In 2021, engineers floated a 74-metre platform off Scotland with 2 underwater turbines, and it's been making power since July

Off the coast of Orkney, in northern Scotland, sits a giant orange platform that is floating on the waves. Underneath it are two huge turbines, turned by the tides. This is the Orbital O2, a floating tidal platform with two turbines and one of the world’s most powerful tidal-energy systems. According to Orbital Marine Power, the O2 has been generating grid-connected power at its Fall of Warness site in Orkney since July 2021, after setting off from Dundee that April.

What makes this floating machine so unusual

The O2 is not attached to the sea floor. It just floats on the water, supported by the use of heavy mooring lines. Two long legs extend from either side of the structure, each with a turbine. Orbital Marine Power states that each turbine spins 10-metre blades that sweep out a 20-metre circle underwater. Each of the two turbines generates 1 megawatt of electricity, for a combined rated capacity of 2 megawatts, enough for about 2,000 UK homes, according to EMEC. The whole machine weighs 680 tonnes, roughly the weight of five blue whales, yet it still floats. The floating design has a big advantage: engineers can lift the legs clear of the water to service the turbines without sending divers into fast, cold currents, which also simplified getting the turbines attached in the first place. That makes repairs a lot easier and cheaper. EMEC, the European Marine Energy Center, said the O2 had to be able to cope with huge forces in rough seas and that each mooring line was strong enough to lift 50 double-decker buses.


Why tides give more reliable power than wind or sun

Solar power is limited to daytime hours only. Wind energy generation is dependent on steady winds. The tide-based system does not share the same limitation because tides occur in regular cycles, controlled by the moon. The study, ‘Power variability of tidal-stream energy and implications for electricity supply,’ published in the journal Energy, measured power output from a 1MW tidal turbine at this same Orkney site and found its variability remained low through each tidal cycle. The study says the predictability of tidal power may be underappreciated in future power grids.

The Orkneys provide an ideal environment to test this idea. The water flows through the channels near the islands at rates that can exceed 3 meters per second, according to EMEC. It is because of the strong and consistent flow of water that the EMEC was set up in this location more than fifteen years ago, and why many tidal energy companies have since brought their machines there for testing.

Clean power with a small footprint

Orbital Marine Power notes that the O2 should help avoid around 2,200 tons of carbon dioxide each year, a figure close to the amount of carbon dioxide produced by a few hundred cars. The turbine was constructed in Dundee, Scotland, and EMEC said about 75% of the parts were supplied by UK firms. The O2 also powers EMEC's onshore electrolyser, generating green hydrogen as part of a demonstration of tidal power's potential for clean-fuel production.

The bigger picture for tidal power

Tidal energy remains small compared to wind and solar energy, both of which had a bigger head start and far more investment. However, more interest is developing for it. A 2021 Proceedings of the Royal Society A study, 'A review of the UK and British Channel Islands practical tidal stream energy resource,' has found that further additions of tidal stream energy capacity in the UK in the coming years may lead to a significant decrease in the cost of tidal energy to make it competitive with gas-based energy production. The same study finds no confirmed collisions of tidal turbines with marine animals so far, only small, measurable changes to the seabed nearby, though the authors note that detecting a collision event at all in a fast-flowing, low-visibility channel like the Fall of Warness is inherently difficult, so this is better read as an early reassuring sign than definitive proof of no impact.

What comes next

Orbital Marine Power is not stopping at one turbine. The company is also leading EURO-TIDES, a project backed by the EU to develop a four-turbine, 9.6-megawatt tidal array at the same Orkney site by 2027. Orbital also has plans for a new design, the O2-X, with several units due in Orkney later this decade. For now, the O2 shows something simple. The rise and fall of the tide is the ocean's oldest rhythm and it can already power thousands of homes.