Power comes in with the tide4 September 2002
Offshore tidal power is a new approach to the familiar use of the oceans' tides for power conversion. Peter Ullman explains why it promises to reintroduce tidal power as an environmentally acceptable, mechanically proven, and economic alternative for renewable power generation
THE PARADIGM for tidal power generation has been the tidal barrage. Although it has been in use for more than 1000 years, the tidal barrage is unsuitable for broad-scale commercial use because of environmental and economic drawbacks primarily due to its shoreline location. Offshore tidal power generation utilises an offshore impoundment structure built of rubble mound construction materials (loose rock, sand, and gravel) sited in a shallow tidal flat with a large tidal range. Placing the impoundment structure offshore helps resolve the environmental and economic problems of the tidal barrage and reintroduces the vast potential of the oceans' tides to the array of generation choices at the dawn of an era in which renewable source power is evolving from a marginal to a mainstream technology choice.
Offshore tidal power generators use familiar and reliable low-head hydroelectric generating equipment, conventional marine construction techniques, and standard power transmission methods. Three projects - Swansea Bay and Fifoots Point, both with a capacity of 30MW and the 432MW North Wales project - are in development in the UK, where tidal ranges are high.
Earliest evidence of the use of the oceans' tides for power conversion dates back to about 900 AD. Early tidal power plants utilised naturally occurring tidal basins by building a barrage across the opening of the basin and allowing the basin to fill on the rising tide, impounding the water as the tide fell, and then releasing the impounded water through a waterwheel, paddlewheel or similar energy-conversion device. The power was typically used for grinding grains into flour. Power was available for about two to three hours, usually twice a day.
However, the power requirements of the industrialised world dwarf the output of the early tidal barrages and it was not until the 1960s that the first commercial scale modern-era tidal power plant was built, near St Malo, France. The hydro mechanical devices such as the paddlewheel and the overshot waterwheel have given way to highly efficient bulb-type hydroelectric turbine/generator sets. The tidal barrage at St. Malo uses 24 10MW low-head bulb-type turbine generator sets. Installed in 1965, the barrage has been functioning without missing a tide for more than 37 years.
The second commercial scale tidal barrage was put in service at Annapolis Royale, Nova Scotia, Canada in 1982 in order to demonstrate the functioning of the Straflo turbine, invented by Escher-Wyss of Switzerland and manufactured by GE in Canada. This 16MW turbine had some difficulties with clogging seals necessitating two forced outages, but has been functioning without interruption since its early days.
Numerous studies have been conducted for large scale tidal barrages in a variety of locations, including the 8640MW Severn tidal barrage (STB) proposal in the UK. A broad range of studies were conducted from 1974-87 on this proposal to dam the Severn Estuary between Wales and England. The tidal range in the Severn is upwards to 12m in places and the potential power from a barrage could provide 12% of the UK's requirements. Major engineering consultancies, large construction companies, several universities, and the UK government's Department of Trade and Industry combined to fund and conduct the 13 years of studies costing almost US$100M.
The STB proposal was shelved in 1987 due to 'economic problems', but the proposal was likely to have been met with fierce opposition from a broad array of environmental groups and local inhabitants. The STB and other large scale tidal barrages suffer from four types of environmental problems:
• Navigation is blocked.
• Fish migration is impeded.
• The intertidal zone is changed and moved.
• The tidal regime downstream is altered.
Offshore tidal power generation has been designed to resolve the environmental and economic problems of the barrage system and put 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, thereby eliminating the environmental problems associated with blocking off and changing the shoreline. Migratory fish simply swim around the structure and ships and boats navigate past it.
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. Turbines are situated in a power house 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 impoundment structure is a conventional rubble mound breakwater, with ordinary performance specifications and is built from the most economical materials. In the event of failure, 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 structure.
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:
• Depth - 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 40m below low water. With every unit of depth, the hydrostatic and hydrodynamic forces increase roughly six-fold. Using a 10m tidal range as a reference point, the force that must be withstood by a barrage in 40m 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 minimise progressive disruption of the intertidal zone that would eventually lead to silting up of the head pond. The offshore tidal power generator is free to utilise both the ebb and the flood tides for generation, thereby doubling the load factor of the barrage. Double the load factor is equivalent to halving the capital cost per unit output.
• Efficiency - 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.
The tidal resource
Tidal cycles are calculated using harmonic constants defined by the rhythmic movements of the sun, moon and earth. This complex pattern has been closely observed for eons and is now known and mathematically predictable, down to the finest detail. It is possible to know the precise tidal level at a specific location at a specific moment 100 years or 1000 years in the future. Wind and weather cause changes under extreme conditions (tidal surges) and these events are not specifically predictable, but the basic harmonic changes in water levels caused by the tides are eminently predictable.
On a global scope the tides are a one meter high bulge in the level of the ocean that moves across the globe every 24 hours and 50 minutes. As this bulge nears land, it is changed in amplitude by the decreasing depth and anomalies of the seabed. At the extremes, some tidal ranges are as small as 15cm and some are as large as 18.3m. Broad-mouthed estuaries create the largest tidal ranges and long straight coastlines tend to have the smallest. The power available (per unit area) in any specific location is a function of the square of the tidal range and thus the largest tidal ranges are the most attractive areas for tidal power generation.
The output of any hydroelectric generating plant is dependent upon the head available [head = pressure x flow] The pressure is determined by the tidal range and the flow is determined by the amount of water available. The amount of water available in an offshore tidal power generator is a function of the area of seabed impounded. It is most economical to build an impoundment structure in a shallow area, so it follows that the most attractive sites for offshore tidal power generation are those where the tidal range is high and there are broad tidal flats at minimal depth.
A tidal barrage can be said to be a single pool/single effect generation profile, because it typically generates only in one direction, on the ebb tide and sluices on the flood tide. The single pool offshore tidal generator operates on both the ebb and the flood tides and can be termed a single pool/double effect generation profile. Generation is maximised when turbine flow is concentrated at maximum head where the highest turbine efficiencies are achieved. By waiting until more head develops with the single pool concept, the same flow can be used at a higher head to develop greater energy. The single pool generation profile produces a load factor of 48% with power available roughly half of the time.
The three-pool generation profile builds on the single pool generation profile by adding two smaller pools intended for capturing smaller heads available during the transition periods (as the tide is either ebbing or flooding). Although relatively less efficient mechanically, it is economically pertinent to have the capacity to generate at as many points in the tidal cycle as possible.
The capital costs of an offshore tidal power generator consist of equipment such as turbine, generator and control system; materials including loose rock, sand and gravel; and soft costs such as design and construction financing.
Equipment costs and soft costs are not site-specific, but the materials cost is site-specific and represents the largest construction risk (ie risk of failure to deliver). Revenues are calculated using existing market conditions at the site. Thomas Thorpe of aea-technology, a consulting firm that specialises in the assessment of energy technologies, estimates that a 30MW offshore tidal power plant, currently in development in Swansea Bay, Wales, will earn an internal rate of return of about 25%, making it a financially profitable project.