Tidal range technology¶
Tidal range power plants exploit high water elevation differences naturally created in certain sites to drive flows through turbines, generating power in a predictable fashion. In recent years, there have been a number of scoping studies focused on harnessing the energy in areas with a large tidal range, driven in part by international commitments to satisfy energy demand from sustainable sources.
Tidal range power plants effectively behave as dams, constructed in areas exhibiting sufficient tidal range as long as it is economically, environmentally and logistically sensible to house turbines for power generation. Plant operation is based on the principle of creating an artificial tidal phase difference, by impounding water over a surface area \(A\). This facilitates a head difference \(H\) which when released drives flow through turbines, tapping into the potential energy that has the maximum described previously in (1).
A tidal range power plant consists of four main components [Bak91]:
Embankment: this forms the artificial outline of the impoundment. The design and layout typically looks to balance the minimisation of construction costs with the maximisation of the enclosed plan surface area.
Turbines: these are integrated into the embankment and are driven by the artificial head differences \(H\) to generate electricity. Turbine sections are usually positioned in the deeper parts of the structure due to turbine size, subject to geotechnical constraints for turbine housings and caissons.
Sluice gates: these are hydraulic structures fitted in concrete caissons that facilitate the transfer of water volumes with minimal obstruction during the operation of the power plant.
Locks: these are commonly proposed in barrages downstream of ports, marinas or harbours to enable vessels to cross through the impoundment, ensuring the continuation of shipping activities when appropriate.
Figure 1: Outline and configuration of a tidal lagoon proposal in the Swansea Bay area within the Bristol Channel, UK.
Tidal range power plants can be either coastally-attached (such as a barrage or a coastal lagoon) or located entirely offshore (as with offshore lagoons). Barrage and lagoon are terms used to refer to tidal range power plants differentiated by the make-up of the impoundment perimeter. If the majority of the perimeter is artificial, then the term lagoon is appropriate, whilst barrage is suitable otherwise. In the case of lagoons, the additional cost for longer embankments relative to the impounded area typically restricts smaller developments that are not perceived as pilot (or “pathfinder”) projects. However, the longer embankments enable the development of lagoons along the coast, offsetting some of the environmental impacts associated with barrages, which typically span the entire width of an estuary.
Tidal range turbines¶
Tidal range power generation is an application of low-head hydropower engineering. In the UK, low-head hydropower was initially considered in 1927 when the Severn Barrage Committee identified the Kaplan turbine, an invention originally developed in 1913, as appropriate for low-head applications. A variant of the Kaplan turbine called the bulb turbine, was then identified as preferable for tidal range schemes. Bulb turbines are designed to be entirely contained within the water delivery tube. A large bulb is centred in the water pipe which contains the generator, the runner and the wicket gates [WA16].
Figure 2: Sketch of a typical bulb turbine arrangement in a tidal barrage.
The performance of turbines is typically defined using hill charts, applying non-dimensional formulae called affinity laws. Hill charts are used to extrapolate how turbines of specific characteristics but varying dimensions would operate under a range of conditions. The methodology to represent a double-regulated turbine has been expanded in [AF12] with an example of a simplified hill chart illustrated in Figure 3. A sequence to predict the hill chart is available in [AKAP18].
Figure 3: Simplified turbine performance. The idealised curves represent the performance of a turbine in the absence of efficiency losses
Tidal power plant designs¶
The design and operation of a tidal power plant are intimately linked. While the constituent elements between designs remain the same (i.e. Turbines, Sluice gates, Embankments), the configuration may vary to deliver considerably different generation profiles. Most proposals involve a single impounded basin (Figure 4a) but variants include two or multiple linked basins (Figure 4b). Single-basin power generation is characterised by one or more distinct and predictable generation periods in each cycle, while linked-basin systems are able to provide a more continuous generation profile.
Figure 4: Tidal range power plant concepts: (a) single-basin and (b) linked-basin operation designs.
Single-basin tidal power plants form the design basis of most tidal range energy projects. Their design entails the distribution of turbines and sluice gates along a single impoundment, enclosing an area that exchanges water volumes with the sea through the turbines for power generations. Over the tidal cycle, the power plant follows a sequence of operational modes which control the hydraulic structures, dictating the flow and power generation as the tide, and thus the relative water levels inside and outside of the impoundment evolve.
Figure 5: Operation of tidal range power plants over a tidal period, illustrating typical modes of operation. (a) Conventional one-way ebb generation, (b) two-way generation and (c) two-way generation with pumping and (d) twin-basin generation. The dark grey sections indicate periods when the head difference drives power generation from the turbines, with a larger area here not necessarily corresponding with a larger overall power generation.
In its simplest form, power generation is one-directional, i.e. it is restricted to either the ebb or flood stages of the tide. For example, in a typical ebb-only generation strategy (Figure 5a) the available modes of operation are Holding at High Water, Ebb generation, Ebb generation with sluicing and Flood sluicing. An equivalent approach is used for flood-only generation. Two-way (or bidirectional) generation (Figure 5b) is a more flexible configuration. However, two-way generation requires hydraulic structures to be designed in a manner that permits power generation on both the ebb and flood tide. Moreover, tidal power plant operation can feature pumping intervals and flexible generation periods (Figure 5c).
Linked-basin systems have been proposed to deliver more consistent and/or controlled power contributions to the electricity grid than single-basin systems, with the potential to provide baseload power to a certain degree. In their simplest form, linked-basin systems feature two lagoons/basins that are internally connected, as in Figure 4b. The operation in time is illustrated in Figure 5d demonstrating how a sufficient head difference (\(H_{HW,LW}\), Figure 5d) can be theoretically maintained to generate power continuously [AKHP20].
The total amount of the energy resource that can be extracted from a tidal range power plant is closely linked to:
turbine technology capabilities (as in Figure 3),
spring-neap (and longer period) tidal variations at the site,
configuration and operation strategy of the structure and its interaction with local hydrodynamics.
When comparing tidal power designs in terms of their energy output, we can appreciate how the design is pivotal in the fraction of the theoretically available energy extracted (Figure 6).
Figure 6: Sensitivity of tidal plant normalised annual energy to tidal amplitude. Representative operational strategies for ebb-only, two-way and linked-basin generation are considered.
We find that the best performing designs in terms of energy output are of the single-basin type [AKHP20], and specifically using a two-way generation approach. Some of the major existing developments (Table 1) have been employing one-way generation due to site specific constraints (e.g. the Annapolis Royal tidal power station in Canada, or the Lake Sihwa tidal power station in South Korea), but more recent proposals aim for two-way generation supplemented with pumping. If controlled optimally, that configuration provides the flexibility for a plant to adapt its operation to better suit the energy demand [HAP19].
| Tidal Power Plant | Country | Year | Capacity (MW) | Area (km2)* | Operation | |
|---|---|---|---|---|---|---|
| 1 | La Rance Barrage | France | 1966 | 240 | 22 | Flexible‡ |
| 2 | Kislaya Guba | Russia | 1968 | 1.7 | 2 | Two-way |
| 3 | Annapolis Royal | Canada | 1984 | 20 | 6 | Ebb-only |
| 4 | Jiangxia | China | 1985 | 3.9 | 2 | Two-way |
| 5 | Lake Sihwa | Korea | 2011 | 254 | 30 | Flood-only |
| * The area refers to the maximum impounded surface area recorded during spring high water conditions. | ||||||
| ‡ Flexible operation refers to the capability of the power plant to switch strategy from ebb-only, two-way, ebb-only with pumping and two-way with pumping. | ||||||
Technology considerations¶
Tidal range energy technology is generally perceived as a relatively mature form of power generation. This is attributed to several long-term, successful tidal power plant examples operating around the world. Nonetheless, there are several factors that will dictate the development of tidal energy schemes in certain areas, even if preliminary resource assessments indicate that there is commercial potential.
Beyond physical constraints, the capital cost and uncertainty over the environmental impacts of large marine infrastructure constitute formidable barriers to development.
Economic considerations
For prospective designs, developers seek to converge towards a reasonable tidal power plant design by balancing a number of interdisciplinary factors. The viability of a tidal power plant proposal that has power generation as its primary function will need to demonstrate its competitiveness relative to other local power generation options. This is typically evaluated through economic metrics such as the Levelised Cost of Energy (LCOE), a value that should be minimised. This becomes a complex endeavour for large scale infrastructure that might need to consider construction and transport logistics, together with environmental compensation schemes and interactions with other marine industries.
Environmental considerations
Tidal power plants act as obstructions to otherwise undisturbed flow, therefore altering coastal hydrodynamics. In turn, hydrodynamic impacts influence coastal processes, leading to environmental and ecological changes. Accurately quantifying the local and far-field impact of prospective designs is crucial for ensuring their feasibility [KABE+12] . Considering the uncertainty surrounding tidal range scheme environmental and ecological impacts, combined with the keen interest in developing sustainable tidal power plant solutions, more research and technological advances are needed to mitigate potential hydro-environmental issues.
Socio-economics and public perception
Tidal range power plants are typically classed as large infrastructure projects situated in coastal and estuarine zones. Proposed designs must accordingly balance and acknowledge the multiple uses of the marine space for economic, social and recreational purposes. As such, feasibility is not merely dictated by maximising the energy extraction or profit from a site, but also the need to demonstrate the scheme’s acceptability relative to other uses of the same marine space occupied by the impoundment. Proponents of tidal range projects highlight the vast long-term and low-maintenance marine energy resource offered, as well as the substantial employment and regeneration opportunities presented. Nonetheless, a key factor for public acceptability regards the impact of a scheme on the local environment. Addressing this issue for marine infrastructure has been integrated into planning law. An example of this is the EU’s Marine Strategy Framework Directive (MSFD) that aims to protect the marine environment across Europe.
Assessment and optimisation¶
The majority of feasibility studies conducted for tidal range energy adopt practices applied in conventional hydropower, coastal and offshore engineering. These include life cycle-assessments [HJS18], construction economics analyses and environmental impact assessments, but can extend to more focussed activities such as technology optimisation for the hydraulic structures proposed in specific sites. However, tidal range energy assessment stands out due to the application of simulation software that provides detailed descriptions of the tidal resource in space and time, as well as insight into the interaction of the plants with that resource [AFBA16]. This process relies on models that predict, and can potentially optimise, the operation of tidal power plants. The applied tools focus on characterising the energy yield of the schemes initially, but can extend to the quantification of the hydrodynamic, environmental and ecologic uncertainties of different designs. Once details regarding the operational performance and impacts are better quantified, these can in turn feed into multi-criteria decision making frameworks (e.g. [WXK+17] ) that narrow down the feasible options based on remaining constraints.
Animations: Simulation of hydro-environmental models for the assessment of tidal range energy schemes. (a) for the Severn Barrage, and (b) for lagoons in Swansea Bay and Cardiff, UK
Simulation software have been developed to quantify the potential and hydro-environmental impacts of prospective proposals. Above you can observe animation segments from models developed to simulate the operation of tidal range energy proposals. While above we concentrate on UK-based schemes, similar designs have been put forward in many countries around the world with equivalent considerations.
Looking ahead there are incentives to render tidal power more attractive by working on solutions that exploit the predictability of the tides and seeing how best these sustainable energy options can work for our society, delivering energy when we most need it, reducing reliance to fossil fuel options and complementing other green options such as offshore wind.
Relevant resources
References
G.A. Aggidis and O. Feather. Tidal range turbines and generation on the solway firth. Ren. Energy, 43():9 – 17, 2012. URL: http://www.sciencedirect.com/science/article/pii/S0960148111006471, doi:http://dx.doi.org/10.1016/j.renene.2011.11.045.
A. Angeloudis, R.A. Falconer, S. Bray, and R. Ahmadian. Representation and operation of tidal energy impoundments in a coastal hydrodynamic model. Renewable Energy, 99:1103–1115, 2016. doi:10.1016/j.renene.2016.08.004.
A. Angeloudis, S. C. Kramer, N. Hawkins, and M. D. Piggott. On the potential of linked-basin tidal power plants: an operational and coastal modelling assessment. Renewable Energy, 155:876–888, 2020. doi:https://doi.org/10.1016/j.renene.2020.03.167.
Athanasios Angeloudis, Stephan C. Kramer, Alexandros Avdis, and Matthew D. Piggott. Optimising tidal range power plant operation. Applied Energy, 212:680 – 690, 2018. URL: http://www.sciencedirect.com/science/article/pii/S0306261917317671, doi:https://doi.org/10.1016/j.apenergy.2017.12.052.
Clive Baker. Tidal power. Energy Policy, 19(8):792 – 797, 1991. URL: http://www.sciencedirect.com/science/article/pii/030142159190049T, doi:https://doi.org/10.1016/0301-4215(91)90049-T.
Geoffrey P Hammond, Craig I Jones, and Rachel Spevack. A technology assessment of the proposed cardiff–weston tidal barrage, uk. Proceedings of the Institution of Civil Engineers - Engineering Sustainability, 171(8):383–401, 2018. doi:10.1680/jensu.16.00015.
Freddie Harcourt, Athanasios Angeloudis, and Matthew D. Piggott. Utilising the flexible generation potential of tidal range power plants to optimise economic value. Applied Energy, 237:873 – 884, 2019. URL: http://www.sciencedirect.com/science/article/pii/S0306261918319093, doi:https://doi.org/10.1016/j.apenergy.2018.12.091.
Margaret Kadiri, Reza Ahmadian, Bettina Bockelmann-Evans, William Rauen, and Roger Falconer. A review of the potential water quality impacts of tidal renewable energy systems. Renewable and Sustainable Energy Reviews, 16(1):329–341, 2012. URL: http://linkinghub.elsevier.com/retrieve/pii/S1364032111004072, doi:10.1016/j.rser.2011.07.160.
Shaun Waters and George Aggidis. Tidal range technologies and state of the art in review. Renewable and Sustainable Energy Reviews, 59:514–529, 2016. doi:10.1016/j.rser.2015.12.347.
Yunna Wu, Chuanbo Xu, Yiming Ke, Kaifeng Chen, and Hu Xu. Multi-criteria decision-making on assessment of proposed tidal barrage schemes in terms of environmental impacts. Marine Pollution Bulletin, 125(1):271 – 281, 2017. URL: http://www.sciencedirect.com/science/article/pii/S0025326X17307002, doi:https://doi.org/10.1016/j.marpolbul.2017.08.030.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.Would you like to contribute to this OER? If so, please get in touch.