Tides of change: can tidal power compete on cost?

14 December 2017 (Last Updated June 23rd, 2020 15:30)

The Scottish Government has approved plans for a tidal energy park off the Isle of Islay but, overall, support for the low-carbon renewable resource remains in doubt. Julian Turner asks Stuart Bradley of the Energy Technologies Institute if tidal energy can compete with wind and solar in terms of cost.

Tides of change: can tidal power compete on cost?
The Alstom Tidal 1MW generator, funded by the ETI. Credit: Courtesy of ETI

Unlike the tides themselves, support for tidal energy in the UK has been anything but predictable since Orkney in Scotland became the site of the world’s first marine energy test facility in 2003.

The European Marine Energy Centre (EMEC) has, to date, deployed more marine energy convertors than any other single site in the world, yet widespread adoption of tidal technology – which uses turbines and generators to convert tidal energy flows into electricity – remains frustratingly slow.

Cost is the most salient challenge. Tidal energy is currently up to ten times more expensive than more established renewables, such as wind and solar, a point brought into sharp relief by the recent postponement of the 30MW Perpetuus Tidal Energy Centre (PTEC) project off the Isle of Wight.

PTEC was unable to rival the prices of offshore wind in the UK Government’s latest contracts for difference (CfD) auction, a blow for tidal energy proponents who still believe the nascent industry can fulfill the government’s ambition of meeting around 12% of the nation’s electricity demand.

“The technology, in this case tidal stream, has been slow to get to market because of various factors, including uncertainty in terms of return for commercial investors and a lack of evidence around the associated risks,” says Stuart Bradley, strategy manager at the Energy Technologies Institute (ETI).

“Tides are predictable centuries in advance, and we also know how strong they will be. However, the capacity factor – meaning the amount of power the turbine harvests compared to its potential – is not as high as offshore wind, for example, say 30% compared to 45% and above.”

Untapped potential: risks and rewards of tidal electricity generation

Tidal electricity can be created from several technologies, among them tidal barrages, tidal fences and tidal turbines. Barrages are dams that use the potential energy created by changes in height between high and low tides in order to turn turbines or compress air and thus produce electricity.

Tidal fences and turbines use the mechanical energy of tidal currents turns turbines connected to a generator. Such techniques have genuine potential in the UK − the country is estimated to possess around 50% of Europe’s tidal energy resource and could produce around 30GW of installed capacity.

The UK, or more specifically, Scotland, is also home to the world’s largest tidal stream project, the 398MW MeyGen tidal array in Pentland Firth, a joint venture between the tidal energy developer Atlantis, investment bank Morgan Stanley and independent power operator GDF SUEZ, now ENGIE.

“Tidal power can be harvested from lagoons – Swansea Bay is a well-known potential development − or tidal streams, such as those being exploited in the Pentland Firth’s MeyGen development,” explains Bradley. “The first concept is well-known,” he adds, “having been used in France since the 1960s and uses fairly conventional technology, similar to low-head hydropower employed in rivers.

“The second is more similar to ships propeller or podded propulsion unit, but in this case, the tidal flow drives a propeller, an electrical machine that produces power rather than consumes it.

“The challenges are technical and commercial, similar to wind, but are more associated with the operating environment,” he continues. “Making the machines able to resist the tidal flow, seawater, turbulence and still be cost-effective is difficult. However, the industry has shown itself to be adaptable, and has used technology from marine and offshore wind to start to become effective.”

A new 30-turbine tidal farm off the Isle of Islay has been approved. Edie.net reports that the Scottish Government recently pledged £7m to help wave energy operators commercialise their technologies.

“Scotland has a clear and well-known plan to exploit marine energy – both tidal and wave,” says Bradley. “From my point of view, it depends on the influence of the devolved government.”

Arrested development: cost and location of tidal projects

And therein lies the largest hurdle to the wholesale adoption of tidal power in the rest of the UK.

The industry is still in its infancy and requires long-term support from the government if it is to avoid being priced out of the growing low-carbon market − support that has so far not been forthcoming.

In the case of the PTEC project in the Isle of Wight, the UK Government opted not to grant a minima − a minimum level of deployment for which money would be specifically allocated for marine energy − in its latest CfD allocation round, meaning that PTEC could not at the time compete on price with wind.

“The advance of offshore wind (OSW) appears to be the biggest challenge to tidal energy,” states Bradley. “It has reduced its cost more rapidly than expected, and together with more manufacturing in the UK, has high public acceptance. Tidal has high acceptance, but there are still concerns about the impact on wildlife.”

In July, investors in the £1.3bn Swansea Bay Tidal Lagoon project, led by insurance firm Prudential, warned that the project could stall unless it gains government approval.

Another issue is that tidal generating capacity works best near inhabited coastlines, yet the most promising resources are isolated from end-consumers. Grid connection is thus complex and costly.

“The best tidal resources are in lower population areas, like Orkney, Hebrides and west Wales, so strong grid connections are vital,” confirms Bradley. “It would be useful if other low-carbon energy sources could also add to that grid use, and even better if that source was complementary, such as biomass.”

Forward thinking: the future of tidal energy worldwide

In the 2016 Budget, £730m was allocated to the next round of CfD auctions, to be used to support “other less-established technologies”. For the UK to remain at the leading edge of tidal energy development, more such support is essential. For his part, Bradley is optimistic about the future.

“In the mid-term, I think that tidal stream will forge ahead, and due to the unclear potential for cost reduction of large civil engineering projects, lagoons might be a longer term prospect,” he says.

“Canada and France will quickly catch the UK up. There has been some technology consolidation −  by Alstom Tidal and Siemens, for example −  but I still think that the energy and determination of Atlantis Resources will come good for the UK, and the chance to export that drive and vigour will mean that the world can exploit commercial-ready tidal resources.

“Tidal energy has significant potential to be a low-carbon energy source in the UK’s energy mix, not as great as offshore wind, but we will still see good deployment rates of up to 10GW to 2030, if the commercial cost trajectory can be maintained and met.”