Tidal energy technology harnesses the ebb and flow of tides to generate energy, driven by the gravitational interplay between the Earth, Moon, and Sun, and in the process provides a consistent and reliable energy source throughout the year.
Unlike solar and wind power, predictability makes tidally produced energy a significant asset for energy security (Johnstone et al., 2013; Neill et al., 2018).
The main approaches to capturing tidal energy are tidal stream and tidal range systems. Tidal stream turbines capture the kinetic energy from tidal currents, operating in naturally fast-flowing currents and taking advantage of the water's density, which allows for significant power generation even at much lower speeds compared to wind turbines on land.
In contrast, tidal range projects, such as La Rance in France and Lake Sihwa in South Korea, use large structures to trap and release tidal waters, generating electricity in the process (Neill et al, 2018).
Tidal stream turbines, which are the main focus of this article, come in different designs adapted to specific marine environments so that energy production is optimised to the characteristics of the site (Figure 1).
Figure 1: Different types of tidal stream turbines. A) horizontal axis turbine; B) vertical axis turbine; C) oscillating hydrofoils; D) enclosed tip (Venturi) E); archimedes screws; F) tidal kites. (https://www.emec.org.uk/marine-energy/tidal-devices/)
- Horizontal axis turbines, resembling submerged wind turbines, rotate around a horizontal axis, efficiently capturing energy from tidal currents.
- Vertical axis turbines, on the other hand, spin around a vertical axis, making them effective in locations where currents change direction frequently.
- Oscillating hydrofoils use wing-like structures that move up and down, generating lift that drives a hydraulic system to produce electricity.
- Venturi effect devices, also known as enclosed tip systems, funnel water through a narrowing channel, increasing its speed and driving a turbine or generating a pressure differential that powers an air turbine system.
- Archimedes screws, inspired by ancient technology, are used in some locations to convert tidal flows into mechanical energy. Each design has specific advantages, making them suitable for various water depths and flow regimes. This versatility of designs reflects the adaptability of tidal stream technology to diverse marine conditions, maximising energy efficiency and effectiveness (Li & Zhu, 2023; Tidal Devices, n.d.).
- Tidal kites glide underwater in a figure-eight pattern, significantly increasing water flow across their onboard turbines and maximising energy extraction.
The world's first commercial-scale tidal stream generator was Marine Current Turbines' SeaGen project in Strangford Lough, Northern Ireland, featuring two 16m rotors with a combined capacity of 1.2MW. Operational from 2008 to 2019, it generated more than 11.6GWh of electricity, powering about 1,500 homes annually (News, 2012; SeaGen Turbine, Northern Ireland, UK, n.d.; Strangford Lough Tidal Turbine, Northern Ireland - Renewable Technology, n.d.).
Nowadays, the third-generation of tidal turbines are capable of generating up to 2MW of power, such as the SeaGen-S 2MW system developed by Marine Current Turbines and later refined by Siemens; this device has been designed to operate in a range of flow conditions with twin horizontal rotors, each with pitch-controlled blades, while the turbine begins operating on its own once the tidal flow reaches an average speed of about one metre per second (SeaGen Turbine, 2016).
The UK's leadership in tidal innovation, with projects like the world's first offshore tidal array (Nova Innovation) underscores the growing maturity of this sector.
Nova Innovation's Shetland Tidal Array, located in Bluemull Sound, Shetland, began with three 100kW turbines in 2016 and expanded to six turbines by January 2023, reaching a total capacity of 600kW (Case Study: Nova Innovation - Shetland Tidal Array | Scotland’s Marine Assessment 2020, n.d.; Nova Innovation – Shetland Tidal Array | Tethys, n.d.).
Also in the UK, projects such as the MeyGen initiative and Orbital Marine Power's O2 turbine in Orkney, Scotland, are paving the way for large-scale deployment. These projects are already supplying power to the grid and demonstrating the potential of tidal stream energy to contribute to national energy demands.
Tidal technology's reliability
For instance, the MeyGen project has generated more than 68GWh of clean energy since the start of the project, highlighting tidal technology's reliability and growing contribution to the renewable energy landscape.
Also in Orkney, Orbital Marine Power's O2 tidal turbine is a 2MW floating device capable of powering about 2,000 UK homes annually, with plans to expand to a 30MW project in the Westray Firth.
Less-invasive installation methods, such as gravity-based foundations, have also been developed in the newer technologies to minimise environmental impacts (Marine Current Turbines, 2016; 'MeyGen', n.d.; Orbital Marine Power | Leaders in Tidal Energy Technology, n.d.; Tidal – Nova Innovation | World Leading Marine Energy | Tidal Energy - Floating Solar - FPV - Marine Renewables - Green - Eco - Sustainable | Edinburgh - Scotland - UK, n.d.).
Based on our research, the UK and USA are leading in tidal innovation, with third-generation turbines now capable of generating energy in currents as slow as one metre per second – a significant improvement over earlier models.
However, our review also demonstrates that tidal turbine technology is still developing; of 105 tidal stream projects, a mere 13% are fully operational. Coastal regions and island nations stand to benefit the most from this predictability and sustainability.
The UK alone could meet up to 11% of its electricity demand through tidal stream energy, equating to ~34 terawatt-hours (TWh) annually (Coles et al, 2021). For island communities, tidal energy can reduce reliance on imported fossil fuels and enhance energy resilience.
Tidal energy’s predictability makes it an ideal complement to intermittent renewable sources like wind and solar, helping stabilise grids and reduce the need for fossil fuel-based back-up power. This reliable energy source can play a crucial role in transitioning to a fully renewable energy system, while also stimulating local economies by creating jobs and fostering innovation.
Tidal energy is a powerful and dependable renewable source with the potential to play a significant role in meeting global energy needs. Its predictability and scalability make it an appealing solution for enhancing energy security, especially for nations with abundant tidal resources.
Continued research, technological advancements, and supportive policies are essential to overcoming the remaining challenges and unlocking tidal energy’s full potential. Embracing tidal energy is not just about cleaner power – it is a crucial step towards a more secure and resilient global energy future, assuming environmental impacts can be reduced to demonstrate its long-term sustainability.
Wildlife interactions with tidal energy devices
Public knowledge and awareness of ocean renewable energy lags far behind onshore technologies, especially in the case of tidal energy (Wiersma & Devine-Wright, 2014).
And because social and environmental considerations must be prioritised to gain public acceptance of these developments (Kolios & Read 2013; Segura et al. 2017), proper consideration must be given to public understanding of this method of extracting energy and potential environmental impacts on marine wildlife.
It is difficult to predict the risk to wildlife from tidal stream energy without setting up monitoring systems at small-scale energy developments. The evidence to date, gathered from monitoring at various global sites states that, for small scale deployments (ie, those that involve no more than one to six tidal energy devices), some potential risks to wildlife can now be considered ‘retired’ because they pose a low enough risk (Garavelli et al. 2024).
Retired risks include noise from devices, electromagnetic fields created by cables to and from devices, changes in benthic and pelagic habitats from anchors, foundations, cables etc, and changes to oceanographic (wave and circulation) patterns (Garavelli et al 2024).
Insufficient data on collision risk between wildlife and tidal devices prevents it being ‘retired’ in this way (Garavelli et al 2024).
Collision risk
Collision risk has been a big focus of research effort into the impact of tidal turbine devices on wildlife and early studies inferred this risk from two significant sources of information:
- Habitat use by a particular wildlife group that overlapped in space and time with areas of high tidal flow (ie where tidal devices might be located in future), and;
- Sensory capability and how turbines might be perceived by each wildlife group so as to evaluate avoidance or evasion behaviour around devices.
For example, harbour seals have been shown to use areas with strong tidal currents to ambush prey, a strategy that is proposed to reduce the energetic cost of foraging for the seals (Hastie et al 2016).
Therefore, this species might potentially be at risk from tidal energy devices within their foraging areas; however, this species also avoids simulated noise of a tidal device from as far as 500m distance (Hastie et al 2018), hence its collision risk may be low since avoidance of tidal devices is predicted to be strong.
Marine mammals
Understanding use of tidal environments by marine mammals is an ongoing research focus as this helps to predict the likelihood of collision risk, but much variation has been seen between studies and sites in the use of these habitats, making predictions difficult (Sparling et al 2020).
For marine mammals (specifically in porpoise, seals, dolphins), when interactions with tidal turbines have been monitored in field experiments, even the species most frequently co-occurring with devices had low encounter risk, due to avoidance of areas where devices were located (Hastie et al 2016, 2018; Gillespie et al 2021).
It is important to note that collision risk may vary between device designs (Sparling et al 2020), also that strike risk may vary with tidal conditions (Joy et al 2018). Also, in at least some groups such as seals, the collision risk may be reduced by fine-scale consideration of device placements within tidal streams versus the animal’s habitat use (Lieber et al 2018).
While a strong avoidance effect may lower the collision risk in certain cases, this carries other potential harms such as a reduced opportunity to feed, which should be addressed especially for large-scale developments.
To assess such impacts, King et al (2015) proposed a method to assess population risk for data-limited species using a stochastic population model, specifically examining the effects of disturbance (for ocean energy developments) on survival and reproduction.
Birds
Mortality from collisions acting on adults of certain groups such as marine mammals and seabirds carries great risk to the overall population because these are long-lived species producing few young, so any additional adult mortality can have serious consequences for population sizes.
Diving seabirds – auks (little auk, guillemots, razorbills and puffin), divers (Gavia spp.), cormorant and shag have been highlighted as a group potentially sensitive to collision risk, based on their population characteristics, habitat use and diving behaviour (Furness et al 2012).
Risk comes from both floating and bottom mounted tidal devices because diving depths may potentially bring seabirds into contact with the latter at some sites. However, this risk of colliding with moving parts may be reduced if diving behaviour is restricted to slacker water periods when devices are not operational, as it may be in some groups (Isaakson et al 2020).
Terns of various species were observed to feed in the wake created by a partially decommissioned tidal device (Lieber et al 2019); however the use by seabirds of tidal races is understudied at present and more insight into this is a priority, especially for vulnerable species such as divers and black guillemots (Sparling et al 2020).
Unfortunately, collisions are very difficult to quantify in seabirds as their movement near devices is rapid and some methods (land-based survey, acoustics) are unable to detect a collision, therefore resolving uncertainty in this area is a research priority.
Fish
Impacts on fish are more tractable experimentally and flume experiments have shown the conditions which lead to worse collision outcomes, eg, when a current is running (Yoshida et al 2020); however wild monitoring experiments have shown avoidance during tidal device operations by fish in riverine habitats (Bevelhimer et al 2017); avoidance from as far as ~140 m away from a device has also been shown in commercially important species like herring and mackerel in coastal locations (Shen et al 2016).
Like marine mammals, noise emitted by tidal energy devices has been suggested to aid avoidance in fish (Shen et al 2016; Grippo et al 2020). Therefore, ability to perceive noise may be useful in terms of reducing collision risk (though noise disturbance then comes into play).
Some studies have argued that interactions with devices is likely to be proportional to patterns of fish movement (Viehman & Zydlewski 2017), so a greater understanding of fish movement will provide data to allow encounter probability to be calculated more precisely (Shen et al 2016).
Impacts on migrating fish (eg herring, salmon, eel) have been highlighted as a research priority, as have impacts on younger animals (eg, Atlantic salmon smolt or eel), which may not be able to avoid turbines in fast-flowing currents in the same way as adults (Garavelli, et al 2024).
Evidence of avoidance during installation operations have shown that this phase should avoid coinciding with big fish migrations, especially for endangered species (Staines et al 2019).
Possible pluses, and scaling-up to tidal energy farms
Finally, some scientists believe that ocean renewable energies even have the potential to create habitat for marine wildlife and urge further research to examine which groups may benefit eg fish or crustaceans (Inger et al 2009; Fraser et al 2018), although it must be remembered that attraction of fish schools may ultimately entice marine mammals or seabirds and therefore increase collision risk (Williamson et al 2019).
Scaling up from single or few devices to farm scale will bring technical challenges due to the complex effects of extracting energy from more than one device.
Vennell et al (2015) showed how scaling up is difficult without interference between devices and the same is true for environmental risks (including large-scale water flow reduction on foraging).
As devices occupy an increasing proportion of a tidal channel, the impacts due to a reduced potential for evasion/avoidance should be considered, accordingly (Copping & Grear 2018).
Operations at farm scales may also bring additional concerns including ecosystem-effects, eg, on primary production, due to altered wind-wake effects, with knock-on impacts in the marine food web.
Such effects were recently shown after decades of wind farm operations in the North Sea (Daewel et al 2022), and these impacts are worrying due to their large spatial scale as well as the time taken to appreciate their effect.
More data are also required regarding risks considered ‘retired’ at small scales, but which might pose unacceptable threats at tidal farm scales in the future (Garavelli et al 2024).
Authors: Alireza Eftekhari and Anne Marie Power, University of Galway.
Figure X: Size comparison of marine renewable energy (MRE) devices (a bottom-based tidal turbine, a floating tidal turbine, and a floating wave energy converter) with other technologies and well-known landmarks. The MRE devices generally represent the largest devices available. Illustration: Stephanie King from Copping (2024).
Figure Y: Stressor-receptor interactions potentially arising from various marine renewable energy devices. Illustration: Stephanie King from Copping (2024).
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