Why I’m still not excited about Tidal Power

Why I’m still not excited about Tidal Power

by Jack Ponton
article from Wednesday 16, August, 2017

THE TIDAL POWER company Atlantis has announced that the four 1.5MW turbines they installed in the Pentland Firth have generated 1GWh of electricity. When the company started installation of the first turbine last September I wrote an article in The Scotsman (below) on the possibilities of tidal power but with the caveat that there were still problems to be solved, and that these possibilities might be more limited than was claimed. I think I was right not to get too excited.

The first turbine started generating two months after installation began. Assuming that the other turbines also needed two months to install and commission there should now have been a cumulative 24 months of operating the 1.5MW turbines. At 720 hours per month the potential generation would have been 26GWh of electricity. 1GWh is just 4 per cent of this.

Of course, tides, like wind and sun are variable and seasonal so the theoretical maximum can never actually be achieved. Allowing for daily variation and assuming that the turbines would be able to produce their maximum rated power even at the neap tide period actual output could be 40 per cent of this maximum. So achieving around 4 per cent still doesn't look too impressive.

The company also claims that this 1GWh (1,000MWh) was enough to 'power' 700,000 homes. This is a puzzling claim. Gigawatt and megawatt hours are energy, not power, which is energy per unit time. Wind turbine developers talk about how many homes so many megawatts will power. So when talking about megawatt hours 'powering' things one must ask 'Power for how long?' The average domestic electricity consumption is 3.3MWh per year so 1GWh would supply only 300 households for a year. Or 'power' 700,000 for less than 4 hours.

If 40 per cent load factor (percentage of nominal power) can in fact be achieved then the 6MW now operational could supply the electricity needed by about 4,500 households. It could completely power only about 1,000 as three-quarters of domestic energy consumption is gas.

Examining the developers’ figures for expected generation from their proposed final 400MW installation however, suggests that they have not designed turbines that could produce their full rated output during neap tides. This is a pity, because the Pentland Firth may be the only location where this might be possible. Their expectation is for a load factor of about 23 per cent, which would nearly half all the above numbers.

This is all costing taxpayers and consumers rather a lot. £53M of their money has gone in grants, £8.8M per megawatt compared with the Hinkley nuclear costs of  'only'  £6.25M per MW and an operational life more than twice as long as the tidal turbines. And while Hinkley's operators will be (over)paid at £92 per MWh tidal power currently collects around a whopping £270 per MWh.

Still, to look on the bright side, these turbines, unlike the wind variety, will loom over no one's garden, their noise will keep no one in the once quiet countryside awake at night, and they will kill no seabirds (dolphins may not be so lucky). Unlike biomass they will destroy no forests. If one must have 'renewable' energy, then this is probably the least environmentally damaging form.

As I explained in my earlier article, the tidal turbines in the Pentland Firth, if combined with technically proven pumped storage for which at least one site is available in Scotland, have the potential to supply as much dependable power as a medium sized nuclear power station. Whether this would be good value for our money is another matter altogether.

Jack Ponton is an Emeritus Professor of Engineering, a Fellow of the Royal Academy of Engineering and the Institution of Chemical Engineers.

From The Scotsman of 3rd January 2017:

Don’t get too excited about Tidal Power

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 6 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 that 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 £23M 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 £18 billion but would remain 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 £35 million which would now be between £450 and £750 million. Grid expansion comparable to the £600 million cost of the Beauly-Denny link would bring the total additional capital cost to be recovered through consumers' bills to around £1 billion. 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 day’s 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.

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