The moon is a long way off, and so, for now, is ‘lunar power’- extracted from the gravitationally induced tides. But we are getting there slowly. Indeed it is sometimes thought that, since they have some similarities, tidal stream turbines, using the fast tidal flows in some estuaries, could replicate wind power’s success, with costs falling as capacity built up. However, so far, progress with what might be depicted as ‘underwater wind turbines’ has been slow , in part due to the high cost of the technology at the initial stage, with early estimates putting the likely cost of power at well over £300/MWh, much higher than for wind.
The pioneering work of Prof. Peter Fraenkel and Marine Current Turbines Ltd in the 1990s, with its two-bladed sea bed mounted 1.2MW Seagen in Strangford Lough in Northern Ireland, was however followed up by SIMEC/Atlantis with three-bladed 1.5MW units, four of which were successfully installed in 2017 in Pentland Firth in Scotland, as the first stage of a planned 398MW Meygen project. Then in 2022, MeyGEN plc was awarded Contracts for difference (CfD) in Allocation Round 4 (AR4), for 28 MW to supply the grid, which will be used to support the construction of Phase 2 which is due to be commissioned in 2027.
In 2023, four more Meygen turbines won CfD contracts in AR5, part of an 11 project expansion of tidal stream support, including other projects in Scotland and some in Wales. Most are them are now-standard bottom mounted turbines, but Orbital’s 2MW pontoon mounted floating turbine also won CfD support. In all, AR4 and AR5 backed 94MW of projects for start-ups by 2028, with a £198/MWh contract strike price being offered for those in AR5.
That’s still much higher than offered to solar PV (£47/MWh) and for wind (£52.29/MWh) in AR5, but there are hopes that tidal stream technology development will reduce costs significantly. A 2024 review by ORE catapult looked to a 67.5% reduction, so that a levelised cost of energy of between £50-110MWh might be attained by 2035, at which point around 1GW of capacity might be installed.
In evidence to a Select Committee, the UK Marine Energy Council (MEC) called for ‘a realistic deployment target of 4 GW in the UK by end of the 2030s, supporting the UK’s 2050 Net Zero obligation’. It claimed that ‘costs could be <£90/MWh with the first 1 GW of deployment. In contrast, offshore wind delivered £125/MWh after the first 2.5 GW of deployment.’ It said that tidal stream power technology could enable more than 6 GW from 30 key tidal sites across all regions of the UK, equivalent to about 11% of the UK’s net electricity and, longer term, the UK’s potential tidal stream resources could, it said, support up to 30 GW.
There is certainly a good reason to look to growth for this technology- unlike wind and solar, tidal power is very predictable, and that is true whether it involves using tidal stream turbines, tidal lagoons or tidal barrages. Of course, Tidal ebbs and flows are, at any one point, cyclic, driven by the lunar cycle (modified slightly by the pull of the sun), so the output of lagoons and barrages, and also individual tidal turbines, will vary with daily lunar cycle and also the monthly/spring-neap cycles. But if you have a series of tidal stream turbines, and/or small barrages or lagoons, sited separately around the coast, the timing of the peak and trough outputs will differ for each one ,and, collectively, they will supply more nearly continuous power.
Tidal stream turbines are at present the hot favourite, but, although they may cost more, tidal lagoons also have their attractions - they can be used for pumped energy storage so aiding grid balancing. So can tidal barrages, and it theory they can be very large (multi-GW scale). However, they are likely to be costly to build (even more so than barrages), and since they block estuaries, they can have much larger environmental impacts than tidal turbines or lagoons - and only two medium scale barrages (of around 240MW) have been built so far, one in France and one in South Korea. But there is still support for small to medium scale projects in the UK e.g. on the Mersey. And also for tidal lagoons, e.g. on Swansea Bay.
For the moment though, tidal stream systems, based on the use of accelerated horizontal ebbs and flow of the tides in constrained channel in estuaries or between islands, have the edge on ‘tidal range’ systems (lagoons and barrages ) which extract energy by creating artificial (vertical) heads of water via impoundments or damns.
*Wave energy is sometimes coupled with tidal energy, even if its source is different - the waves are the result of wind moving over the sea. They persist for a while after the wind has died down, so waves are stored wind power. At one time, stimulated by the pioneering work of the late Prof. Stephen Slater, much was expected of wave energy. However, although there are in theory many more suitable sites than for (geographically defined) tidal projects, the open sea wave environment is very chaotic, with multiple energy vectors, making it hard to extract power. Several devices have been destroyed by storms at sea.
By contrast, it’s much easier extract energy from the relatively smooth ebbs and flows of the tides a few metres under the surface. So it’s not surprising that wave systems have not developed as fast as tidal systems - so far none have won CfD support. But there are nevertheless still several wave systems under development in the UK and elsewhere, and, as the technology improves and new ideas emerge, we might be looking to 6GW or more by 2050, according to a 2023 study by Edinburgh University - the same as for tidal streams. We will have to wait to see how it goes. It has certainly attracted a lot of ingenuity over the years – with all manner of floating duck, oscillating column, tethered buoy, hinged flap, and segmented snake designs being tested. Hopefully we’ll get there in the end.
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