Tidal Energy

ME927 Tidal energy Gravity and the tides

Earth Moon

 Lunar cycle has a period of about 12h 25min.  Tidal range would be very small (about 0.5 m) if the earth were covered in water. But the land masses interfere, and create large ranges in some parts of the world.  Two power extraction methods: tidal range and tidal stream.

ME927 Tidal energy 2 Tidal range: single-effect barrage system

S: sluice gates T: turbines S Sea Basin T

Sea Basin

Mean level z1 Z z2

Datum

ME927 Tidal energy 3 Operation of a single effect barrage system

pumping

1 0.8 0.6 0.4 0.2 0 -0.2 0 750 -0.4 -0.6 -0.8 Filling Standing Standing -1 Standing Pumping Power generation

 Basin is filled through the sluices until high tide; sluice gates are closed.  There may be pumping using grid electricity to raise the level further.  Turbine gates closed until the sea level falls to create sufficient head across the barrage.  Gates opened and turbines generate until the head is again low.  Sluices are opened, turbines disconnected and the basin is again filled.

ME927 Tidal energy 4 La Rance tidal barrage system, France

Power: 240 MW (from 24 turbines) Annual generation: 500 GWh Tidal range: 8 m Capacity factor: 28% Length: 700 m

 Location: Brittany, France  Completed 1967  Capital cost €94.5 million  Operated initially as an experimental power plant  Owned and operated by Électricité de France

ME927 Tidal energy 5 Proposed Severn barrage, UK

 Length: 16 km  Tidal range: 10 m  Rated power: 7 – 12 GW

Artist’s impression

Arguments against:  Estimated cost: £34 billion  Scale and impact would be unprecedented  Major environmental impacts  Other significant resources exist in the UK

ME927 Tidal energy 6 Tidal stream power extraction

 Conversion technology still immature.  Proposed designs offer a wide variety of configurations and mooring arrangements.  Three major contenders: . horizontal-axis turbines . vertical-axis turbines . oscillating hydrofoils  Pentland Firth estimated at ~1.9 GW or ~40% of the total Scottish demand.

Aqua-RET 2012

ME927 Tidal energy 7 European Marine Energy Centre (EMEC) http://www.emec.org.uk/

 Established in 2003, located in Orkney  Provides developers with accredited, open-sea testing facilities  Maintains 14 grid-connected test berths and associated infrastructure

ME927 Tidal energy 8 Stingray

Artist’s impression

 Employs an oscillating hydrofoil.  Hydraulic power take-off system.

ME927 Tidal energy 9 Stingray trial in Yell Sound, Shetland, September 2002

 Device produced 90 kW output in a 1.5 m/s current.

ME927 Tidal energy 10 Testing of prototype turbine near Lynmouth, Devon.

 Project managed by Marine Current Turbines Ltd.  Artist’s impression of a twin 2-  300 kW rating - 2 blades, rotor diameter 11 m. bladed installation.  No yaw mechanism and no electrical connection to  Rotors contra-rotate to shore. minimise the reaction torque on the structure.

ME927 Tidal energy 11 Strom AS, Norway

 Rated at 300 kW, rotor diameter 20 m.  Deployed in fjord near Hammerfest in 2003.  Turbine has a tripod supporting framework with gravity ballast.  Joint venture with ScottishPower to deploy a proposed10 MW farm in the Sound of Islay, Scotland.

ME927 Tidal energy 12 Open Hydro  Hub-less design with a permanent magnet generator around the rotor rim.  Rigidly mounted, accepts bi- directional flow.  Prototype device has been evaluated at EMEC.  Further demonstrator planned for Bay of Fundy, Nova Scotia.

artist’s impression

ME927 Tidal energy 13 Lunar Energy

 Bottom-mounted, bi- directional device.  Turbine and generator can be extracted from main structure for servicing.  1 MW prototype tested at EMEC in 2007.  8 MW farm proposed in joint venture with EON UK.

ME927 Tidal energy 14 SMD Hydrovision

artist’s impression

 Main picture shows 1/10 scale turbine on test site at NAREC UK.  Features a moored buoyant structure for use in deep water.  Full scale device has 2 rotors of 15m diameter, rated at1 MW.

ME927 Tidal energy 15 SeaGen  Designed by Marine Current Turbines Ltd.  Rated power output 1.2 MW.  Twin 16m diameter rotors on a piled supporting column.  Installed in Strangford Lough, Northern Ireland, 2008.

ME927 Tidal energy 16 Voith Hydro

 110 kW prototype.  To be tested in S Korean waters in late 2010.  1 MW version to be produced in 2011 for trials at EMEC.

ME927 Tidal energy 17 Atlantis Resources Corporation

 Australian group with links to Norway and the UK.  Bi-directional, single- rotor machine, 18m diameter.  Atlantis AK1000 1 MW turbine to be tested at EMEC in 2011.

artist’s impression

ME927 Tidal energy 18 MeyGen

 Consent obtained for installation in the Pentland Firth, between Orkney and Scottish mainland.  AR1000 turbine has a rotor diameter of 22.5 m, weighs 1,500 tonnes, and is rated at 1MW.  Aspiration: 400 turbines http://www.bbc.co.uk/news/uk-scotland-24100811 generating 398 MW. viewed September 2013

“August [2017] proved to be a world record month, providing enough energy to power 2,000 Scottish homes from just two turbines [700 MWh]. “ (David Taaffe, Project Director as quoted in the Independent)

ME927 Tidal energy 19 Alstrom

 22 m long nacelle  weight 150 tonnes  3 pitchable blades, 18 m dia.  buoyant to allow towing  deployed in a water depth of about 40 m  rotates around vertical axis Deployment at EMEC  reached full nominal  power of 1 MW in January 2013 at EMEC  endurance and reliability tests underway

ME927 Tidal energy 20 ESRU/ Nautricity: CoRMaT

 Contra Rotating Marine Turbine (CoRMaT).  2.5 m diameter contra-rotating turbine prototype.  Tow-tank tested and sea trialled in the Firth of Clyde and Sound of Islay.

ME927 Tidal energy 21 CoRMaT characteristics

Two closely spaced dissimilar rotors move in opposite directions.

Reduced Capital Cost . No expensive moorings or pilings required . Commercially viable even at small generating scale

Reliability . Direct drive generator eliminates need for gearbox . No complex blade pitch control

Ease of Maintenance . Easy to deploy and recover . Small number of simple sub-assemblies

Efficient . Increased energy capture compared to single rotors . Always optimally oriented to tidal flow . Increased deployment density due to decreased wake effects

Wide operating envelope . Suitable for deployment in water depths from 8- 500m where maximum tidal energy harvest is likely

ME927 Tidal energy 22 CoRMaT test results

1: Tow-tank tests confirm 2: Sea trials confirm 3: ... and enhanced power neutral buoyancy dynamic stability ... output

nominal plus2deg plus4deg Poly. (nominal) Poly. (plus2deg) Poly. (plus4deg) 50

0.5 40

0.4 30 Hub 20 Vgen (V) 0.3 Pitch (deg) 10 Roll (deg) 0.2 0 Power coefficient Power 0.1 0 20 40 60 80 100 -10

0 -20 2 4 6 8 10 12 Tip speed ratio Time (seconds)

ME927 Tidal energy 23 CoRMaT 750 kW device manufacture

 GFRP Blades – Airborne, Netherlands  Contra-rotating radial PMG – Smartmotor, Norway

ME927 Tidal energy 24 CoRMaT assembly and pre-testing

ME927 Tidal energy 25 CoRMaT deployment at EMEC – September 2013

ME927 Tidal energy 26 Challenges … horizontal axis turbine evolved from wind technology … oscillating aerofoil driving hydraulic accumulators MCT’s SeaFlow

MCT’s EB’s Stingray SeaGen … contra-rotation, tethered  Reduce capital cost.  Limit corrosion and abrasion.  Maintenance and safety issues.  Power take-off at low rotation speed.  Gearing reduction/elimination.  Power transmission/grid access. ESRU’s  Land access and use. CoRMaT  Phased operation of different sites.  Maritime & aquaculture impact.  Overcome vested interest.  Key question: given the daily and monthly velocity variations, can phased tidal stream sites be employed to provide predictable, firm power?

ME927 Tidal energy 27 Synchronised tidal stream farms  3 Scottish coastal sites investigated: • Cape Wrath • Crinan at the Sound of Jura • Sanda off the Mull of Kintyre

 Hourly stream velocities taken from Admiralty charts

2.75 Cape Wrath Crinan Sanda 2.5 2.25 2 1.75 Cape Wrath Crinan Sanda 1.75 1.5 1.5 1.25 1.25 1 ` Note the significant variation 1across the Tidal velocity (m/s) velocity Tidal 0.75 lunar cycle and departures from 0.5 sinusoidal behaviour. 0.75 0.25 ` 0.5

0 (m/s) velocity Tidal 0 3 6 9 12 15 18 21 24 0.25 Spring Time (hours) 0 0 3 6 9 12 15 18 21 24 Spring and Neap tide daily Neap velocity variations at the 3 sites Time (hours)

ME927 Tidal energy 28 1.75 Cape Wrath Crinan Sanda Synchronised power output 1.5 non-uniform 1.25 velocityvariation 1

0.75 ` Assumptions: 0.5 da el it ( s) /s (m ty ci lo ve al id T

0.25

0  Turbines operate in an open stream 0 3 6 9 12 15 18 21 24 environment. Time (hours)

 Dynamic loading ignored (as caused by 14000 Available pow er 13000 Turbine pow er output velocity shear, stream misalignment or wave 12000 11000 action). 10000 9000

8000 device  Turbulence effects ignored. 7000 6000 characteristic  Turbine has a cut-in stream velocity, with 5000 4000

3000

enforced idleness at slack water. 2000

1000  Above a rated stream velocity power is held 0 0 50 100 150 200 250 300 350 400 constant. Time (minutes)

 Device sized for maximum power extraction. 20000 Site 1 Site 2 Site 3 Total 3 18000  Available power given by P = ½ ρ A V (ρ the 16000 fluid density, A rotor swept area and V stream 14000 power output 12000 velocity). 10000 8000 Power (W/m2) Power  Turbine Cp= 0.3, cut-in @ 1 m/s, cut-out @ 6000 4000

2.5 m/s). 2000

0 0 50 100 150 200 250 300 350 400

Time (min) ME927 Tidal energy 29 Site and aggregate power output

350 Cape Wrath Crinan Sanda Total output 325 300 275 250 225 75 Cape Wrath Crinan Sanda Total output 200 175 150 125 Power (kW) Power 100 50 75 50 25 0

spring (kW) Power 25 0 3 6 9 12 15 18 21 24 Time (hours)

neap0 0 3 6 9 12 15 18 21 24 Time (hours)

 Spring tides: a significant base load is evident - about 1/3 of peak.  Neap tides: the outputs are much lower at about 1/4 of peak.  Changes between successive cycles are evident - Sanda is cycling at a higher frequency, a phenomenon that will reverse at another point in the lunar cycle. ME927 Tidal energy 30 Power output fluctuation over the lunar cycle

325 Options: 300  size turbines to restrict the 275 maximum output to that 250 experienced during neap tides; 225 200  size turbines for the average 175 monthly output and introduce 150 other sources of energy to meet 125 Power (kW) Power the shortfall; 100

75  size turbines for the spring tide

50 condition and introduce long

25 term (weekly) energy storage 0 so that the excess capacity 0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 432 456 480 504 528 552 576 600 624 648 672 Time (hours) during near-spring tides can be stored for later use.

 Variation between spring and neap tide power production shown here over a half-month period.  Fluctuations in output due to the lunar cycle affect all sites simultaneously.

ME927 Tidal energy 31 Conclusions

 Some level of base load provision can be achieved via the phased operation of dispersed tidal current power stations.  Difficulties arise due to the non-uniformity of site velocities.  The natural variations that occur between successive tidal cycles (daily and monthly) produce a significant irregularity in aggregate power output.  The predictability of tidal power output may be regarded as a major asset in energy supply management.  The twice-daily cycle may be smoothed by the use of hydraulic pumped storage, or the phased operation of conventional hydro power plant.  The lunar cycle induced variation in power output may be accommodated by complementary sources of energy or energy storage.  Linking widely dispersed sites will place extra demands on the network.  Accurate data are needed to predict the performance of systems of this kind.

 See tutorial questions for typical tidal range and stream calculations.

ME927 Tidal energy 32