June 26, 2026 ยท Tags: energy, physics, ocean, tidal power, renewables
Water is 800 times denser than air. That fact is the reason tidal power works, the reason the machines look nothing like wind turbines, and the reason the economics remain so difficult.
The Cubic Law That Makes Tides Interesting #
Power in a moving fluid follows a simple cubic relationship: double the velocity, and you get eight times the energy. The formula is the same one that governs wind turbines (P = 0.5 x density x area x velocity-cubed), but seawater's density changes the scale. A 2 meter-per-second tidal current carries roughly the same energy density as a 56 m/s wind. Tidal turbines are smaller, spin slower, and produce power in conditions that would barely register on an anemometer.
The physics also sets hard limits. The Betz limit caps any open-flow turbine at about 59% efficiency. Channel geometry tightens things further. Modeling of the Pentland Firth, the strait between mainland Scotland and Orkney, shows that extracting the full theoretical resource would reduce water flow by 60%, with catastrophic effects on sediment transport and marine habitats. In practice, developers constrain flow changes to around 10% to stay within environmental boundaries.
What the Machines Actually Look Like #
The dominant design is the horizontal-axis turbine, essentially an underwater wind turbine bolted to the seabed. MeyGen's Phase 1 in the Pentland Firth runs four of these at 1.5 MW each, and has produced over 70 GWh since 2018. Orbital Marine Power takes a different approach. Their O2 is a floating platform with twin rotors that hang beneath it on retractable legs. When a storm hits or maintenance is needed, the whole thing tows itself to port. Their next machine, the O2-X, stretches the blades to 13 meters, the longest in the industry, which increases swept area by 70% to over 1,000 square meters.
Then there's Minesto, whose Dragon 12 in the Faroe Islands looks nothing like a turbine. It's a kite-shaped device that "flies" through the water in figure-eight patterns, sweeping a much larger area than a fixed rotor could. It reached 1.2 MW and is heading toward a 10 MW array.
China's LHD Zhoushan station has been running for seven years, and their Endeavour turbine has been in continuous grid-connected operation for over 30 months, producing 4.5 GWh.
Why It Costs So Much #
Current levelized cost for tidal energy sits around $360/MWh. For context, offshore wind is in the $50-100 range, and solar is cheaper still. The cost gap comes from three places: manufacturing turbines that survive saltwater corrosion and extreme loading, installing heavy machinery on the seabed in fast-moving water, and maintaining equipment that you can only access during slack tides.
Projections put the cost at $170/MWh by 2050, but that depends on learning rates holding at 15%. Drop the learning rate to 10%, and the subsidy investment required quadruples. The UK's Contracts for Difference scheme has awarded 121 MW of tidal capacity by 2029, with dedicated budget rounds. Whether that spending actually drives costs down depends on whether the industry can standardize manufacturing and build out a supply chain that doesn't yet exist.
The One Thing Tides Do Better Than Everything Else #
Tidal cycles are predictable decades in advance. You can look up the exact time and height of every high tide at any port on Earth for the year 2050, and those numbers will be accurate to within minutes. No weather forecast, no cloud cover model, no wind variability. That predictability has real grid value, even if the energy is expensive per megawatt-hour.
Europe has a 165 MW pipeline of publicly funded ocean power projects planned over the next five years, with 152 MW in tidal stream alone. Orbital is building six next-gen turbines for Orkney between 2026 and 2028. The Bay of Fundy, which has the highest tides on the planet, is getting its first floating tidal array through a Canadian partnership. The industry is no longer hypothetical. It's just not cheap.
Why This Matters #
The physics of tidal power is elegant and punishing. Water's density makes small turbines productive, but that same density breaks anything that fails. The resource is finite and geographically concentrated in narrow channels and straits, which means there's a hard ceiling on global capacity. Within those constraints, though, tidal energy offers something no other renewable can: perfect, long-term predictability. Whether the economics catch up is the open question.