Renewable energy - could tidal power supply all the UK's electricity?

I seem to spend an increasing amount of time engaged in debates with my work colleagues about renewable energy. Usually we exchange a few half-remembered and unverified 'facts' until two or more clearly contradictory statements emerge, at which point we retreat from the topic, muttering about checking facts and getting  to the bottom of the matter eventually. I have decided that the only way to get reliable information on renewable energy is to do the calculations myself and publish the results on this blog.

Let's start what will be an occasional series of posts with an examination of a statement that I was presented with last week.

'All of the UK's electricity could be supplied by tidal power.'

I would love this to be true, but my informant was unfortunately unable to find a reference to support his original assertion.  What he did supply was this:


'Studies have suggested that one-third of the UK's total electricity needs could be met by tidal power alone, and Alex Salmond, Scotland's first minister, predicted that the Pentland Firth region, where the north-east Atlantic meets the North Sea, will become the "Saudi Arabia" of marine energy.'


'The Pentland Firth has been described as the greatest untapped source of energy in Scotland, and has the potential to generate vast quantities of tidal, wind and wave power.

It is estimated that around 8 TWh could be generated by tidal power in the Pentland Firth, representing 8% of total UK electricity consumption of 350 TWh.'

So straight away we've gone from tidal being able to supply all of the UK's electricity to being able to supply a third. This got me rubbing my hands together in anticipation of unmasking further exaggeration and misrepresentation. Furthermore, the eagle-eyed amongst you will have noticed that the quote above isn't even consistent - do they mean that the Pentland Firth could supply 8 TWh or 8 % of 350 TWh? 8 TWh or 28 TWh? 

I find all this talk of TWh difficult to visualise, so I am going to borrow David Mackay's trick of converting into units that are more human in scale, kilowatt hours per person per day (kWh pppd). To provide a bit of context, electricity consumption in UK is around 16 kWh per person per day, a tiny fraction of our total energy consumption of 195 kWh pppd.

So in my more friendly units, 0.4 or 1.3 kWh pppd may be generated from tidal power in the Pentland Firth. I decided to check what David Mackay had estimated. To my surprise he estimated - based on what seem to be reasonable assumptions - the tidal energy potential in the Pentland Firth to be a whopping 3.5 kWh pppd!

Now the kilowatt hour per person per day calculations are all based on the population of the UK (60,000,000), not of Scotland (5,200,000). Substituting the population of Scotland into the calculation changes everything - 1.3 kWh pppd (UK) turns into 15 kWh pppd (Scotland). The Pentland Firth, exploited in line with the predictions above, is sufficient to completely satisfy the electricity demands of Scotland! If Mackay's more optimistic estimate is correct the figure is 40 kWh pppd (Scotland), in other words one tidal site could supply 20 % of Scotland's total energy demands, enough to meet all our current electricity demands and to convert most of our ground transportation to run on clean electric power.

When I started my calculations I expected to find that the benefits of tidal energy had been exaggerated just as much as those of wind. I have found the opposite to be the case. My confidence in Alex Salmond in particular has taken a knock thanks to his handling of the Trump episode and what I regard as excessive support for onshore wind developments, but it appears that he has been well advised about the potential benefits of tidal power. The main message that I am taking from the above is that renewable energy makes much more sense in the context of an independent Scotland than in the context of the UK. 

All in all this might be enough to make me reconsider my decision to spoil my paper in the upcoming Scottish elections.