On Mon, 28 Nov 2005 13:01:03 +0000, The Natural Philosopher
wrote:

On Mon, 28 Nov 2005 12:35:40 +0000, Matt wrote:

On Mon, 28 Nov 2005 11:27:55 +0000, The Natural Philosopher
wrote:

On Fri, 25 Nov 2005 23:50:09 GMT, dennis@home wrote:

The electricity isn't wasted at night.

Actually it is.

The power stations are essentially idling at the great traffic lights of
consumer demand. Since they take a LOT of hours to come on line, they have
to be left at idle.

With the exception of (the majority of) UK nuclear plants that have
specific problems with load changes down and then rapidly up, the
installed generating capacity in the UK (even the 500's and 660's coal
plants installed in the 60's and 70's) are perfectly capable of being
shut down or run on very reduced load overnight and return relatively
rapidly to load next morning. They don't like doing it and it is
wasteful in some respects - hence why some power stations have in the
past bid into the system at zero cost (they got paid the system
marginal rate) which works right up until the point at which the load
drops unexpectedly, the system marginal bid is zero and nobody gets
paid - it has happened!

Exactly they don;t like it and its wasteful.

Thert is a difference between shutting down - going 'cold' - and going to
'standby' , A BIG difference. Some power stations take a week to bring up
to speed from 'cold'. Standby means they are simply turning over, possibly
not connected to the grid at all, but still with heat being generated and
watsed and combustion going on.

It takes HOURS to get a steam boiler up to pressure. Only gas turbines are
relatively quick.

You effectively "box" the boilers up (no fans, all dampers closed, no
oil burners, main steam valves shut, all drains shut) and the boiler
pressure decays away *very* slowly, shut down past the evening peak
and then around 4am before demand starts picking up next day, run the
fans, kick in a few oil burners, blow down your drains and then add a
mill or two, keep a close eye on the turbine clearances and vibration
and you can easily go from this semi-hot state with absolutely *zero*
heat input to full boiler firing and maximum generator load in around
2 hours.

The 120MW units were doing it just like this way back in the 60's, the
500's and 660's were capable (and in some cases regularly doing it)
back in the 80's. Well before gas fired generation arrived it was
taken as read, almost regardless of unit cost that as larger gas
cooled reactors came on line two shift operation of the large coal
generators would become an essential requirement. Considerable
investment was made in updated instrumentation and control systems
(and sometimes boiler metallurgy and combustion engineering) to
facilitate that change.

The only time a power station would ever take a week to run up from
cold would be after a major overhaul when there would be numerous
checks to carry out. In normal operation, from room temperature
ambient metal temperatures on the boiler and turbine it is perfectly
feasible to reach full load in around 24-36 hours. I've seen a 500MW
unit come back from an annual overhaul involving a lot of boiler
retubing, with major turbine, alternator and auxiliaries work, have a
boiler pressure test late Friday, do a safety valve float Saturday and
be at full load 6am Monday (it was off load Tuesday with a boiler tube
leak though!)


--