My Presentation ay the MREC in Cambridge, MA in Nov 2010

1. Harnessing and Converting Wave
Power in Mild, Moderate, and Extreme
Wave Climates
Presenter: Michael Raftery, M.E.
Research Engineer
Stevens Institute of Technology
Office: +1 201 216 8704
Testing and Research Funded by
The Office of Naval Research

2. Problems
• Mild wave slopes do not provide sufficient
acceleration of power take-off components for
existing systems to operate efficiently
• Buoyancy and stability related catastrophic
failures have occurred during prototype
deployments
• Power take-off and electricity generation are
strongly coupled in existing systems
• Storm events result in extreme structural loads
all wave climates

3. Develop Solutions
• Capable of increasing wave slopes and
concentrating wave power
• Capable of eliminating single point, buoyancy
and stability related failures
• Capable of decoupling power take-off from
electricity generation with energy storage
• Capable of avoiding extreme anchor loads
during storms

4. Quantify Mild and Moderate Waves
• Average annual significant wave heights of
0.75m or less = mild wave climate
• Average annual significant wave heights of
0.75-1.5m = moderate wave climate

5. Wave Tuning Research
• A variable-depth, fully-submerged tension
leg platform (TLP) was tested in the wave
tank facility at Stevens Institute of
Technology (Stevens) to quantify the
relationships between platform depth,
wave period, wave height, mooring
stiffness, and the changes in wave form.
The test matrix was scaled at 1:10 based
on average waves off NJ/NY/NE

6. Wave Tuning “Shoaling” Feature
TLP Shoals
10cm Wave over
a 20cm Buoy
Buoy Motion
Increases with
Wave Steepness

7. Mild Wave (H =5.1cm)

8. 5.1cm, 2.21s Wave – 15cm Platform Depth
Wave Heights:
•Before Platform:
•5.1cm (2.0 in)
•Over Platform:
•12.7 cm (5.0 in)
•Wave Height
Increase: 150%

9. Mild Wave Results
H =0.051m (2.0in)
Cg = 1.97 m/s at 1.98m tank depth
E = 3.27 J/m^2
P = 6.43 W/m
Shoaled Wave
H = 0.127m (5.0in)
Cg = 1.14 m/s at 0.15m platform depth
E = 20.25 J/m^2
P = 23.07 W/m
259% increase in power density

10. Moderate Wave (H = 10.4cm)

11. 10.4cm Wave – 15cm Platform Depth
Wave Heights:
•Before Platform:
•10.4 cm (4.1in)
•Over Platform:
•17.8 cm (7.0 in)
•Increase in Wave Height:
71%
•Note: waves dropped
below the wave wire over
the platform resulting in
an apparent “flat” bottom

12. Moderate Wave Results
H = 0.104m (4.1in)
Cg = 1.97 m/s at 1.98m tank depth
E = 13.58 J/m^2
P = 26.74 W/m
Shoaled Wave
H = 0.178m (7.0in)
Cg = 1.14 m/s at 0.15m platform depth
E = 39.78 J/m^2
P = 45.32 W/m
69% increase in power density

13. WEHD-Full Scale Design

14. WEHD – Redundant Systems

15. Scaling Power Take-Off
• Mild Waves 0.51m, 7s = 2.04kW/m
• 1.27m Shoaled power = 7.30kW/m
• Moderate Waves 1.04m, 7s = 8.47kW/m
• 1.78m Shoaled power = 14.34kW/m
• Note: Froude scaling is used as power
take-off estimates are based on the wave
making resistance of a surface float

16. Energy Storage System
• 2 – 10m wide buoys = 20m width
• Storage volume, 80 – 300L Accumulators
• = 24m3 = 12m3 working volume
• Storage pressure = 3000psi (200 bar)
• Adiabatic Storage capacity = 303kWh
(1.09GJ)

17. Charge Rate at 35% Efficiency
• 20m x 7.30kW/m x .35 = 51kW
• 20m x 14.34kW/m x .35 = 100kW
• Mild waves charge the energy storage
system in 6 hours
• Moderate waves charge the energy
storage system in 3 hours

18. Discharge Rates
• 75kW for 4 hours
• 150kW for 2 hours
• 300kW for 1 hour
• 600kW for 30 minutes
• 1.2MW for 15 minutes
• 2.4MW for 7 minutes 30 seconds
• 4.8MW for 3 minutes 45 seconds

19. Anchor Load Avoidance
• Anchor loading can be reduced by
lowering the platform near the sea floor

20. 10.4cm Wave – 110cm Platform Depth

21. 10.4cm Wave – 110cm Platform Depth
Wave Heights:
•Before Platform:
•10.4 cm (4.1 in)
•Over Platform:
•10.4 cm (4.1 in)

22. Northeast Wave Energy Region

23. 44008,66m depth, 54NM offshore

24. WEHD Benefits for Existing Designs
• WEHD platforms can provide
steeper waves to existing
systems such as the
“Powerbuoy” developed by
Ocean Power Technologies
(OPTT), and the “Pelamis”
developed by Pelamis Ltd.
• Wave systems can damp waves
approaching offshore structures

25. Contact Information
Michael Raftery, M.E.
Research Engineer
Stevens Institute of Technology
Davidson Marine Laboratory
711 Hudson St.
Hoboken, NJ 07030
Email: michael.raftery@stevens.edu
Phone: 201 216 8704
Fax: 201 216 8214