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17 May 2019updated 13 Apr 2023 4:36pm

“Deep Green“: how a Swedish startup is making waves underwater

Six kilometres off the coast of Holyhead, North Wales, the Swedish startup Minesto is flying a very unusual kite. For one thing, the kite is around 100 metres below the sea. Stranger still, the company believes it may one day power thousands of homes.

The kite is “a type of tethered turbine that produces power from slow tidal streams,” explains Minesto’s communications manager, Magnus Matsson. The company calls the system that it operates “Deep Green”.

Tidal power refers to energy obtained from changing sea levels (when the tide is in or out). It has strong renewable energy credentials as it is a far more consistent and predictable source than wind or solar, which are at the mercy of the weather. Tidal turbines are similar in appearance to wind turbines, but their rotating blades are driven by water currents. Conventionally, these turbines are static structures that work most effectively in fast-moving water with strong currents, because there is more kinetic energy to turn the blades, but Minesto’s innovation is designed to operate in calmer waters.

Deep Green looks like a small plane, but the principle is closer to a kite. The 12m-long “wing”, which carries a turbine and generator and control system encased in a small box, is attached to a concrete foundation on the seabed by a cable between 80 and 120m long, and “flies” on the hydrodynamic lift provided by slow tidal currents.

Matsson explains that the kite follows “a figure-of-eight trajectory”, which it is guided along “using a predetermined navigation programme, and rudders. “As the wing flies in the current,” he says, “water flows through the turbine and electricity is produced in the generator. Then that electricity is transferred through a cable in the tether attached to it. The electricity goes via the cable to the seabed foundation, and then eventually by another, longer, cable back to the grid on the shore.” Wiring, Matsson says, is encased in “plastic materials such as polyethlyne, which are then reinforced by something stronger to stop the water getting to it, like steel”.

Minesto began installing the infrastructure for its Deep Green pilot project at Holyhead in May 2018. Less than six months later, in October 2018, Minesto successfully generated electricity via slow-current water for the first time. As yet there is no onshore grid facility, so the electricity generated by Deep Green is transferred to an offshore buoy, which acts as a “floating micro-grid system, facilitating grid compatibility testing by analysing electricity” generated by the wing device. Its DG500 model can generate 500kW of power – equivalent to around 1,800 solar panels.

“We’re still at a testing phase”, says Matsson. “We’ve made adjustments to the design [of the wing] to see what shape works best. In the long term, we’d want to have multiple devices, similar to a wind farm, all feeding into the grid.” Following the successful deployment of the DG500 model, Minesto intends to install further devices and gradually expand the site to an “array” of up to 10MW total capacity.

The power available from tidal currents is considerable and consistent, as sea levels change twice a day. According to Matsson, the company’s site off the shore of Anglesey holds a total potential of “approximately 80MW” installed capacity. “Such a tidal energy farm,” he says, “would generate electricity equivalent to more than 60,000 Welsh households.” A large number of “kites” would be needed, however. “A rough comparison would be that it takes three or four Deep Green systems to generate a similar amount of power as the average wind turbine.”

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Matsson explains that the fact that the wing is moving underwater means that the electricity produced by its generator is “a lot more than if the turbine was stationary”, as most tidal power technologies are. “Most large, horizontal-axis tidal energy turbines require currents of more than 2.5m/s to actually cost-effectively produce electricity.” But Deep Green can produce electricity from currents moving as slow as 1.2m/s, and in depths of 60-120 metres, “because it moves faster than the tide flows, hence speeding up the water flow that the turbine experiences”. This makes Deep Green more felxible than static turbines, because “there are many more potential sites available for this type of technology.”

The brainchild of former Saab engineer Magnus Landberg, the idea behind Minesto’s Deep Green technology came from a search for lightweight alternatives to wind turbines, which “usually weigh hundreds of tonnes”. Matsson explains that “as sea water is about 800 times heavier than air, there was the idea to use a carbon fibre [which does not corrode in salt water] blade, or wing, to support a small turbine and generator to produce power from tidal streams.” The size of the Deep Green wing, he claims, means that the device can weigh “up to 15 times less per MW” than competing tidal power technologies. The detachable design concept, meanwhile, means that any maintenance can be done in a “more cost-effective” manner on the shore, rather than using complex equipment in the water.

The invention was transferred from Saab to students at Chalmers University of Technology in Gothenburg to study the technical and commercial viability of the concept in 2007. In May 2015, Minesto received €13m in investment from the European Regional Development Fund, through the Welsh European Funding Office, to support the development of Deep Green.

Why did Minesto choose Wales to trial and launch its technology? “We considered a range of slow-current sites around the world, but eventually identified Wales as the most suitable.” The UK’s territorial waters include around half of the European tidal resource, Matsson says, and the “resource conditions” off Holyhead were ideal for testing. Furthermore, the Welsh government has shown “great enthusiasm for renewables projects”, encouraged by the “potential for local economic growth”.

Considering Deep Green is a project still in its infancy, does the UK’s decision to leave the European Union complicate matters? “The funding we have already received is ring-fenced, so we aren’t at a risk of losing that. We were exploring opportunities outside of Europe [in Taiwan, for example] before Brexit happened. I guess Brexit has made us think about those opportunities a bit more pressingly.”

What about wildlife? Numerous wind farms have fallen foul of the risks they might pose to birds; does Deep Green face similar challenges with fish and marine mammals? The Environmental Impact Assessment and Habitats Regulations Assessment conducted at the Holyhead site have “so far found nothing that says that our devices can’t operate in unison with the marine environment and wildlife,” Matsson says. “All energy technology has some impact, but we are convinced that ours is one of the more lenient ones. Obviously this will be further studied as we deploy more machines, but in Northern Ireland [where Minesto also conducts some of its R&D and device testing], we have seals in the area, and we have never had an incident with the mammals there.”  

Minesto’s ambition to “power towns and cities” using slow-current water, Matsson admits, “is still a while off”. In the “short to medium term”, the company works in “parallel tracks to establish its product” in the market. Later this year or early next year, a smaller-scale model will be deployed off the Faroe Islands, which Matsson says will be important commercially “as there is an incredible need globally to supply off-grid users, like island communities, with clean and predictable power.”

But bigger megawatt-scale units are the company’s real ambition, Matsson adds. “We are working on the next generation of utility-scale power plants for further installations so that we can start climbing down the cost curve.”

This is where Deep Green, as with other new energy technologies, may need more government help. “Revenue support schemes,” says Matsson, “have enabled wind power and solar energy to mature and establish themselves in the energy market… The world needs to move towards 100 per cent renewable energy systems to combat climate change. To do that, more solutions than wind and solar are required.”