Growing up in Pembrokeshire, I spent quite a bit of time around the coast.
One of my first jobs was clearing tables at The Sloop Inn in Porthgain.
If you’ve ever been there, you’ll know the harbour has a habit of playing tricks on visitors.
You’d look out across the water and see fishing boats gently floating at their moorings.
Come back a few hours later…
The water had vanished.
The boats hadn’t gone anywhere.
They were simply sitting on the harbour floor, leaning against the mud, waiting for the tide to return.
As a kid, I never really questioned it.
That’s just what the harbour did.
Looking back, it’s quite incredible.
Twice a day, billions of tonnes of seawater quietly arrive…
Then quietly leave again.
Entire harbours fill.
Entire harbours empty.
Without a single engine.
Without a single pump.
Without anyone switching anything on.
Every time you’ve visited the seaside, you’ve probably witnessed one of the most predictable energy systems on Earth.
The misconception
Most people think tidal power is simply another version of wave power.
They’re actually very different technologies.
Wave power is driven primarily by the wind.
If the wind stops, eventually the waves disappear.
Tidal power is different.
Its energy comes from the gravitational pull of the Moon, assisted by the Sun.
That means engineers already know almost exactly when tomorrow’s tides will arrive.
Next week’s.
Next month’s.
Even the tides years from now can be predicted with remarkable accuracy.
Nobody has to forecast the Moon.
Where does the energy come from?
Every day, the Moon’s gravity pulls on the Earth’s oceans.
The Earth rotates beneath this enormous gravitational bulge, producing the familiar rise and fall of sea levels we call the tides.
In narrow channels, estuaries and coastal inlets, that moving water accelerates dramatically.
Those locations become ideal places to harvest energy.
Unlike fossil fuels, there is no fuel delivery.
Unlike wind, there are no weather forecasts.
Unlike solar, there are no clouds to worry about.
The Moon simply keeps turning up for work.
Two very different approaches
Modern tidal power generally falls into two categories.
Tidal range
These systems use the changing water level itself.
Large barrages or lagoons trap seawater behind a wall.
As the tide rises and falls, water flows through turbines, generating electricity.
They’re essentially hydroelectric dams built in coastal estuaries.
Tidal stream
Instead of trapping water, tidal stream systems place underwater turbines directly into fast-moving tidal currents.
Think of them as underwater wind turbines.
The water flows past the blades.
The blades rotate.
Electricity is generated.
Water is around 800 times denser than air, so even relatively slow tidal currents contain enormous amounts of energy.
The hidden supply chain
Like wave power, tidal doesn’t really have a fuel supply chain.
The resource arrives naturally.
Instead, the industry depends on:
- Marine civil engineers
- Offshore construction companies
- Turbine manufacturers
- Heavy lifting vessels
- Cable laying ships
- Diving teams
- Environmental scientists
- Grid connection specialists
- Offshore maintenance crews
The challenge isn’t finding the energy.
It’s building equipment capable of surviving underwater for decades.
Insurance and operational risk
From an insurance perspective, tidal power shares many risks with offshore wind.
But it introduces a few unique ones of its own.
Corrosion
Saltwater attacks almost everything.
Steel.
Electrical connectors.
Bearings.
Protective coatings.
Every component must survive constant exposure to one of the harshest environments on Earth.
Marine growth
Nature has a habit of claiming new structures remarkably quickly.
Barnacles.
Mussels.
Seaweed.
Marine growth increases drag, alters performance and makes maintenance significantly more difficult.
Fatigue loading
Unlike wave energy, tidal flow is incredibly predictable.
But it never really stops.
Twice every day, turbines experience changing loads as currents accelerate and slow.
Millions of loading cycles accumulate over their operational life.
Designing for fatigue becomes critical.
Navigation
Many tidal projects occupy busy waterways.
Commercial shipping.
Fishing vessels.
Recreational boats.
Everything has to coexist safely.
Marine navigation risk therefore becomes another important design and insurance consideration.
Why don’t we build more?
This was the obvious question for me.
The fuel is free.
It’s completely predictable.
It never needs importing.
So why isn’t tidal everywhere?
The answer comes back to geography.
Good tidal resources only exist where several factors align:
- Large tidal ranges
- Fast currents
- Suitable seabed conditions
- Grid connections
- Environmental approval
- Commercial viability
Nature decides where tidal power works.
Engineers simply make the best of the opportunities they’re given.
The strategic lesson
Looking back, I probably spent years watching one of the world’s most reliable renewable energy sources without ever thinking about it.
Those fishing boats in Porthgain weren’t just floating up and down.
They were quietly demonstrating one of the most predictable natural cycles on the planet.
Most renewable energy technologies spend their lives reacting to the weather.
Tidal power doesn’t.
It follows something far older.
The slow, relentless pull of the Moon.
Sometimes the most dependable piece of energy infrastructure isn’t built by engineers.
It’s been orbiting above our heads for over four billion years.
Gareth Winterman