water in the dam (H), and the efficiency of the plant (e). Most hydro stations will have an efficiency of between 0.87 and 0.91, so there isn't a lot in it. There is also a constant that man can't change, ie Gravity (G).

0.87-0.91 - sounds like a bloody high efficiency to me. I would have thought it to be quite a bit lower than that... (or do you mean 0.87%-0.91% which would seem very low on the other hand.)

But I guess, since there isn't any significant heat involved, that the efficiency could potentially be much much higher than for any thermal powerplant.

Aren't there also some energy loss due to friction in both penstock and tailrace? I would guess that this loss would be higher for higher velocities...

As for gravity - hypothetically you could chuck some superdense material at the end of the tailrace and that should increase your local gravitational pull

And yes, I am being silly...

[McDuck]

[QUOTE=Mikkel;1609010]Aren't there also some energy loss due to friction in both penstock and tailrace? I would guess that this loss would be higher for higher velocities...

[QUOTE]

There are special coatings on the inside of the penstokes and tailraces meaning that they dont have much.

[Jantar]

0.87-0.91 - sounds like a bloody high efficiency to me. I would have thought it to be quite a bit lower than that... (or do you mean 0.87%-0.91% which would seem very low on the other hand.)....

Hydrelectric plant is about the most efficient form of conversion that there is. Turbine efficiencies are often over 92%, and generator efficiencies are often around 96%. Transformer efficiency is close to 99% because hydro plant use excitation to control power factor, rather than transformer tapping.

That is why hydro engineers keep laughing when claims are made about E3P or Otahuhu B being the most efficient plant around.

[Edbear]

[QUOTE=McDuck;1609021][QUOTE=Mikkel;1609010]Aren't there also some energy loss due to friction in both penstock and tailrace? I would guess that this loss would be higher for higher velocities...

There are special coatings on the inside of the penstokes and tailraces meaning that they dont have much.

Having worked in them, I was involved in "painting" the coatings inside them and it was a real experience to climb down the ladder and walk up the race to the gate, noting the trickle of water coming under the gate and contemplating the enormous pressure on it from the other side, and then walking all the way down to the turbine, clambering through the turbine, (HUGE!), and out into the station. Those pipes are between 15' - 20'ft in diameter if I remember correctly! Fascinating places!

[Mikkel]

There are special coatings on the inside of the penstokes and tailraces meaning that they dont have much.

Interesting - must be quite a task to keep those coatings in good condition at all times

Hydrelectric plant is about the most efficient form of conversion that there is. Turbine efficiencies are often over 92%, and generator efficiencies are often around 96%. Transformer efficiency is close to 99% because hydro plant use excitation to control power factor, rather than transformer tapping.

That is why hydro engineers keep laughing when claims are made about E3P or Otahuhu B being the most efficient plant around.

That's bloody impressive! I assume that the reason behind this is that you convert potential energy directly into kinetic energy and skip the thermal energy step... (Higher temperature -> higher entropy)

Having worked in them, I was involved in "painting" the coatings inside them and it was a real experience to climb down the ladder and walk up the race to the gate, noting the trickle of water coming under the gate and contemplating the enormous pressure on it from the other side, and then walking all the way down to the turbine, clambering through the turbine, (HUGE!), and out into the station. Those pipes are between 15' - 20'ft in diameter if I remember correctly! Fascinating places!

Nice

You just hope they haven't forgotten your walking around in there with your paintbrush Would be the mother of all waterslides though

I definitely would like to visit and have a look around one of the larger hydro powerplants!

[Edbear]

...
You just hope they haven't forgotten your walking around in there with your paintbrush Would be the mother of all waterslides though

I definitely would like to visit and have a look around one of the larger hydro powerplants!

Your imagination does tend to play up a bit at those times.... There's no quick way out of there! Plus I was fairly young then too.

[ManDownUnder]

I think I've got it... I unplugged my laptop... and it just kept on going!

Why didn't I think of that earlier. So obvious and think of the power saved

[Fooman]

That is why hydro engineers keep laughing when claims are made about E3P or Otahuhu B being the most efficient plant around.

Interesting point about gas turbines: overall simple, open cycle thermal efficiencies are between 30 to 40% conversion of chemical energy into mechanical energy (i.e. power at a shaft that can drive a generator or a fan or a pump). Now, the actual efficiency of the turbine end is quite high - around 90%+ conversion of thermal energy into mechanical energy. Where does the rest of it go? Into driving the compressor. A 45 MW LM6000 (like southdown or unit 6 at huntly) actually generates ~100 MW of mechanical energy, but 50 to 60 MW is needed to run the compressor section. And a lot of that is actually supplied by the first stages of turbine blading. Each 1st stage blade (worth about $20,000 US each), a hunk of nickel alloy about the size of two thumbs placed together, is pulling about 750 hp out of the gas stream at about 1100 C.

A colleague once described the working life of a turbine blade as holding a double decker bus while it is glowing red hot!

Cheers,
FM