LONDON — For countless centuries the power of the tides has been known and understood. It is perhaps remarkable that this prodigious energy been used to generate electricity only within the last half a century. In a bid to rectify this apparent omission, over recent years significant research and development dollars have been ploughed into the development of numerous novel marine hydrokinetic technologies, such as tidal current turbines, which are only now emerging commercially.
There is, however, a tried and tested tidal generation technology that offers considerable scope for further development. Tidal barrage or enclosure-type installations rely on the changing head of water that tides bring, and as such employ well-proven and extremely reliable conventional hydropower turbines. Typically, these have been Kaplan-type machines, which recently celebrated a hundred-year track record. Kaplans and their variants, such as bulb turbines, are also extremely efficient and are suitable for use with a relatively low head and over a wide range of flow conditions, ideal for such applications.
Although only a few tidal barrage projects of any size are currently operating, the precise predictability of the tides and the reliability of these technologies, coupled with the vast quantities of energy potentially available, have prompted continued interest.
Bigger and Better
The world’s most successful tidal energy installation, and still one of the largest, is La Rance, a 240-MW installation on the Rance estuary at Ille-et-Vilaine in Brittany, France, operated by Electricite de France (EdF) . Plans to build La Rance began in 1943 and the plant was commissioned in November 1966.
The 240-MW La Rance tidal project in Brittany, France. Credit: EDF
Comprising of a 330 meter-long dam enclosing a 22 square kilometre (km2) basin and a large reservoir of 184 million cubic meters, the project exploits an average of more than 8 meters of tidal range. It generates around 540 GWh annually from its 24 bulb turbines, rated at 10 MW and weighing 470 tonnes each with a 5.4-meter diameter. The use of bulb turbines allows generation to take place on both the ebb and flood cycle of the tide, i.e. when the lagoon is filling and emptying, and across a wide range of flows. At La Rance the flow range is between 4000 and 18,000 cubic meters (m3) per second. Pumping is also used in order to increase productivity in a similar operational mode to pumped storage hydro.
According to EdF, ebb generation results in 60 percent of the energy generated, flood generation for 2 percent–6 percent, pumping is responsible for 15 percent–20 percent, and free flow through the turbine orifices accounts for 20 percent of the total energy produced.
After 40 years each of the 24 units had run 222,690 hours on average, with a cumulative gross output of about 21.6 TWh. Cathodic protection is used for all the turbines, the gates and the metallic parts of the lock; as a result none of the 24 bulb machines have required replacement.
Nonetheless, a program for the renovation and modernization of the plant has recently been undertaken and will run up to 2022. From 2013, a four-year program will see the renovation of five units, including replacement of the stators and refurbishment of the rotors. A further 10 turbines will be renovated subsequently while the control system will be entirely upgraded as well as the power plant auxiliary systems.
La Rance is without doubt the oldest tidal barrage scheme, but it is no longer the biggest following the April 2012 commissioning of the 254-MW Sihwa Tidal Power Plant project in South Korea.
Found in the mid-west of the Korean Peninsula in Gyeonggi Province and not far from the city of Siheung, Sihwa is a single-effect flood type arrangement and cost some U.S. $355 million to develop. The project is located in the 12.7 km-long Sihwa embankment, which was completed in early 1994, separating the West Sea from Banwol Bay and creating a 56.5 km2 lake. However, a dramatic decline in water quality prompted a decision to allow seawater to circulate, some 60 billion tonnes annually.
The embankment now houses additional sluice gates as well as 10 bulb turbines of 26 MW each and with a 7.3-meter runner diameter. Generating some 552 GWh annually, like La Rance corrosion protection is again cathodic.
Owned by the Korea Water Resources Corporation (K-Water), which carried out the feasibility study for the construction in 2002, the primary contractor was Daewoo Engineering & Construction with turbines supplied by Andritz Hydro following their 2006 acquisition of VA Tech Hydro. It qualifies under the UN CDM program, generating an estimated 315,000 tonnes of carbon dioxide emissions savings annually.
Further Tidal Barrage Opportunities
With the development of Sihwa under its belt, South Korea is pressing ahead with its plans to develop additional tidal barrage generation capacity, exploiting the topography of its western coast where tidal ranges can average 6 meters or more.
One site already under consideration is Garolim Bay, where a 520-MW installation has been proposed. Again located on the west coast, the tidal range in the 18 km-long bay is more than 4.5 meters on average. A number other tidal barrage projects also are being proposed in South Korea, including one near Incheon with a potential capacity of 700-1,000 MW.
Beyond Korea’s ambitious plans and progress, a number of other countries are exploring the potential of tidal barrage technology. For instance, currently the world’s third largest such project and the only one located in North America is the 18-MW Annapolis Royal Tidal plant in Canada’s Bay of Fundy. This project, which has been operating since 1984, was developed as a test installation to determine the effects of such a plant and is a single-action type. It produces 80-100 MWh a day, according to owners Nova Scotia Power.
Other countries blessed with large tidal ranges and suitable geography include China, Russia, the U.K., the U.S., India and Mexico, and a number of small projects have been established. For example, China has developed the 3.2-MW Jiangxia Tidal power station, which uses five turbine units. Meanwhile, Russia developed its experimental 400-kW Kislaya Guba project in 1968.
In the U.K., the Severn Estuary with its 14-meter tidal range has been the site of proposed tidal barrage schemes for well over 100 years and with a potential generating capacity estimated at more than 8 GW, some 5 percent of current U.K. requirements. However in September 2013 the U.K. government rejected one of the more recent proposals for a $40 billion Severn Barrage project on both economic and environmental grounds. The plan by Hafren Power Ltd featured an 18 km-long barrage.
Nonetheless, new proposals for a 240-MW tidal lagoon project in Wales’ Swansea Bay were submitted for consideration in February 2014. Developers Tidal Lagoon Power Ltd. say the $1.2 billion project featuring a 9.5 km-long sea wall enclosing a 9.5-km2 lagoon will produce an estimated 400 GWh annually. Unlike barrages, tidal lagoons can be constructed with natural materials along a coastline and generate power with the tides. There is minimal environmental impact as sea life can swim around or through the structure. Given planning approval is granted, construction is expected to begin by 2015 with commissioning anticipated by 2018. Tidal Lagoon Power says that it will be the first of a number of similar installations planned by 2023.
Elsewhere in the U.K., feasibility studies have considered tidal barrage schemes in the Eastern Irish Sea, in the northwest of England, including the Solway Firth and the estuary of the River Mersey, among other locations. However, a 2011 feasibility study for a tidal power scheme in the Mersey Estuary by Peel Energy found that the high construction costs — estimated at £3.5 billion ($5 billion) — mean that the project, which could produce around 1 TWh annually, is unlikely to go ahead without a change in renewable energy support mechanisms. Anthony Hatton, Peel Energy’s development director, explains: “In the longer term, once the upfront capital costs have been paid off and for the rest of its 120-year life, the cost of electricity would be very competitive. But the preferred scheme is unlikely to attract the necessary investment while the emphasis in the financial sector and renewable energy incentives is on technologies that provide short to medium term returns.”
It is evident that both economic and environmental considerations present challenges to the development of tidal barrage projects. Nonetheless, it is also evident that with a global potential estimated at several hundred GW, tidal barrage technology offers a flood of opportunity.