Large Tidal Turbine Farms:A tale of two NZ channels

R. Vennell, Tuning turbines in a tidal channel, Journal of Fluid Mechanics, 2010. R. Vennell, Tuning tidal turbines in-concert to maximise farm efficiency, Journal of Fluid Mechanics, 2011R. Vennell, Estimating the Power Potential of Tidal Currents and the Impact of Power Extraction

on Flow Speeds, Renewable Energy, 2011

Ross Vennell Ocean Physics Group, Department of Marine Science, University of Otago

ross.vennell@otago.ac.nz http://www.otago.ac.nz/oceanphysics

Sea-Gen

Two types tidal power

http://en.wikipedia.org/wiki/Tidal_power

1960’s, Worlds Largest 240 MW plant on the Rance River, France

Require large tidal range > 5m

Rare!!

1) Tidal Barrage

2) Tidal Current PowerRequires currents around 2m/s

Common in straits and channels around the world

High density energy at predictable times

Tidal Current PowerTidal Turbines- wet wind turbines?

www.marineturbines.com1.2MW at 2.25 m/s

Verdant Power – New York’s East River Open Hydro (Ireland)

– Canada

Kobold Vertical Axis Turbine– Straits of Messina, Italy

Large Tidal Turbine Farms Different to Wind Farms

Wind Farms are tiny compared to volume weather systems which drive then

->Farm does not affect free-stream flow

NZ Met. Service

Tidal Turbine Farms must be densely packed within channel

• Strong interaction between power extraction and flow-> affects free-stream flow

• Power extraction slows currents along entire channel!

How does power output scale with farm size?

1MW 100 MW’s?

Tidal current research and development

Most: CFD modelling and building single turbines

Few: estimating the limits of production from a given channel

No one: connected the dots by determine how much power a given number of turbines can deliver from a channel

Power extraction slows the flow

-> power does not scale linearly!!

Upper limit for Production in Channels

Number of Turbines ->

FarmPowerProduction

Installed Capacity

Channel’s Upper Limit or Potentialrequires a “wall of turbines”

Decreasing Flow->

Flow will bypass turbines through any gaps needed for navigation!

Maximum realisable with gaps

Gaps to allow Navigation along ChannelBypassing flow and Mixing Losses

Mixing Losses

Bypassing Flow

Turbines

Channel Shoreline

Yes there are equations!

Two examples

EnergyScape, 2009

Kaipara Harbour

Cook Strait

Kaipara HarbourChannel

• 15 km long channel• 25 m deep• 2.5 km wide

Estuary

• 950 km2

• 400km2 dry at low tide

• 1.5-2.7m tidal range

Kaiprara Harbour Entrance At Peak Flow Averaged over Tidal Cycle

Upper Bound or Potential

570 MW 240 MW

Requires Turbines to Fill Cross-section

250 turbines + 40% flow reduction

Filling 10% of cross-section and 10 rows

100 MW 45 MW

Requires 250 turbines + 5% flow reduction

Filling 30% of cross-section and 10 rows

300 MW 130 MW

Requires 740 turbines+ 17% flow reduction

Based on 1.7m/s peak flows and 18m diameter turbine blades and assumes turbines are optimally tuned for the channel.

Power production will be smaller as these values as they don’t allow for

• Mechanical loses in gear boxes

• Electrical conversion and transmission losses

• Energy losses due to drag on turbine’s support structure (?)

• Effects of upstream rows and their turbulence on turbine efficiency (?)

• Energy dissipation with the shallow Harbour due to bottom friction (?)

Cook Strait Channel

• 100 km long channel• 150+ m deep• 25 km wide

High tide at one end when almost low tide at the other

Cook StraitAt Peak Flow Averaged over Tidal Cycle

Upper Bound or Potential

36,000 MW 15,000 MW

Requires Turbines to Fill Cross-Section

15,000 turbines + 34% flow reduction

Filling 10% of Cross-Section and 10 rows

1,800 MW 800 MW

Requires 15,000 turbines + 0.5% flow reduction

Filling 30% of Cross-Section and 10 rows

8,300 MW 3,500 MW

Requires 44,000 turbines + 4% flow reduction

Based on 1.1 m/s peak flows and 18m diameter turbine blades and assumes turbines are optimally tuned for the channel.

Effect Of Current Speed on Turbine Output1.2MW

2.25 m/sRated Current

0.5MW

1.7 m/sKaipara

Power Productionof Sea Gen

Current Speed

0.14MW1.1 m/sCookStrait

Power V 3

Low currents low output per turbine large numbers of turbines required.

Fillin

g mor

e of C

ross

-secti

on

Cook Strait Numbers Unduly Pessimistic

• Install in high flow regions to reduce turbine numbers• These regions will move as a result, but should give higher flows

that 1.1m/s cross-sectional average velocity.

Peter McComb- MetOcean Solutions

Summary• A compromise between Power Production and

1) The fraction of the cross-section turbines are permitted to occupy2) An environmentally acceptable flow reduction

• For Kaipara, 250 18m diameter turbines give an average of 240 MW if channel cross-section filled with turbines and a

40% flow reduction 45 MW if only 10% of cross-section filled and a

5% flow reduction • For Cook Strait low average flows mean large numbers of

turbines are needed, however targeting high flow regions would require far fewer turbines and yield 1-2GW

R. Vennell, Tuning turbines in a tidal channel. Journal of Fluid Mechanics, 2010. R. Vennell, Tuning tidal turbines in-concert to maximise farm efficiency, Journal of Fluid Mechanics, 2011R. Vennell, Estimating the Power Potential of Tidal Currents and the Impact of Power Extraction

on Flow Speeds, Renewable Energy, in press

ross.vennell@otago.ac.nz www.otago.ac.nz/oceanphysics