Tidal and Wave Energy: Harnessing the Power of the Ocean

The world's oceans cover approximately seventy-one percent of the earth's surface and contain an enormous store of kinetic and potential energy in the form of tides, waves, currents, and thermal gradients. This ocean energy — driven by the gravitational pull of the moon and sun (for tides), by wind transferring energy to the water surface (for waves), and by solar heating of surface waters (for thermal gradients) — represents one of the most underexploited renewable energy resources on the planet. Theoretical estimates of the global technically exploitable ocean energy potential are measured in thousands of terawatt-hours per year — many times current global electricity consumption — yet as of the mid-2020s, ocean energy contributes less than one percent of global electricity generation.

The slow development of ocean energy relative to wind and solar reflects the extraordinary engineering challenges of deploying, operating, and maintaining machinery in the marine environment. The ocean is perhaps the most hostile environment on earth for mechanical and electrical equipment: corrosive seawater and biological fouling attack materials and surfaces; waves and storm surges impose dynamic loads far beyond those experienced by any land-based structure; cables and foundations must be designed for decades of service on the seabed; and access for maintenance is difficult and expensive, especially in the powerful tidal streams and exposed Atlantic swell conditions where the best resources are found. The machines that will commercially exploit ocean energy must be robust enough to survive conditions that have sunk ships and destroyed structures, while remaining economically competitive with wind and solar power whose costs have fallen dramatically in recent decades.

Despite these challenges, tidal energy — particularly tidal barrage technology at exceptional tidal range sites — has demonstrated commercial viability since the 1960s. The La Rance tidal barrage in Brittany, France, which has generated electricity continuously since 1966, remains the world's most enduring demonstration of tidal power and has produced more electricity from the ocean than any other facility in history. A new generation of tidal stream turbines — underwater devices analogous to wind turbines but driven by tidal currents rather than wind — are demonstrating commercial viability at sites with strong tidal flows in Scotland, Canada, France, and elsewhere.

Wave energy, despite decades of research and hundreds of device concepts, remains in the pre-commercial development stage, with no technology yet proven at the scale and economics needed for widespread deployment. The history of wave energy is strewn with promising devices that failed structural tests, proved more expensive than projected, or were abandoned when funding ran out — yet the fundamental physics of wave energy extraction is well understood, and the resource is enormous. The technological and economic challenges of wave energy are real but not necessarily insurmountable.

Ocean thermal energy conversion (OTEC) — using the temperature difference between warm tropical surface waters and cold deep waters to drive a heat engine — and osmotic power (using the salinity difference between fresh and salt water) represent additional forms of ocean energy that have been demonstrated but not yet commercialized.

The Physics of Tides: Gravitational Forces and Tidal Ranges

The tides — the twice-daily (in most locations) rise and fall of sea level — are driven by the gravitational attraction of the moon and, to a lesser extent, the sun on the earth's oceans. The moon's gravitational pull creates two bulges in the ocean — one on the side of the earth facing the moon, and one on the opposite side — which the rotating earth sweeps through, producing two high tides per day. The sun's gravitational effect on tides is approximately forty-six percent as strong as the moon's.