Marine renewables, also known as ocean energy, refers to a broad range of technologies that extract energy from the ocean; this energy can be in the form of ocean waves, tidal movements or thermal gradients. Marine renewables are, in general, at a relatively early stage in their development and, as such, the methods of converting these potential energy sources into useful electrical power are still highly diversified, with many technologies competing for commercial viability.
When using a tidal barrage system the technology required to convert tidal energy into electricity is very similar to hydropower as water flows into and out of gates through turbines set along a dam or barrage built across a tidal bay or estuary. A 240MW commercial tidal barrage has been operating for more than 40 years in France (EDF); however globally there are few other examples of this technology.
Tidal barrage technologies exploit the movement of tidal waters induced by the interaction of the gravitational fields of the earth, moon and sun. A barrage, or dam, is built to enclose a body of water, or reservoir, that will fill and drain with the movement of the tides. The necessary large ranges in tidal height occur in limited places due to a favourable combination of bathymetry (subsea contours can act to damp or distort the tide as it moves; magnifying it at certain locations), geography and location. Tidal ranges have the benefit that they are generally extremely predictable: the movements of the sun and moon, and thus tides, can be accurately determined many hundreds of years into the future. This makes it relatively simple to determine the amount of power that can be generated at any particular time.
The method of extracting power from the water in the reservoir is very similar to that employed in conventional hydroelectric power. To create the necessary pressure for power generation, water is allowed to flow into or out of the reservoir through open gates before the barrage is closed/sealed. As the tidal cycle progresses a difference in water level, or head, is created between the inside and outside of the reservoir. The water is then permitted to flow from the high to low level through conventional turbines in the barrage. There are a number of approaches as to how this flow of water can be created and utilised:
The reservoir is filled on the high tide then closed while the tide recedes. Generation then occurs as the reservoir drains to the sea before the incoming tide refills it. The La Rance power station in France operates on both ebbs of the daily tidal cycle (EDF).
This can be considered as the inverse of ebb generation whereby the reservoir is sealed at low tide once the water has flowed without restriction out of it. Then as the tidal height rises on the seaward side the turbines are started and water flows into the reservoir. This is less efficient than ebb generation due to the phenomenon whereby the reservoir fills during generation, reducing the available head/pressure difference.
Tidal barrage systems can also be built with two reservoirs. With this scheme one reservoir is filled at high tide while the other is used to generate at low tide. Turbines connecting the two basins allow generation to occur between the full and empty reservoir. This has the advantage of extending the times in which power can be produced; bringing the output profile closer to the desirable baseload, as opposed to the predictably intermittent ebb or flood generation schemes. However the high cost of construction limits its application. A two-basin system came close to construction in North West Australia before being rejected (Government of WA, 2000).
Tidal lagoons have also been proposed as one potential way of overcoming environmental concerns with tidal barrage schemes. Rather than using the natural course of a river or estuary as the reservoir, a tidal lagoon requires that a fully enclosed area be fenced/barraged which can have its tidal height controlled independently of the surrounding water body. This would prevent the broader area from being effected by the control of the tides; however the system would occur significant additional capital cost in order to fence in such a large area of water.
There is also the potential with some of these schemes to run the turbines in reverse to pump water into the reservoir/lagoon to increase the available energy supply. This has been proposed to make use of other intermittent renewable energy sources as a form of pumped storage.
There are limited opportunities for tidal barrage power due to the strict limitations on location. Not only must a site have a significant tidal range from low to high (La Rance has 13m range while the rejected site in Australia had in excess of this) but it must also have a suitable area that can be economically turned into a reservoir without undue environmental impact. The World Offshore Renewable Energy Report 2004-2008 (DTI, 2004) determines that although there is in the order of 3000GW of gross tidal power in the oceans, less than 3% is located in areas suitable for power generation and of this, only a subset would suit tidal barrage schemes. Eastern Canada, North West Australia, certain estuaries in the UK and areas in China and Korea, amongst others, are known to have the necessary large tidal fluctuations. The only detailed resource information that could be found was for the UK where there is an estimated 5.6GW of installed capacity available (CERG, 2009).
As discussed above, the area should not only have a good tidal resource (in the form of a large tidal range) but also a suitable estuary or river that can be used as the containing reservoir. This can have significant effects on the flow characteristics and any proposed barrage scheme would require an extensive consultation process with all stakeholders including shipping/transport/fishing vessels and environmental groups. The restriction to craft leaving or entering the estuary would be controlled by a lock (due to the differing water heights at many times) and this may place a large restriction on the number or size of craft.
The installation of a tidal barrage system would typically involve a very large programme of civil works. Large numbers of huge concrete caissons would need to be produced locally and installed along with the fabrication and installation of the sluices/gates and locks. Much of the hydroelectric generation equipment such as the turbines would be imported in most countries but would require engineers experienced with hydroelectric power schemes to plan and implement the installation. Deployment methods must either be extremely robust to survive the fast flows or very rapid in order to deploy caissons during a brief period of low flow (slack tide).
Maintenance of tidal barrage systems is less of an issue and better understood than other forms of marine energy. The long experience with hydroelectric turbines and the inherent survivability of the concrete caissons means that maintenance requirements and costs would be relatively low for tidal barrage systems.
The installation of a tidal barrage system in a modern setting would now require an extremely rigorous programme of consultation and environmental impact assessments. The multi-year consultation periods of the proposed Severn Barrage in the UK, before environmental monitoring has even begun, hint at the huge amount of regulatory and legal requirements that must be satisfied for such large, and typically controversial civil projects. The regulatory framework in any particular country would very likely be untested in regards to tidal barrage power specifically; however it could be expected to be similar to any existing framework for large conventional hydro schemes.
The experience with public acceptability in relation to tidal barrage technologies has been mixed. Schemes such as La Rance which have operated reliably for long periods now have almost certainly improved perception amongst those that are aware of it; however there is often strong resistance to new schemes. The proposed Severn Barrage in the UK has proved to be a divisive and controversial project with disagreement between environmental groups and also amongst the public (The Independent, 2009). Getting public acceptance for a tidal barrage project would form a significant part of the challenge of implementation.
The 240MW barrage system at La Rance is by far the best known scheme and is an order of magnitude larger than anything else currently commissioned. There is also a 20 MW station in Annapolis Royal, Canada, and some much smaller systems in the Bay of Kislaya in Russia, in Jangxia Creek, China and in South Korea. All of these projects are in the order of 25 – 40 years old.
Although many tidal barrage systems have been investigated over the past 40 year, almost none have been built due to the high capital costs and potential environmental impacts (SEDO, 2001). The Severn Barrage, which could have a capacity of up to 8GW is undergoing a full public consultation in 2010.
At this stage there seems to be a very limited role for tidal barrage generation in the short to medium term energy mix due to the high cost and environmental impacts (see below). However should the Severn Barrage receive approval and gain public acceptance then the global potential for tidal barrage power may be revised.
The environmental impacts of tidal barrage systems are similar to those of large-scale hydroelectricity. Effectively a large dam is built across an estuary, with similar ecological impacts to large dams on rivers. The main issue is the way in which the water flow changes due to the reduced tidal range which can lead to excessive silt build up in the river / estuary, damaging riparian communities and potentially requiring regular dredging (SEDO, 2001).
The reduction in tidal range also changes the size and distribution of the ecosystem around the fringes of the reservoir and the reduced flow could increase the concentration of suspended particles/pollutants in the reservoir which collect along the river and that would have been previously flushed out (Laughton, 1990).
Recent studies of proposed barrage schemes in the Bay of Fundy, Canada, have found that they would have a high environmental impact (Gordon, 2008).
Tidal barrage systems directly contribute to climate change mitigation by providing a completely renewable energy source free of GHG emissions (beyond the significant initial GHG gases associated with construction due to the large volumes of concrete required). However, the relative lack of suitable energetic, environmentally suitable sites and political will means the total installed capacity of tidal barrage technologies will likely remain unchanged (barring possible developments in the UK) in the short to medium term, meaning that their overall contribution to mitigation in the next decades will be small.
The installation costs of tidal barrage systems are very high compared to natural gas and other renewable energy technologies (Diesendorf, 2000). In spite of low operational costs the typically very high upfront capital costs make the delivered power relatively expensive at anything more than a modest discount rate (SEDO, 2001). This kind of investment profile with a very large upfront cost and a long payback period is unappealing to private investors. In the long term the delivered cost of energy at good sites is likely to be lower than other forms of marine renewable; however it is difficult to predict delivered energy costs because of this long wait for return on investment and the resulting sensitivity to monetary market fluctuations and discount rates.
In the meantime, this combination of borderline economic feasibility and environmental concern makes further barrage systems unlikely without direct government intervention and funding.
[This information is kindly provided by the UNEP Risoe Centre Carbon Markets Group.]
As of March 2011, there is one registered tidal barrage project in the CDM pipeline using the methodology ACM02: Consolidated baseline methodology for grid-connected electricity generation from renewable sources. This project is the Sihwa Lake Tidal Power Station, which is under construction in South Korea.
Diesendorf, M. 2000. Debate over Tidal Power in Western Australia, available at: www.sustainabilitycentre.com.au/TidalPowerWA.pdf, last accessed 17/05/2010
Claverton Energy Research Group (CERG), 2009. Tidal Barrage power generation potential in England, available at: www.claverton-energy.com/tidal-barrage-potential-in-england.html, last accessed 17/05/2010
DTI 2004, The World Offshore Renewable Energy Report 2004-2008, available at: www.berr.gov.uk/files/file43217.pdf, last accessed: 10/05/2010
EDF, The Rance tidal power plant, power from the ocean, available from: www.edf.fr/html/en/decouvertes/voyage/usine/usine.html, last accessed 17/05/2010
Gordon, D.C. 2008. Intertidal ecology and potential power impacts, Bay of Fundy, Canada, Biological Journal of the Linnean Society, vol. 51, no. 1-2, pp. 17-23.
Government of Western Australia (WA), 2000. Comparison report on Derby Tidal Power and gas proposal released, available at: www.mediastatements.wa.gov.au/Lists/Statements/DispForm.aspx?ID=109902, last accessed 17/05/2010
The Independent, 2009. The great divide: Green dilemma over plans for Severn barrage, available at: www.independent.co.uk/environment/green-living/the-great-divide-green-dilemma-over-plans-for-severn-barrage-1516790.html, last accessed 17/05/2010
Laughton, M.A. 1990. Renewable Energy Sources, Elsevier Applied Sciences, Oxon, UK
Sustainable Energy Development Office (SEDO), 2001. Study of Tidal Energy Technologies for Derby, available at: www.clean.energy.wa.gov.au/uploads/Derby%20Tidal%20Energy%20Study%20-%20Executive%20Summary_21.pdf, last accessed 17/05/2010