Marine energy — tides, waves and salt

Prototypes, steel, cables, corrosion, ecosystems and maintenance at sea.

A machine where everything tries to break it

Two resources, two ways of capturing them

The sea offers two physically distinct energy sources. Tides are extraordinarily predictable — they depend on the position of the Moon and Sun, calculable with precision years in advance — and can be harnessed in two ways: a tidal barrage that impounds water between high and low tide and turbines it on release (as at La Rance, in France, operating since 1966), or tidal stream turbines, submerged devices that work similarly to a wind turbine but driven by water instead of air — water is about 800 times denser than air, allowing much more compact turbines for the same power. Waves, by contrast, are far more variable and unpredictable in the short term, and are attempted to be harnessed with a wide diversity of converter designs, none of which has yet become an industry standard.

Building something that survives the harshest environment in engineering

Any marine energy device must simultaneously withstand corrosion from salt water, marine organisms attaching to any submerged surface, mechanical fatigue from constant water movement, and occasional storms with forces far exceeding normal operating conditions. This requires high-strength steel, specialized concrete, composite materials, seabed anchoring systems, and kilometers of submarine electrical cable to carry the generated electricity to shore. Installing and maintaining these devices requires specialized ships and equipment, often the same used by the offshore infrastructure industry, with considerably higher operating costs than equivalent onshore installations.

During operation: predictable but demanding to maintain

Once installed and running, a marine energy device consumes no fuel, and in the case of tides, generation is so predictable it can be planned with precision years in advance — an advantage no other variable renewable source, such as solar or wind, can offer. The real challenge lies in maintenance: accessing a submerged or partly submerged device to repair it requires ships, divers or specialized robotic equipment, and can be impossible for weeks in adverse sea conditions. Cables and anchors on the seabed can alter local benthic habitat, and noise, electromagnetic fields generated by the cabling, and the turbines' own movement can affect marine wildlife, though the magnitude of that effect depends heavily on the specific technology and installation site.

Why this source remains, for the most part, a testing ground

Unlike solar or onshore wind, already fully commercial and deployed at gigawatt scale worldwide, most marine energy technologies — except tidal-barrage, the oldest and most proven — remain in demonstration phase or early small-scale commercial deployment. That a prototype successfully generates electricity for months or years does not yet prove that specific technology is cost-competitive, reliable long-term against the sea's wear, or manufacturable at scale at a reasonable price — three separate questions, and usually only the first one is already answered when a pilot project is presented as a success.

How to read a marine-energy promise without getting swept up in enthusiasm

Before judging any marine energy project as an already-mature solution, it is worth asking: how many real operating hours has the device logged, not just how long has it been installed?, has it already survived a season of strong storms?, how much net energy has it delivered, after subtracting the consumption of maintenance vessels?, what real availability does it have versus downtime for breakdown or bad weather?, what environmental permits has it passed?, and is there already a decommissioning plan for end of life? Marine energy may come to occupy valuable niches where the resource is especially good and predictable, but presenting it as an infinite ocean of energy, without mentioning the finite and fragile machines needed to capture it, is exactly the kind of oversimplification this platform wants to avoid.

Still a niche, not a general solution

In favor: tides are the most predictable variable renewable energy source that exists, and the world's wave-energy resource is, in theory, enormous.

To watch: installation and maintenance cost far higher than equivalent wind or solar, technological maturity still low except for tidal-barrage, and a track record of pilot projects that have not always survived their first big storm.

The indicator that best separates a mature technology from a promise: real hours of continuous operation under normal sea conditions, not just the existence of a prototype that worked during a controlled test.

Where it makes sense, and where it does not: coastlines with very strong, predictable tidal currents, or with especially energetic and constant wave action, may justify the extra investment this technology demands. On most of the world's coastlines, offshore wind — already commercially mature — remains, today, the more sensible offshore option.

The numbers, with their source

Almost sixty years of history... and still small-scale

The La Rance tidal barrage, on the coast of Brittany (France), has been operating since 1966 — the world's first commercial-scale tidal project — with a capacity of 240 MW, and for 45 years was the largest facility of its kind on the planet, until South Korea's Sihwa Lake plant (254 MW) overtook it in 2011. That the oldest, most proven technology in this whole leaf has been running for nearly sixty years, and the world's two largest tidal-barrage plants together still amount to barely half a gigawatt of installed capacity, gives a sense of how limited actual marine-energy deployment remains compared with wind or solar, whose global capacity is measured in hundreds of gigawatts.

Source: Wikipedia — Rance Tidal Power Station.

A tidal-stream project, step by step

The MeyGen project, in the Pentland Firth strait (northern Scotland), is one of the most advanced tidal-stream turbine deployments in the world: it reached its first phase of 6 MW (four 1.5 MW turbines) submerged on the seabed, with an expansion plan that could reach up to 398 MW in future phases if the first ones prove reliable and profitable long-term. The scale difference between this first phase and the final plan — more than 60 times larger — illustrates well how this technology is actually developed: in small, verifiable phases, not with large deployments from day one.

Source: MeyGen (Wikipedia); MeyGen Tidal Energy Project (Tethys).

What this table is missing

There are not yet life-cycle emissions, mortality or land-use figures for marine energy as well-established and comparable across studies as those existing for solar, wind or hydropower — precisely because the number of large-scale operating installations, with enough years of track record, remains very small. That absence of mature data is itself relevant information about the real state of the technology.

Sources