Tidal Energy vs Wave Energy

Two resources with different parents

Tidal energy is astronomical. The Moon and, to a lesser extent, the Sun pull unevenly on the Earth and its oceans, raising a bulge of water that the rotating Earth carries beneath. In open ocean the rise and fall is modest; where a coastline funnels it into a strait or channel, the flow becomes a current fast enough to drive a turbine.

Wave energy is meteorological, and second-hand. Wind blowing across open water transfers energy to the surface, and that energy travels as swell — often thousands of kilometres, arriving long after the storm that made it has gone. A wave field is wind energy that the ocean has collected, stored and delivered.

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Tidal streams: a familiar equation in a heavier fluid

A tidal stream turbine is, in engineering terms, a wind turbine in water. The available power through a swept area is the same expression:

P = ½ρAv³

with two substitutions that change the picture entirely. Seawater has a density of about 1,025 kg/m³ against air's 1.225 — roughly 800 times more mass per unit volume. Against that, tidal currents are slow: a strong site runs at a few metres per second, where a wind site is quoted at several times that speed.

Velocity is cubed, so the slower flow costs a great deal — but not 800 times. The density term still wins comfortably, which is why tidal rotors are small relative to wind rotors of comparable output.

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Everything else carries over too. The flow is open, so the same ceiling holds — the Betz limit is not a wind result, it is an open-flow result. Blade count, tip-speed ratio and wake behaviour follow the reasoning set out for wind rotors, with cavitation added as a constraint air never imposes.

A second family works on tidal range rather than tidal streams: a barrage or lagoon impounds water at high tide and releases it through turbines as the tide falls. That is a head-driven machine, closer in principle to micro-hydro than to a wind turbine, and not subject to the Betz ceiling at all.

Waves: power per metre of crest

Wave energy is not quoted per square metre of anything. It is quoted per metre of wave crest, because a wave front delivers energy along its length as it advances.

For waves in deep water, the standard result quoted in ocean engineering references and in IRENA's ocean energy work gives the power per metre of crest as approximately:

P ≈ (ρg²/64π) · H²T

where H is the significant wave height and T the wave period. Put in seawater density and gravity and the constant collapses to a memorable rule of thumb: about 0.5 kW per metre for each H² T, with H in metres and T in seconds.

A moderate sea of 2 m waves at 8 s carries roughly 0.5 × 4 × 8 ≈ 16 kW for every metre of crest. A hundred-metre-wide device in that sea has more than a megawatt passing it — which sounds decisive until you apply the same formula to a storm.

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Because height is squared, a 6 m sea at 10 s carries about 180 kW per metre — more than ten times the moderate case. The resource is not merely variable; its dynamic range is enormous, and that is the engineering problem.

Predictability, and what it is actually worth

This is where the two technologies genuinely diverge, and it is not a small distinction.

Tides are deterministic. The positions of the Moon and Sun are known indefinitely, so tidal currents at a given site can be predicted years ahead to within minutes and a few percent. A tidal plant can tell a grid operator what it will generate next March.

Waves are forecast, not predicted. A few days of skill is normal, and the forecast is probabilistic. That is much better than nothing — it is enough to schedule maintenance and to warn of survival conditions — but it is weather, with weather's uncertainty.

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Tidal output is still periodic: strong flows four times a day around mid-tide, slack water in between, and a spring–neap cycle over a fortnight. The useful property is not steadiness but schedulability — a grid can plan around a generator whose output is known, in a way it cannot around one that is merely likely, and storage such as pumped hydro can be scheduled around its gaps.

Survival, not generation, sets the design

Both technologies are dominated by a constraint that has nothing to do with producing electricity: staying intact, the same reason a wind turbine shuts down in a gale.

The asymmetry is worth stating plainly. A wave device must survive seas carrying an order of magnitude more power than the ones it harvests: a machine built for its worst day and operated on its ordinary ones. A submerged tidal turbine has an easier time — the surface may be in chaos while the flow it works in stays orderly, and the schedule tells crews when they can reach it.

Device families, and what each is betting on

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Tidal stream has largely converged on horizontal-axis turbines on seabed foundations — the same convergence wind went through, for the same reasons. Tidal range uses low-head turbines in a barrage or lagoon, an old technology whose difficulties are environmental and geographic rather than mechanical.

Wave energy has not converged, and the variety reflects a genuine question: what do you react against? A point absorber reacts against its own inertia or the seabed. An oscillating water column uses a trapped air pocket, so the turbine never touches water. An attenuator reacts one floating section against the next. Overtopping devices fill a reservoir above sea level and run a low-head turbine, turning a wave problem into a water-turbine problem.

Each bet answers the same trade: how to be compliant enough to move with ordinary waves and stiff enough to survive extraordinary ones. That tension, more than any efficiency figure, is why marine energy has matured so much more slowly than the technologies on land.