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Tides are regular and reliable, and the raw material is free and ‘clean’, so why are we yet to see such a steady, dependable source of power being used effectively for commercial electricity production?
The answer often given is the significant upfront capex required to design and construct tidal power facilities. The high initial cost, coupled with expensive ongoing maintenance requirements, a lack of developed supply chains, minimal support from governments to scale up and competitiveness with other technologies, has meant that tidal power projects have failed to move into the mainstream.
However, advances in technology, the desire from financial institutions to finance ‘greener’ projects and the aim of many countries to be carbon-neutral by 2050 may be just what is needed to turn the tide for this energy source.
How it works
Tides can be categorised as inexhaustible, predictable, frequent and stable. They provide an opportunity for maximum generation every 12 hours, with lower generation availability at the six-hour mark in between. There are three different methods for harnessing tidal power: tidal barrage, tidal lagoon and tidal stream (note that power generation from waves is a different type of prospect). All three involve the driving of submerged turbines but each use different methods, and each has its own advantages and disadvantages.
Types of static tidal power generation
Tidal barrages are built across tidal estuaries and ‘hold’ high tides before releasing them through turbines to generate energy. Examples include the Rance Tidal Power Station in France, which has been in operation since 1966 and generates 240MW, or more recently, Sihwa Lake in South Korea, which was constructed in 2011 and generates 245MW.
Tidal lagoons involve building sea walls to create a lagoon, with turbines embedded in the sea walls. These generate electricity as the tide comes in to fill the lagoon and again as the tide goes out and the lagoon drains. A tidal lagoon plan was proposed off the south coast of Wales, and other such projects are under consideration in the UK, but none have received UK government financial support.
Lastly, tidal stream projects involve strategically placing individual turbine units into tidal streams in order to capture the kinetic energy of the tides. The MeyGen project (off northern Scotland) is planned to create an array of turbines submerged on the seabed—imagine an underwater offshore windfarm—with the initial stage being the installation of several submerged turbines, which in 2019 was reported to have generated over 13.8GWh of renewable electricity.
Movable tidal power generators
There are a number of projects in their infancy worldwide that involve the design and construction of movable tidal turbine structures. These can be strategically placed to take advantage of tides in certain areas, and moved, relocated and transferred if desired, subject to obtaining the necessary environmental consents, planning permissions (or other local building permitting requirements).
Any move will also be dependent on the availability and proximity of grid connections—and, of course, negotiating the related offtake arrangements—and will likely need the consent of all parties and stakeholders involved in the project.
Designs range from an underwater ‘kite’ which houses turbines and can move or ‘swoop’ within the waters to maximise its power generation (the Deep Green design by Minesto) to dynamically positioned turbine units which are submerged beneath a barge or vessel (for example, Orbital Marine Power’s Orbital O2 2MW). Many prototypes are under development, demonstration and testing, and have the benefit of lower capital costs. They can be used as part of an array, or individually.
Availability of finance
Although the capital costs of static tidal power generation remain significant, it is usually argued that the ongoing operating and maintenance costs can be lower than for other offshore energy projects.
However, the movable models of tidal power generation provide an attractive alternative. These movable turbines have the advantage of being viewed as individual assets, rather than a fixed infrastructure investment, which can open up new sources of funding.
Traditional financiers of vessels, particularly those linked to the offshore oil and gas industry, such as floating production, storage and offloading units and drilling rigs, are finding themselves under increasing pressure to support more carbon-neutral projects, and the financing of tidal power generation units could fit neatly within their lending principles.
Viewing each movable turbine as an asset, rather than a long term fixed infrastructure/power project, could enable initial smaller scale funding to prove the concept, which then might be more readily scaled up into a fleet or array of floating turbines.
The mobile aspect of these small-scale power generation units could allow the owners and financiers more flexibility to move and redeploy the asset(s) and, in the event of default, to repossess and sell the asset(s). This could lead to lower margins for financing the movable turbines. Oil majors looking at increasing their investment in cleaner energy may consider whether movable tidal generation units can be used alongside certain field developments or to support vessel power.
Environmental concerns
It should be noted that concerns are raised in relation to the damage that may be caused by underwater turbines required for harnessing tidal power. In particular, tidal barrages have been seen to contribute to the potential loss of habitats, such as mudflats and salt-marshes. In addition, fish and mammals may suffer harm by collision with the turbines, although fish passes may be constructed.
Movable tidal generation units might provide less of an environmental concern compared to permanent structures, but environmental impact assessments (or their local equivalent) would need to be carried out on a case-by-case basis, in accordance with local regulations. Another environmental advantage could be, even when a project is up and running, that the formation or location of the units are capable of being changed if necessary to reflect changing environmental conditions.
The future?
Much will depend on the development of technology, but movable tidal energy units would certainly allow for a change in the way that tidal power has been developed to date. Combined with financial institutions that understand the financing of floating structures looking to deploy their capital on cleaner energy, and the clear desire for carbon-neutrality by many governments, this could just be the impetus needed to focus on harnessing the power of the sea.
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