Wave and Tidal
The tide is the only renewable that is perfectly predictable, decades in advance, because it is driven by the moon. And after fifty years of trying, it still barely works.
The effect compounds within years. Put it in place and it keeps working.
Origins
Tide mills are medieval. The Domesday Book records them. You trap the incoming tide behind a gate, let it out through a wheel as it falls, and grind your grain twice a day, on a schedule set by the moon. There is a working tide mill in Woodbridge in Suffolk on a site that has had one since 1170.
So the idea is not new, and the resource is not small. The oceans contain a staggering amount of energy: waves carry the accumulated force of wind across thousands of kilometres of open water, and tides move the entire ocean twice a day under the gravitational pull of the moon.
And for fifty years, engineers have been trying to build machines to capture it, and the sea has been breaking them.
This is the honest story of wave and tidal power, and it deserves to be told plainly. It is not a story of suppression by oil companies, or of a technology waiting for its moment. It is a story of an extraordinarily hostile environment. Salt water corrodes everything. Storms deliver forces orders of magnitude beyond the average conditions a machine is designed for. Marine growth fouls every surface. Maintenance requires a boat, a weather window and a great deal of money. And the ocean does not stop.
Dozens of promising companies have gone bankrupt. Prototypes have sunk. It has been called the graveyard of good intentions, and the description is fair.
What it actually is
Two quite different technologies get bundled together, and they should not be.
Tidal power is the more promising, and its great virtue is predictability. The tide is driven by the moon and the sun. It can be forecast, precisely, for decades in advance. No other renewable can say this: the wind is a guess, the sun is a probability, the tide is an appointment. A grid operator can know exactly what a tidal array will produce on a specific afternoon in 2043.
Tidal stream turbines — essentially underwater wind turbines sitting in a fast current — are the most credible approach, and they are beginning to work. Water is roughly 800 times denser than air, so a slow current carries a great deal of energy, and the machines can be far smaller than wind turbines for the same output. Arrays in Scottish waters have now run for years and delivered real electricity to the grid.
Wave power is harder, and it is harder for a specific and brutal reason: the ratio between average and extreme conditions. A machine must be economic in ordinary swells and must survive a once-in-fifty-year storm delivering many times the force. Build for the storm and it is too expensive. Build for the average and it is destroyed. This dilemma has killed most of the industry and it has not been solved.
Meanwhile offshore wind, sitting in the same water, got cheap. That is the competition, and it is winning.
The numbers
The predictability, which is unique. Tides are driven by celestial mechanics and can be forecast decades ahead with near-perfect accuracy. No other renewable resource has this property, and in a grid struggling with intermittency it is genuinely valuable.
The density. Water is roughly 800 times denser than air. A tidal turbine in a slow current can produce as much as a far larger wind turbine, in a much more compact machine.
The cost, honestly. Ocean energy remains among the most expensive renewables, far above wind and solar, and it lacks the manufacturing scale that drove those costs down. The learning curve has not begun in earnest because there is no volume.
The graveyard. The list of wave energy companies that raised significant capital, deployed a prototype and failed is long, and it includes genuinely good engineering. This is not a slur. It is the record, and any honest assessment has to start from it.
The survivability ratio. This is the number that kills wave power. A device must be economic in average conditions and survive extremes many times larger. No other energy technology faces a gap of that magnitude between its design case and its survival case.
And where tidal is actually working. Tidal stream arrays in Scottish waters have now operated for years and delivered power to the grid. It is a small industry. It is no longer a hypothetical one.
Why it matters
The moon pulls the ocean twice a day, and it has done so for four and a half billion years, and it will keep doing it long after we are gone.
There is something in that which is hard to let go of, and it explains why engineers keep coming back despite the wreckage. Every other energy source is a story about extraction or interception — digging something up, or catching something falling. Tidal power is a story about orbital mechanics. You are taking a small share of the angular momentum of the Earth-Moon system. The energy in your kettle came from the moon.
The tide has come in on the same shore for the whole of human history. Your ancestors watched it, and set their lives by it, and built mills on it eight hundred years ago, and it will come in tomorrow at a time we can already state to the minute.
We include this page fully aware that it may not work. Wave power in particular has broken a generation of good engineers and a great deal of good money, and offshore wind ate the opportunity while it was struggling. It is entirely possible that the honest verdict on wave energy is that it was a beautiful idea the sea would not permit.
But the tide is still there. Perfectly predictable. Twice a day. Forever. If we ever learn to take a little of it without being smashed, it will be the most reliable energy any civilisation has ever had.
What it actually takes
Surviving the sea, which is the entire problem. Not generating in the sea — that is straightforward. Surviving it, for twenty-five years, with acceptable maintenance costs, in salt water, through storms, while marine growth colonises every surface. Every failure in this sector traces back to this one thing.
Volume, which does not exist. Solar and wind got cheap through manufacturing scale. Ocean energy has no volume, therefore no learning curve, therefore no cost decline, therefore no volume. It is a genuine chicken-and-egg trap, and only sustained public investment breaks it.
Being honest that offshore wind won. Offshore wind occupies the same water, uses a mature supply chain, and is far cheaper. Ocean energy must find a niche that offshore wind cannot serve — deep water, remote islands, perfectly predictable output for grid balancing — or it does not have a case.
Separating tidal from wave. They are conflated constantly and they should not be. Tidal stream is working, modestly, now. Wave energy is not. Bundling them lets the failures of one poison the prospects of the other.
Ecological caution, before scaling rather than after. Turbines in tidal races sit in exactly the fast, nutrient-rich channels where marine life concentrates — marine mammals, seabirds and fish use those places because the current is there. The impacts are not yet well understood, and the industry should want to find out first.
Where it matters most
Scotland is the world capital of tidal stream. The Pentland Firth is one of the strongest tidal races on Earth, and the arrays there have moved the technology from hypothesis to operating record.
The Bay of Fundy in Canada has the largest tidal range in the world — some sixteen metres — and has consumed a series of turbines that the current simply destroyed. It is the most powerful site available and the most punishing.
Northern France has the Rance barrage, generating since 1966, by far the longest-running tidal power station in the world. It also permanently altered the estuary it sits in, which is the trade a barrage always makes.
The North Sea and the Atlantic coasts of Portugal and Ireland have among the best wave resources on Earth — and are also where the storms are, which is the whole problem in a single sentence.
Remote island communities everywhere are the most plausible near-term market: very high electricity costs, diesel generators, and a great deal of ocean.
How to tell it’s being done well
Is it tidal or wave? Tidal stream is beginning to work and has an operating record. Wave energy does not. Conflating them is the most common error in this field and it flatters the weaker one.
Has it survived a winter? The sea is the test and the test takes years. A prototype that generated through a calm summer has not been tested at all.
What is the maintenance plan? Everything in salt water needs servicing, and servicing needs a boat and a weather window. A device that cannot be maintained affordably will fail economically even if it survives physically.
What is in that channel? Tidal races concentrate marine life precisely because the current does. Understanding that before scaling is the difference between a good technology and a new problem.
What you can do
Anyone
- The tide is the only renewable resource that is perfectly predictable decades in advance, because it is driven by the moon rather than the weather. That is a genuinely remarkable property.
- Wave energy has broken a generation of good engineers and a great deal of good money. That is the honest record and it should be known before anyone invests more.
Island and coastal communities
- Remote islands running on diesel are the most plausible near-term market for ocean energy: high electricity costs, and a great deal of ocean.
- Ask what is in the channel. Tidal races concentrate marine mammals, seabirds and fish precisely because the current is there.
Policymakers
- Fund tidal stream, which is working, separately from wave, which is not. Bundling them wastes money and obscures a real result.
- Ocean energy has no volume, therefore no learning curve, therefore no cost decline. Only sustained public investment breaks that trap, and it may not be worth breaking.
- Require ecological assessment of tidal races before scaling, not after.
Business and investors
- Offshore wind occupies the same water, has a mature supply chain and is far cheaper. Ocean energy needs a niche offshore wind cannot serve, or it does not have a business.
- Predictable output has genuine value to a grid operator, and it is the one thing tidal offers that nothing else does.
Who is working on this
We are researching which organizations in our directory of 8,493 actively work on this solution, and we only list an organization once we have verified it. That research is ongoing. In the meantime, search the directory yourself:
Questions
Why is tidal power interesting if it barely works?
Because it is perfectly predictable. The tide is driven by the moon and the sun and can be forecast decades ahead with near-perfect accuracy. No other renewable can do that: the wind is a guess and the sun is a probability, but the tide is an appointment. In a grid struggling with intermittency, that is genuinely valuable.
Why has ocean energy struggled for fifty years?
Because the sea destroys machines. Salt water corrodes everything, marine growth fouls every surface, maintenance requires a boat and a weather window, and storms deliver forces orders of magnitude beyond ordinary conditions. It is not suppression or bad luck. It is an extraordinarily hostile environment, and dozens of good companies have gone bankrupt learning that.
What is the difference between wave and tidal?
They are conflated constantly and should not be. Tidal stream turbines, essentially underwater wind turbines in a fast current, are beginning to work and have a real operating record in Scottish waters. Wave energy is much harder and has not been cracked.
Why is wave power so difficult?
Because of the ratio between average and extreme conditions. A wave machine must be economic in ordinary swells and must survive a once-in-fifty-year storm that delivers many times the force. Build for the storm and it is too expensive; build for the average and it is destroyed. No other energy technology faces a gap of that magnitude, and it has not been solved.
Why not just build offshore wind instead?
Largely, that is what happened. Offshore wind sits in the same water, has a mature supply chain, and got dramatically cheaper while ocean energy was still failing. Ocean energy now needs a niche that offshore wind cannot serve, such as deep water, remote islands, or perfectly predictable output for grid balancing.
Does tidal power harm marine life?
Possibly, and it is not yet well understood. Tidal races concentrate marine mammals, seabirds and fish precisely because the fast current brings nutrients. Putting turbines in exactly those channels needs proper ecological assessment before the industry scales, not after.
Sources
- Project Drawdown - Ocean Power (Drawdown Explorer) Framework and classification. Cited, not reproduced.
- IRENA (2025) - Renewable Power Generation Costs in 2024
- IEA-OES - Ocean Energy Systems annual report
- European Marine Energy Centre (EMEC), Orkney
- IPCC (2022), AR6 Working Group III - Energy Systems
The solution taxonomy follows the framework popularised by Project Drawdown. The analysis above is our own; for their carbon modeling and rankings, visit them directly.