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Concentration of CO2 in the Atmosphere

Tidal Power

The 16-metres-in-diameter turbine is lowered into place from a platform designed by Open-Hydro. Image: Cape Sharp Tidal.

The 16-metres-in-diameter turbine is lowered into place from a platform designed by Open-Hydro. Image: Cape Sharp Tidal.

By Joan Rech

Canada’s Bay of Fundy has extreme tides – which may be an understatement. At the east end of the bay near Truro (Nova Scotia), the vertical difference between high and low tide exceeds 16 metres (54 feet). Currently 50% of Nova Scotia’s electricity is generated from burning coal. The province has committed to obtaining 40% from renewable sources by 2020. Can the bay’s powerful tides be one such source?

To understand the present, it is necessary to review past activity on the Bay of Fundy, specifically on the Annapolis River. The area was settled by Europeans – French – in the 17th Century. Based on their experience in Europe, they recognized the agricultural potential of the marshlands around the river and built dykes to drain the marshes. The dykes were earthen and sod berms with basic wooden sluiceways with a one-way valve or flap at the end. The flap allowed fresh water to drain but prevented salt water from flowing in. Within two to three years, the salt water had been flushed from the marshes leaving very fertile and productive farmland.

The turbine

The turbine

Fast forward to the 1950’s. The province was responsible for maintaining the dykes. The Maritime Marshlands Reclamation Authority, rather than maintain the network of dykes, built a dam and a set of sluice gates to regulate the incoming tide. The dam transformed a productive tidal wetland into a freshwater lake. Salt water species died off. Without spartina grass to hold the banks in place, erosion from strong west winds increased dramatically. It was, in the words of one area resident, an environmental disaster.

The energy crisis in the 1970’s led to the idea of a mega-project: a dam in the upper part of the Bay of Fundy which might meet most of the electrical needs of the province. Since no turbine yet existed to meet the requirements of the site, a prototype was built and installed in the existing dam on the Annapolis River. This turbine, built by Esher/Wyss, is able to resist corrosion by saltwater and damage from sediment. It is controlled by a computer which varies the strength of the electromagnet with the varying pressure of the water as the tides rise and fall. The level of the water is controlled by the sluice gates. When the incoming tide reaches the level of the lake, the gates are opened. After the lake rises approximately 0.8 metre, the gates are closed. When the tide falls below the lake level, the water is directed through the turbine, generating electricity. It operates in one direction only, when the tide is flowing out. It runs approximately five hours during a 12 hour, 25 minute tide cycle, and at its peak, it generates a maximum of 20 megawatts.

The Annapolis Tidal Generating Station has been operating since 1984. To be clear, it is a tidal-hydro station. Electricity is generated not from the energy of the tides but from the difference in level between the lake behind the dam and the estuary in front of the dam. It is the only one of its kind in North America, and it likely will be the only one of its kind. It was built only because the dam already existed.

In 2007, Nova Scotia’s Department of Energy initiated a process to look at the tides themselves as a potential source of energy. Determined not to repeat the mistakes of the past, community outreach and participation were part of the process from its inception. A panel of stakeholders (including area residents and businesses, municipal governments, fishermen, representatives of the Mi’kmaq community) were assembled to develop guidelines. The panel agreed on three principles:

1. Nothing will be installed that cannot be removed (i.e., NO MORE DAMS).

2. Installation will be done incrementally.

3. The effects on currents, sedimentation and marine life will be thoroughly monitored.

This led to the creation of the Fundy Ocean Research Centre for Energy (FORCE), a test center for in-stream tidal in the Minas Passage, a channel 5.5 km. wide between the main bay and the Minas Basin. It was chosen because there are strong currents at depths between 30 and 55 metres, and because it is an area of relatively low biodiversity. The sea floor is bedrock; fish use the passage but do not spawn there. As a non-profit organization, FORCE is authorized by Nova Scotia Environment to lease berths (essentially study areas) for research projects. The first turbine was lowered into place to record data in November, 2009. Post-recovery examination revealed that it had failed. The fins had been damaged by Fundy’s strong current. OpenHydro, the Irish tidal power technology company that built the turbine, was undeterred. There was, they pointed out, more energy there than had been anticipated, and they proceeded to reengineer the turbine.

Cape Sharp Tidal, a joint venture between Emera, the parent company of Nova Scotia Power, and OpenHydro, is a lessee. They aim to determine whether grid-connect in-stream turbines can produce energy silently, invisibly, and without harm to the environment. The first of two grid-connected 2 megawatt turbines was deployed in November, 2016. It is an open centre turbine which rotates on a horizontal axis. It has permanent magnets embedded in its outer rim, which rotate past the generator coils, which are housed within the duct. It is approximately 16 metres in diameter. A large triangular base brings the total weight of the unit to approximately 1,000 tonnes, and its weight holds the device securely to the bottom of the channel. Electrical control modules convert the output of the device to alternating current in synchronization with the grid, and underwater transmission cables deliver the power to a substation on shore.

turbine3The turbine is lowered into place from an OpenHydro-designed barge and deployment/recovery platform which allows for micro-siting within the berth area. It operates at a slow speed, 6 to 8 rotations per minute. Because of the slow speed, there are no pressure changes in its vicinity. In addition, it uses no oil, so there is no potential for spills or contamination. The turbine operates continuously, both when the tide is flowing in and flowing out.

The turbine has a projected life span of 15 years with maintenance scheduled every 5 years. It is estimated that the two 2-megawatt turbines will replace the consumption of 2,000 tonnes of coal annually, the burning of which would result in 6,000 tonnes of greenhouse gases.

The turbine was retrieved in June 2017 for minor repairs and upgrades which are still in progress. A second 2-megawatt turbine is scheduled for deployment in the spring of 2018.

For more information on the project (including detailed information on environmental monitoring), go to www.capesharptidal.com. For more information on the Annapolis Tidal Generating Station, visit http://bit.ly/AnnapolisTidalStation. The Annapolis Tidal Generating Station Interpretive Centre is on Route 1 east of Annapolis Royal. It is open mid-May through mid-October.

My thanks to Les Smith, Manager of Interpretive Services at the Centre, who provided information for this article.

Joan Rech is part of the New York G.E.T. distribution team and enjoys visiting Canada.

 

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