The relatively low labour rates and a commitment to sustainable energy make the idea of placing marine turbines for power generation in the mouth of a natural harbour not as unrealistic as it might seem in other countries. New Zealand produces over 60% of its electricity from renewable energy, primarily from hydroelectric and geothermal sources.

The Kaipara harbour in northern New Zealand has a number of factors making it suited to marine turbines. The tide itself is not that large, averaging nearly seven feet (2.10 metres), but the harbour is about 40 miles (60 kilometres) long and an immense volume of water passes through a relatively narrow channel about four times a day.

The water, which is equivalent to flooding the island of Manhattan to half the height of the Empire State Building (600 feet / 185 metres), or filling a cube 2200 yards square (2 x 2 x 2 kilometres), reaches around five miles per hour (8 kph) through the channel. The lack of commercial shipping in harbour is also important, allowing turbines to be sited about 24 feet (7 metres) under the low tide mark giving free passage to leisure craft above. The harbour is situated just north of New Zealand’s largest city, Auckland.

Crest Energy has applied for permits from the New Zealand government authorities. The application process should be complete in late 2007. Assuming a positive outcome, the preferred plan (although there are several options) is to raise capital through a public offering of shares, and to proceed with construction of the key three elements: installation of 200 marine turbines in the mouth of the Kaipara, placing two five mile (7 kilometre) DC cables across the harbour from the shore to the turbines, and the construction of DC-AC converters and substation on land. The turbines are expected to produce around 200MW, enough to power NZ 250,000 homes.

Of the three key elements, most of the technology is accepted and robust. HVDC cables, converters and substations are common. The DC generators on the turbines use self-excited magnets with simple motor windings, and are also common. DC is important to match voltages and currents to periodic tidal generation, independent of the national grid for synchronisation and reactive power compensation. The only part of the project that is leading edge are the turbine blades and structures themselves. These can be constructed in steel, concrete or modern composites.

Crest Energy seek to offer opportunitities to current and potential marine turbine suppliers. The turbines need to be completely submerged at all times and scaleable from around 15 to 25 yards tall (16-24 metres).

The project has many facets aside from the obvious technology issues. The Kaipara Harbour is a beautiful and delicate environment (see Gallery ), with many large mammals including whales, dolphins, and orcas. Huge numbers of wading and diving birds feed in the harbour. To minimise possible impacts on the environment the cables follow a longer but safer route, and will be buried at least three feet under the harbour floor. High voltage DC radiates about 5% of the EMF of AC (sharks, skates and rays can be disrupted by electrical radiation).

The total costs over ten years are about US$400 million offset by modest but growing revenues from year three. Annual revenue is theoretically US$70 million at current wholesale electricity prices, and of course more at retail prices.

Tidal power is predictable, sustainable, silent and invisible. Tidal power complements other energy sources and will, eventually, be important to maritime nations.