TL;DR:

  • The UK holds roughly 50% of Europe’s tidal stream resource, concentrated in areas like the Pentland Firth, Orkney, and the Channel Islands
  • Unlike wind and solar, tidal energy is precisely predictable — tides follow lunar cycles known decades in advance
  • Commercial-scale tidal projects are finally moving forward after years of engineering and cost challenges, with Contracts for Difference support unlocking financing

When people think about the UK’s renewable energy future, they picture offshore wind farms stretching across the North Sea. But beneath the surface — sometimes literally — a different energy source is beginning to emerge: tidal stream, which harnesses the kinetic energy of moving water as tides flow in and out.

The UK has one of the best tidal stream resources in the world. Strong tidal flows through narrow channels and around headlands create conditions that are genuinely exceptional, particularly in the Pentland Firth between mainland Scotland and Orkney, the Alderney Race in the Channel Islands, and stretches of the Welsh coast. The Crown Estate estimates the UK’s technically extractable tidal stream resource at around 34 GW — enough to power millions of homes if even a fraction were developed.

What Tidal Stream Actually Is

Tidal stream turbines look something like underwater wind turbines — rotor blades mounted on a nacelle, anchored to the seabed. As tidal currents flow past, the rotors spin and generate electricity. The key difference from tidal barrages (the old approach, typified by the La Rance facility in Brittany) is that tidal stream doesn’t require a dam or barrage. The turbines sit in the water column and exploit the natural flow without blocking it.

The physics are straightforward. Water is about 800 times denser than air, which means a tidal turbine can generate the same power as a much larger wind turbine. A 1 MW tidal turbine has blades roughly 20 metres in diameter; a 1 MW wind turbine typically needs blades over 50 metres long.

The Predictability Advantage

This is where tidal stream differs from every other renewable energy source except hydropower: the output is almost completely predictable.

Tides are driven by gravitational forces from the Moon and Sun. These forces follow well-understood cycles — the lunar month, the spring-neap cycle, the seasonal variation in the Sun’s influence. Tidal predictions for any location can be calculated years in advance to within minutes. A tidal stream turbine array’s output profile for 2040 can be estimated today with high confidence.

This matters enormously for grid balancing. Wind and solar require backup capacity or storage because their output is variable and only partially forecastable. Tidal energy’s precise predictability means it can be integrated into the grid as near-firm power, scheduling output in advance and complementing the intermittent renewables that dominate the energy mix.

In the UK, the Pentland Firth is particularly attractive because tidal flows in the area are roughly out of phase with tidal flows in the Irish Sea and around Wales. Arrays in different locations can be operated to provide a smoother combined output than any single site would deliver.

MeyGen: The World’s Largest Tidal Stream Array

The MeyGen project in the Pentland Firth is the most advanced commercial tidal stream development in the world. Operated by Simec Atlantis Energy, MeyGen has four turbines installed in the Inner Sound channel, with a combined capacity of around 6 MW. The project has been operating since 2016 and has generated substantial amounts of electricity while demonstrating that tidal turbines can survive the punishing conditions of a high-energy tidal site — strong currents, cold water, salt, and continuous mechanical stress.

The long-term plan for MeyGen is to expand to around 400 MW, which would make it a genuinely significant power source. Phase 1C, which would add further turbines, has secured planning permission. The key enabler is Contracts for Difference support, which the UK government has included in recent CfD allocation rounds specifically to support tidal stream.

The inclusion of tidal stream in CfD allocation round 6 (AR6) marked a turning point. Previous rounds excluded tidal stream on cost grounds, meaning developers had to fund projects without the long-term revenue certainty that offshore wind developers rely on. With CfD support available — providing a guaranteed strike price for electricity generated — the economics of tidal stream projects become financeable.

Orbital Marine Power and the O2

While MeyGen uses seabed-mounted turbines, Edinburgh-based Orbital Marine Power has taken a different approach: a floating tidal stream platform. The Orbital O2, deployed in the waters off Orkney, is a 2 MW floating structure with two 1 MW turbines mounted on arms that hang into the water. The O2 has been operating at the European Marine Energy Centre (EMEC) in Orkney, which provides world-class test conditions and grid connection for marine energy devices.

The floating approach has practical advantages. Installation and maintenance are easier than for seabed-mounted turbines — the platform can be towed to port for servicing rather than requiring specialist marine operations in fast-flowing tidal streams. It also makes the technology deployable in deeper water and at sites where seabed conditions make fixed foundations difficult.

The Cost Challenge

Tidal stream hasn’t scaled faster for a real reason: it’s expensive. Offshore marine operations in high-tidal environments are technically demanding. Early projects faced sobering cost overruns. Current costs for tidal stream electricity are broadly in the range of £150–£300 per MWh, compared to around £50–£70/MWh for offshore wind.

The path to cost reduction mirrors offshore wind’s trajectory. Offshore wind in the early 2010s cost over £150/MWh and was dismissed by many as perpetually uncompetitive. It fell to below £50/MWh through a combination of learning by doing, supply chain development, larger turbines, and standardised installation techniques. Tidal stream developers argue the same process will apply — but the starting installed capacity is tiny compared to where offshore wind was when its cost reduction curve really accelerated.

Government support via CfD, plus the Marine Energy Programme funding through Innovate UK and the Crown Estate’s commitment to marine energy leasing, is intended to provide the development runway to drive costs down.

What Role Could Tidal Play?

Realistically, tidal stream will not be a major contributor to total UK electricity generation in the 2020s — the installed capacity is too small. But the longer-term potential is genuine. National Grid scenarios for net zero increasingly include tidal stream as a source of predictable, low-carbon generation that complements the variability of offshore wind and solar.

For the specific communities around tidal sites — Orkney and Caithness in particular — tidal stream has a more immediate local significance. The presence of EMEC in Orkney has made the islands a global hub for marine energy testing, attracting research, jobs, and expertise that wouldn’t otherwise be there.

If you want to follow the sector’s development, EMEC and the Offshore Renewable Energy (ORE) Catapult publish regular updates on marine energy progress. The story of tidal stream energy is moving — slowly, but unmistakably — from demonstration to deployment.