A tidal power plant is perhaps the most capital-intensive type of power station yet envisaged. It involves building a low-head hydropower scheme in the tidal reaches of an estuary, an environment where construction is, at best, difficult. Construction schedules are long so lengthy up-front loans are required, with a considerable gap between granting of the loan and income from the plant.
There is so little experience with this type of project that no useful conclusions can be drawn from experience. However several projects have been examined and costed, particularly in the UK and more recently in Australia. These provide some economic guidance.
The best site in England is the estuary of the River Severn. It has been extensively studied. The design favoured by the Severn Barrage Development Project in a 1989 report involved construction a power station with an installed generating capacity of 8640 MW. This was expected to take 10 years to build at a cost of around $17 billion at 1994 prices, a unit cost of $1970/kW.
A smaller project, on the River Mersey in northwest England, has also been examined in some detail. A plant with a proposed generating capacity of 700 MW was expected to cost around $1.5 billion to build, at 1994 prices, a unit cost of $2150/kW. This scheme would take 5 years to complete.
Capital costs for these two schemes are in line with the cost of similarly sized traditional hydropower projects. But tidal power has two special features which must also be taken into account. First, the load factor is low. A plant operating on the ebb tide will only generate power for half the time. Typical load factors for tidal power plants are around 23%. Efficiency can be improved slightly by pumping water from the sea across the barrage at high water to increase the head of water. However this involves additional capital expense for pump turbines.
The second special feature associated with tidal power relates to the time at which power is generated. Generation is restricted to the period between high tide and low tide. This period will occur at a different time each day.
As a consequence, the primary role of a tidal power plant is likely to be to replace generation from conventional fossil-fuelled power stations. When a tidal plant is generating, fossil fuel consumption can be cut back. When it stops generating the conventional plants must be brought back into service.
There is a way of re-timing the output of a tidal plant, but that involves building an allied electricity storage station. This would permit tidal power to be delivered either at a steady rate, or at times when the plant is not actually generating. However the addition of a storage facility pushes up the cost of the tidal project.
These features mean that the electricity generated from a tidal power station tends to be expensive. UK estimates, based on figures published by the Energy Technology Support Unit, suggest a generation cost of around $0.1/kWh assuming a discount rate for loan repayment of 8%. The cost of electricity roughly doubles if the discount rate is 15%.
In Australia, the government of Western Australia commissioned a report into a tidal power plant at Derby.1 The report found that the most cost-effective option was a 5 MW tidal plant which would cost A$34 million. The cost of power would be A$0.41/kWh. In this case the plant was intended to replace power generated using diesel engines, which is an expensive source. However even with a renewable energy credit, the project was judged too expensive.
As both the UK and Australian examples indicate, on a purely economic basis tidal power looks uncompetitive today. But other criteria should be taken into account when determining the true cost of a tidal power plant. The lifetime of a tidal barrage is probably 120 years; and that is a conservative estimate. Turbines will probably need replacing after 30 or 40 years. Thus, once loans have been repaid, the plant will still have a long life during which it can be expected to generate cheap electric power.
Today, however, the capital intensity is crucial. And without some sort of government support or encouragement, tidal power does not look attractive. Private sector companies have been heavily involved in studies of tidal projects in the UK, but none has yet been tempted to commit itself to construction. The Derby project in Australia was also put forward by the private sector, but this was rejected by the state government in favour of fossil fuel power. While the Australian developers remain hopeful that they can develop a project elsewhere, the outlook for tidal power is generally poor. Changes in the financial and political climate may make tidal power look more attractive in the future. But for now most projects look set to remain paper studies.
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