Grid system operators can only perform their magic of continuously matching supply to fickle demand if they have access to sufficient dispatchable – controllable – generation. Assurance of this is given by the “capacity credit” (or capacity value) of the generation method. Cc is the fraction of installed capacity available with high probability at peak demand. Solar is completely absent at winter peak, and tidal stream – though always there – occurs in peaks and troughs shifting in a fashion not correlated to the demand cycle. So both are discounted for Cc purposes.
We will assume that 0.5GW of the peak demand is locally sourced and used. That leaves biomass, hydro, pumped hydro and wind power, having powers of, say 0.5, 1, 1 (assuming the Ben Cruachan extension is completed) and 3GW respectively, to give the grid-distributed public supply winter peak demand of 5.5GW.
For the biomass and hydro stored energy systems we could, for the purposes of the present discussion, reasonably assume 100 per cent availability. Variable wind power at as high as 50 per cent or more contribution or “penetration” to the grid supply is a bit of an unknown. But from what I have read, the Cc of a mixed off-and-onshore system in a small country like Scotland at this level would be no higher than about 7 per cent (which is probably optimistic as UK wind is frequently no more than 1 per cent of its installed capacity).
So, for our assured 3GW peak we would need an installed capacity of at least 43GW – which is absurd. In addition to the environmental impact, the capacity required (and cost) for security of supply would be 11 times the average consumer demand which actually pays for it.
We are not of course “electrically insulated”, but part of the UK National Grid with up to 4GW connectivity. So provided the rest of the UK has an adequate mixed supply, we can have a sensible generation level and import/export to maintain grid stability. But that would not be a 100 per cent Scottish renewables system, would it ?
Incidentally we now have, I believe, about 8GW of installed wind capacity which with hydro and nuclear on average amounts to much more than average demand and well above the base load of about 2GW.
A final thought. A turbine grabs about 50 per cent of the wind energy passing through it and turns it into electricity. Our already installed wind farms thus alter the natural wind pattern over about 5 per cent of Scotland. If that’s not man-made climate change then I’m a Dutchman!
l Dr Andrew McCormick, BSc PhD, is a retired research scientist. He lives in Terregles, Dumfries