By Dave Elliott
IRENA, the International Renewable Energy Agency, puts the technically available global tidal resource at near 1 TW. That is for all types of tidal system, those using vertical tidal ranges (barrages and lagoons) and those based on tidal streams, using the horizontal ebbs and flows (tidal current turbines). In practice, local limitations, access problems and other constraints will limit what may actually be achievable. So how much might be available?
Large barrages are seen as expensive to build and likely to have major environmental impacts. But although large barrages are less favoured these days, there is over 500 MW of mostly barrage capacity in place globally, and some new tidal range projects are planned or proposed, for example several new barrages (around 3 GW in all) in South Korea and a 250 GW lagoon in the UK (across Swansea Bay), with maybe more to come, including a lagoon off Cardiff (maybe 2.5 GW). Tidal lagoons should have much less environmental impact than cross-estuary barrages, but the cost of energy may be higher. The Swansea project is claimed to need around £168/MWh (€233), more than for offshore wind, although larger projects would be cheaper, comparable to on-shore wind.
It has been estimated that, overall, tidal range systems might supply 15% of UK electricity. Similar contributions might be made elsewhere. About 60 GW’s worth of potential medium-to-large tidal barrage sites have been identified by WEC globally, as well as up to 100 GW of very large sites in Siberia. With some large very speculative projects like this included, IRENA says there are over 100 GW of tidal range projects under consideration around the world. But it is not likely to be big in the near term.
By contrast, there are many tidal current turbine designs under test around the world, and one in semi-commercial operation (MCT’s SeaGen) in the UK. However, costs are still relatively high and only around 200 MW is expected to be in place globally by 2020. Nevertheless, IRENA says the long-term total global resource for tidal stream energy is much larger than for tidal range, although the full global resource hasn’t been looked at yet in detail outside Europe, where the resource has been estimated as at least 12 GW. But speculatively, Alstom has talked in terms of 100 GW globally. That may be an underestimate, but it depends on the cost.
IRENA suggests that, on the basis on some of the European studies, by 2020 tidal projects might be generating at between €0.17/kWh and €0.23/kWh although, it says, current demonstration projects suggested the levelised cost of energy (LCOE) would be in the range €0.25-0.47/kWh. The UK government’s Contracts for Difference subsidy system offers marine renewables like tidal stream projects a strike price of £305/MWh (€0.42/kWh or $0.44/kWh) and a protected allocation of contracts for up to 100 MW has been ring-fenced up to 2019. That should encourage new projects to come forward and help prices to fall. Even so it will be a while before costs come down.
One thing that might give tidal power an edge is if it could supply continuous output. Tidal barrages, lagoons and tidal current turbines all have different characteristics, but share the same basic problem – reliance on a variable energy source. However, unlike wind and wave energy flows, tidal flows are not erratic, solar/weather driven. There may be some small interactions between the winds and waves and the tidal flows (sometimes positive ones), but the tides are fundamentally lunar driven. Nevertheless, although the energy availability in tides is therefore highly predictable, it does vary. This is a big problem for large barrages: they deliver their outputs in large bursts, on a lunar cycle, not often in phase with demand.
A series of smaller barrages and/or lagoons, if geographically dispersed so as to take advantage of the delay in high tides at different locations, could limit this problem, but that option would arguably be much easier to develop with a network of many small modular tidal current turbines around the coast. For the UK, around 200 MW might provide a continuous supply of about 45 MW according to a study by Hardisty. Certainly having a large 8 GW barrage, like the Severn scheme, on the network, with large bursts of power, would make it harder to balance the overall system. The same would, of course, be true for major localised concentrations of tidal current turbines, for example, as might be the case in the Pentland Firth off Scotland, where the resource is very large. All other things being equal, for effective balancing, a distributed network of similar-sized units would be best.
It may be easiest to do this just with tidal current turbines, suitably distributed, but as Yates et al have indicated, it might also be possible to have a mixed system, with some small barrages and lagoons as well as tidal current turbines optimised to offer the best overall output and balancing mix. Extending this further, the potential for balancing would increase if tidal projects across the EU were also included in the network, with Simon Waldman from Heriot-Watt University, UK, suggesting that ‘at spring tides, the full EU resource can provide a continuous output of approximately 40% of the maximum output, while UK sites alone can only provide a continuous output of approximately 27% of their maximum’. So a large proportion of the maximum possible output from the total capacity on the network would be available all the time, making it possible to play a role alongside other firm energy sources in meeting demand peaks and supply shortfalls.
Clearly there would be a need to analyse the best mix and in reality this system cannot be optimized in isolation from the wider grid system (e.g. in the UK, and in the EU) as a whole, which will have other variable inputs, for example from wind and solar, as well, possibly, as storage capacity. That adds yet another dimension. In principle, tidal barrages and lagoons could operate as pumped storage systems, using surplus grid electricity from wind or solar, much as with hydro pumped reservoirs. Segmented barrages and lagoons with multiple basins could enhance this function, and also, by judicious phasing of filling and emptying, allow individual tidal range systems to deliver more nearly continuous output: see the helpful Korean animation at https://www.youtube.com/watch?v=lnHwb8BKJzU Moreover, with regionally distributed multi-basin tidal range pumped-storage systems included in a national tidal network, the potential for more continuous output availability from the full system should increase even further (2).
Given these potential developments, it may be that barrages will become more popular, although they still have to face the environmental impact issues. Smaller barrages may have less problems of this kind and they may also be less of a problem with lagoons and certainly less in the case of tidal current turbines, which involve only marginal interference with water flows. However, if sites are carefully chosen, it may be possible, and useful, to come up with an appropriate mix including smaller barrages.
- This is edited from parts of my much longer paper ‘Tidal power- still moving ahead’ in a Energy Technology overview to be published by Nova. For an interesting critical view on the prospects for lagoons, including multiple site balancing options, see: http://euanmearns.com/a-trip-round-swansea-bay/