The Trouble With Tidal

Why the most predictable kind of renewable energy is also the hardest to use.

Could you steal the gold from Poseidon’s teeth? I found out how, and it’s harder than it looks.

I’m referring, of course, to tidal energy, in which I received a thorough blooding a decade ago, having joined a startup with the Very Good Idea of harvesting the raw power of tidal races to generate stable, predictable green energy. It’s a fine idea and you can immediately see the appeal: Unlike solar, which goes out with a bit of cloud, or wind turbines, on which you might as well flip a coin, tidal energy is as dependable as the orbit of the moon. It’s predictable 3,000 years in advance, you can set your watch by it, and it’s powered by the greenest energy of all; the kinetic energy of our big ol’ orbiting rock.

It is also, unfortunately, very, very difficult to achieve. It relies on fine engineering, at great expense, deployed into an environment that wants to kill everything in it.

So how do we do it?

Read on, and discover the secrets of generating green energy… the hard way!

1: Poseidon’s Teeth.

Across the world there are areas where natural geography creates channels or pinch-points in natural tidal flows, concentrating them and greatly increasing both their vertical range and velocity. One example of such a ‘tidal race’ is the Bay Of Fundy in Nova Scotia, where every six hours moves a water flow equivalent to three times all the rivers in the world combined. Other tidal races include the waters around the tangled archipelago of the Orkney Islands near Scotland, or the waters near the island of Alderney in the English channel, close to France. These tidal races worldwide, if peppered with tidal turbines, could potentially provide 1,000 TeraWatt-Hours of electricity a year, or a thousand-thousand-thousand-thousand kilowatt-hours. Or 20% of the electricity use of the United States.

And a tidal race is brutal. Almost un-dive-able in many areas, they are challenging for surface vessels too, with current flows of up to 4 metres per second at their peak. Lest it be forgotten, because water is a thousand times more dense than air, a tidal flow of 4m/s exerts a force on fixed subsea structures equivalent to a 400km/h tornado wind on land. This is not engineering for the faint-hearted!

But first things first: How do we generate power from it, anyway?

There are two fundamental ways of generating tidal power: Barrage and freestream turbine. A barrage is functionally similar to a dam, in that it features a barrier between a captive body of water (a tidal lagoon) and the open sea, in an area with a high tidal range. The barrier has a series of channels containing turbines, and sluice gates that can control the flow of water. When the incoming or outgoing tidal flow has created a high enough vertical range, the gates open and power generation commences.

A freestream tidal turbine, by contrast, is more subtle: It is a turbine, usually mounted subsea but occasionally floating or moored, which sits in the open water region of a tidal race and generates power directly from the flow in the same manner as the wind turbines we all know. 

Each method has its own strengths & weaknesses. This week we'll look at the method I have personal experience in, the freestream tidal turbine…

2: King Midas, water into gold.

More intuitive than a tidal barrage, the tidal turbine simply stands or floats moored in the path of a tidal race and absorbs the full brunt of it, which spins a series of rotors and powers a generator. The generator outputs electricity, of course, but this is not the end of the story, for the energy produced by the turbine is rough & raw, completely incompatible for a national grid and needs stabilizing and frequency matching to a level of quality that can be safely exported. This is done through a current convertor & turbine control unit which can be co-located, located on a tower or on shore. 

So far, so simple, but despite this apparent simplicity this is by far the least popular way of generating tidal energy: The big tidal plants are all barrages so far. This may of course change, but as of now the design maturity of free-mounted tidal turbines is limited, and many approaches are still being tried out.

There are still design forks to be navigated, from assembly & deployment to rotor design, types of generator, maintenance access & location of the turbine control centre. Let’s navigate all of these, and give some kind of explanation as to why this form of power generation is so damned difficult to pull off.

3: Blades & Generation.

Two clear forks in turbine design relate directly to electricity generation. Namely, how many blades the rotor should have, and what kind of generator to use; podded geared generators versus direct drive permanent magnet turbines.

Beginning with the blades: Most tidal turbines have either two or three blades, and the trade-off here is between pure hydrodynamic efficiency and structural loading. Essentially, the pressure difference between the up-flow and down-flow directions on a blade, as well as moving the rotor, also creates flow swirl around the tip of each rotor blade, which affects the incoming current and the blade angle of attack, generating induced drag much as on an aircraft’s wing. Because co-located blades interact with each other, you would therefore expect that optimal hydrodynamic efficiency is achieved with a minimum number of blades: Two. 

And while that is true, there are other factors. For one, it’s not all about hydrodynamic efficiency; once you’ve committed to sinking a thousand tons or more of tidal turbine structure, you might as well squeeze out as much power from that installation as you can, even if adding a blade brings some declining returns. To pull as much power out of two bladed installations as possible you’d need to rotate them fast, which makes cavitation a risk for larger installations. 

Secondly there’s the factor of twist and resonance: With a wind turbine, if the wind direction suddenly changes then the differences between upwind and downwind lift from the blades creates a twisting moment on the support structure, which becomes dangerous with two-bladed designs due to the potential for dynamic resonance. Three blades avoids some resonance modes for wind turbines, and while tidal turbines don’t twist they can create pitching moments very easily due to large velocity gradients near the sea floor. 

As ever, it’s all compromise. Smaller installations and floating turbines get on well with two blades. Large seabed mounted ones tend towards three. 

For generation, the mainstay is a traditional geared, podded generator: A multi-wound rotor & stator combo connected to a gearbox that spins the rotor up close to grid generation frequencies. The downside of such a thing is the need for foolproof multi-year waterproofing and corrosion resistance, generally by a pressurized oil/ grease reservoir that prevents water ingress. The alternative less, common, generator type is a direct drive permanent magnet system, which is what we used in my time in OpenHydro: It’s a flooded design, so doesn’t concern itself with water ingress, and eschews a gearbox, relying on the raw speed of the rotor moving past the coils to generate. Theoretically simpler, the DDPM approach tends to be much harder to build and asks for exacting build tolerances on very wide diameter assemblies. In practice, it’s not an ideal solution.

But we didn’t know that at the time, and it does make for a very cool looking design.

4: Bearing bother.

We shall, just for a moment, take a close look at the OpenHydro turbine that I worked on, which conveys the very special challenges of a flooded DDPM generator approach.

Basically, OpenHydro had a clear design philosophy, which was to minimize the amount of moving parts underwater. This makes sense, since the sea is a violent, corrosive brute that kills machinery, and of course the smallest number of moving parts in a turbine is one.

To achieve this meant a DDPM design with no gearbox, so the permanent magnets, mounted on the outside of a rotor, would run straight past a series of epoxy-secured coils and thereby generate electricity. Both magnets and coils were mounted in water proof, cathodically protected, corrosion resistant housings, but the DDPM approach drove three design features which were an absolute pain to deal with:

Firstly, the magnets had to be on the outside of a giant ring structure, because efficiently generating the current required the magnets moving past the coils with great velocity. This meant a giant toothy doughnut design, with rotor blades facing inwards on a neutrally-buoyant O-shaped rotor with bearings and magnets on the outside!

Secondly, the rotor and stator had to be huge, to maximize magnet-coil passing speed: A 14m diameter rotor on our 2 megawatt turbine design.

Thirdly, the distance between magnet and coil, where they passed, had to be minimised. A water gap of just over half an inch, and a bearing gap of much less, on a 14 metre diameter centre-less assembly is no mean feat. Mounting tolerances were controlled to within +/- 0.5mm radial position with a virtual centre located by regular laser positioning surveys throughout assembly. This was, understandably, a right royal pain in the arse to manage, especially when our prototypes were being put together in environments without temperature control. Plus the stator & rotor were assembled in different facilities entirely, and expected to be transported to meet, and fit, together. 

We made it work, but it wasn’t easy. At all!

5: Staying the beast: Mounting to the seabed.

A turbine is of no damn use if it moves around, so we have to mount it. There are several competing ways to do this.

A monopile is, as the name suggests, a single big pile mounted into the seabed. It's driven in by a vessel, such as a construction barge, with heavy lifting equipment and a pile driver: Literally a big weight that rises and falls, rises and falls, smashing the pile into the seabed like the wrath of a minor God. Monopiles are often used to support piers, or wind turbines in shallow waters, and it can be ‘fun’ to be around a pile driving operation, as the regular BANG! BANG! carries for miles underwater.

On the face of it, a well-driven pile should be a good mount for a tidal turbine too, except for one minor flaw, which is that tidal races simply won't give you the time to do it. The slack water period in a tidal race can be as little as 45 minutes, and that's just not enough time to sink a pile capable of supporting a tidal turbine. As a result, mooring the brutes this way requires jack-up barges; fiercely expensive pieces of heavy equipment that jack themselves up off the seabed at speed to provide a stable build platform for the pile sinking operation.

Doable, but not straightforward. Or cheap.

The Seagen turbine, a pile-secured beauty in Strangford Lough, Scotland, used a novel way to get around this: Initially mounting a working platform on top of a barge-placed structure with low water resistance, the working platform was then used to drill guide holes for four 9m depth pin piles which would mount the subsea structure at it’s corners into the seabed, using the initial barge-placed structure as a platform for pile driving. In this way, the work could be spread out over multiple slack water periods and the turbines only added after full structural reinforcement was complete. This was an artifact of Seagen’s unique design, which was a surface-piercing tower with two turbines mounted on arms from a central collar, which could rise above the water for maintenance. Placing it all in one go would have been difficult.

The third, and by far most common, method of turbine deployment is a gravity base: Simply put, mount the turbine to a colossal heavyweight base that will secure itself to the seabed by friction alone. This has the advantage of very rapid deployment, as a gravity base can be sunk and positioned within a single slack water period and immediately followed by cable-laying. It does, however come with a significant drawback…

…The gravity base has to be bloody massive!

Remember, a tidal turbine needs to take spring tide flows of 4m/s, plus extra allowances for wave action, plus allowances for storm surge. Add all these safety factors together and a 2 megawatt tidal turbine, such as the ones I worked on with OpenHydro, needed a 1,300 ton monster gravity base; three–legged, mild steel filled with poured concrete, squatting on the seabed like a Jules Verne creature made real in steel.

It gets worse: Add the weight of the turbine itself (200-400 tons, depending on design), plus turbine control centre and other paraphernalia, and you can be looking at 1.7 to 2 kilotons of subsea structure for rapid precision deployment. 

It gets worse still: While most tidal turbine manufacturers, such as SAE Renewables/ Atlantis Resources, designed their turbine with self-centreing mounting features to be fitted and removed quickly from a pre-placed gravity bed, the OpenHydro turbine, due to it’s unique shape (a giant toothed doughnut!) had to be mounted portside and deployed in one go. This meant that the entire 1,700 tons of turbine & grav base, plus cable management and all the rest, would need to be hauled up in the event of maintenance. We needed to design a special deployment barge just for this, to enable such a feat of precision positioning to be performed within the razor-thin margins of a slack water period. Tick-tock-tick…

No pressure.

Or, of course, you could take the unique approach pioneered by Orbital Marine Power with their floating moored turbine design: To eliminate the need for either a special deployment barge or high cost dynamically positioned vessels, these guys just allowed their turbine to float like a ship, moored to a set of anchor points made of simple interlocking 64 ton sections that can be handled by regular cheap work boats. It also means that their design sits high in the water, where tidal flows are fastest, meaning more power from less rotor, and is easy to access for maintenance or towing to port for overhauls.

There is a lot to admire about that kind of thinking, but right now the most common approach is gravity base.

But once the turbine is down there, it needs to actually produce usable power. This brings us to the next design fork…

The turbine control centre.

6: Current Control Conundrums.

Whether a wind turbine farm or a tidal turbine farm, you need something that’s going to control settings, dump excess current, feather blades (where possible), apply brakes, set start-up sequences for low flow and so-on and so-forth. You also need a current management system that will convert the raw untamed output of a tidal turbine, reacting solely to the changing flow in its environment, and make that into something a national grid would regard as usable. This is the turbine control centre (TCC), and the next design fork is where to put it…

The first and most obvious solution is shoreside, in a purpose-made building. This is the cheapest way to build current management gear, the easiest to access for maintenance and the best at avoiding corrosion and through-life issues, but to make up for it you need lots and lots of cables, since every turbine will now need its own line to shore to connect with the TCC, and cable laying gets expensive and awkward when you have to do a lot of it. Maybe best, then, that this solution is just for near-to-shore and prototype scale deployments. 

The next alternative is common in offshore wind farms, which is to house the TCC offshore in a surface-breaking tower in the tidal generation region. The tower would likely be supported onto piles driven into the seabed, and can support current control equipment, condition monitoring & control for the turbines, a communications link to the shore (for remote access) and an export line for the saleable power generated from the tidal field. The tower needs to be robust, high enough to avoid the worst of the sea spray and accessible for personnel. It is, inevitably, harder to build than other solutions, but allows for simpler cable management from an offshore field.

The final TCC design option, and the one that OpenHydro went with, was the unique solution of a submerged TCC unit connected to the gravity base of one of the turbines themselves, with other local turbines networked into it. This minimizes cable lay & tower build hurdles, but instead creates a design challenge: Creating an automated, long-life system of massive complexity that can function for many years underwater with minimal to zero maintenance (as any maintenance on it will bring down a tidal farm, or a large portion of it).

There’s a reason the engineers who worked on our submerged TCC won an award for their efforts. 

As ever, because the tidal industry remains young and immature, there is no single agreed “best way”: How could there be, when the economics of this bizarre corner of the energy industry has yet to be fully ironed-out?

7: The Ocean Hates You.

Also yet to be ironed out is how to cope with the simple, violent entropy that is life at sea, for the sea contains many dangers.

As we found out by harsh experience, not only is it just an issue of managing harsh currents and storm surges, but even normal corrosion puts huge demands on design & build of complex moving structures: You can't make everything out of stainless steel or aluminium, glass fibre composites are expensive at scale, and not the best under point loading. So the hundreds of tons of cheap mild steel in every turbine was coated in two or three coats of thick marine grade paint.

In fact, long before we got to painting we had to take other measures, because eventually all paint chips, and corrosion gets under the surface, slides into your DMs, and breaks your heart. To guard against paint failure we had a second level of corrosion protection: Vast zinc blocks were bolted into the superstructure of the turbine, forming the anode of an electrically connected galvanic corrosion system, so it would rapidly decay instead of the steel of the turbine. Thus in turn meant that everything, down to individual bolts, had to be electrically connected to the superstructure with a welded wire… many, many hundreds of times. Where this couldn’t be done, such as on the huge tension bolts fixing the two-ton glass fibre composite blades in position, viciously expensive materials such aa Titanium 6-4 needed using.

As for the coils in the flooded generator section, each were precision assembled to precise fraction-of-a-mm curvatures and encased in moulded epoxy. Once again, mounting bolts were electrically connected and painted, because the loss of even a couple on a coil assembly could begin a kinetic chain reaction that would tear the turbine apart. The exact same logic was applied to the horrifically powerful Neodymium permanent magnets that ringed our rotor, each firmly encased in its own resin coffin.

Oh, and just to give you an idea of how powerful those magnets are, heed this lesson: If two of them come together for any reason then -BANG!- after a tremendous impact, which liquefies anything caught between them, it takes a forklift truck to separate them. We tried it.

And our health & safety manager, demonstrating their strength with a wrench to some auditors, lost the end of his thumb in the process. Sprayed blood all over the auditors, too.

We placed these things underwater, millimetres away from contact surfaces, and spun them around really fast. No stress.

But I digress…

It gets even worse. After the corrosion protection system was in place and the paint dried, another coat of specialist paint needed adding, this time a chemically-tinged anti-biofoul layer to stop anything growing on the turbine.

Yes, that's right: Even barnacles & seaweed are enemies when you choose to build mechanisms in the sea. You wouldn't expect anything to be able to grow in the currents we operated in but believe me, life finds a way.

-And before you ask, no our turbines didn't make fish into sushi.

That's not even the end of this chapter, because the ocean's hatred of all things human extends to the sea state, which can make turbines unreachable by surface vessels, the currents, which are un-diveable, and all manner of debris that endeavours to silt up every exposed cavity with muck. The very flooded cavities you need for maintenance access when you bring the turbine to the surface.

Oh, and most of the sea floor is unsuitable for your turbine, so you need to pay for expensive ocean floor & current surveys long before you do anything at all.

So to summarise, we were making intermittent, uncontrollable electricity the hard way, with huge complex machines that are difficult to build, then dropping them into an environment that exists to kill machinery.

No problem.

8: Making The Impossible? Possible.

We almost did it. Pre-series turbines were built, sank, spun. The standard product design and factory concept was revved-up into real concrete and a factory was unveiled in Cherbourg, France, a few hundred metres from the quayside. A town of heavy engineering expertise, grown from naval nuclear projects, was ready to take on the tides and then the world.

The effort failed. Tides of finance reclaimed OpenHydro, and the greatest and most awe-inspiring of the DDPM turbine designs sank without a trace.

But tidal stream power is not dead, and other companies are carrying the mantle, fitfully.

Can it be done right, by someone else?

It's possible, but an evolution in approach is required, and it's already starting. The expensive subsea surveys that we spent so much money on can now be done remotely by drone boats at a fraction of the cost: An approach, fittingly, pioneered by XOcean, a startup created by the former employees of now-bankrupt OpenHydro.

Precision fabricated mild steel assemblies can be rethought, either by simplifying the design to reduce the number of welds, changing to composite material layup or automating what welding remains. You can fabricate cheap if you have abundant low-cost labour to play with, but the regions of the world most interested in tidal power are wealthy places where this isn’t the case. Automation is a prerequisite, which in turn means scale and repeat orders.

The manual laser-guided assembly of bearings, magnets & coils used on our pre-series prototypes was to be replaced, in the full commercial product, with simple machined mounting features using a giant vertical axis machining rig: More man-hours removed.

The deployment & recovery can be made cheaper by purpose-built deployment barges: Far more cost effective than hiring out dynamic-positioning vessels with heavy lift capabilities and a murderous daily charge rate. OpenHydro had the right idea on this with its specially designed OH Installer, but we never got the bulk orders and the scale to get payback on that. C’est la vie.

Heavy lifts in all forms need to be minimized. Large cranes cost a lot to use directly, and even more indirectly, as they force you to work quayside, where the cost rate is many times higher than an inland factory. Concrete pouring is cheap though, so you can visualize using gravity bases that are almost entirely concrete, poured & set under the waterline before deployment and reducing heavy lifting to a minimum. 

Moving maximum activity inshore is also crucial to making tidal power work. Expect quayside work to cost several times more than an inland factory, and offshore work to cost several times more again: Maximizing the amount that can be done in hub factories before deployment is crucial to cost-reduction, and that means a simple, modular design broken into lightweight subcomponents that are easily fitted together.

Maintenance needs to be automated or minimized: Every interaction offshore means high expense, so getting data on system reliability in the tidal zones, and improving the time between maintenance operations, is absolutely critical. These turbines need to be almost drop & forget to be worthwhile.

And get rid of DDPM permanent magnet designs entirely: We tried it, and it’s too much trouble. No, thank-you!

All of this is a tall order, but the technological capability exists, and elements of most of these strategies have been attempted piecemeal. The prize will go to whoever bravely ties them all together into an industrial system that works and generates lots of power, cheaply.

Who’s up for that?

The most obviously “up for that” companies right now are SAE Renewables, whose Meygen project off the coast of Scotland is the largest in the world, and its primary turbine supplier Proteus Marine Renewables: The Phase 1 pilot is fully operational, with four deployed turbines and 6MW of capacity. Phase 2 contracts have been awarded for a further 59MW over the next few years, and a final phase build-out of 398MW is contingent on the success of that. With each turbine and each new MW of capacity, learner effects strengthen, automation and redesign becomes more cost effective, and the industrial flywheel spins up faster. It's taken a long time to get to this point, but Godspeed to Meygen.

I am loathe to pick winners in this industry, as solar & wind have probably got so much first-mover advantage that tidal can never catch up… but I have a soft spot for the O2 turbine. The Orbital Marine O2 turbine is a floating design with, -as mentioned earlier-, a clever anchor method that eliminates costly specialist ship hire entirely. The 74 metre long floating turbine itself can keep specialist gear accessible above the water line and weighs “only” 680 tons.

OK, so that’s actually bloody massive, and puts you straight back to quayside heavy lifting agan, but in principle there’s no reason that it couldn’t be made a whole lot lighter, with a glass fibre composite hull or even a modular build structure. 

Hell, put it together the right way and you could even launch it off a slip-ramp like a boat!

Like I said, with sufficient ingenuity you have options. Think fast.

9: Afterword

I love the romance of tidal power, I do. But then, I love the drama of Sisyphus as well, pushing his boulder endlessly up a hill as punishment for the hubris of thinking he can outsmart Zeus and cheat death. 

Up and down… unending toil. Maddening punishment.

Tidal power has an air of the Sisyphean. It, too, is trying to cheat a god, and as punishment is rendered into an impossible task. Perhaps we’ll manage it one day; perch the rock on the top of that hill. Pull cheap abundance from the tides. 

Steal gold from Poseidon’s teeth.

But for now, while we strive, we know that we haven’t outsmarted the gods quite yet.

Keep on trying…

Images and data in this article credit to OpenHydro Technology, Proteus Marine Renewables, Alstom and the papers Practical Turbine Design Considerations: A review of technical alternatives and key design decisions leading to the development of the Seagen 1.2MW tidal turbine (Peter Fraenkel)/ A review of tidal current turbine technology: Present and future (Faisal Wani).