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centralised electricity distribution system?

 
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Joe Otten



Joined: 29 Mar 2007
Posts: 3

PostPosted: Tue Apr 03, 2007 3:07 pm    Post subject: centralised electricity distribution system? Reply with quote

This is an argument I keep hearing:

"...nuclear locks us into a centralised electricity distribution system for another 40 years, reducing the appetite for distributed and micro-generation..."

Can anybody explain this to me? I would have thought that renewables would place a higher burden on the grid because of the intermittency. Electricity will have to be moved from where-ever the wind is blowing to where it isn't.

And does it really make sense to "promote" microgeneration by installing insufficient grid capacity to supply some locations with enough grid power?
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clv101
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PostPosted: Tue Apr 03, 2007 3:38 pm    Post subject: Reply with quote

I think this argument comes from the silly people at either the Green Party or Friends of the Earth (I forget which) who criticise the large amount of wasted energy in the current electricity generation and imply that a large centralised grid is to blame. They quite rightly point out that some two thirds of primary energy used in generating electricity goes up cooling towers or makes the sea just a little bit warmer. This is then incorrectly blamed on using a centralised grid.

The centralised grid only wastes some 7% at most and the bulk of that isn?t in the 400kV long distance lines, it's in the lower voltage regional grid. The large losses of primary energy are largely an inescapable consequence of using a heat engine ? it?s the Carnot cycle.

I centralised grid is a good thing as far as I can tell and something I?m happy to have available forever more, it doesn?t diminish the effectiveness of microgeneration at all.
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monster



Joined: 11 Jun 2006
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PostPosted: Tue Apr 03, 2007 7:01 pm    Post subject: Reply with quote

'the silly people at either the Green Party or Friends of the Earth'

clv101 the 'silly people at the green party', are the only political party with representation at council level, european parliament, and GLA who formally recognise and actually have policies on peak oil unlike the other big three.....

There is nothing in Green Party policy that goes against centralised grids just promoting more decentralised power generation
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Joe Otten



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PostPosted: Tue Apr 03, 2007 7:16 pm    Post subject: Reply with quote

It seems a bizarre way of talking. To me centralised generation would mean one big power station, and what we have is very decentralised already.

If you have a city that has a demand of more than 2 GW, it is hardly less centralised to build lots of little plants next to it than fewer big ones.

I would guess we all agree that more CHP would be good, and micro where it is effective.

But surely the question of what size you build your coal or gas or nuclear or wind generators is a matter of economics: capital costs, thermal efficiency and all that sort of thing. Are there any figures to suggest they are being built too big?
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monster



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PostPosted: Tue Apr 03, 2007 7:39 pm    Post subject: Reply with quote

The advantages of decentralised plant don't just refer to reduced transmission losses, and I agree centralsied vs decentralsied isn't always the best term.

Larger plants eg 1000MW+ tend to be less flexible as they take longer to ramp up and down - on the whole, so are less efficient when demand and supply of electrcity are variable especially when variable output from renewables is included.

CHPs can be switched on and off more quickly so supply is less 'lumpy'. Take Western Denmark for example it has five large 'centralised' coal fired plants, but over 100 gas and biomass CHPs. Most coal plants have a 'sweet spot' where efficiency is at its highest, so if they are ramped above or below this point they are less efficient. An easier thing is to just switch off a CHP.


One problem with switching off CHP is you lose the heat, so if there are no other ways of providing heat CHP sometimes has to stay on too. Denmark is getting around this problem by switching to electric water heating either domestically or in CHP plants themselves. This will allow greater wind penetration - the goal is 50% by 2025. In other words when its windy electricity is supplied by wind and also water is heated by electrcity. When wind speeds are low the CHPs are switched on and run on gas (or biomass). This is a brief summary of a complex situation.

http://www.newenergy.info/index.php?id=1350

Generally transmission losses are less and also decentralised supply generally refers to CHP which is inherently more efficient as the waste heat is utilised for district heating.

The other point is that reserve power in the grid is usually equal the the biggest power plant on the grid ( in case it goes off line unexpectedly - which is often the case with nuclear plants). In england we usually have about 1350MW reserve. If a bigger plant was on the grid eg 1500MW this may need to be increased. In other words bigger is not necessarily better.

This link shows output in East and West Denmark for centralsied, decentralised and wind output in real time.....
http://www.energinet.dk/Integrationer/ElOest/ElsystemetLigeNu/energinet1.swf
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Bandidoz
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PostPosted: Tue Apr 03, 2007 10:30 pm    Post subject: Reply with quote

monster wrote:
I agree centralsied vs decentralsied isn't always the best term.
It shouldn't be mentioned at all. That tangential extrapolation thinking driven by self-contradictory reports from Greenpeace leads to daft ideas such as "1 million domestic wind turbines", damaging the credibility of the Green Party (etc) from the viewpoint of those with an engineering background.

The comparison should be thermal (with no heat recovery) -vs- non-thermal (or thermal with heat recovery).
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Joe Otten



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PostPosted: Wed Apr 04, 2007 9:55 am    Post subject: Reply with quote

Thanks monster,

I see what you're getting at. But doesn't it remain the case that a good mix of generation types is optimal. OCGT is quick to switch on and off too, and that is why we have it. So I would have thought there would still be a sizeable role for thermally efficient slow on-off plant at or closer to baseload, at least as big as the excess of demand for power over heat. (In the summer?)

The use of this analysis as an argument against nuclear is to suggest that there is a binary choice between two very different strategies for delivering power rather than a change of emphasis and a shift towards more CHP.

And of course with greater 'centralisation' of electricity grids, the reserve can be shared over a larger area, and therefore used more efficiently. Does the N+1 strategy really still apply?
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