But if wind and solar are to provide a greater share, they will need support from more advanced energy-storage technologies, because the sun is not always shining and the wind does not always blow. This is one reason why another potential source of renewable energy may be in use on a larger scale by 2040 – a remarkably reliable one, to boot – albeit one that has proved very difficult to harness on a large scale up to this point: the tides of the sea.
"Tidal energy has the massive benefit of being completely predictable," says Alisdair McLean, executive director of Canada's Offshore Energy Research Association (OERA). He notes that although tidal power would also need to be supported by energy storage technology, that energy would only ever need to be stored for around six hours, until the tide turned, whereas it may need to be stored for longer periods to support wind and solar.
But deployment of both wind and solar power has accelerated hugely in recent years, helping to ensure that costs associated with these energy generation methods has fallen significantly. By contrast, use of the various techniques and technologies used in different forms of tidal energy generation are far less widespread, and few have been used in a genuinely large-scale commercial project – yet. Were such a project to be constructed, costs would be much higher than for a project using wind or solar.
There is certainly a lot of energy available in the sea, in theory. The EU estimates that 100GW of marine/tidal energy generation capacity could be in operation around the globe by 2050, equating to about 100 conventional power stations.
Broadly speaking, there are two ways to exploit tidal energy. Tidal range energy projects use hydropower turbines, built into tidal lagoons or barrages. The other method, tidal stream energy generation, uses arrays of individual devices, either attached to the sea bed or floating in the water. Progress towards construction of large-scale projects of either kind has been slow for three main reasons: the engineering challenges involved in using the tide to generate electricity, availability of finance; and the direction of government policy.
One recent example of government policy turning against tidal energy was the June 2018 decision by the UK government not to support construction of the Swansea Bay tidal lagoon project in Wales, a planned tidal range facility backed by the devolved Welsh government. Construction of the lagoon would have cost £1.3bn, but the 16 turbines housed within a 9.5km breakwater would have added 320MW of generating capacity to the UK's electricity grid; enough to power 155,000 homes.
The developer seeking to build the lagoon, Tidal Lagoon Power (TLP), made some bold – and disputed – claims about the economic benefits the lagoon would bring, claiming it would contribute £316m in GVA to the Welsh economy during construction, then ££76m of GVA per year thereafter.
TLP sought a 90-year contract for difference (CfD) with the UK government, with an average guaranteed price for electricity generated of £89.90 per megawatt hour (MWh), falling from £123 in the first year of the contract to £43 by its end. But the government decided this did not represent good value, with business secretary Greg Clark saying the cost "would be so much higher than alternative sources of low carbon power that it would be irresponsible to enter into a contract with the provider."
Despite the government's misgivings, it is reasonable to suppose that the project would have worked as intended. There are already examples of similar projects elsewhere. EDF's tidal energy plant in the Rance estuary in northern France has been in operation since 1966, has an installed capacity of 240MW and produces 500GW of electricity per year. At Sihwa, on the west coast of South Korea, a 254MW tidal energy facility has been in operation since 2011, generating more than 550GW a year.
Those arguing for greater investment and government support for tidal stream energy projects say they would stimulate job creation, while providing work for professional services such as surveying, along with the opportunity to export equipment and expertise. Research produced by the Offshore Renewable Energy (ORE) Catapult suggests the UK's tidal stream industry could generate net benefits of £1.4bn and support 14,500 jobs by 2040.
Unfortunately, UK government policies to date have failed to provide a huge amount of support for tidal power. Ringfenced subsidies for commercial-scale projects were removed in 2016, which means UK tidal energy projects can now only bid for contracts for difference against other renewable energy projects. In practice, they cannot compete at present with the much lower cost of energy offered by wind or solar generation.
But if there were significant changes in UK government policy, ORE Catapult claims that a successful roll-out of commercial scale tidal stream projects would yield significant cost reductions, from about £300/MWh to less than £100, comparable to the price of offshore wind. These reductions would be achieved through economies of scale, accelerated learning, and a fall in the cost of capital as a maturing industry became more attractive to debt finance. Investors are also likely to find such projects increasingly attractive as the green bonds market develops, and sustainable investment becomes a more important topic for investors.
In the meantime, progress continues to be made by tidal stream technology companies, including some working with the European Marine Energy Centre (EMEC) at Orkney, off the northern coast of Scotland. Across the Atlantic, the other prime location for testing the technology is the Bay of Fundy in Canada, where a number of companies are working towards deployment of commercial scale arrays, helped by subsidies from the provincial government of Nova Scotia.
The key engineering challenge that these companies have to face is a harsh operational environment. Some early prototype tidal stream devices were simply destroyed by the sea. An effective turbine must somehow combine a high level of operational efficiency with exceptional durability. But there is not yet complete agreement on the best way to do this. Andrew Scott, CEO and director at tidal stream device company Orbital Marine Power thinks the fact that the basic design of a wind turbine was settled at an early stage helped accelerate the development of wind energy. "That kind of convergence helps focus research and development and investment," he says. "With tidal, R&D efforts are still focused across a range of solutions."
Nonetheless, the scale on which some of those solutions are being deployed is increasing slowly. SIMEC Atlantis is building the MeyGen tidal stream array on the sea bed between the island of Stroma and the Scottish mainland. The first turbines installed have a generating capacity of 2MW, but further planned phases would eventually deliver a capacity of 400MW.
Other tidal stream technology companies have taken a different approach. Orbital is deploying a 2MW turbine at the EMEC that floats on top of the sea and can power 1,500 homes. Minesto's, by contrast, is driven by an underwater "kite" (see box, below). The company has received Welsh government support to submerge 20 of the units off the coast of Anglesea – enough to power every home on the island.
If large-scale tidal energy does become a mainstream proposition, attention may then turn to another, more difficult method of power generation: putting turbine-type devices into ocean currents.
Tidal energy technology company Minesto is investigating this possibility, because it's Deep Green turbine could be used to harness energy from slower moving bodies of water. A kind of underwater "kite", the turbine is anchored to the seabed and swirls through the tide in a figure of eight pattern, increasing the speed at which water passes through it.
"Harnessing the currents in the Holy Grail, because they are constant, 24 hours a day," says Minesto communications manager Magnus Matsson. "The ocean currents flow too slow for any other technologies to exploit, so we are the only ones addressing this potential source of energy."
Among the locations that might be suitable for this type of energy generation are the sea bed south of Florida, and coast of Taiwan and Japan. Current there are particularly predictable and stable, and the topography of the sea bed might be conducive for building an array of devices which may be capable of drawing energy out of the ocean.
There is also activity under way in France. In November 2018, SIMEC Atlantis formed a joint venture with the Normandy regional government to build 20 2MW tidal stream turbines, which could eventually add 2GW of generating capacity to the French grid. The first phase of the project is due to be completed in 2021.
Back in south Wales, in February 2019 TPL claimed that several companies, among them LandSec, the Berkeley Group and Cardiff Airport, had expressed an interest in signing power purchase agreements to finance the Swansea Bay project. But there has been no further news on a possible resurrection of the plans, and none of the companies listed above would comment on this article.
It's a state of affairs that frustrates those who wish to promote renewable energy generation, of all kinds. David Clubb, director at Renewable UK Cymru, is among those who have criticised the government's decision to withdraw support for the lagoon. "I would be very surprised if you don't see significant tidal range developments in my lifetime," he says. "If that's the case, why are we not going ahead now, and capturing the benefits of the supply chain?"
There is a great deal of technical activity under way in relation to tidal energy in the UK, Canada and France, but this is coupled with frustration about a lack of full-blooded support from central government. Some of those working for UK-based tidal energy generation companies contrast this with the huge amounts of money the UK government has committed to the construction of new nuclear power stations, a programme not without problems and risk. They also draw comparisons with the UK's failure to invest in wind energy at an early stage of its development, which meant the country lost an opportunity to become a world leader in the technology, allowing other countries to take leading roles in what is still a rapidly expanding industry. They warn that something similar could happen with tidal.
That would be good news for those countries, of course. Ultimately, as we seek to reduce our reliance on fossil fuels, the further development of tidal energy in any form and any location will surely be good news for everyone. There is a growing sense, that the day will come, in the not too distant future, when the tide finally turns in favour of this form of energy generation.
Illustration by Karolis Strautniekas