New Zealand has large ocean energy resources but does not yet generate any power from them. TVNZ reported in 2007 that over 20 wave and tidal power projects are currently under development. However, not a lot of public information is available about these projects. The Aotearoa Wave and Tidal Energy Association was established in 2006 to "promote the uptake of marine energy in New Zealand". According to their 10 February 2008 newsletter, they have 59 members. However, the association doesn't list its members. From 2008 to 2011, the government Energy Efficiency and Conservation Authority is allocating $2 million each year from a Marine Energy Deployment Fund, set up to encourage the utilisation of this resource. The greater Cook Strait and Kaipara Harbour seem to offer the most promising sites for using underwater turbines. Two resource consents have been granted for pilot projects in Cook Strait itself and in the Tory Channel, and consent is being sought for a project sites at the entrance to the Kaipara. Other potential locations include the Manukau and Hokianga Harbours, and French Pass. The harbours produce currents up to 6 knots with tidal flows up to 100,000 cubic metres a second. These tidal volumes are 12 times greater than the flows in the largest New Zealand rivers.
Tidal power Tidal power is generated by capturing some of the energy in the tides as they cycle forth and back, twice each day. Tidal devices can be weir or dam like structures (barrages), used to hold the tide back, or turbines anchored within the tidal stream. By world standards, New Zealand's tides are, for the most part, moderate. The tide usually ranges between one and two metres. Tidal currents are usually around two kilometres per hour (one knot). Some exception are in and around Cook Strait, where tidal currents can be much stronger, and at the entrance to some harbours, particularly Kaipara Harbour. Headlands and constrictions like these focus the currents, giving energy levels reaching 750 W per square metre. Tides are controlled mainly by the gravitational pull of the Moon. About once a day the Moon rotates around the Earth, attracting as it travels the bulge of water called the high tide that also travels around the Earth. There are actually two high tides, because the Earth and Moon, as a system, both rotate about a common centre of mass. This centre is two-thirds out from the centre of the Earth, not at the centre of the Earth. The effect of the Earth spinning about this centre is that it behaves as a centrifuge, resulting in a second high tide bulge in the ocean most distant from the Moon. A second influence on the tides occurs because of gravitation from the Sun. Gravitation from the Sun has less influence than the Moon, because it is so much further from Earth. However, the Sun influences the tidal range. When the Sun, Earth and Moon are aligned in a straight line (at new and full moon), their tidal effects combine, producing the particularly high and low tides called spring tides. When the Sun is at right angles to the Moon, the effects are partially cancelled, producing the small tides called neap tides. New Zealand has a relatively small tidal range, usually less than two metres. However, some of the larger harbours on the west coast of the North Island, in particular the Kaipara, experience significant currents as the tides rise and fall. In harmonic tidal analysis, tides are represented as the sum of many periodic components. 62 tidal constituents are typically large enough to be considered for use in predicting marine tides, though only a subset are required for practical accuracy at a given location. The gravitation of the Moon and Sun are the most important.
A third influence occurs because the Moon orbits at an angle to the equator. This means that if one of the bulges travelling around the Earth is above the equator, then the other bulge is below the equator. It also follows that some places will have one daily diurnal tide, while other places will have semi-diurnal tides twice a day. For example, there is a diurnal tide in the Ross Sea near Antarctica every 24.84 hours. The height of this tide dwindles to almost zero in a cycle which takes 13.66 days. New Zealand's tides are semi-diurnal. The primary cause, the lunar tide, is labelled the M2. The M stands for the Moon and the 2 stands for twice a day. A fourth influence occurs because the orbit of the Moon around the Earth and the orbit of the Earth around the Sun are elliptical rather than circular. The effect of this is that the time between high tides changes a little from day to day. The Moon takes about 24.8 hours to orbit around the Earth, so it takes half this time, 12.4 hours, for the M2 tides to occur. The tides can be predicted far in advance, because the Moon and Earth have orbits that are predictable. The National Institute of Water and Atmospheric Research (NIWA) run a tidal computer model specific to New Zealand. The actual tide pattern and timing is determined by the nature of the resonances in each ocean basin with the various frequencies of the gravitational influences, over many cycles. New Zealand's situation (like Iceland's) is a small island in a large basin, and the peaks and troughs of the M2 tides sweep continuously anticlockwise around New Zealand. When it is high tide on the west coast, it is low tide on the east coast, and vice versa: the straightforward notion of tidal bulges aligned with the Moon is insufficient. These currents are most noticeable in straits such as Cook Strait and in Foveaux Strait. A notable example is French Pass, just off the greater Cook Strait, where, despite the low tidal range, tidal streams can reach nearly eight knots.
Manapouri
Since the construction of the Manapouri power station, there has been about five MW of tide-determined generation. The tailrace tunnel exit by Dusky Sound debouches at sea level, and thus the effective head of the power station is affected by the level of the tide there. If the turbines are operated at a fixed flow aperture, the power produced is not constant but follows the tide, an effect that can be seen in the following graph. Note that the timing follows the tides around the clock, not the usual twenty-four cycle of electricity usage. The Opunake power station has its tailrace exiting to the beach but its operation is intermittent so if there is any tidal effect on generation there, it is unclear.
Cook Strait
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