Power shares

[Nor]

The dam would be made of earth, and plenty of bentonite on the east coast to seal it if necessary.

Land is cheap and already cleared down the East coast from @ young Nicks Head to the coast of southern Wairarapa.

The power house would be located down close to sea level, and off peak power would be used for the pumping..
This could be augmented by some wind turbines built over the lake, rather than over the sea like Denmark does, and/ or higher ground around the lake.

If ground was chosen in a series of valleys, there would be inflow of fresh water and rainfall, same as any other dam as well.

Am I making a case, or just a nut case? lol

It would probably get blocked up with slash in no time, in that location.

[Snoopy]

A question for those who have enough knowledge to reply.

Now that Lake Onslow is not on the agenda, what is the viability of using the ocean as a lake adjacent to the coast, and creating a pumped hydro scheme by pumping water up to a man made lake on an adjacent plateau of say 35 plus metres height, or more if necessary and the site provided it?

I'm aware sea water is more corrosive, but use concrete pipes and races. and liners can be used on steel parts.

I can think of places around NZ, particularly in the North Island to avoid reliance and loading on the Cook Strait cable and SI grid, that would offer a major site, and plenty of scope for smaller ones if local power authorities want to set them up.

Compared to some pumped hydro schemes overseas, one of the 'lakes' is already 'built', and would never run dry, compared even to NZ ones that rely on rainfall or snowmelt.

Going back to the basic physics of this, water can supply power by utilising::

A/ Potential Energy (water falling from a great height) OR
B/ Kinetic Energy (lots of water moving really fast)

Lake Onslow is 700m above sea level and Lake Dunstan is 200m above sea level. So we are looking at a head of 500m for water to fall. The formula for potential energy is 'mgh' (mass x gravitational acceleration x height ). Your proposed 35m high dam walls Getty would house a volume of water that is on average 17.5m high. This is much less of a head than available with Lake Onslow. So to make it as useful as Lake Onslow you would have to compensate by increasing the mass of water available by a factor proportional to the lost height: 500/17.5 = 28.6 times.

Lake Onslow covers an area of 380 hectares. So your equivalent pumped seawater reservoir would have to cover an area of 28.6 x 380 ha = 8580 hectares. To get some idea of that area, the Auckland central business district CBD covers 433 hectares in a triangular area, bounded by the Auckland waterfront on the Waitematā Harbour and the inner-city suburbs of Ponsonby, Newton and Parnell. So you are looking at an area of 20 Auckland Central business districts which would require a truly enormous structure to wall in.

Furthermore unlike the Clutha River which has a natural discharge rate of 614 cubic metres per second, a giant inland inshore reservoir from the sea would have an underlying natural flow rate of - zero. All the water would have to be pumped into the reservoir. So there is no net natural kinetic energy, akin to a naturally flowing river, that adds to the kinetic energy that can be recovered from such stored water. (The kinetic energy recovered from such water is only equal (actually a bit less because of frictional losses) to the kinetic energy put into the water as it was pumped into the reservoir).

I think you can see that the construction of such an ''inland sea reservoir" would be an enormous civil engineering task, likely costing multiples more than the dumped Onslow project. And looking at the energy flows you would need to fill it to get a significant output in times of low rain, I don't think the costs involved make sense.

SNOOPY

[Getty]

Excellent detailed reply thanks Snoopy.

My focus was on the freebie provided by the sea, combined with a lake about the same size as Onslow.

The main missing ingredient is the lack of height difference.

Looks like I'm dead in the water.

Thank you for taking the time to give a comprehensive answer.

[Doug]

Lake Onslow covers an area of 380 hectares.
SNOOPY

That's about the current Lake Onslow area. The project included a new dam that expanded the area to up to 7100 ha.

[Snoopy]

That's about the current Lake Onslow area. The project included a new dam that expanded the area to up to 7100 ha.

The reason why I said Lake Onslow was 380ha in area was that I pulled that figure from one of the matter of fact regional council websites. I haven't studied the plans for a theoretical completed and raised Lake Onslow. But I am happy to take your surface area figure at face value Doug. So how does this revelation change the argument?

I am not sure of the increase in water associated with that surface area change. But if we use the increase in surface area as a proxy for the increase in water volume, then we are looking a a lot more water stored at that higher 'potential energy head' to make the Onslow project viable.

So if 380ha of Onslow is equivalent to 8580ha of our 'theoretical coastal sea water reservoir substitute', then our sea water reservoir size will have to expand to (7100/380)x8580ha = 160,310ha with this new information. The area of Lake Taupo is 61600ha. So all we have to do is to build some capacious enough earth walls to hold 160310ha/61600ha = 2.6 Lake Taupos and we are sweet! How do we fit 2.6 Lake Taupos into downtown Auckland though? And much would building such walls cost? Maybe we could get Donald Trump Engineering to bid on the job?

SNOOPY

[xafalcon]

Going back to the basic physics of this, water can supply power by utilising::

A/ Potential Energy (water falling from a great height) OR
B/ Kinetic Energy (lots of water moving really fast)

SNOOPY

Slightly correction and clarification

Water held at height in Lake Onslow only has potential energy. The water in the lake is not falling or generating any electricity. As the name implies, it is potential (in this case to generate electrical) energy

To extract that energy in a turbine, the potential energy must first be converted to kinetic energy = moving/falling water being accelerated by gravity - penstock friction (which is lost as heat). Kinetic energy is the energy contained in a moving object (water in this case), usually its momentum (related to both its velocity and mass)

The falling water momentum (kinetic energy) is transferred to the turbine and attached generator, which converts the kinetic energy into electrical energy (and some more heat)

[Snoopy]

Slightly correction and clarification

Water held at height in Lake Onslow only has potential energy. The water in the lake is not falling or generating any electricity. As the name implies, it is potential (in this case to generate electrical) energy

To extract that energy in a turbine, the potential energy must first be converted to kinetic energy = moving/falling water being accelerated by gravity - penstock friction (which is lost as heat). Kinetic energy is the energy contained in a moving object (water in this case), usually its momentum (related to both its velocity and mass)

The falling water momentum (kinetic energy) is transferred to the turbine and attached generator, which converts the kinetic energy into electrical energy (and some more heat)

Fair enough correction, you are right xafalcon, When I wrote about the 'kinetic energy' I was actually thinking of the energy passing by Onslow each day in the Clutha that is used to power the pumps that top up Onslow. I said 'kinetic energy' distinguish it from the potential energy held within Onslow.

With Getty's 'seawater lake' system there is no 'river on the spot' to power the pumps. So all the power to operate those pumps would have to come in by wire from another location. This could be done. But it would be a less tidy system solution compared to Onslow.

SNOOPY