Pulling power

The ebb and flow of the tides could be harnessed for use in a network of ‘hubs’ around the UK to provide a powerful energy resource, says Tom McKay.

The energy stored in the oceans is vastly more than Man could possibly use. The oceans convert the gravitational forces from the celestial bodies into mechanical energy, felt by the sea as it bulges to form the ebb and flow tides.

This kinetic energy can be transformed following similar rotor/turbine principles used to extract energy from the wind. Seawater is 800 times denser than air, therefore the energy available across a representative cross-section is also 800 times greater at the same speed. This means the swept area necessary to generate a given output from a tidal turbine will be substantially less than it is for a wind turbine.

A typical tidal current turbine might develop its rated power at little more than two m/s compared with 12m/s needed to achieve full-rated power for modern wind turbines. The logic follows that a one megawatt tidal turbine rotor would be less than 20m in diameter, compared with 60m for a one megawatt wind turbine.

As the sun rises and sets and the moon comes and goes, tidal velocity drops and gains throughout the lunar day (24 hours and 50 minutes). Thanks to the sun and moon’s regular and predictable relative movements around the Earth, the electricity generated by a device can be predicted long into the future — unlike the unpredictable weather phenomena that drive the wind and waves. This is a powerful tool for negotiating with utilities in terms of matching the contract of the generator with future electricity supplies for the operator, thus enabling improved economic modelling of a system and increased security of supply for the future.

The dream is to assemble arrays of tidal stream rotors, generating hundreds of megawatts, built up in ‘hubs’ around the UK and gently brought on-line as grid infrastructure is strengthened.

By deploying groups of generators in different locations around the coast, daily fluctuations in power output can then be smoothed out, achieving peak power output at a different time according to the local tides. In the UK, there is a difference of several hours between the high tide on the east and west coasts.

Realistically, the tidal energy resource that could be extracted is confined to a few regions in the world that have an exceptional resource. Estimates of a significant UK tidal resource of 22 TWh — representing about half the European extractable resource — correspond to about seven per cent of the UK‘s electricity demand.

The immediate future for commercial operation of tidal stream devices will see the costs of capital and operating expenditure fall at all plant capacities. This is because the near-shore locations will result in lower cable and burial/protection costs and reduced complexity of the foundations and moorings.

It is to be expected that new-generation technology will incur longer construction periods and lower load factors compared with conventional fossil plant, resulting in longer payback periods, with unit cost highly sensitive to the discount rate employed.

Project risks are associated with the civil engineering works required, and delays to the construction schedule having an eventual effect upon the interest charged during the construction period.

Furthermore, installations where the downstream current velocity is altered significantly across the width of an estuary may have consequences for the transport of sediment and ecosystems.

As power flux of flow is proportional to the cube of the velocity of the water in the flow, tidal schemes based on free-flow currents are expected to be certified to limit energy reduction in the flow to around 10 per cent. This equates to a flow velocity reduction of 3.5 per cent which is not expected to have a significant effect on transport of sediment and downstream ecosystems.

Tom McKay is an energy planner with international energy engineering consultant PB Power.