An oceanographic study conducted in one of the world’s most powerful tidal streams (where currents exceed 8 knots) seeks to clarify an essential issue that the tidal energy sector will need to address in the coming years.
Tidal energy, or “energy from the tides,” is power generated by the natural rise and fall of sea levels during tidal flows. This energy source is widely recognised as the most regular and predictable form of renewable energy. Unlike solar, wind, or wave energy, tidal energy is independent of weather conditions and instead relies on the gravitational forces exerted by the Moon and Sun on Earth’s oceans. As such, it allows for long-term and highly accurate forecasting of energy production [source: Ocean Energy Europe].
Abundant, clean, and predictable, tidal energy currently presents an advantageous alternative to fossil fuels. It has a lower environmental impact compared to other renewables, primarily because energy converters (tidal turbines or Tidal Current Energy Converters – TCECs) are located underwater and «can sometimes be integrated into existing offshore infrastructure. This approach facilitates simpler connections to local or national energy grids» [source: “Tidal current energy harvesting technologies: A review of current status and life cycle assessment” – Renewable and Sustainable Energy Reviews, June 2023].
Now, let’s take a closer look at the methodologies and systems enabling energy generation from tidal flows, along with the global potential of this resource.
TAKEAWAYS
Tidal energy: generation methods and systems
Harnessing energy from tidal currents is a concept known for over a thousand years. In ancient Europe, for instance, it was used to drive water wheels in mills to grind grain. However, the first large-scale feasibility studies for tidal hydroelectric power plants were conducted between 1924 and 1977 in the USA and Canada. The first fully operational tidal power plant was established much later, in 2007, at Strangford Lough, Northern Ireland, while «the first tidal barrage was built in 1966 at La Rance in France. This plant remains in operation, with a generating capacity of 240 megawatts. It held the title of the largest in the world until 2011, when a 254 MW facility opened in South Korea» [source: National Geographic].
There are several methods to capture tidal energy. The first relies on tidal turbines – either floating or seabed-mounted – positioned precisely where tidal currents are strongest. These turbines resemble wind turbines in both appearance and function, although marine turbines are significantly smaller, requiring multiple units to produce the same energy as a single wind turbine.
Another approach uses large barriers known as tidal barrages, installed across rivers, bays, and estuarieswith strong tidal flows. The La Rance model in France is a leading example. «Turbines within the barrage allow the basin to fill during incoming tides and empty during outgoing tides, generating electricity in both directions».
Lastly, tidal lagoons operate similarly to barrages but are constructed along coastlines. They employ artificial walls «to partially contain a large volume of incoming tidal water, with built-in turbines to capture its energy». However, with no functioning models as yet (though some are under development in the UK, China, and North Korea), the feasibility of energy production through tidal lagoons remains unproven [source: Pacific Northwest National Laboratory – PNNL].
The potential of tidal energy
The “Ocean Energy – Stats & Trends 2023” report, published by Ocean Energy Europe in April 2024, highlights Europe’s continued «technological leadership» in tidal energy, boasting an «installed cumulative capacity three times greater than all other countries combined». According to the report, since 2010, Europe has installed tidal energy infrastructure with a combined capacity of 30.5 MW, generating around 93 GWh of electricity annually. In 2023 alone, four new systems were brought online, adding 280 kW to the region’s electricity supply.
In this landscape, the UK and France lead the market. Specifically, the tidal climate around Scotland’s islands supports «the highest tidal energy production in the UK».
Analysts at Ocean Energy Europe also note that recent EU funding has unlocked 14 MW of new tidal power projects. This support is essential, as «continued EU grant funding will be key to driving competition through new implementations, ensuring that development of innovative technology continues. The current pipeline is expected to expand and could reach 700 MW by 2028».
Outside the EU, the US is leading in cumulative installed capacity, with 10.9 MW since 2010 and an additional 95 kW added in 2023. Notably, «the US government has increased its annual ocean energy budget for the third consecutive year, with a total of $520 million allocated over the past five years».
Meanwhile, China has formally announced a «large-scale deployment of ocean energy in its five-year plan», projecting significant steps towards industrialising the sector over the next half-decade.
Challenges in tidal energy
While tidal energy is among the most predictable of marine renewable resources, highly turbulent and complex tidal flows can complicate energy capture, impacting tidal turbine performance. This issue is discussed in a study from the University of Plymouth’s School of Biological and Marine Sciences and School of Engineering, Computing and Mathematics, titled “Sheared turbulent flows and wake dynamics of an idled floating tidal turbine,” published in Nature Communications.
The authors anticipate significant growth in offshore renewable energy infrastructure along the UK coastlinein the coming decades. This trend underscores challenges around safeguarding the long-term integrity of these technologies. They argue that «advancements in field measurements of tidal flows» are needed to better understand the interactions between currents and turbines. Improved monitoring aims to optimise turbine efficiency and prevent slowdowns or disruptions caused by unpredictable currents.
The study begins with projections suggesting that energy from tidal flows, using horizontal-axis underwater turbines, could «meet up to 11% of the UK’s current annual electricity demand, equating to 34 TWh/year». However, the researchers caution that «tidal flow sites are highly energetic environments,where intense forces from rapid flows (current speeds >2 m/s) and high turbulence (intensity >10%) pose challenges for reliable energy extraction».
While the average speed of tidal flows is generally predictable, regions with more constrained flows – such as channels, basins, and promontories, where tidal energy is densest – generate «highly dynamic flow environments that are not as regular or easily measurable».
In these areas, accelerated currents create significant variability in both average speed and turbulence. «For example, fast currents near promontories and islands can produce vortices and wakes bounded by sharp horizontal shear, also known as the ‘cross-flow velocity gradient’ or ‘shear layer’», which negatively impacts turbine operations.
Use of drones and acoustic doppler current profilers to map powerful water flows
To address the unpredictability of highly dynamic ocean flows near islands, channels, and basins – which complicates tidal energy extraction – UK researchers propose measuring the average depth velocity of these flows. This is achieved by combining aerial drones footage with Acoustic Doppler Current Profilers (ADCPs) installed on the hulls of floating turbines. ADCPs, or hydroacoustic current meters, estimate water current velocity over specific time intervals by utilising the Doppler effect, measuring sound waves scattered by particles within the water column. Equipped with piezoelectric oscillators, these devices transmit and receive acoustic signals for precise flow mapping.
The team has field-tested this measurement approach by mapping the complex tidal flows encountered by the O2 tidal turbine, recognised as “the world’s most powerful.” The O2 turbine is located in the heart of the Orkney Islands, Scotland.
«Unlike conventional tidal current turbines, the O2 floats on the water’s surface, anchored to the seabed via mooring cables. Connected to the national grid through the European Marine Energy Centre (EMEC), this platform stretches over 70 metres and is projected to potentially power 2,000 homes annually in the UK».
The O2 tidal turbine case study
The University of Plymouth-led study focused specifically on measuring water flows and wake dynamics around the floating turbine array of Scotland’s O2 installation. The researchers note that the turbine array is positioned within relatively strong horizontal shear layers, «influenced by the proximity to the wake of Eday Islandand the associated shear lines». This setting contributes to the complex tidal currents affecting the installation.
Drone footage and hydroacoustic data from ADCPs reveal that the transverse impact of shear lines where the turbine array is located produces a vertical shear profile. This profile is characterised by reduced current speed in the upper water column and stronger velocities during low-tide flows.
These findings further confirm that vertical shear profiles in real-world complex ocean flows – “real-world” meaning field-measured rather than computer-simulated – «can deviate from conventional power law distributions» when intersecting with horizontal shear layers created by vortices and wakes typical of constrained sites with tidal obstructions.
The researchers add, «Our surface ocean flow measurements, obtained using drone-based video techniques,demonstrate the prevalence of turbulent flows near the horizontal shear line, resulting in a reduction in tidal current velocity».
In summary, the study has documented – and quantified – significant variations in tidal flow speed caused by intersections with unpredictable, strong crosscurrents. These interactions may negatively impact the performance and energy output of tidal turbines, posing challenges for the consistent generation of clean, renewable energy.
Glimpses of Futures
If we genuinely aim to harness the substantial benefits of clean, renewable, and inexhaustible tidal energy by 2030, the observation, analysis, and in-depth understanding of all variables in marine environments – like those described in real-world, field-based studies rather than simulations – are essential tools.
With a view to anticipating possible future scenarios, let’s apply the STEPS framework to examine the potential social, technological, economic, political, and sustainability impacts that advancements in research like this could bring.
S – SOCIAL: the oceanographic research conducted by the UK university has quantified a reduction in current speed during high-tide flows in installations where turbines are exposed to strong horizontal shear layers. This reduction in tidal flow translates into a dip in tidal energy production. In a future scenario where the technology and methods for gathering such data advance, the research community could gain increasingly detailed insights into complex ocean currents. This would enable adjustments to tidal energy generation systems, such as changing the quantity, type, and positioning of turbines, to better balance the interaction between these machines and the marine ecosystem. The goal would be to ensure continuous operation of all devices, avoiding drops in output, slowdowns, or interruptions.
T – TECHNOLOGICAL: from a technological perspective, the combination of aerial drone video footage and hydroacoustic current profilers for estimating water current velocity has promising applications in other fields. For example, observing and measuring surface ocean flow via drone-mounted cameras could, in the future, integrate AI-driven video analysis techniques, enabling automated, overhead assessment of tidal turbine arrays. This would provide an even broader and more precise view of turbulent flows in the targeted area, optimising the monitoring of tidal energy parks.
E – ECONOMIC: as mentioned, EU funding in the past year has unlocked 14 MW of new tidal energy parks through projects such as the “Sustainable European Advanced Subsea Tidal Array” (SEASTAR) and the “European Tidal Energy Pilot Farm Focused on Industrial Design, Environmental Mitigation and Sustainability” (EURO-TIDES), both running from December 2023 to February 2029 in the Orkney Islands, Scotland. SEASTAR will provide a 4 MW facility with 16 tidal flow turbines, while EURO-TIDES will deliver a 9.6 MW installation of four orbital turbines. In this context, EU financial support serves as a vital catalyst for sector development. Progress in research on the interaction between tidal turbines and complex ocean currents could help mitigate risks of stalling, production declines, slowdowns, and interruptions in tidal energy generation systems, thereby reducing waste and losses due to underperformance.
P – POLITICAL: ocean energy technologies, including tidal energy, are key components of the EU’s Blue Economy, which aligns marine activities with the objectives of the European Green Deal. Over the past decade, the EU has invested more than €4 billion in ocean energy research projects. Additionally, through the Strategic Energy Technology. Plan, the EU has set ambitious cost-reduction targets for ocean technologies over the coming decade, aiming to foster their development, adoption, and market competitiveness. Findings from the oceanographic study discussed here – documenting reduced current speeds during high-tide flows in installations where tidal turbines are exposed to turbulent currents – mark the start of research focused on performance loss in tidal energy systems due to challenging marine conditions, with a view to developing solutions that mitigate these issues.
S – SUSTAINABILITY: in terms of sustainability, tidal energy raises several considerations. While often cited as the least environmentally disruptive form of renewable energy (due to the submerged placement of tidal turbines), there are concerns about its impact on marine flora and fauna. Future research – such as that conducted by the University of Plymouth researchers – into the effects of complex ocean currents on tidal energy installations will expand knowledge of the interactions between turbines and marine ecosystems, potentially fostering a more sustainable balance between renewable energy generation and habitat preservation.
