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Energy from the ocean occurs as wind-driven waves, as wind-driven ocean currents and as lunar activated ocean tidal changes. There are a range of methods whereby the various forms of energy may be extracted from the ocean. At several locations around the world, ocean currents pass through narrow oceanic straits where submersible marine hydraulic turbines may be installed to generate power. Water flow speed invariably increases when ocean water flows from a wide and deep channel into a narrow and shallow channel. Submersible turbines operate at higher efficiency when the water passing through them flows at a higher speed.

The combination of the West Wind Drift and Falkland Current pushes ocean water through a narrowing channel called Falkland Sound that separates West Falkland from East Falkland. The channel is about 1-mile wide at its narrowest (and shallowest) point, where an undersea hydraulic turbine may be installed. The West Wind Drift flows through similar channels between islands located to the south of Tierra del Fuego on the Chilean side of the border with Argentina. At such locations, power may be generated from ocean currents at high efficiency (over 25%) and the installations would have the potential of becoming viable and cost effective. This method of generate power from the ocean may be modified so as to involve a tunnel dug under a short stretch of land that separated 2-bodies of ocean water.

In Newfoundland, Canada, a 5-mile wide stretch of land separates the northeast corner of Placentia Bay from the southeast corner of Trinity Bay. A 5-mile long tunnel may be built below sea level and under this stretch of land to connect the 2-bays. An ocean current called the North Atlantic Drift along with winds called the Westerlies push ocean water into Placentia Bay to a greater height than which occurs in Trinity Bay. A turbine installed in a tunnel that connects these 2-bays could generate several hundred kilowatts of power for most of the day. Several megawatts of power could be generated when the ocean tide rises sooner in Placentia Bay and also to a greater height than it does Trinity Bay. It is possible that the cost of building the tunnel and installing turbines at that location may cost less than the cost of installing bi-directional submersible water turbines across the 40-mile wide entrance to Placentia Bay. Below is a table of the typical times and heights of tides in Placentia Bay and Trinity Bay.

At another Eastern Canadian location, a 16-mile stretch of land separates the east-end of Chicnecto Bay which is located at the east-end of the Bay of Fundy near Amherst, Nova Scotia from Northumberland Strait. An underground tunnel could be built below sea level and under the land that lies between these 2-bodies of water. A hydraulic turbine driving electrical equipment may be installed in the tunnel. The nature of the ocean currents and prevailing winds causes water levels in the Bay of Fundy to be slightly higher than water levels in Northumberland Strait. At high tide, the difference in the height of water between these 2-bodies of water becomes very greatly pronounced. During low tide, the tunnel turbine would produce very small amounts of power. After the tide has risen, the power being generated by the tunnel turbine would greatly increase.

The technology to build tunnels in the earth is well proven and is continually being advanced and improved. The cost of building a 16-mile tunnel below sea level and under the land between Chignecto Bay and Northumberland Strait may be competitive with the cost of building a control dam and installing submersible hydraulic turbines across the 40-mile wide entrance to the Bay of Fundy. The turbine in the tunnel could operate at over 80% energy conversion efficiency in faster flowing water while many submersible turbine designs would operate at near 45% efficiency in slower flowing water. Over time, the higher efficiency of the tunnel turbine may be able to generate enough revenue from the sale of electricity so as to be able to justify a higher initial cost.

At present, a small tidal power generating station has been built at the mouth of the Saint John River that flows south into the Bay of Fundy. Another small tidal power station is being installed at the entrance to the Minas Basin, an inlet located on the south side of the Bay of Fundy. As high tide approaches, the flow of ocean water from Chignecto Bay into the tunnel may be reduced until water levels reach maximum tidal height at Saint John and in the Minas Basin. At this point, ocean water would resume flowing through the turbine in the tunnel between Chicnecto Bay and Northumberland Strait and continue to generate power. The operation of a turbine in an interconnecting tunnel would be quite different to the operation of bi-directional submersible turbines.

The interconnecting tunnel would enable power to be generated from ocean energy in the Bay of Fundy at times other than when the tide changes. This energy would be the effect of the ocean current and the Westerlies. The combination of the tunnel turbine(s) operating in co-ordination with the tidal power stations at Saint John and at the Minas Basin could make for viable and cost-competitive ocean energy power conversion in the Bay of Fundy. At an earlier time, the idea of connecting 2-bodies of water via a tunnel may have been regarded as impractical. Advances in tunnel building technology and hydraulic turbine technology could make the tunnel approach competitive in locations like the east-end of the Bay of Fundy and between Placentia Bay and Trinity Bay. Below is a table of typical tidal heights and times in the Bay of Fundy and Northumberland Strait.

On the other side of the world, New Zealand's West Coast has 2-sites that share similarities to those in Eastern Canada. The East Australian Current flows parallel to New Zealand's West Coast while the Nor'Wester winds blow directly toward it and pushes ocean water into Kaipara Harbour and Manukau Harbour. The east-end of the Kaipara Harbour is located some 12-miles from New Zealand's East Coast while nn 8-mile wide stretch of land separates the east-end of Manukau Harbour from Hauraki Gulf that is also on the East Coast. Building tunnels under these stretches of land would allow a base level of electric power to be generated from energy in the ocean. The narrow entrances to both Kaipara Harbour and Manukau Harbour can allow for less costly installation of control gates to hold in the ocean water after the high tide ends. When the tide is low, the control gates would remain open so as to allow wind driven ocean currents to enter the harbours and generate low levels of power.

The harbours at Auckland, New Zealand and Saint John, Canada are frequented by commercial and recreational marine traffic. If submersible bi-directional turbines were to be installed at the entrances to Manukau Harbour and the Bay of Fundy, there would need to be enough clearance to allow container ships with 45-feet keels to pass over the turbines at low tide. This means that a substantial cross section area of the flow of ocean water could not be used to generate electric power. Under such circumstances, the tunnel turbine system could compete against submersible turbines in terms of total power output and also in terms of energy conversion efficiency.

The tunnel turbine approach may also be able to compete against submersible turbines in terms of cost at locations where the entrances to suitable bays is very wide and the tunnel distances are comparatively short, such as the example at Placentia Bay and possibly at the Bay of Fundy. Researchers who study power conversion from the ocean may need to undertake a comprehensive technical and economic analysis of the respective merits and drawbacks of each of these technologies. The findings of such research will determine the optimal installations for Placentia Bay and the Bay of Fundy in Canada and also Kaipara Harbour and Manukau Harbour in New Zealand.