Test Tidal Power Technology in Cook Strait

Victoria Vaganov University of Hawaii

Dr. Alex Malahoff GNS Science

Good Conditions (1)

•  Dependence on geography

•  Powerful currents are found where flows are forced through CS

•  Power ∝ velocity3

GDP derived velocity vectors. Source: Chriswell and Rickard 2006

Good Conditions (2) Viable locations for tidal power (a.k.a. tidal current power or tidal in-

stream power) are dependent on geography. Ideal locations exist where two large land masses restrict flow. This happens to be the case in Cook Strait, CS, which is located between the North and South Islands of New Zealand (NZ).

In the waters around NZ, the general direction of circulation in the ocean is towards the east. Strong circulation currents coupled with a unique tidal cycle makes CS a region of high kinetic energy. Mass conservation and continuity dictate that the location with the smallest cross sectional area will have the largest velocities. Therefore the width as well as the depth of the Strait are important factors. The most powerful flows are expected in the narrowest point of the Strait. It is important to note that on the Tasman Sea side the depth is approximately 100 meters and once the narrowest point is crossed the depths quick drop to approximately 1,000 meters in Cook Strait Canyon, a submarine canyon.

Power in flows is proportional to velocity cubed. So, small changes in velocities produce large variations in available power. Observations of current velocities range in ~1 m/s at the bottom of the water column to ~3.5 m/s near the surface (Vennell & Collins 1991).

Extraordinary Conditions (1)

Extraordinary Conditions (2) The previous plate was taken from a TRANSPOWER brochure.

The Cable Protection Zone (CPZ) is shown in red. The cable landing location on the East side of the Strait is Oteranga Bay; on the West side of the Strait is Fighting Bay. Many activities are restricted in the cable protection zone and all ocean energy development operations need to be cautious in these waters. The cables are a hazard but the electrical substations at both ends are a great resource to ocean energy developers in the region.

The presence of a high voltage transmission system directly adjacent to the high energy region of the environment is ideal. It is also terrific that the resource, which is adjacent to an already present high voltage transmission system, is next to the capital of NZ. The population in the region of Wellington is about half a million. Nearly all Wellington residents live within 3 km of the coastline. The electrical infrastructure and the human resource in the area provide extraordinary conditions for the development of an ocean energy system in CS.

Transmission Infrastructure (1)

•  Key for offshore electrical power production

•  AC and DC in area provides flexibility

Source: Transpower 2009

Transmission Infrastructure (2) Here is another look at the transmission infrastructure in NZ.

Haywards electrical substation is where direct current, DC, is converted to alternating current, AC, for distribution through the North Island. If power is being diverted to the South Island then AC current is converted to DC at Haywards before it is sent down to the South Island.

NZ uses 230 volt power at 50 Hz. You use AC current regularly but DC current is used for bulk transmission of energy from distant generating stations. The use of high voltage, HV, current is necessary to prevent large transmission losses. The advantages of HVDC over HVAC systems for bulk transmission include higher power ratings for a given line (important since installing new lines and even upgrading old ones is extremely expensive) and better control of power flows.

The presence of Haywards electrical substation which handles AC and DC at the foot of the ocean energy resource is terrific and it will give power engineers some flexibility with the design of ocean energy systems in the region.

CPZ in CS (1)

CPZ in CS (2) The previous plate is a NIWA map showing

bathymetry in the centre of CS. The approximate boundaries of the CPZ are drawn in red. The CPZ takes over much of the narrowest passage in the Strait. The narrowest point to the Strait is approximately 12 nautical miles or 22 kilometres. However the steep drop off of the shelf would limit the distance away from shore where power systems could be placed.

As the Strait widens, it deepens, south of the CPZ. Therefore to take advantage of the highest flow rates (most powerful flows) power systems should be placed north of the CPZ.

CPZ in CS (3)

CPZ in CS (4) Here the green line is the approximate location of the 100 meter

depth counter and the blue line is the approximate line of the 200 meter contour. The 100 meter contour coincides with the shelf edge in most areas shown here.

Studies on the seabed composition in the CPZ reported a westward fining of sea sediments (Carter). Pebbles and gravels are seen on the sea bed on the east and more sands and fines are seen on the west of the Strait. This sediment characteristic indicates that flows are too strong on the east side of the Strait to allow fines to settle.

The currents on the Wellington side of the Strait side move quicker and therefore have more energy than the eastern part of this point in the Strait. The high velocities/energies of the flows on this side, allow the flows to hold a larger sediment load; this is why the westward sediment fining was observed by studies around the cable.

Consequently, a power system placed on East side of the Strait would be expected to be more productive than one closer to the South Island.

Human Resource (1)

•  Key to the development of energy resource •  Long standing hydro electric generation

history •  Many skilled professionals in the hydro

electric industry could cross over and help move the development of tidal power. – Unique and invaluable trait of the CS

location for tidal power technology deployment

Human Resource (2) The cable was put across the Strait to move electricity from the

hydro electric plants in the South Island to the North Island, where most of the demand lies in the Auckland region. The cable extends from Benmore to Haywards.

NZ has a long standing hydro electric generation history. NZ has many skilled professionals in the hydro electric industry who could cross over and help move the development of tidal power. This is a unique and invaluable trait of the CS location for tidal power technology deployment.

The jobs created by ocean energy projects will require environmental, engineering, legal and other specialists. TRANSPOWER, government agencies and others have decades of experience with engineering projects in the Strait. The challenges ocean energy systems will face could not be overcome by a better equipped group than NZ nationals.

Permitting and Timing (1) Initial steps in development will include permitting. The history of

engineering projects in CS provides a protocol for permitting projects in the area.

Devices not expected to impact the environment can be placed in CS for 31 days without special permitting (Department of Conservation). Tidal power device are not expected to make significant impacts on the environment. No measurable effects to the environment would be expected in the short time interval of one month.

A testing period for a tidal power technology device would fit well into the 31 day permit free period. Thirty days is the exact span of time needed to make a good evaluation of technology in the tidal environment. A 30 day period of observation is necessary to differentiate between the different tide effects, e.g. M tide vs S tide. The different tides are the M, S, N, R, O, P etcetera. The M tide or moon tide is approximately 80% of the tidal force.

There are other benefits of this permitting protocol but they are beyond the scope of this work.

Permitting and Timing (2)

•  Devices not expected to impact the environment can be placed in CS for 31 days without special permitting

•  Exact span of time needed to make a good evaluation of technology in the tidal environment.

•  A 30 day period of observation is necessary to differentiate between the different tide factors, e.g. M tide vs S tide

Conclusions (1) In conclusion, a viable operational site for ocean

energy projects is on the NE side of the CPZ, just north of Cape Terawhiti. This location offers a great human resource and has access to the electrical infrastructure of the country, as well as the energy resource of strong currents in the area.

A favorable turbine would operate directly in the high energy flows through CS at depths no greater than 100 meters. Any successful system would be modular with deployment/retrieval times that would not exceed 1 day. Also, allowable systems would not pose a threat to marine life in the area.

Conclusions (2) •  Operational site on NE of CPZ

–  Just North of Cape Terawhiti •  Energy resource •  Human resource •  Electrical infrastructure

–  Favorable turbine would • operate at <100 m • Quick (1 day) deployment/retrieval time • Safe for marine life

References •  Chriswell, Stephen M. and Rickard, Graham J. 2006. “Comparison of

model and observational ocean circulation climatologies for the New Zealand region.” Journal of Geophysical Research, Vol 111, C10011, loi:10.1029/2006JC003489.

•  Transpower. “A Guide to Transpower 2009. Keeping the energy flowing.” Available at www.transpower.co.nz/f2484,12225290/12225290_guide-to-

transpower.pdf Accessed May 7, 2009. “Cook Strait Cable Protection Zone Brochure”. Available online at

www.transpower.co.nz •  Wellington City Council. Facts and Figures posted online May 1, 2009.

Available at http://www.wellington.govt.nz/aboutwgtn/glance/index.html

•  Vennell, R. and Collins, N. 1991. “Acoustic Doppler Current Profiler Measurements of Tides in Cook Strait, New Zealand.” Coastal Engineering - Climate for Change” 10th Australasian Conference on Coastal and Ocean Engineering, Auckland, 2-6 December 1991.

Test Tidal Power Technology in Cook Strait

Victoria Vaganov University of Hawaii

Dr. Alex Malahoff GNS Science