Salt and Battery: Wave energy & you!

Can we power our world from the ocean wave?

Who wants to steal gold from Poseidon's teeth and swipe Neptune's belt buckle? We can already tap the dead breath of ancient fossils, and the magic of the reactor core, so why not generate power the hardest way of all!

Solar and wind don't count. They're sissy ways of casting kilowatts and act like a bad flatmate: Great for making promises, but never good for the rent. If I want to eke out ergs from unreliable mother Earth, I want to do it in the most macho way possible, so let's, ah, dive in and see what wave power is all about, shall we!

1: Ride the wave! Or stand in its way.

Wave power is a compelling energy source on paper. It's simultaneously more and less variable than its solar or wind powered contemporaries, while sharing some challenges with tidal power, which I've written on at length before.

How can it be “both more and less variable”? Make sense Jordan, you fool!

Fair enough. It's less variable in that there's almost always some usable wave energy at a given location, whereas solar turns off at night and the wind dies whenever it pleases. It's also vastly more variable because an extreme storm-driven wave can hit with fifty times more power than a regular everyday one.

Fifty is a lot. Civil engineers may have to deal with factors of safety of 5 to 10 (I.e designing a structure for 5-10 times its normal loading) and weight conscious aerospace components might make do with 1.5-3, but fifty is beastly.

You might expect, therefore, to see some big heavy tank-like constructions in this sector, and you'd be right.

But on the other hand, waves are always available and because they build slowly out at sea you can get a pretty decent forecast of the sea state. Moreover, unlike wimpy wind or solar, wave power is concentrated!

A typical European offshore Atlantic wave has an average power intensity of between 30 and 70 kilowatts per square metre. If we assume that a third of this (10-23 kW/m^2)is extractable, then how does that compare with offshore wind?

A big 8 megawatt offshore wind turbine has a blade swept diameter of 165 metres, giving a swept area of over 21 thousand square metres. If we assume a 35% capacity factor (average output of 2.8 megawatts), then you're extracting an average of 0.13 kilowatts per square metre, or less than one percent of what a wave generator can do with equivalent space. In theory.

Put like that, wave power sounds pretty appealing. What's the catch?

Before we get there let's talk a little about how waves are formed in the first place, because special factors like tides or earthquakes aside, they're mostly formed by the wind, making energy from the waves a sort of second-hand wind energy, or 3rd-hand solar energy.

Find a calm body of water: A full bath will do. Now blow gently but consistently on the surface and you'll see little ripples forming. These ripples, by dint of their shape, then allow the water to catch more wind, and the added energy makes them grow, and grow again. Keep doing it for long enough, with strong enough winds over a long enough distance and the waves so formed can become enormous, which is why the really big waves are formed by storms and consistent wind systems in big areas of open ocean.

A wave is not, however, a sideways movement of water, even though it looks that way. Instead, it's a reciprocal circular oscillation, transmitting energy but not necessarily net mass.  The efficiency of this process means that large waves can cross thousands of miles. A storm in the Atlantic can cause big coastal waves on a calm day, three days later, with not a gust or cloud in the sky.

So a big wave is a formidable beast. How do we cage it?

There are dozens of different ways to tap into the fury of the sea, but in general they’re split into three main classifications: Terminators, attenuators and point absorbers.

A terminator is a device which in some way stands in the way of the wave, gritting its teeth and taking the brunt of the impact, like many of us have on holiday, digging our toes into the sand and going into Portugal’s Atlantic-coast waves shoulder-first. When we do this we usually end up on our arse, pummelled by churning froth, and so this is one of the more King Canut-style power generation methods. Ballsy but brutal.

An attenuator rides the wave, floating and letting itself be pushed up & down, and side-to-side, deforming its profile and letting that generate power through hydraulics or servos or whatever.

A point absorber, the most subtle and practical (or just cowardly?) form of wave device, lets the wave wash over it and generates power from the rhythmic increase in pressure as the water column thickens overhead. 

In each one of these there are a hundred different varieties, so let’s jump into a few of them for some real-life examples.

Shown here is the ‘Limpet’, the world’s first wave power plant to be connected to a grid and operate commercially. It was installed on the Scottish island of Islay, which is also known for making the best whisky on the planet and I will not be convinced otherwise! 

The limpet is a terminator device that works on the principle of an oscillating water column, forcing incoming waves under a concrete channel into a huge plenum chamber, where the water level rises and falls, creating a rise & fall in air pressure that drives a Wells air turbine and generates electricity. In this way, the primary mover uses air rather than water, and spreads out the rhythmic impulse generated by the waves somewhat. This is assisted by the Wells turbine itself, which is a unique design with perpendicular blades that rotate in the same direction no matter if the air is coming into the turbine or out of it. This comes at a trade-off in turbine efficiency, at between 40% and 70% efficiency compared to the much higher efficiencies achieved by ducted one-directional turbines, which can exceed 90% in ideal conditions. The loss of efficiency is a trade-off that allows for mechanical simplicity and a more constant rate of revolution from wave to wave: Both useful in a harsh, variable environment where corrosion is an issue.

The Limpet operated between 2000 and 2011 and generated a max 250 kiloWatts of power. Not much, but adequate for an experimental installation on a tiny island, and simple enough to be run remotely by Wavegen personnel in Inverness or Belfast. 

Happily, the technology behind the Limpet was passed on to another, with the inauguration of the Mutriku breakwater plant in Spain, which has been operating since 2011, generating 296kW from sixteen small turbines. 

Another way of generating wave power through the ‘Terminator’ method (a macho name) is Aquamarine’s ‘Power Oyster’, which has a less macho name. This comprised a semi-buoyant breakwater oscillator which stood in the way of waves and drove a subsea piston that pushed seawater through a long flowline to a generating unit on the shore. This generating unit would use a Pelton wheel generator to create electricity from the high-speed seawater.

A Pelton wheel, by the way, is one of those little bucket-wheel style devices that you might have seen as a child. It looks laughable, but it actually works well with low-volume, high-speed water flows with significant hydraulic head. The Power Oyster, which was installed at EMEC, the European marine Energy Centre in the Orkney Islands off Scotland, was effective at translating low speed, high volume wave energy into a high-pressure hydraulic flow, and drove a 315 kiloWatt generator. The mounting of the primary generation gear onshore also means that the massive power capture unit itself can be simple & robust, which is what you want in wave power. 

The downsides of this scheme include the massive size and difficulty of installation of the Power Oyster itself, though to their credit they did manage to test a second, beefier version: The originally-titled Oyster 800 was installed in 2012 and generated 800 kiloWatts for tens of thousands of operational hours. 

So that’s terminators covered; what about Attenuators? 

Probably the most famous example of a wave attenuator device is the Pelamis device, which formed the world’s largest wave farm in the world, generating 2.25 MegaWatts off the coast of Portugal until violent weather wrecked it. The Pelamis was a charismatic beastie, one hundred and twenty metres long and floating like a giant sea-snake on top of the waves, generating power by the twisting and bobbing between different sections, which pressurized an internal hydraulic system to generate power. 

There are some advantages to this: The use of hydraulics is an effective power take-off system, and the use of a device that responds to wave curvature rather than gross height or power means that it was effective at pulling usable energy from small waves, though at the expense of throwing away gains for the biggest of them. Several versions of the Pelamis device were tested in Scotland and Portugal before the company finally went under, which will become a theme: Wave energy is hard energy, and it takes casualties.

And of course remember that the Pelamis devices in Portugal, despite looking like massive sea monsters, were crippled by high seas. After all of that blood & sweat, Poseidon went and killed the kraken.

(Before someone points it out: Yes I know the Kraken isn’t a Greek myth. I only know that because I looked it up. It turns out that the Kraken is a Norse legend, but nobody knows any Norse gods apart from Zeus and maybe his dad, so we’ll say no more about this…)

Finally, point absorbers. These can either be submerged or floating buoys and can physically move or stand still while waves wash over them. There’s a lot of design variety here, but the common theme is relative simplicity and sometimes appealingly compact profiles, the better for large scale deployment. 

Some float under the water, some float on top of it, some are fixed to piers and let the waves splash underneath them. But all face common engineering challenges, and we’ll start with the first and most obvious.

Waves are variable, and they’re slow. So painfully slow. Slow enough that as a child I’d play race-the-waves on choppy winter days, watching the winter squalls crash with a terrifying seismic THUMP! on the cliff defence walls in Scarborough and running between the splash zones, hoping to cross the giant puddles quickly before the next wave came and soaked me to the bone. I didn’t always make it, but I mostly managed. Because waves are slow.

Electricity isn’t. We need a steady fifty Hertz drumbeat on our national grid, pulsing up & down fifty times a second in perfect, nationwide synchrony, and this is a world away from the steady slosh and pull of the ocean wave, so how the hell do we get from one state to the other?

Let’s talk power take-off systems…

2: Harnessing The Beast & Making It Gallop: Turning Heave into Hertz.

How do we transform the salty kiss of seawater, which is slow, unappetizing and unappealingly wet, and turn it into the organized thrum of electricity? There are many ways, and all try to both increase the frequency of the impulse and stabilize it across the rise & fall of the sea.