French Pass has the fastest tidal flows in New Zealand
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.[1][failed verification] 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.[2] However, the association doesn't list its members.[3]
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.[4]
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 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.[5] Headlands and constrictions like these focus the currents, giving energy levels reaching 750 W per square metre.[6]
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.[5]
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.[5]
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.
Altogether there are sixty-two recognised natural influences on the tides, though only some will be significant at a given location. The gravitation of the Moon and Sun are the most important.
Global surface elevation of the M2 ocean tide (NASA)[7]
This computer animation shows the peaks and troughs of the M2 tides sweeping anticlockwise around New Zealand. When it is high tide on one side of Cook Strait it is low tide on the other side. For this reason, Cook Strait can experience exceptionally fast tidal flows.
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.[5]
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.[5] The National Institute of Water and Atmospheric Research (NIWA) run a tidal computer model specific to New Zealand.[8]
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.[9] 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
Generation and tide height
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
Tidal patterns in Cook Strait. The south part (Nelson) has two spring tides per month, versus only one on the north side (Wellington and Napier).
Cook Strait has tidal flows amongst the strongest in the world, even though it has a smaller tidal range than most places in New Zealand. This is because the main M2 lunar tide component which circulates anti-clockwise around New Zealand is out of phase at each end of the strait. On the Pacific Ocean side the high tide occurs five hours before it occurs at the Tasman Sea side. On one side is high tide and on the other is low tide. The difference in sea level can drive tidal currents up to 2.5 metres per second (5 knots) across Cook Strait as well as into the Tory Channel.[5][10] An unusual complication is that although there are two spring tides a month on the south side, the north side has only one spring tide a month, as shown in the plot. A further consequence of these opposed tides is that there is almost zero tidal height change at the centre of the strait. Although the tidal surge should flow in one direction for six hours and then the reverse direction for six hours, a particular surge might last eight or ten hours with the reverse surge enfeebled. In especially boisterous weather conditions the reverse surge can be negated, and the flow can remain in the same direction through three surge periods and longer. This is indicated on marine charts for the region.[11]
There are numerous computer model representations of the tidal flow through Cook Strait. While the tidal components are readily realizable,[12] the residual flow is more difficult to model.[13]
In April 2008, a resource consent was granted to Neptune Power for the installation of an experimental underwater tidal stream turbine in the strait. The turbine has been designed in Britain and will be built in New Zealand at a cost $10 million. Fourteen metres in diameter and constructed of carbon fibre, it will be capable of producing one megawatt. It will be placed in eighty metres of water, 4.5 kilometres due south of Sinclair Head, in waters known as the “Karori rip”. Power from the turbine will be brought ashore at Vector'sIsland Bay substation. The turbine is a pilot, and will be sited in slower tides for testing. Neptune hopes to generate power from the unit by 2010. The company has claimed there is enough tidal movement in Cook Strait to generate 12 GW of power, more than one-and-a-half times New Zealand's current requirements.[10][14][15][16] In practice, only some of this energy could be harnessed.[17]
On the other side of the strait, Energy Pacifica has talked for some time about applying for resource consent to install up to ten marine turbines, each able to produce up to 1.2 MW, near the Cook Strait entrance to Tory Channel. They claim Tory Channel has tidal flows of 3.6 metres per second with good bathymetry and access to the electricity network. No application had been lodged by March 2011.[10]
The power generated by tidal marine turbines varies as the cube of the tidal speed. Because the tidal speed doubles, eight times more tidal power can be produced at spring tides than at neap tides.[10]
The entrance to Kaipara Harbour, one of the largest harbours in the world, is a channel to the Tasman Sea. It narrows to a width of 6 kilometres (3.7mi),[19] and is over 50 metres (160ft) deep in parts. On average, Kaipara tides rise and fall 2.10 metres (6.9ft). At high tide, nearly 1000 square kilometres are flooded. Spring tidal flows reach 9km/h (5 knots) in the entrance channel and move 1,990 million cubic metres per tidal movement or 7,960 million cubic meters daily.[20]
In 2011, Crest Energy, a power company, received resource consent to install about 200 underwater tidal turbines for the Kaipara Tidal Power Station, which would use the substantial tidal flows moving in and out every day near the harbour mouth to produce electricity for approximately 250,000 homes.[21]
Crest plans to place the turbines at least 30 metres deep along a ten kilometre stretch of the main channel. Historical charts show this stretch of the channel has changed little over 150 years. The output of the turbines will cycle twice daily with the predictable rise and fall of the tide. Each turbine will have a maximum output of 1.2 MW, and is expected to generate 0.75 MW averaged over time.[20][22]
The peak level of generation for the combined turbines is about 200 MW. This exceeds the projected peak electricity needs of Northland. It would have environmental benefits in offsetting annual carbon emissions from a thermal-based, gas turbine generator of 575,000 tonnes of carbon.[20] The project is costed at about $600 million and to be economic would have to be scaled up rapidly to near full capacity.[23]
However, while the Department of Conservation has approved the project, and has made substantial environmental monitoring conditions part of the consent, the project also has objectors on the grounds of claimed influences on the local ecosystems and charter fishing.[24] Appeals before the Environment Court were concluded in 2010, with a favourable decision released in February 2011.
Wave power
Wave power involves converting the energy in ocean surface waves into electricity using devices either fixed to the shore, the seabed or floating out at sea. Wave energy varies with time, depending on when and where the winds and storms that drive the waves occur. Tidal energy is more regular and predictable.
Two wind zones affect New Zealand. South-east trade winds dominate in the north, enlivened by an occasional cyclone from the tropics. The rest of the country is dominated by the roaring forties, a broad band of westerly winds that span the middle latitudes of the southern hemisphere. The roaring forties extend over most of the southern part of the Tasman Sea and the Southern Ocean. These winds produce some of the stormiest seas in the world, with maximum wave heights regularly exceeding 4 metres.[25]
On average, ocean waves in New Zealand deliver about 25kW to each metre of coastline.[6] The west and south-west coasts have the country's most energetic waves. Even on windless days, swells that were generated in the Southern Ocean still arrive. Less wave energy arrives at the north-east coast, because it is sheltered from the south-west waves (click the link on the right for a diagram).[25] The amount of energy in a wave is proportional to the square of its height, so a two-metre wave contains four times the energy of a one-metre wave.
Wave Energy Technology - New Zealand (WET-NZ) is a Government-funded research and development collaboration programme between Industrial Research Limited, a Crown Research Institute, and Power Projects Limited, a privately owned Wellington-based company. The programme seeks to develop a wave energy device that generates electricity from both the kinetic and potential energy available in open ocean waves. In 2010 WET-NZ received resource consent for half-scale prototype testing at two test sites.[26] The device is now called Azura and is being tested in Hawaii.
1966: The world's first tidal barrage project goes online at La Rance, France, with a capacity of 240 MW.
2003: Seaflow, the world's first underwater turbine prototype, goes on stream off north Devon with a peak capacity of 300 KW.[27]
2008: The world's first commercial wave farm goes on line in Portugal. It uses Pelamis devices and has a peak capacity of 2.25 MW.[28][29]
2008: SeaGen, the world's first commercial scale tidal stream energy generator, goes on stream in Strangford Lough, Northern Ireland with a peak capacity of 1.2 MW.[30]
2008: Neptune Power given resource consent to build a pilot turbine off Sinclair Head in Cook Strait.[32]
2008: Energy Pacifica applies for resource consent to install up to 10 marine turbines, each able to produce up to 1.2 MW, near the Cook Strait entrance to Tory Channel.[10]
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and are also caused by the Earth and Moon orbiting one another.
A strait is a landform connecting two seas or two water basins. While the landform generally constricts the flow, the surface water still flows, for the most part, at the same elevation on both sides and through the strait in both directions. In some straits there may be a dominant directional current through the strait. Most commonly, it is a narrowing channel that lies between two land masses. Some straits are not navigable, for example because they are either too narrow or too shallow, or because of an unnavigable reef or archipelago. Straits are also known to be loci for sediment accumulation. Usually, sand-size deposits occur on both the two opposite strait exits, forming subaqueous fans or deltas.
Cook Strait is a strait that separates North Island from the South Island of New Zealand. The strait connects the Tasman Sea on the northwest with the South Pacific Ocean on the southeast. It is 22 kilometres (14 mi) wide at its narrowest point, and is considered one of the most dangerous and unpredictable waters in the world. Regular ferry services run across the strait between Picton in the Marlborough Sounds and Wellington.
The Pentland Firth is a strait which separates the Orkney Islands from Caithness in the north of Scotland. Despite the name, it is not a firth.
Tidal power or tidal energy is harnessed by converting energy from tides into useful forms of power, mainly electricity using various methods.
Tory Channel is one of the drowned valleys that form the Marlborough Sounds in New Zealand. Inter-island ferries normally use it as the principal channel between Cook Strait and the Marlborough Sounds.
Kaipara Harbour is a large enclosed harbour estuary complex on the north western side of the North Island of New Zealand. The northern part of the harbour is administered by the Kaipara District and the southern part is administered by the Auckland Council. The local Māori tribe is Ngāti Whātua.
Marine currents can carry large amounts of water, largely driven by the tides, which are a consequence of the gravitational effects of the planetary motion of the Earth, the Moon and the Sun. Augmented flow velocities can be found where the underwater topography in straits between islands and the mainland or in shallows around headlands plays a major role in enhancing the flow velocities, resulting in appreciable kinetic energy. The Sun acts as the primary driving force, causing winds and temperature differences. Because there are only small fluctuations in current speed and stream location with minimal changes in direction, ocean currents may be suitable locations for deploying energy extraction devices such as turbines. Other effects such as regional differences in temperature and salinity and the Coriolis effect due to the rotation of the earth are also major influences. The kinetic energy of marine currents can be converted in much the same way that a wind turbine extracts energy from the wind, using various types of open-flow rotors.
The European Marine Energy Centre (EMEC) Ltd. is a UKAS accredited test and research centre focused on wave and tidal power development, based in the Orkney Islands, UK. The centre provides developers with the opportunity to test full-scale grid-connected prototype devices in wave and tidal conditions.
The following outline is provided as an overview of and introduction to Oceanography.
SeaGen was the world's first large scale commercial tidal stream generator. It was four times more powerful than any other tidal stream generator in the world at the time of installation. It was decommissioned by SIMEC Atlantis Energy Limited in summer 2019, having exported 11.6 GWh to the grid since 2008.
Marine Current Turbines Ltd (MCT), was a United Kingdom-based company that developed tidal stream generators, most notably the 1.2 MW SeaGen turbine. The company was bought by the German automation company, Siemens in 2012, who later sold the company to Atlantis Resources in 2015.
The Aotearoa Wave and Tidal Energy Association (AWATEA) is a New Zealand organisation established in 2006 to promote renewable energy from marine sources. This includes energy from tides, waves and ocean currents.
Marine energy or marine power refers to the energy carried by ocean waves, tides, salinity, and ocean temperature differences. The movement of water in the world's oceans creates a vast store of kinetic energy, or energy in motion. Some of this energy can be harnessed to generate electricity to power homes, transport and industries.
A tidal stream generator, often referred to as a tidal energy converter (TEC), is a machine that extracts energy from moving masses of water, in particular tides, although the term is often used in reference to machines designed to extract energy from the run of a river or tidal estuarine sites. Certain types of these machines function very much like underwater wind turbines and are thus often referred to as tidal turbines. They were first conceived in the 1970s during the oil crisis.
A tidal barrage is a dam-like structure used to capture the energy from masses of water moving in and out of a bay or river due to tidal forces.
The Kaipara tidal power station was a proposed tidal power project to be located in the Kaipara Harbour. The project was being developed by Crest Energy, with an ultimate size of 200MW at a cost of $600 million. Consent for part of the project was granted in 2011, but it was put on hold in 2013 and has not progressed since.
The Alderney Race is a strait that runs between Alderney and Cap de la Hague, a cape at the northwestern tip of the Cotentin Peninsula in Normandy. A strong current runs through the race north of the Passage de la Déroute, a treacherous passage separating the Cotentin from the Channel Islands. The current is intermittent, varying with the tide, and can run up to about 12 knots during equinoctial tides. The French call it Raz Blanchard. In Norman French it is called L'Raz.
A hapua is a river-mouth lagoon on a mixed sand and gravel (MSG) beach, formed at the river-coast interface where a typically braided, although sometimes meandering, river interacts with a coastal environment that is significantly affected by longshore drift. The lagoons which form on the MSG coastlines are common on the east coast of the South Island of New Zealand and have long been referred to as hapua by Māori people. This classification differentiates hapua from similar lagoons located on the New Zealand coast termed waituna.
Many tidal stream generators have been developed over the years to harness the power of tidal currents flowing around coastlines. These are also called tidal stream turbines (TST), tidal energy converters (TEC), or marine hydro-kinetic (MHK) generation. These turbines operate on a similar principle to wind turbines, but are designed to work in a fluid approximately 800 times more dense than air which is moving at a slower velocity. Note that tidal barrages or lagoons operate on a different principle, generating power by impounding the rising and falling tide.
↑ Bowman, M.J., A.C. Kibblewhite, R. Murtagh, S.M. Chiswell and B.G. Sanderson (1983) Circulation and mixing in greater Cook Strait, New Zealand. Oceanologica Acta 6(4): 383-391
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