Tidal power is the most reliable renewable energy source we have, because it is driven by the predictable gravitational forces of the moon, the sun and the planets.
This reliability makes it even more surprising that tidal power contributes only a few megawatts of generation capacity in the UK.
Here, we assess why the industry hasn’t yet got off the ground....
The Severn Barrage
The Severn Estuary has one of the largest tidal ranges in the world, up to 14m, capable of generating 13.5 GW, which is equivalent to about 10 nuclear power stations.The idea of generating electricity from a tidal barrage across it has been around since 1925, when it was prevented from going ahead because of cost. Fast forward 87 years, and we are pretty much in the same position.
Barrages only work in very few geographically suitable locations, but currently exist in France, Canada and South Korea. La Rance barrage in Brittany, France has been generating 600 MWh per year since 1966.
The reason that the Severn barrage project has been on the table for so long is that it has the potential to generate up to 11% (though current plans are for ~5%) of the UK’s current electricity usage of 360 TWh per year.
However it’s cost is it’s biggest impediment. It would be a huge project, larger in scope than all the currently operating barrages. Cost wise, it is like committing to building more than 10 nuclear power stations all at once.
Due to cost, construction risk, and the alteration of a natural habitat DECC’s 2010 feasibility study ruled out the barrage for the foreseeable future - though the government isreportedly considering the option again.
Tidal Stream
If you take a tidal barrage, then remove all of the structure except the turbines, then you are left with what is termed a tidal stream generator. These harness the same tidal flows as a barrage, but have the advantage that they don’t require the complete blockage of a channel or estuary.
The amount of electricity that could be harnessed from tidal stream turbines has been widely debated. An extensive report by Black and Veatch and the Carbon Trust estimate 20 TWh/year (approximately 5% of UK electricity usage) could be harnessed by tidal streams. The chief scientist at DECC, David Mackay, roughly estimated that about 100 TWh/yearcould be up for grabs.
The reason that Black and Veatch’s estimate is so much lower is that they assume that shipping, fishing, and conservation areas will take priority over tidal stream turbines. They calculate that if priorities turn to energy generation, then up to 29 TWh/year could be harnessed.
The estimate also includes a number of economic predictions, such as how technology costs will change, which are inherently uncertain. Furthermore, the report suggests that the majority of locations will have tidal currents that are too small to be economic.
Technology
The idea is stalled though by uncertainty the best turbine design. There are about 12 small competitors in the UK furiously trying out different variants of tidal stream technology.
The problems start with the equations that describe the operation of turbines.
Due to the difference in density between water and air - the drag forces applied to tidal turbines for a given power rating are much higher than that for wind turbines.
Along with the difficulty and expense of servicing and installing large pieces of machinery at sea, the result is that tidal stream generators require a great deal of engineering design and prototyping before they can be deployed safely and with confidence into the harsh marine environment.
Even then, many tidal projects have suffered serious technical difficulties very soon after being deployed.
What this means is that in an economic environment in which there is little project financing or grant support the industries deployment has been held back.
Clearly the technology is not mature - however, large scale wind turbine technology was in a similar position 25 years ago.
Investment
In 2010 DECC commissioned a report looking into why investment wasn’t reaching the tidal industry. Interviews were carried out with banks, corporates, venture capital and angel investors.
The same reasons for lack of investment were repeated by all investors: the risk in the technology and the timescale of return on investment.
With the exception of a few angel investors the private money that is around will only be invested once the technology has been developed, i.e. when there is one type of turbine on the market and it is ready to deploy.
Even the competitions introduced to fix the problem set the bar too high.
The long-running Saltire competition, which has pledged £10 million to the first company to generate 100 MWh from the sea.
But if a company can generate 100 MWh by either harnessing wave or tidal energy, it will already have an electricity-making (i.e. money-making) machine capable of being deployed, but to get there, to get to 100 MWh, that requires investment.
The Kay Review, commissioned by Vince Cable at the Department of Business, Innovation and Skills concluded that UK investment is far too focused on short term gain relative to our competitors.
Although the government is investing £20 million in wave and tidal technology, the industry still needs private investment, which it isn’t getting.
With this kind of investor psychology it isn’t surprising that the UK has not led the way on developing any of the main renewable technologies, (i.e. wind (Danes), solar (Spaniards) or hydro (Norwegians)), instead, it has waited for other countries to take the lead, and then got on board once the technology is ready for the market forces.
The potential of tidal remains significant - but the outlook for tidal making a significant contribution to electricity generation in the coming years looks bleak.
