We must not
 be swept away by politicians’ current enthusiasm for tide power

Jack Ponton, FREng
Jack Ponton, FREng
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Some of our politicians have recently shown much excitement about the prospects for power from tides around Scotland.

“Saudi Arabia of renewables” has even resurfaced in the media. While it is true that tidal power could have advantages over onshore wind in particular – turbines under the sea would not have the divisive and destructive effects on communities that putting wind turbines outside peoples’ homes has had – it is really too soon to get excited about what is an essentially unproven resource.

Power from tides, like that from wind or sun, would be both intermittent and seasonal. However, tidal power is a more plausible source of reliable electricity than either of these. The short-term variation of tidal flow, between high and low tide, is six hours compared with half a day for solar and unpredictable periods of several low wind days. The “season” for tidal flows, between spring and neap tides is seven days, compared with summer to winter variation for solar. Finally, for the Pentland Firth even the neap tide season could provide a sensible level of power, unlike solar in wintertime Britain.

It would be possible in principle to couple tidal generation with currently practical forms of storage to provide effectively continuous power. To give an average of 1.2Gw, equivalent to Hunterston, over the neap tide season would require 2.83Gw of tidal turbines. Over about half the six-hour period there would be more than 1.2Gw generated and this would have to be stored and recovered when tidal output dropped. The total storage needed would be about 3Gwh, which is well within the capabilities of existing pumped storage.

This sounds quite promising, so why shouldn’t we be getting excited? For a start, this is technology which has yet to be developed or deployed. The largest, 2Mw, tidal turbine in the UK has yet to be installed. The largest in operation is a 250kw device, and the largest “array”consists of two 100kw turbines. We do not know just how much power it would be possible to extract from the Pentland Firth, estimates range from 1.9Gw to 3.5Gw. To provide an effective 2Gw would require 5Gw of installed turbines, 2,500 of the largest currently available. The practicality of deploying and maintaining these in one of the wildest stretches of our coastal waters has yet to be established. It is hard to imagine that they would not have an impact on shipping or marine life.

The costs are unknown but the Scottish taxpayers’ grant of £23 million to Meygen for “up to” 6Mw suggests that to provide the same effective capacity as Hinkley C nuclear station would need 8,250 turbines at a cost of around £22 billion. Hinkley will cost £18bn but would be operational for at least 60 years, whereas the predicted life of the tidal turbines is 25 years.

Although the SNP Government has generously allowed us to pay for these first turbines this capital would presumably be provided by the developer.

What is significant is the cost of the electricity to the consumer. At present this would be dominated by the mandated subsidy for tidal power which is five times that for onshore wind.

So the cost on the grid would be around £270 per megawatt hour, nearly three times that for Hinkley. But this would only be part of what consumers would have to pay.

Under present arrangements consumers, and not the developer, are liable for the costs of storage or backup to make the supply reliable and grid expansion to bring power from the most remote part of Britain. The estimated cost of a new pumped storage scheme, for which a site at Loch Lomond was surveyed in 1971 was then £35m which would now be between £450 and £750m. Grid expansion comparable to the £600m cost of the Beauly-Denny link would bring the total additional capital cost to be recovered through consumers’ bills to around £1bn. A nuclear power station on an existing site like Hinkley would incur none of these extra costs.

Finally, it is probable that only one tidal site, the Pentland Firth, has strong enough tides in the neap season to provide continuous power with any practical storage method. The speed of the neap tides there is actually greater than the spring tide speed at all the other likely sites. This would mean that these would require several days’ storage, the same, insoluble, problem that makes wind power unable to provide a secure supply.

So tide-generated electricity could cost several times as much as the most expensive alternatives. Its potential contribution to a secure supply might be no more than that of a single medium-sized power station.

Not something to get excited about.

l Professor Jack Ponton, FREng