1. An Overview of the Technology and Economics of Offshore Wind Farms
James F. Manwell, Ph.D.

2. Typical Offshore Windfarm
20, 2 MW Turbines
Middelgrunden Wind Farm (off Copenhagen, Denmark)
Photo: J. Manwell

3. Winds off Massachusetts
Excellent wind resource off the coast
Wind speeds highest furthest from shore
Map: True Wind Solutions, with support from Mass. Tech. Collaborative, Northeast Utilities, CT Innovations

4. Typical Week of Wind in Nantucket Sound

5. Water Depth Moderate depths (less than 100’) presently required
Shallow water (less than 50’) preferred
< 120 ft
< 90 ft
< 60 ft

6. Typical Wind Turbine Converts energy in wind to electricity
Major components
Rotor
Hub
Blades
Gearbox
Generator
Tower

7. Offshore Wind Farms Multiple wind turbines Bottom mounted foundation
Electrical grid between turbines
Power cable to shore
Infrastructure for operation & maintenance

8. Conceptual Design of Typical Offshore Wind Plant
Foundation
Bottom mounted up to ~ 60 ft. depth
Floating structure in deep water

9. Conceptual Design of Typical Offshore Wind Plant
Submarine cable to mainland for power and communication

10. Conceptual Design of Typical Offshore Wind Plant
Barge with crane for installation

11. Support Options for Offshore Wind Turbines

12. Electrical Cables Typical cable layout Cable cross section
Cable trencher
Cable laying ship
Illustrations from

13. Installation
Photos: Courtesy GE Wind

14. Determinants of Cost of Energy
Total installed costs
Turbines, Foundations, Electrical System
Installation
Energy produced
Wind resource
Turbine operating characteristics
Turbine spacing
Operation and Maintenance (O & M)
Scheduled maintenance and repairs
Financial considerations (interest rates, etc.)

15. Factors Affecting Cost of Energy
Number of turbines
Size of turbines
Distance from shore
Water depth
Mean wind speed
Turbine reliability and maintainability
Site accessibility

16. Typical Offshore Capital Costs
Turbine costs (inc. tower): $ /kW
Cable costs: $500k-$1,000,000/mile
Foundation costs:
Costs depend on soil and depth
North Sea: $ /kW
Price increases ~15%-100% when depth doubles (from 25 ft to 50 ft)
Total installed costs: $1200-$2000/kW

17. Offshore Capital Cost Breakdown
Turbine (w/out tower): 17-40%
Tower and foundation: 28-34%
Electrical grid: 9-36%
Other: 6-17%

18. Energy Production Wind resource Turbine power curve Capacity factor
Actual energy/maximum energy
Typical values offshore: 35-45%
Availability
Fraction of time turbine can run

19. Typical O & M Costs 1.0 – 2.0 US cents/kWh O & M increases with
Increased distance from shore
Increased occurrence of bad weather
O & M decreases with
More reliable turbine design
Greater number of turbines

20. Cost of Energy Cost of energy (COE), $/kWh, depends on:
Installed costs, C
Fixed charge rate, FCR – fraction of installed costs paid each year for financing
O & M
Annual energy production, E
COE = (C*FCR+O&M)/E

21. Simple Payback Simple alternative economic measure
Simple payback period (SP), years, depends on:
Installed costs, C
Annual energy production, E
Net price obtained for electricity, P
SP = C/(E*P)

22. Value of Energy Bulk energy sold at wholesale
Internalized social benefits
Wind energy production tax credit (PTC)
Renewable energy portfolio standards (RPS) certificates (RECS)

23. Social (External) Costs of Electricity Production
Costs not accounted for directly in fuel price or production costs
Examples:
Air pollution health affects
Damage due to global warming
Typical estimates:
Coal: 2-15 cents/kWh
Gas: 1-4 cents/kWh

24. Actual Costs of Energy, Existing European Projects - 2001
Turbine size: 450 kW-2000 kW
Number of turbines: 2-28
Wind speeds: ~7.5 m/s
Water depth: 2-10 m
Distance from shore: 250 m-3 km
Cost of Energy: cent (EC) /kWh ( ≈ 5.3 – 11.2 US cent/kWh)

25. Costs as a Function of Distance and Total Size
1997 European study:
7.5 MW wind farm, 1.5 MW turbines,
5 km from coast – 4.9 US cent/kWh
30 km from coast – 6.9 US cent/kWh
200 MW wind farm, 1.5 MW turbines,
5 km from coast – 4.1 US cent/kWh
30 km from coast – 4.4 US cent/kWh

26. Sample Economic Assessment
Assume
Installed cost: $1500/kW
Capacity factor: 40%
Availability: 95%
Value of Energy: 8.3 cents/kWh, based on:
Wholesale: 4 cents/kWh
PTC: 1.8 cents/kWh
RPS: 2.5 cents/kWh
Operation & Maintenance: 1.5 cents/kWh
Fixed charge rate: 14%
Simple payback = 6.6 years
COE= 7.8 cents/kWh

27. Technical Considerations with Sites Further from Shore
Greater energy production
More extreme environment
Greater cable length
Deeper water, larger foundation costs
Technology development useful to reduce costs
Floating supports for deep water

28. Deep Water Possibilities
Delft University, 2001
UMass, 1974

29. Summary
Offshore wind energy is a reality in shallow water, close to shore
Cost of energy higher than from conventional sources, ignoring externalities
COE competitive, including RECS and PTC
Technology for moderately deep water still expensive
Technology for deep water, far from shore remains to be developed