Offshore tidal power generation resolves the environmental and economic problems of the barrage system and puts tidal power generation back amongst the choices for commercial-scale renewable power generation. Rather than blocking an estuary with a barrage, offshore tidal power generators use an impoundment structure, making it completely self-contained and independent of the shoreline (visualize a circular dam, built on the seabed), thereby eliminating the environmental problems associated with blocking off and changing the shoreline. Migratory fish simply swim around the structure[i] and ships and boats navigate past the structure. The optimal site for offshore tidal power generation is the shallow water of near-shore areas, while shipping lanes require deeper water. The offshore siting is the distinctive characteristic of the design and one of the fundamental claims of its patents[ii]. Turbines are situated in a powerhouse that is contained in the impoundment structure and is always underwater. Power is transmitted to shore via underground/underwater cables and connected to the grid. The structure need not be more than a few yards beyond the low tide level and the optimal site is one that is as shallow as possible, thereby minimizing the cost of building the impoundment wall.
The impoundment structure is a conventional rubble mound breakwater, with ordinary performance specifications and is built from the most economical materials. In the event of a failure of the structure, the consequences do not include safety issues or collateral property damage. The most likely cause of a failure would be a strong nearby earthquake and the most likely type of damage would be a breach of the impoundment structure. Thus, the principle consequence of failure would be economic (temporary interruption of service) and, therefore, economics are the primary driver in choosing the materials[iii] and construction method.
Building a complete impoundment structure offshore may seem to be more expensive than building a relatively short barrage which uses the natural contours of the existing shoreline to do most of the containment “work.” The barrage is much shorter than an impoundment structure with the same output capacity, but the barrage is a much larger structure. The cost per unit output of the offshore tidal power generator is less than that of the barrage for the following reasons:
Hydrostatic and hydrodynamic forces increase markedly with depth. The impoundment structure is built on near-shore tidal flats proximal to the low tide level and avoids deeper areas. In contrast, the barrage must span an estuary and must cope with whatever depths exist on the site. In the case of the STB, the depths are up to 40 meters below low water. With every unit of depth, the hydrostatic and hydrodynamic forces increase roughly six-fold. Using a 10 meter tidal range as a reference point, the force that must be withstood by a barrage in 40 meters depth water is roughly 1296 (6 x 6 x 6 x 6) times the force that must be withstood by the impoundment structure built near low water.
· Load Factor
Barrages must generate primarily in one direction (on the ebb tide) in order to minimize progressive disruption of the intertidal zone that would eventually lead to the silting up of the head pond. The offshore tidal power generator is free to utilize both the ebb and the flood tides for generation, thereby roughly doubling the load factor of the barrage. Double the load factor is equivalent to halving the capital cost per unit output.
Both the impoundment structure and the barrage are intended to hold back water. The power of the tides lies only in the tidal range, the difference in water levels between high tide and low tide. The impoundment structure is built so as to perform only that function, whereas the barrage also holds back all the water below low water level and all the water in the intertidal zone. None of this water produces any power, yet it is very costly to contain.
· Generation Equipment
The offshore tidal generator uses conventional low-head hydroelectric generation equipment and control systems. The equipment consists of a mixed-flow reversible bulb turbine, a generator, and the control system. Low-head hydroelectric generation equipment has been in existence for more than 120 years and state-of-the-art equipment is mature, mechanically efficient (96+%), familiar (over 100,000 units in use world-wide), reliable, and durable (the equipment comes with performance guarantees and a design life of over 50 years.) Manufacturers/suppliers include Alstom, GE, Kvaerner, Siemens, Voith, Sulzer, and others.
[i] The barrage obliges the out-migrating and returning fish to pass through the turbine. The offshore impoundment structure presents no more of a hazard to fish than would a new sandbar and fish instinctively avoid swimming through a turbine.
[ii] US Patent # 5,426,332 in 1995 and US Patent # 5,872,406 in 1999
[iii] Loose rock, concrete, and marine sheetpiles are among the types of appropriate materials for the impoundment structure.