Monday 27 February 2023

Tidal Power: A Mystery

Earlier in the month on a thread about hydrogen, somebody raised the topic of tidal energy and I said we'd cover that in a specific post.  I voiced my perennial puzzlement that no material progress has been made - anywhere in the world - towards effectively generating electricity from tidal power.  A breakthrough on this would be important for the UK particularly, as we have some of the greatest tidal ranges anywhere on the planet.  Why would we expect it to be a good energy source?  And what's gone wrong?  Here's some background.

The theory:  Tidal energy has been harnessed, for hundreds of years (there's an old tidal mill in Bow, East London).  Obviously, vast quantities of "free" energy are sloshing around when tides rise and fall tens of feet, regularly and reliably.  But can they be captured efficiently, and used to generate electricity - even if in a rather peaky profile?  You'd think so: there's not much that isn't known about tidal flows; and in the maritime nation that is the UK, we have longstanding expertise in marine engineering, civils, turbines and, since the 1960s, offshore installations.  What could be missing?

The practice:   Well, since tidal power schemes have been trialled in a stunning variety of different specific designs for 15 years or more, quite evidently a good design is what's missing.   Quite a lot of money, both public and private, has been thrown at this - much of it in benighted Wales, where anyone offering schemes that might generate jobs is pretty much given carte blanche, however dubious their credentials - to no practical avail.  Unless, that is, you consider it worthwhile to be able to rule out certain apparently prospective designs that have been tested and failed.

The scheme that made the most waves (if you'll excuse the idiom) is probably the Swansea Bay Tidal Lagoon.  The physics looked simple enough, and it was very energetically pushed by its promoter, one Mark Shorrock.  Choosing my words carefully, let's say that Shorrock is a showman and somehow persuaded a lot of folks in South Wales, not only to sign up for his elaborate PR schemes but to pitch in with their own money - from small punters putting in a few hundreds, to big players putting in millions.  Shorrock spent the lot, tens of millions in total, much of it going to, errr, himself by way of "management fees".  If it had gone ahead, his scheme was structured financially to yield him personally a vast sum upon completion - not completion of the project but of its financing package.  

Anyhow, after eight years or so of constantly rising costs he'd still failed to get several of the big regulatory approvals needed and the government called time on his application for public money.  (He now has a very large house in Gloucester ... and peddles solar panels in Vietnam.)

He never satisfied the marine authorities on critical matters like silting-up, and disturbing the flows (& wildlife) of the Severn Estuary.  He also came unstuck on wild claims like: if you have enough tidal lagoons situated at key points around the UK coast (he was "planning" five of his own), the regional variations in tidal peaks and troughs all balance themselves out and you get baseload electricity.  This was demonstrated to be, errr, Not True.

Other higher-tech schemes, like this whacky one, have shaken themselves to pieces when trialled at even small scale, and have been abandoned (after many millions of public money have been spent, and Welsh dreams shattered once again).

*   *   *   *   *

As a non-engineer I am bemused because, well, it all seems so logical.  The raw energy is there, alright.  Maybe we do simply await that really cunning design that squares all the circles.

Does anyone have a sound,  knock-down, science-and-engineering argument as to why tidal power is a chimera?   



DAD said...

Ask the EDF (Electricitie de France) about their Tidal Generator at St Malo. Built (IIRC) in the 1960s and still going strong, BUT never has tidal energy been repeated in France.


E-K said...

“The fundamental question is one of economics,” says Brian Polagye, Associate Professor of Mechanical Engineering and Director of the Pacific Marine Energy Center at the University of Washington. Because of the early stage of the technology, tidal power is an expensive source of energy: according to a 2019 study, commercial-scale tidal energy is estimated to cost $130-$280 per megawatt-hour,1 compared to $20 per megawatt-hour for wind.2 High upfront costs of building plants, expenses associated with maintaining machinery that can survive corrosive seawater, and the pricey engineering work that goes into them make up a significant portion of that cost discrepancy. Polagye adds that the supply chain for tidal power also isn't yet capable of providing necessary components and technologies at scale to make this energy source and, as of now, "everything’s pretty custom.” In fact, the market discrepancy between tidal and other, more mature, renewable energy systems is actually growing because the cost of generation from wind and solar generation continues to drop.

Plus the Greenies hate what it does to the environment. Disturbance of migratory flows, noise and electro-magnetic pollution.

E-K said...

I know that anything 'marine' say, a cleat for a boat comes it at 3x the cost of anything for the garden.

Anonymous said...

Having installed a lot of bright shiny expensive stainless steel "rust free" installations in plants along the west coast (likely Ineos now) - it doesn't remain rust free (and working) for long. It looked like it was going to be an expensive court case concerning warranties but somehow the customer blinked first.

It may have been our metallurgists provided the key phrases for the warranty - and we had some very good metallurgists.

Suggest its not civil or electrical engineering that is the limits here, but the metallurgy.

Wildgoose said...

Tidal Energy is like Nuclear Energy - the cost is all up front in building the thing in the first place.

Unlike nuclear energy, tidal energy is only suitable at a limited number of sites. And corrosion from the sea is a bigger problem than corrosion from stray neutrons.

And that's without taking in to account all the genuine environmental concerns.

Which means the real question to answer is Why choose a novel Tidal Energy plant over a modern Nuclear Energy plant?

dearieme said...

I've seen the scheme at Rance. I've been in the tidal mill at Woodbridge. The latter is the better visit - delightful town, Woodbridge. But I digress.

First "tides rise and fall tens of feet, regularly and reliably".

That's what we're used to in Britain but on many shores it just ain't so. You get humungous tides in the Bay of Fundy but much of the rest of the North American Atlantic Coast gets feebler tides. Look at these figures for NYC for instance.

Boston's a bit better but compare it to "The UK has particularly good tidal range 7-10m not being uncommon."

Once you accept that there are only a few convenient contenders for tidal power you must also accept that there will be virtually no economies of scale. Then there's the features it shares with wave power: high energy densities and a corrosive working fluid imply high capital costs. Your equipment has to be strong enough to take the battering from the sea.

Such a pity: the idea of submerged turbines in the Pentland Firth attracts me no end but will they make money? Would the same thing work in the Cook Strait?

Anonymous said...

I spent 3 years working as an engineer on diamond recovery plants in Namibia. We had to use sea water to run them as we were in the desert. Salt water eats steel, the maintenance work carried on round the clock. I suspect this might have something to do with it. What happened to the French plant, I think it is still going but they have never repeated it?

Jan said...

There is research going on in Orkney at the European Marine Energy Centre but I can't find much about how they're getting on with a quick google search:

L fairfax said...

@EK said it really well.
I do remember reading about Salter's duck

I guess that it doesn't really work well.

microdave said...

It's often said that combining multiple tidal generators around our coast would smooth out the intermittency problem. Some years ago Euan Mearns and the late Roger Andrews did a lot of research into this, and concluded that the opposite was true:

"I built a generation model based on 7 lagoons spread along the English coast in areas with the highest tides. The model shows that the ideology does not work in practice. In fact, UK tidal lagoons will produce more intermittent electricity than any other form of renewable generation providing four spikes separated by four periods of zero production each day"

If you type "Tidal" into the search box on his blog, you'll find links to several more posts.

jim said...

Not my field but I will have a go. The primary problem is the energy density and its statistical variability. Matching this to feasible structures, in particular the mass and size of generators makes any design 'difficult'.

The starting point for design problems is the mass and size of a decent sized alternator - say 200MW. 100 tonnes or so of iron and copper if it spins at the usual 3000rpm. Wave speeds and tidal flows mean a lot lower speed which takes us into much bigger (heavier) alternators or DC generators - aka windmill types - where we swap one problem for another.

Fundamentally electric current comes from the rate of change of magnetic flux lines cutting through coils of copper wire. Design choices are bounded by feasible magnetic field strengths for the magnets, fast spinning rotors determined by turbine speeds and mechanical strength. Or we can go to Hadron Collider type magnets and maybe slower rates of change - but even that ratio is only about 10:1 when we need a ratio of about 200:1 to match the slow motion of sea waves. Even the larger wave machines that bob up and down usually have some sort of gearbox to speed up the motion. Not an attractive idea at MW power levels. Bottom line, sea waves are slow, alternators prefer fast. And no one likes rust.

Anonymous said...

And then your lagoon silts up.

(I'm very pleased neither Swansea Bay or the Severn Estuary went ahead, as I love both places. Severn estuary is a pretty wild place still, especially at low tide, be it Purton and its hulks (and excellent pub) on one side, or Lydney Harbour on the other, where the exit lock gate drops 30 feet onto mud flats. With a lagoon yachties would have taken the shoreline over and Sharpness would turn into Poole)

Anonymous said...

Thought occurs - Falls of Lora, where the fjord which runs half way to the A82 in Glencoe empties itself into the sea at Connel Bridge as the tide goes out.

Even without spring tides there's a lot of energy there. And it's by a road. Must be a good reason why no one harnesses it.

auralay said...

It is possible to produce fairly steady, reliable tidal power using two lagoons and careful juggling of the levels between the two. I am pretty sure this was used in a medieval tidal mill at Carew Castle, West Wales.
I found this modern proposal in Scotland.

Anomalous Cowshed said...

What would J F Pownall do?

How much power could you get out of the Grand Contour Canal, and for how long? Head would be around half of the Hoover Dam.

E-K said...

I like Jim's point.

If you are talking about slow energy you need BIG sails, blades, floats and gearing to create speed at a small spindle.

Sackerson said...

Wildlife and habitat issues, sea-rusting and barnacles... not so easy.

rwendland said...

> With a lagoon yachties would have taken the shoreline over and Sharpness would turn into Poole

Yes, and more. When I looked at a Severn barrage financial plan many years ago, part of it was the creation of two quite large towns at either end of the barrage. The large increase in land value from farming to residential was part of the barrage financing - it assumed this increase in land value would somehow entirely go toward the cost of the barrage, reducing loans and finance costs.

Anonymous said...

If you are talking about slow energy you need BIG sails, blades, floats and gearing to create speed at a small spindle.

Or a choke.

rwendland said...

ND> Does anyone have a sound, knock-down, science-and-engineering argument as to why tidal power is a chimera?

Just on tidal flow (underwater turbines), not knock-down, but serious is that power in the flow is proportional to the *cube* of water speed. This tends to make tidal flow systems very peaky in power output, which makes getting decent ROI difficult.

If you had a perfect underwater turbine utilising all the water power, it generates over 80% of its maximum power for ~1.5 hours in every ~12.5, and over 50% ~2.75 hours in 12.5. Generally the ebb tide is much weaker than flood, so you get nowhere near maximum on ebb. In fact sometimes underwater turbines don't work on the ebb flow (so they don't need to rotate or work in reverse).

However because of this underwater turbines are often designed to peak flat at about 20% of maximum water flow power, so they can sustain generator maximum power about 3.5 hours in every 12.5, about 28%, still not great, or over 50% of their power about 5 hours in 12.5.

So they have physics limits a lot of people don't realise.

dearieme said...

I think the Cook Strait idea may suffer from competition: NZ already has lots of hydro power (unless the mad witch who ran the place decided to close it) so that would make it hard to compete. Some geothermal power too.

Near the sea bed there's a reasonably steady reflux flow of water westwards out of the Straits of Gibraltar but it seems unlikely that it could provide cheaper electricity than, say, gas from Algeria.

Nick Drew said...

Many thanks, all - this non-engineer has learned a lot here.