Offshore tidal power - beyond the barrage5 June 2002
OTP is a new approach to a much debated but rarely realised renewable resource. Based on a self-contained impoundment structure, it holds out the promise of an environmentally acceptable, mechanically proven and economic alternative for green power generation. Potential sites where plans are most advanced include southwest Alaska, Puerto Montt, Chile, and Swansea Bay in the UK.Peter Ullman, Tidal Electric Swansea Bay, London, UK
Offshore tidal power is an innovative approach to a familiar low-head hydroelectric power conversion project that uses durable and mature components and construction methods to produce cost-advantaged commercial-scale electrical power in a highly predictable and dispatchable fashion. It consists of low-head hydroelectric generating equipment (mixed-flow reversible bulb-type turbine generator sets) installed in a rubble mound impoundment structure situated in shallow waters a mile offshore where mean tidal ranges are a minimum of 15 feet. The impoundment structure is subdivided into multiple cells that provide multiple output pulses and create a more continuous output and greater opportunity to respond to price signals from the grid.
A simulation model produces a basic precisely predictable load factor of 57-62 per cent (depending upon the 18.6 year nodal cycle of the tides) that will be enhanced by less predictable utilisation of pumped storage, wave overtopping, and storm surges. Financial models of three projects in development in the UK suggest internal rates of return (IRR) of 32 per cent under the existing New Electricity Trading Arrangements (NETA).
No projects have been built and all projections are subject to adjustment once performance data are obtained, but the developers expect that the income stream of the project will be enhanced by conventional pumped storage techniques and by participation in the balancing mechanism, 'insuring' unpredictable wind and/or wave projects against non-delivery penalties.
The public is calling for the increased use of renewable source power while at the same time increasing its demand for power. Public policy makers are gradually shifting the subsidies and tax benefits from fossil fuels and nuclear to the renewables sector in an effort to level the playing field for emerging power sources. Specifically, the UK requires that 10 per cent of its electricity must come from renewable sources by 2010 and has provided economic incentives that support the development of renewables. Distributors too are involved. Effectively, those that fail to use renewables will be paying fines to competitors who do. OPT therefore offers an economic and reliable alternative to electricity providers and a real motivation to use renewables.
The site-specific environmental impact of the impoundment structure is positive. Nearshore and intertidal zones are delicately balanced environments where many species interact. Any structure installed among the forces generated by it will change them. The proposed structure has the positive effect of creating a habitat for biodiversity in an otherwise relatively barren area. The structure is built from locally-obtained rock, sand, and gravel and will quickly naturalise as native plant and animal species take up residence in and around it. Materials are delivered by rail or conveyor belt, avoiding heavy road traffic.
The coastal process changes are of particular concern because the impoundment structure changes the current and wave regime in the area. The key potential problems are the depositing of sediments where they are not wanted, such as shipping lanes, and removing sand from places such as beaches. In places like the UK where coastal problems exist the proposed projects could ameliorate them by directing accelerated flow along shipping lanes to provide a natural dredging action, and away from beach areas, relieving existing coastal erosion. Coastal process modelling will assist the design process.
The conventional approach to harnessing tidal energy is the barrage. Historically, devices such as an overshot water wheel or a paddle wheel have provided the power for milling grain and so on.
There are two commercial scale tidal power barrages in the world today: a 240 MW project in France (1965) and a 16 MW project in Canada (1982). The EdF-owned project in France has been functioning without interruption since 1965 despite some minor difficulties (siltation problems in the 60s and some corrosion of blades recently). The Canadian barrage demonstrates the use of the STRAFLO, an innovative rim-type generator invented by Escher-Wyss of Switzerland. Both barrages provide roads across the estuary and attract great numbers of visitors.
The scale of the modern demand for elecrical power has challenged the technical limtations of the barrage system. To increase the capacity of a low-head hydroelectric project, the amount of water passing through the turbines must be increased, requiring a very large head pond. Larger capacity equipment and multiple units are a simple solution to the equipment requirements: the French project uses 24x10 MW bulb-type turbine/generator sets. It is the size of the head pond that creates problems for large-scale barrages. Specifically:
• Navigation is blocked
• Fish migration is impeded
• The inter-tidal zone is changed and moved
• The tidal regime downstream is altered
Every recent UK barrage proposal has also included a road across the top of the structure as a benefit to transportation and a trade-off for the problems created. Unfortunately, the inclusion of the road doubles or triples the cost of the structure, which has rendered recent barrage proposals unsuccessful.
The technical case
The OTP proposal includes the successful technology employed in the world's 45 000 large dams, 98 per cent of which are low-head hydroelectric. The corresponding generating equipment reached maturity in the 1920s when efficiencies as high as 98 per cent were reached. Further improvements in durability and materials have enhanced their performance. Financial performance guarantees virtually eliminate equipment-based risk for the 50-year+ design life. Similarly, using rock to hold back the sea is a familiar application and considered conventional and with minimal technical risk. Settlement, piping, scouring, and seepage are all known processes that are included in the civil design engineering. There are no exposed technical risks, as the risk of defects in manufacture is covered by manufacturer's guarantees. The equipment consists of a mixed-flow reversible bulb turbine, a generator, and the control system. Manufacturers and suppliers are well known - Alstom, GE, Kvaerner, Siemens, Voith, Sulzer, and others.
Wind and weather cause tidal surges under extreme conditions and these are not specifically predictable, but the basic harmonic changes in water levels caused by the tides are eminently so. The earth's complex tidal pattern has been closely observed for eons and is now mathematically predictable, down to the finest detail. It is possible, if it strikes one's fancy, to know the precise tidal level at a specific location at a specific moment 100 or 1000 years inthe future.
Combining reliable equipment with a reliable power source creates a predictable output curve and allows the operator to deliver power as contracted and avoid the stiff fines for failure to deliver. Output can be be increased through the use of a multi-cell impoundment structure and pumped storage.
The commercial case
Capital costs consist of equipment, material, and 'soft' costs. If turnkey suppliers are engaged they would have to provide financial performance and delivery guarantees. Equipment costs are not site-specific, but the materials cost is, and represents the most significant construction risk. Costs of materials include acquisition, transportation, and placement. Multiple redundant suppliers will be required to assure delivery schedules are met. Using indicative bidding on both the equipment and materials, a 6 per cent discount rate on a 70/30 debt/equity ratio over a 10 year amortisation period, and revenue of 5p per kWh (3p for the Renewables Obligation Certificate and 2p for the power) at a load factor of 62 per cent, the internal rate of return (IRR) is just above 30 per cent as calculated for the 30 MW project in Swansea Bay, Wales. Revenues are calculated without the additional expected revenue from using the balancing mechanism, pumped storage, and dispatching to price signals. Chantrey Vellacott are the chartered public accountants who authored the financial model, incorporating UK accounting and taxation provisions.
According to Thomas Thorpe of AEA Technology, an expert quoted in the UK government's recently published PIU energy study, the Swansea Bay power plant currently in development should produce positive rates of return at typical prices paid for electricity and high rates of return at renewables prices. AEA's report on perceived risk includes the following:
• No insuperable problems are foreseen in implementing the concept
• The environmental impact of the scheme is likely to be acceptable. It is possible that the structure could have environmental benefits, such as providing a wildlife habitat.
• This kind of rubble mound breakwater has been used extensively. Provided it is designed using appropriate codes and rules, the technology should prove trouble free.
• The proposed Kaplan low-head hydroelectric turbine is a mature technology. Its installation in sea water would require the judicious selection of materials but should not prove a problem.