Tidal Power Energy North Atlantic Energy Structures

Harnessing the power of the moon through tidal energy! North Atlantic Energy Structures Inc. has patented a concept for a green power energy bridge. Although this bridge is designed primarily for use between the island and the continental North American portions of the Province of Newfoundland and Labrador, Canada, there may also be world-wide cold ocean environment applications of this technology. Spanning a distance of up to 18 kilometres across the Strait of Belle Isle, this bridge is comprised of concrete caissons and spans across its midsection.

The Green-Energy Bridge is a concept proposed to convert tidal currents into clean renewable energy. Patent rights have been licensed to North Atlantic Energy Structures (NAES), a technology development company located in the National Research Council of Canada's Institute for Ocean Technology (NRC-IOT), St. John's Newfoundland Labrador, Canada.

A Green-Energy Bridge would consist of numerous caissons; each made of high-strength reinforced concrete. A typical caisson would measure 40m high by 40m wide by 120m long. To install a caisson, it would be floated into position, and then ballasted down onto a rock weir. Once its base slab was firmly seated onto the rock weir, a perimeter of additional rock would be placed on the cantilevered edge of the base slab, securing each caisson into position.

Initially, the Green-Energy Bridge was intended to be a fixed link; necessary to join the Island of Newfoundland to the mainland of Canada. The water passage that separates Newfoundland from its other half (Labrador) is the Strait of Belle Isle. However, due to the immense current that flows through the Strait, the caissons had to be designed to allow large quantities of water to flow through.

To allow a massive quantity of water to flow through the caissons, each was designed with a parallel array of water-flow chambers. A typical caisson would contain 20 chambers with portals to allow the water in and out. A typical portal would be 10 meters high by 5 meters wide. During the preliminary design, it was quickly realized that a very large amount of energy could be extracted from the water flowing through the caissons; approximately 10MW per caisson, hereinafter called green-power caissons! Using such green-power caissons, a Green-Energy Bridge across the Strait of Belle Isle (18 km) would generate approximately 700MW of clean renewable power.

NAES is of the opinion that the most practical way to extract large amounts of energy from the relatively slow flowing body of water moving through the chambers is to use large paddlewheels. Paddlewheels are highly efficient in slow moving water. They are also environmentally practical as paddlewheels are relatively passive to the environment; fish actually swim through underflow paddlewheels without harm. The larger the wheel, the safer and easier it is for the fish to swim through with the flow of water.

As water flows through each chamber, it would rotate the energy conversion wheel (paddlewheel). The energy conversion wheels would transfer the force of the water into rotational energy and simultaneously from rotational energy into mechanical energy, such as hydraulic or pneumatic pressure; the fluid pressure would then be used to transfer the power to a generator drive. The whole process would be relatively elementary.

To further work with the environment, green-power caissons would be spaced approximately 50 meters apart. This would create large openings of water between them. The 50m channels would allow fish and whales to swim through a Green-Energy Bridge unrestricted. These large openings would also allow the water not directed through the caissons, to easily flow around them. Thus, a Green-Energy Bridge will not create a barrage situation.

Water Flow Study Through and Around Green-Power Caissons

In 2002, a dynamic water-flow study using Flow 3-D was undertaken to numerically model the water flowing through and around a green-power caisson. The results concluded that the potential of extracting renewable energy from the tidal currents of the Belle Isle Strait is immense. Using an average current velocity of 1.25m/s, (taken from existing data) the flow rate through the water-flow chambers was determined to be 2.5m/s. The flow-rate of excess water around the green-power caissons was determined to be as high as 4m/s.

The dynamic water-flow study also measured the torque on an 18 meter paddlewheel that was set at various angles to the flow. The average force on the 8.5 meter paddles was determined to be 300 kN. The average torque at the axis of the paddlewheel was calculated to be 1500 kN-m. These relatively large values correspond well with the amount of energy that would be extracted from the flow of water using the equation Pw = 0.5 ?A (Vff3-Vpaddlewheel3). Where ? is the density of sea-water, and A is the cross-sectional area of the flow, Pw is in watts. Note that the velocity of the water has a cubed effect on power.

Although the water-flow study calculated a velocity of 2.5m/s in the chambers, it is considered conservative as the rock weir foundation and perimeter rock was not taken to consideration. The underwater weir and perimeter rock will help direct the full depth of the water passage into the chambers of the green-power caissons. The constricting of the water depth significantly increases the velocity of the water approaching the green-power caissons. It is estimated that the rock weir foundation and perimeter rock will increase velocity by as much as 50%. Note that if the velocity through a typical chamber was 3.75m/s, the power in that body of flowing water would be 1358 MW. The proportion of power extracted from that body of moving water would be determined by the strength and reliability of the paddlewheel design.


NRC Pack-Ice Study

To minimize the resistance to pack-ice, the roof elevation of green-power caissons are positioned near the low-tide level and, the leading and following edges are inclined into the water at a low angle. At low-tide, this provides a low-angle ramp for slabs of ice to be pushed onto and over the roof. At mid-tide, an ice slab ½ meter thick would barely touch the roof.

In 2004, NRC's IOT performed a slab-ice interaction study on the inclined edges (ice diverter shields) of a green-power caisson. The numerical study concluded that the ice diverter shields provided virtually no resistance to ice slabs being diverted over and across the roof. The study also concluded that the forces in the inclined shields and vertical chamber walls resulting from a ½ meter thick sheet of ice were very low; relatively speaking.

Further to the numerical study, a physical study of slab-ice being forced against an inclined face was recently conducted at IOT's ice-tank. The set-up of the physical model was similar to that which will occur on the inclined ice-diverter shields of green-power caissons. With respect to the interactionary forces resulting from slab-ice being pushed against the ice diverter shields, the results of the physical modeling confirmed the predictions made by the numerical study. The combined physical and numerical modeling results are therefore very encouraging toward the continued development of green-power caissons.

The ice diverter shield has also been conceived to resist the potential forces of ice ridges; although ice-ridges are relatively uncommon in the fast moving waters of the Belle Isle Strait. With the rounded leading edge of an ice-diverter shield located approximately 5 meters below the water's surface, a fulcrum is created which would tip an ice ridge over or, possibly shear-off the ice ridge completely. The toppled ice would then be pushed over the roof by the force of the ice sheet behind it. Note however, that for ice ridges, the ice diverter shields have not yet been modeled.

Icebergs

Although not large, there are icebergs in the Strait of Belle Isle. However, since the maximum sized iceberg is relatively small (50,000 tonne) the force from a potential impact can be tolerated within the existing walls of green-power caissons. The leading edge and shields would transfer the impact load through to the chamber walls which in turn would transfer the load through to the base slab. Some additional reinforcement and minor dimensional changes may yet be required. Those requirements will be known after working through the detailed design regarding the potential of iceberg loads.

To assist with iceberg resistance, a significant amount of additional rock would be installed on the north side of a Green-Energy Bridge. The strategically placed rock would create an underwater iceberg berm. Such an underwater berm would reduce the water depth to approximately 40 metres. Thus, the maximum sized iceberg that could come in contact with a caisson would have to float in 40 meters of water (25,000 tonne); not a big iceberg at all.

Other Attributes of Green-Power Caissons

As previously noted, the roof elevation of green-power caissons would be set to the low-tide water level. However, by submerging the caissons, the chambers would fill with water. This would create a significant resistance to paddlewheel rotation. To remove this resistance, the top half of a chamber would be pressurized with air to drive the level of the water down to just above the axis of the wheels and the tops of the portals (10 to 15 psi). Thus, in a typical green-power caisson, the tops of the portals would be located approximately 10 meters below the surface of the water. This positioning of the portals also prohibits floating debris from entering the paddlewheel chambers.
Since the power inherent in a body of moving water is determined primarily by the cube of its velocity (Pw = 0.5 ?AV3), increasing velocity significantly increases the energy density of that body of water. To calculate the amount of power extracted from a flow, one would simply subtract the power inherent in the outflow from that of the free flow in each chamber, Pw = 0.5 ?A (Vff3-Vpaddlewheel3). As an example, a paddlewheel that reduces the velocity of water in a typical chamber from 2.5m/s to 1.25m/s would be extracting approximately 350 kW from the flow. From this, one can easily see that the design of the paddlewheels may yet dictate the amount of energy that can be extracted from such large bodies of moving water. The principle constraint will be to transfer a very large force through a very slowly rotating wheel. A common design may yet require a paddlewheel to transfer between 500 to 1MW of power.

The principle design constraints of the energy conversion wheels (paddlewheels) are anticipated to be structural strength, practicality, constructability, reliability, and cost. Because the paddlewheel's will be protected inside the green-power caissons, the primary forces applicable will be those resulting directly from the force of the water and the resisting forces of the mechanical energy converters; the hydraulic pumps or pneumatic compressors.
As previously alluded to above, each paddlewheel would be connected to one or more air compressors or hydraulic pumps, which in turn would drive one or more generators. However, to generate power from water flowing in either direction, the compressors or hydraulic pumps would have to be of positive displacement; not dependant on the direction of rotation. The paddles of the paddlewheel's are therefore designed to accommodate bi-directional flow.
To increase the velocity of the water through the chambers, the sidewalls outside the portals are positioned like wedges to help direct and condense the large current into the chambers. As noted previously, the rock weir and installed perimeter rock also contribute to increasing the water-flow velocity in the chambers. The rock would be positioned as a continuation of the lower wedge. The combined effect on water-flow velocity is an increase of between 2 to 3 times that of the normal current.
By installing green-power caissons 50 metres apart, conventional reinforced concrete beams can be easily installed to support a bridge between the caissons. The bridge abutments would be made from extending two adjacent chamber walls through the roof and up to the desired elevation. A typical abutment would have a footprint of 5m wide by 10m long. A typical green-power caisson would have three abutments; one in the middle and two at the ends. The distance between abutments would be the same, 50 meters.
To install consecutive bridge sections, the pre-stressed beams would be installed from land or a previously-constructed bridge deck in a leap-frog construction sequence. Such leap-frog construction of a bridge will eliminate expensive marine installation costs.
To allow ships to pass, a navigation span would be required near the middle of a Green-Energy Bridge. A conventional concept design has been proposed; however, the actual design of the navigation span would be site specific.
Where a bridge is not required on a series of green-power caissons, wind power generators could be installed on a sole middle abutment. Since a typical caisson would have a net weight in the vicinity of 200,000 tonnes, such caissons would make good foundations for wind power converters.
Green-power caissons would be constructed in a two-phase operation; initially in a graving dock and then completed at a deepwater quay. Once complete, each caisson would be towed to a site for installation onto an underwater rock weir as noted above.

The Strait of Belle Isle

The Strait of Belle Isle is a relatively narrow passage of water that connects the Gulf of St Lawrence with the mighty North Atlantic. It also separates the Island of Newfoundland from the mainland of Canada. At the underwater crown of the Strait, the water depth is relatively even at approximately 60 meters; a good location for a Green-Energy Bridge. The distance across the Strait at the crown is approximately 18 km.

Water flow studies performed near the crown have recorded relatively steady mid-stream flow rates of 5 knots; approximately 2.5 meters per second. It is interesting to note that the velocity profile of the water is not considered to vary significantly. The result of the relatively consistent velocity profile is that there is virtually no mud on the bottom of the Strait. The strong tidal current keeps the bottom swept clean.

Conclusion

The ability to convert tidal flow energy into renewable power will provide future generations with a greener environment in which to live. Through the Green-Energy Bridge design, that greener tomorrow can also join two land masses.

The developer of the Green-Energy Bridge, North Atlantic Energy Structures, has identified many other locations throughout that would benefit from a Green-Energy Bridge. Such projects would generate new sources of clean renewable energy from tidal currents for Canada and other places in the world.

Newfoundland Accommodations