On 25/11/2011 12:45, The Natural Philosopher wrote:
Roger Chapman wrote:
On 25/11/2011 11:23, The Natural Philosopher wrote:

It gets worse. The backup likewise has to be as good as the worst case
wind output. One of the things that has become established over the last
year or two is that national wind output can, on occasion, drop to as
near zero as makes no difference.

If we are becalmed in a winter blocking high and also very cold which
causes high demand for heating too. Last year was a good example. You
basically have to provide enough other capacity to handle that worst
case scenario or drop people off the grid in rolling blackouts.

Solar PV is even worse. It's guaranteed to do this every night.

Solar PV doesn't really need any hot reserve, let alone spinning
reserve. With a multitude of very small generators over an extended
area the actual output could be predicted to very close limits merely
on the basis of cloud cover and date.

it doesn't matter.

You STILL have to ramp up gas to cope at sunset, and that gas needs to
be warmed up on the way, and while it ramps up, its wasting gas, and
when it ramps down, its losing stored heat.

Actually I don't think you do - at least not with the amount of solar
power that is installed in the UK at present. The load demand curve is
on the decline by sunset anyway except perhaps in the far north and deep
midwinter so as long as the amount of solar dropping off is less that
the amount of load being shed it acts to smooth things out.

It makes **** all difference if you KNOW what variation you have in
advance - you still have to DEAL with it.

If you know that both load and generation are going to go down it is a
lot more helpful than if they are anti-correlated.

That is the reason why at lower latitudes solar PV can make a lot more
sense (though solar hot water is better) - peak demand there for AC is
strongly correlated with sunshine and having an array on the roof helps
decrease inbound radiative heat load. A win win situation. I have to say
it is madness subsidising them in the UK and Germany.

Solar PV may do little to help with the 5.30 pm peak but almost all of
its output (all for most of the year) will fall within the high demand
zone during the day unlike the output of windmills which is much less
predicable and close to random in distribution through 24 hours.


Wrong again, the forecasts are accurate to 70% typically.

It is not the unpredictability of intermittency that is the problem its
the variation, and, in particular, the slew rate.

The slew rate on solar will be fairly soft since unless they are
tracking arrays they taper off at a predictable rate at sunset. And peak
demand is during daytime so they are positive load correlated. Their
total output is a useless fraction of load barely worth the effort of
measuring but that is a different matter entirely.

--
Regards,
Martin Brown