Neptune Power Limited is pressing ahead with a proposal to generate electricity from a trial generator in Cook Strait.
The proposal is to anchor a trial generator to the floor of Cook Strait at a depth of about 80 metres, sufficiently deep to avoid being a hazard to shipping passing through the area.
The benefit of deep tidal generation is that it avoids the large fluctuations that can occur when generating electricity from wave activity near the surface. While the variations caused by storms and other weather effects near the surface is less, it is thought that extreme weather events can impact the tidal currents that keep the deep turbines turning.
It is thought that as much as 10 GigaWatts of energy could be generated from two areas in Cook Strait if they were fully populated with turbines.
In a proposal lodged with the Ministry of Economic Development in 2007, Neptune Power estimated the cost of a trial at about $11 million, including the purchase and installation of the turbines, connection to the grid and monitoring equipment to study the environment and viability of the strait for generation. The trial would involve the installation of a pair of ½ MegaWatt buoyant generators anchored at two points to massive concrete blocks on the sea-bed.
One of the concerns about submerged turbines is the possibility of their becoming massive mincing plants, shredding fish and other creatures passing through the area. However, it is thought that the slow rotation rate of the turbine blades is sufficient to allow fish, whales and other deep-sea creatures to take evasive action when they encounter the devices.
Deployment of technology in Cook Strait has highlighted unforeseen hazards in the past. Strong tides causing flows of pebbles and small rocks have damaged cables in the past, requiring their replacements to be armoured against damage. In the late 1990s an armour-plated fibre-optic cable was severed in mysterious circumstances, and it is thought that illegal fish-trawling activity might have been responsible.
If the trial goes ahead, the turbines will be floating in Cook Strait part-way between the seafloor and the surface, presenting a unique opportunity to learn about the hazards that exist at those depths.
Generating electricity from tidal currents is still an experimental science, and a trial in Cook Strait will greatly improve our knowledge of the risk and commercial potential involved. Two other locations have been identified for trials, Foveaux Strait and the Manukau Harbour, but these are shallower environments and more susceptible to weather and shipping hazards.
Tidal generation is more forecastable than wind generation, and it is thought that deeper generators have a longer duty cycle because they are less susceptible to weather variations. Nevertheless, the environment has its own challenges with corrosion, seismic activity and debris flows being issues for further study. A cost comparison with wind generation will be made possible once a trial is underway.
If tidal generation proves to be commercially viable, New Zealand’s hydro-electric generation will still be a crucial part of our energy supply, providing the majority of our base-load generation at times when wind is light and tides are on the turn.