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Storing wind-generated energy as gravitational potential energy? - Page 29

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Posted by The Natural Philosopher on December 9, 2008, 12:34 pm
Neon John wrote:

Nope. load average is 30% at best. On offshore.

The wind output is very variable. And the average is not steady. Some
days, the whole country is doing 100%, other days, the whole country is
doing less than 10%.

We are not geographically large enough to always have a gale somewhere
when the rest is becalmed, and even if we were, the cost of taking the
power from the windy north of the country (where dynamo Dave lives) to
the south is very high.

The eco nuts are no proposing that we cover the Sahara with solar panels
and build a 1TW capacity undersea cable to power N Europe..

Not sure what happens at 'sunset in the sahara' though.

Bit of a waste of a cable that only works half the time..

Posted by Dave Liquorice on December 7, 2008, 10:00 pm
On Sun, 07 Dec 2008 19:27:00 +0000, Andy Champ wrote:

The cynic in me says that if the plant is capable of producing power but
can't because there isn't any wind that would count into the "availabilty


Ball park, aroound 1/3rd of rated capacity is what is generally accepted
as "acceptable" from a wind turbine.


Posted by The Natural Philosopher on December 9, 2008, 12:28 pm
 Andy Champ wrote:

And need about ten times as much backup as a nuclear power station.

BTW nuclear power stations CAN be modulated. Switching off completely is
hard, but some designs are very easy to 'turn up the wick' on.

pebble beds look very promising in this regard.

Maybe nuclear john can comment on the response time of a nuclear set to
demand change?

The ultimate point is that you always need variable output: storage
systems can cope with very short term fluctuations, and bringing extra
capacity in can cope with very long term ones (like summer to winter )
in between you have to modulate what you have to cope with daily cycles.

Posted by Neon John on December 10, 2008, 9:13 am

Even old nukes can change fairly fast (minutes), though the operators avoid
that on a metal fatigue basis.  Let's take Sequoyah, the plant that I trained
to operate before moving on to engineering and wider job opportunities.  A
licensed reactor operator has to know every wire, valve, switch and pump in
the plant and be able to recite the details anytime a superior or an NRC
inspector decides to ask.

My training was in the 70s so bear with me, as many brain cells have met their
maker since then :-)

The control sequence is thus.  Reactor power is controlled to maintain a
constant tiny delta-T (16 deg F comes to mind) across the steam generators.
That in turn varies with the steam load which is in turn varied with the
throttle valves on the steam turbine.  The valve is modulated by a load signal
from the electrical side.

The turbine control system at Sequoyah was originally designed to be
controlled by the central dispatcher and there is a simple analog input (10-50
ma DC signal) for that purpose.  (In my dreams I've often visualized having a
joystick connected to those terminals, running the plant up and down just for
fun :-)

It was long ago decided that remote control of a nuke would not be permitted
so now the operator sets a load setpoint on the turbine control panel
(normally 100%) and the system maintains that load.

The system can dump load very rapidly.  The condenser bypass system, designed
to dump excess energy directly to the condenser to avoid having to change
reactor power too often, is capable of 100% capacity.  That is, the turbine
load can be throttled from 0-100% literally in seconds using the condenser
dump if the reactor is at 100% power.

Even though fuel cost is practically nil compared to the revenue generated, it
is not desirable to operate the plant in this mode, so normally the condenser
dump valves remain closed and the reactor power is modulated according to the
electrical load.

Load shedding can happen within seconds.  This does involve the use of the
condenser dump valves, and even the atmosphere vents, but only until the
reactor power can be reduced.

Load acceptance is a bit slower, but not as slow as the time constant of the
reactor might lead one to believe.  There is significant thermal inertia in
the reactor primary system and in the secondary steam systems (reheaters and
ancillary equipment) so that the turbine can accept load faster than the
reactor can respond, but at the expense of efficiency.  Steam temperature
drop, moisture content increases and efficiency drops, but only for a few

I'm afraid to try to quote reactor time constants from memory but they're in
the small minute range.  The actual nuclear reaction can be varied at pretty
much any arbitrary rate but the system is limited by heating and cooling rates
of other hardware.

Executive summary: even an old nuke can load follow but at the expense of both
efficiency and equipment lifetime.  Therefore it's cheaper just to base load
the nukes and let other sources load follow.  

By the way, gas turbines  and hydros aren't the only load following devices
that can respond rapidly.  Small coal-fired plants (100 to 300 MW range) can
load follow about as fast as a gas turbine or a Hydro Plant.  In fact, since
most gas turbines are now combined cycle (which means that the plant has both
a gas turbine and a steam system fired from the waste heat of the turbine)
their load following ability is somewhat limited by the steam plant.  I say
"somewhat"because the control system will vent turbine exhaust the atmosphere
rather than failing to follow the load signal.  Efficiency drops while that
he's being wasted but the demand is satisfied.

This is one reason why I've advocated smaller nuke plants.  A 100 to 300 MW
nuclear plant could load follow just as well as a similar sized fossil plant.
It is likely that a combination of very large plants and small load following
plants will be built in the future, once a licensing logjam is removed.  I see
that happening in the next 10 years.  The smaller plants will likely be shop
built and sent to the site as modules where they are assembled into the plant.
assembly may take only a few months.

It will obviously take some time for the market to develop and the factory
shops to be built to enable this technology, but it will happen.  Modular
nuclear plants are a major topic of discussion within the industry and the
trade press right now.

true.  It's probable that battery/inverter technology will soon come into the
load follow repertoire.  As I mentioned in another post, sodium sulfur
batteries are now being pilot tested in several utilities.  Though terribly
expensive at this point, they're still probably no more expensive than a
combined cycle gas turbine and will surely come down in cost as production
ramps up.  There's no real high technology involved in this kind of battery
like there is in lithium batteries so ramping up production will simply
involve building bigger production lines.

It is an interesting time for the utility business, both in the real world and
in the Chinese proverb way of interesting.


John De Armond
See my website for my current email address
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
I love cats ... they taste just like chicken.

Posted by The Natural Philosopher on December 9, 2008, 12:11 pm
 Neon John wrote:

Newer windmills are better than that. But the so are newer nuclear sets.
I make it about 1000 windmills = one nuclear station. To be RELIABLE
(see below)

The load average  of large European windmills is 30%. Thats average.
Sadly the means to worst case on any given set of days, is far far worse.

Without storage or backup I estimated that a functional wind solution
requires between a 6 and 10 times overcapacity of windmills and wire
interconnects over a very large geographical area.

I.e at lest 6-10 times more wire - copper and/or aluminium - than a nice
steady nuclear station.
Windmills may be efficient in terms of cost of the actual energy, but in
every other sense..use of land area, use of materials - they are a
fecking nightmare once you look at the OVERALL picture. Not just 'how
much it costs to generate electricity from this windmill ASSUMING IT IS

You are in the nuke indistry.. calculate for me how many watts per
square meter of land a nuclear set produces ?

The very BEST 'renewable' energy is a solar furnace in a desert, at
maybe 15W/sq meter of land. Everything else is in the 0.1-5W/sq meter
sort of area.

The UK runs on about 10% of the energy that actually falls on the land
surface of it..mutatis mutandis, that means that around 20% of the total
land area of Britain would need to be covered in 'renewable power'
stations to generate the current needs of the population. And whilst we
might be able to do on maybe half what we burn now, we cant do on 10% of it.

The sheer construction size of the renewable solutions exceeds the
amount of houses roads railway factories and airports that currently
exist by a comfortable margin. And with all this power being relatively
unreliable, you need a massive grid to balance it.
The ultimate conclusion of the renewable energy lobby would be a country
looking like a giant industrial landscape, with windmills, solar panels,
and electricity pylons criss-crossing it at 100% density, and the whole
coast surrounded by flapping windmills covering all the coastal waters.

whereas 100 nuclear power stations each the size of a medium factory
dotted round the country would do a far far cheaper job with far far
less environmental impact.

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