Renewable sources have a big part to play in future energy supply. But how big? Sometimes their supporters can be guilty of exaggeration.

Take last week’s report from the Intergovernmental Panel on Climate Change (IPCC), which provided the basis for a press release dated May 17. We’ll overlook the fact that the press release appeared a full month before the report that gave it substance, though it’s an egregious breach of scholarly good manners, appropriate castigated by Climate Audit here. 

The press release claimed that “close to 80 per cent” of the world’s energy supply could be met by renewables by 2050. Its authority is Chapter 10 of an immense report called Special Report on Renewable Energy Sources and Climate Change Mitigation.

Chapter 10 reviews a number of future energy scenarios, which estimate that as little as 15 per cent or as much as 77 per cent of energy might be met by renewables by 2050. So the press release takes the outlying estimate, 77 per cent, and leads with it. Naughty. That’s the sort of thing journalists do.

It gets worse, as has been reported by Climate Audit and in The Independent and The Sunday Times. One of the authors of Chapter 10 is an employee of Greenpeace International, Sven Teske, and the 77 per cent claim originally appeared in a report he co-wrote. How agreeable to be able to review your own work and give it alpha-plus! And then to issue a press release congratulating yourself.

Mr Teske wrote the original report in 2008, in cooperation with the European Renewable Energy Council, a trade association for renewable energy companies. In effect, it was a shop-window for renewables. No harm in that, except that one doesn’t expect reports of this sort to turn up later as part of a supposedly impartial review designed to help policy-makers decide just how useful renewables are likely to be.

I can’t improve on the conclusion drawn by Steve McIntyre of Climate Audit, who writes: “The public and policy-makers are starving for independent and authoritative analysis of precisely how much weight can be placed on renewables in the energy future. It expects more from IPCC Working Group 3 than a karaoke version of a Greenpeace scenario.”.

And so to matters more domestic. Alex Salmond, the Scottish First Minister, is also an enthusiast for renewables. Since first taking office the target for electricity production in Scotland from renewable sources has been increased from 50 to 80 per cent by 2020, and recently he’s taken to saying he’s going for 100 per cent by the same year.

He did so recently at the opening of a new windfarm in South Ayrshire. See, for example, The Herald and the Daily Telegraph, June 14 2011. “Scotland is on course to generate all its electricity from renewable sources by 2020” was The Herald’s intro.

To be fair to Mr Salmond, this isn’t quite what he said. That was: “I am confident we will achieve our target for Scotland to generate from renewables the equivalent of 100 per cent of our own electricity needs by 2020”.  

That is different, for reasons that will become apparent. Scotland is well-placed for renewable energy, with its long-established hydro-electric plants and more recently, its windfarms. In 2008, hydro, wind and biomass between them generated 8.8 TWh (terawatt hours) of electricity. Total production in Scotland was 48.2 TWh, of which a net 18 per cent was exported. (see table, below)

So output less net exports was 39.5 TWh. Of this, the renewables listed above contributed 22 per cent, though the Scottish Government’s Draft Electricity Generation Policy Statement 2010, from which this table comes, makes it 25 per cent. How they do this isn’t clear, but I think it is by including pumped storage (1 TWh) as a renewable. Then the figure works out at 24.8 per cent. Actually pumped storage isn’t a renewable in my book unless all the electricity stored is generated renewably.

Note what is happening here, because it’s clue to the form of words used by the First Minister. Of total generation, renewables excluding pumped storage generated 18.3 per cent. This is upped to 25 per cent by assuming that all the net electricity exported to England and (via sub-sea interconnector) to Northern Ireland, is generated by coal, nuclear or gas turbine stations, and that all the electricity used to pump water uphill to replenish the pumped storage schemes is generated by wind. It’s hard to see how either can be justified.

So let’s look at the projections for 2020 in the same document. By then, wind (onshore and offshore) biomass, wave, tidal, and hydro are expected to amount to 28.9 TWh. Total output is expected to be 53.6 TWh, so renewables will represent 54 per cent, assuming all these plants come on line and work as designed.

But the table claims 76-80 per cent. How? First, subtract exports (22 per cent in 2020), bringing the Scotland-only figure down to 41.8 TWh.  Crediting renewables with 1.6 TWh of pumped storage ups the renewables figure to 30.5 TWh. That is 73 per cent of Scottish usage, not 76-80 per cent. So there must be some other statistical twitch I’m missing.

This 2020 figure is the one Mr Salmond now wants to increase to 100 per cent. Most of the increase in output would come from wind, with modest additional contributions from wave, tidal and biomass plants. But actually it’s technically impossible to run an electricity system so heavily biased towards wind power, because there are days when it’s cold but the wind doesn’t blow, even in Scotland, or it blows too hard for the turbines to operate.

What would have to happen under the 80 per cent scenario would be the large-scale importation of electricity from England. This would be happening more than 60 per cent of the time, according to the 2010 Scottish Government policy statement from which all these figures come (see page 20). For 1-2 per cent of the time, the imports would be so large they would exceed the current and planned interconnector network.

The alternative, which would work much better and which is actually Scottish Government policy, would be to continue to invest in gas-turbine or coal-fired plants with carbon-capture. They aren’t renewable sources, of course. But they are vital to keep the lights on. The only way they might be avoided is by the development of large-capacity systems to store the wind turbines’ electricity when the wind is favourable, for use when it isn’t. Apart from pumped storage, whose expansion is limited by a shortage of good sites, there isn’t such a technology.

That is why Mr Salmond expresses himself so carefully. It’s technically impossible, using mainly wind power, for Scotland to generate all its electricity from renewables. And it can only achieve the claims expressed in the way that he and the policy statement do by making the assumptions that all the electricity exported is non-green, and that the green status of the electricity that is imported is doesn’t matter.