Tidal power ain't all that's claimed for it

Tidal power ain't all that's claimed for it

by Jack Ponton
article from Thursday 8, November, 2018

IT IS DEPRESSING how the media routinely publish press releases from renewable energy developers and lobby groups as if they were scientific fact rather than exercises in public relations. An example of this appeared in The Scotsman on 31 October entitled 'World first as Scottish tidal turbines connect to grid' and subtitled 'Tesla tech turns tide power into green baseload'. Neither of these statements is true.

The 'world first' claim would come as a surprise to EDF, whose tidal power station at La Rance in Brittany has been in operation since 1966, and closer to home, to Meygen whose turbines in the Pentland Firth were fully operational in April this year. However, this is a minor sin of self-promotion compared to the risk that our technically illiterate politicians might take at face value the claim that this is 'baseload' power.

Coal, gas and nuclear can provide 'baseload' generation because they are able to produce nearly their full rated output to the grid on demand. Solar cannot do this as the sun does not shine at night and hardly shines at all in the winter. Wind is highly variable, unpredictable and may die away almost completely for days at a time.Tides do indeed have the advantage of predictablility, high tide to low tide is every six hours every day. This however means that a tidal turbine will go from maximum output to zero output and back to maximum every six hours. On its own, tidal power definitely does not meet the criteria for 'baseload'.

However, this is where the developer's claim that they can make tidal power baseload comes in. Tesla batteries will be charged up at the peak of the tidal cycle and discharged to the grid at the bottom, providing continuous power. This is a perfectly sound idea and would be quite straightforward to implement. The scheme is described as being rated at 300kW, which would normally refer to the maximum power output of the turbines. Assuming they attain this at the maximum point of the tidal cycle the output averaged over the whole cycle would be about 127kW. To even this out would require 236kWh of storage which could be provided by seventeen Tesla 14kWh batteries at a cost of about £91,000. 

So what is the problem with these claims? 

Firstly, the cost. This is a very small installation. To replace Hunterston or Torness, 1.2GW, would require 2.83GW of tidal turbines, nearly 10,000 times as much. It would require about 3GWh of storage. To provide this with batteries would cost more than £600 million. However providing storage by a pumped hydro scheme such as Cruachan would cost 'only' around £90 million. Batteries are not a cost effective way of storing electricity in these quantities and are unlikely to become so in the foreseeable future. 'Tesla tech' is irrelevant to large scale power generation.

But this is not the main problem. There is also a tidal 'season'. Spring and neap tides occur at 14 day intervals. The maximum velocity of the tidal stream falls by about 60% from spring to neap. The power which can be generated however, depends on the cube of the velocity, so 60% velocity means only about 20% power. If the 300kW turbines are intended to give their maximum power during the spring tides, when they will produce on average 127kW, then 14 days later this will have fallen to around 25kW! The average power output over the 14 day cycle would be only about 76kW. But this would not be 'baseload' power as it would not be continuously available on demand. To turn it into baseload would need more storage, and this would need to be enough to store seven days worth of electricity rather than just six hours. A very rough estimate (the idea is so impractical that it isn't worth a better one) would suggest about 8,500kWh of storage would be required, and that batteries to provide this would cost about £3 million. To put these sums in perspective, a 100kW diesel generator would cost about £25,000.

The Scotsman article appeared under the byline of the paper's 'environment' correspondent, but was in fact just a regurgitation of statements by the CEO of Nova Innovation, the developer who has just colllected £272,600 of the Scottish taxpayers money to fund the project, and a spokeperson of WWF Scotland, a renewable energy lobbying organisation. This same text has appeared practically word for word elsewhere. A poor excuse for responsible journalism from The Scotsnman which likes to think of itself as Scotland's national newspaper.

Jack Ponton FREng, Emeritus Professor of Engineering

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