What is Tidal Power?

THE SCIENTIFIC PERSPECTIVE

Earth’s tides are a function of the gravitational pull of both the Sun and Moon. While the Sun is a much larger body than the Moon, exerting a gravitational pull on the Earth that is 177 times that of the Moon, it is the proximity of the Moon to the Earth that accounts for the majority of tidal activity. More simply put, the closer proximity of the Moon to the Earth has a much greater gravitational effect on the development of tides than the larger mass of the Sun. The blue bulge shown around the Earth illustrates the effect of a spring tide, when the Sun and Moon are aligned and the tides are the highest.

THE HISTORICAL PERSPECTIVE

Harnessing the power of tides likely originated with the tide mill – a waterwheel driven by the release of rising tidal waters collected in a millpond behind a dam, providing mechanical energy to mill grain, and later to drive simple machinery. The tide mill originated in the Middle Ages and possibly as far back as Roman times. Prior to the Industrial Revolution, over 750 tide mills were operating in England, France and North America. By the end of the Industrial Revolution, the power of tides was no longer simply the raw kinetic energy found in tidal waters, but energy converted by turbine/generators into a more useful and universal source of power – electricity.

APPLICATIONS OF TIDAL POWER

Modern means of harnessing the tides center around two principal methodologies: Tidal Range (right) and Tidal Stream (below). Tidal range facilities predominated during the 20th Century with tidal stream becoming de rigueur in the 21st Century. At the beginning of the 20th Century, engineering visionaries L.B. Bernshtein and R. Gibrat furthered tidal power research and development, as well as its application. Bernshtein’s efforts led to the 1968 completion of the 1.7 MW tidal range station at Kislaya Guba near Murmansk on the Arctic Ocean, and Gibrat’s efforts culminated in the construction of the 240 MW tidal range facility at La Rance, France in 1966.

Both facilities continue to operate successfully, with the owner/operator of the La Rance Tidal Power Plant – Electricite de France (EDF) – boasting that the price of electricity generated from the 60-year old facility is less than $35/MWhr! Other tidal power stations constructed and operating are the 20 MW Annapolis Royal Power Station in Annapolis, Nova Scotia completed in 1984, and the 260 MW Sihwa Tidal Power Station in South Korea completed in 2009. Further tidal range projects have been proposed in the Severn Estuary in the southwest of England and in the Bay of Fundy, Nova Scotia, but have yet to materialize. The reasons for the lack of greater activity in the tidal range deployment are discussed below.

In tidal range, a basin is separated from the sea by a marine enclosure in two configurations: (1) a barrage, or (2) a shore-connected lagoon (see figures above). An offshore lagoon (3) is possible but is likely uneconomical. The enclosure includes a powerhouse containing hydroelectric turbine generators. The rise and fall of the tides creates a difference in water levels between the sea and the basin (differential head), which drives water through the turbines to produce electricity.

Tidal stream (or hydrokinetic) devices, on the other hand, are free standing turbines anchored to the sea floor in tidal waters. Tidal stream or hydrokinetic devices are essentially underwater wind turbines. These devices capture the kinetic energy of tidal flows just as wind turbines capture the kinetic energy of the wind. Several individual experimental tidal stream devices have recently been deployed in North America and Europe with mixed results. Prototype tidal stream devices have typically been very large relative to their electric output.

THE PROBLEMS WITH TIDAL STREAM POWER

(A) Very high cost per kilowatt hour of energy output. The typical domestic retail electricity rate in Nova Scotia is $138 / MWh, while the tidal stream feed-in tariff in Nova Scotia is $652 / MWh – 5 times the current rate! (see table at right). In its Environmental Assessment (EA) of the ORPC in-stream tidal project in Cobscook Bay, Maine (FERC Project No. 12711-005), FERC estimates the cost of power to be $1,062.25 / MWh, while FERC estimates the cost of power from the Admiralty Inlet Tidal Project (FERC Project No. 12690-005) in Washington State to be $8,552.27 / MWh! The low energy density of tidal stream devices requires the deployment of either massive machines or multiple arrays of machines covering the proximate sea floor at various depths in order to generate a modest amount of energy. The table below (after G) compares the size and output of tidal stream turbines to tidal range turbines. Until turbines become cheaper than cement, tidal range will always have a significant cost/price advantage! With high cost and low output, hydrokinetic devices will have a difficult time approaching the economics of tidal range facilities – if ever.

(B) The difficulty and high cost of connecting a tidal stream facility to the grid. Each turbine will need to be connected to a central interconnection point at sea and then relayed to a further interconnection point or switchyard on land. The cost of such efforts underwater is likely to be at least double the cost for interconnecting a tidal range facility.

(C) Increases in sedimentation. The rate of sedimentation in a basin is extremely sensitive to flushing and residence time. A reduction in flow (and a concomitant increase in flushing and residence time) leads to a rapid rise in the rate at which sediment is deposited from the water column onto the seafloor. This high degree of sensitivity is the basis of land reclamation practiced in the Netherlands and in South Korea where dikes are intentionally built to reduce the flow of water and collect sediment. The extraction of energy by hydrokinetic devices reduces flow, flushing and residence time, inevitably leading to the deposit of sediment.

(D) The cost of maintenance. In order to perform maintenance on the large number of turbines in a tidal stream array, one or more barges with crews would need to be employed continuously raising, maintaining and redeploying turbines – think of a lobster fisherman and his traps.

(E) Reduction in tidal amplitude. In the process of extracting energy from tidal flow (19% to 25% being the likely maximum) hydrokinetic turbines reduce the tidal amplitude by 30 % to 40 %. Tidal amplitude represents the height and breadth of the tides. This reduction in tidal amplitude results in the permanent loss of intertidal zone – the shore area which is alternately submerged and exposed with the rise and fall of the tides. Intertidal zones are among the most productive and biologically important marine habitats. Such losses would create ecological mayhem. The amplitude reduction can be minimized to about 8% by reducing energy extraction by 50%. In macrotidal estuaries such as the Bay of Fundy or the Severn Estuary with tides of 9 to 12 m, a loss of 8 % leads to a loss of about 1 m of tide height. Because estuaries tend to have gently sloping mudflats, a loss of 1 meter of tide would still result in the loss of vast areas of the ecologically vital intertidal zone. Further, the “mitigation” comes at a cost of underutilizing the existing resource by 50 % – only 12.5 % of the available energy, rather than 25%, is used.

(F) Increased flushing and residence time. Recent studies conducted on behalf of the US Department of Energy indicated that flushing time (a measure of the time required to “clean out” a basin) and residence time (the time that a parcel of water remains in the basin before being flushed) are both extremely sensitive to reduction in flow volume. Large hydrokinetic arrays reduce flow volume without remedy. Halcyon tidal range facilities avoid this problem by using its turbines as high speed pumps to maintain flow volume. It is estimated that a 10 % reduction in flow volume leads to a 50 % increase in flushing time. It is a basic principle of eco-hydrology that a poorly flushed estuary is biologically less robust than a well flushed one. Lengthening the flushing and residence time have profound negative impacts on the marine ecology, including eutrophication (oxygen depletion). Avoiding these negative impacts may require a further major reduction in the extraction rate of hydrokinetic devices to a value well below even 10 % of the available energy. Of course, output at this level, given the cost of constructing and operating a hydrokinetic system, will be produced at prohibitive cost.

(G) Limited number of suitable sites. Hydrokinetic projects require large flow velocities. Unfortunately, experience has shown that deploying in-stream tidal devices in such an environment requires device robustness that further increases the cost, maintenance and price of energy produced from such devices, while shortening their useful lives. These large velocities occur in narrow channels leading to large estuaries, and the scarcity of these geographical features limit the number of suitable sites. Due to this scarcity, hydrokinetic devices are increasingly being located in rivers rather than in marine environments.

While tidal stream power has attracted federal and state development funding of late, it is too early to tell whether the flaws noted above will ultimately prevent commercial deployment of tidal stream arrays. In stark contrast, tidal range power has the benefit of 50 plus years of empirical evidence on which to base its claims; and with the overlay of the ‘Halcyon Solution’ improvements, it becomes a far more effective and efficient means of harnessing tidal energy than tidal stream.

THE PROBLEMS WITH TIDAL RANGE POWER AND THE HALCYON SOLUTION

The typical tidal range facility employs a barrage with turbines at its center and relies on ‘Ebb Generation’ to produce power, as noted above. The barrage consists of an embankment with the turbines fixed in concrete caissons towards the center of the barrage. The embankment is constructed of rock, earth or a combination thereof with a massive base rising in the form of a pyramid for strength and stability. The cost of constructing this type of barrage, whether ‘dry’ (through the use of coffer dams) or ‘wet’ (in the water without coffer dams) is expensive, increasing exponentially with both depth and length. The process of completing this type of construction is also very time-consuming, only adding to the overall cost. These are the principal reasons why the typical tidal range barrage is constructed at the narrow mouth of an estuary. Such placement has led to serious environmental harm to the estuary basin and the intertidal zones.

Notwithstanding the high cost and environmental harm, existing tidal range facilities have proven operationally successful and, over the long-term, economically viable. Further, because a tidal range facility concentrates tidal flows through the turbine caissons at its center, it is far more efficient and, while expensive, still far less expensive than a tidal stream array.

Our ‘Halcyon Solution’ to the economic and environmental challenges of tidal stream and tidal range power is explained here.


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