Friday, 23 September 2011

Biased reporting: tidal power sensation (a rant)

In my book I take great care to be objective. That means not just presenting accurate data but presenting it in a transparent manner. I don't compare apples and bananas. If only all authors did the same.

Many people seem to think (and try to persuade me) that tidal energy is the obvious solution to our energy problems. Admittedly it is reliable and environmentally friendly (though this has yet to be tested on an industrial scale). However, in practice it can't provide more than a small fraction of our energy needs.

The latest New Scientist this week  (17th September) features an article on developments in tidal stream power being tested in the Orkney Islands (click here but you need a subscription). Tidal stream power is from the flow of water due to tides, rather than the rise and fall in water level. The magazine cover gives the article the headline Oceans of power – Harnessing the planet's mightiest force. That sounds promising don't you think?

 The bit of the article that makes me most cross is the sentence Tidal currents could theoretically provide up to 5% of the UK's power, as compared to wind which now contributes just over 3%. What is that supposed to mean? That tidal power has twice the potential of wind? It doesn't. What that sentence means is that even in the most fanatical scientists' wildest dreams tidal currents could not provide more than 5% of the UK's (current) power consumption, whereas wind power already supplies more than half that.

The difference between the theoretical potential and the practical potential for a  technology is usually at least an order of magnitude. For tidal power:
  • The theoretical potential is how much energy could be extracted from the tidal flow without breaking any known physical laws.
  • The technical potential is how much energy we could get with feasible technology  – so without using fantastical materials like unobtainium which is infinitely strong, weighs nothing and never corrodes.
  • The practical potential further limits us, for example to locations which are accessible, not dangerous to sensitive wildlife or shipping, not too deep or too shallow or too far from the shore. Also there are limits in availability of materials: we can't in practice monopolise the world's steel production for the next 100 years to make only tidal power turbines.
  •  Finally there are financial limitations, which restrict us to locations where the flow is such that there would be  a reasonable return on investment, even with appropriate subsidies.
The DECC report 2050 Pathways Analysis explores alternative scenarios for how our energy needs could be met by 2050 with reduced carbon emissions. Its top estimate for energy from tidal stream is 70 TWh/year. For wind (onshore plus offshore) it envisages 560 TWh/year as practically feasible, albeit extremely ambitious: that would be  about 30% of current consumption [1], and is many times more than can be hoped for from tidal stream power. So, when you are reading about the exciting potential of new energy technology, try not to be beguiled by journalistic hype.

[1] Digest UK Energy Statistics 2011
Final energy consumption for 2010 was 159.1 million tonnes of oil equivalent which is about 1850 TWh.

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