Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 88

Offshore vs. Onshore Wind Power Station Costs
Many utility-scale wind turbines are now being installed in offshore wind power plants
in Europe, as illustrated in Figure 2-4 and listed in Table 3-9. According to the European
Wind Energy Association ( EWEA ) if certain institutional barriers are removed, up to 40 GW
( gigawatts ) of offshore generating capacity could be operating in the European Union by 2020,
producing 4 percent of Europe's electricity supply [EWEA 2007]. In the Eastern U.S., the
amount of space available for off-shore wind turbines is many times larger than for on-shore
wind power stations [Musial and Butterield 2004]. Two economic issues affecting the cost of
energy from offshore wind turbines will be addressed here, namely (1) estimated comparative
costs of offshore and onshore components, and (2) estimated beneits of potentially higher en-
ergy production as a result of lower wind shears over water.
Comparative Cost Estimates
Early estimates of the additional costs that builders of offshore wind turbines might face
were made by Madsen and Svenson [1995 and 1997] who concluded that an offshore system
would cost approximately 40 percent more than an equivalent system onshore. The added
costs were primarily for the foundation (six times as expensive), the grid connection (three
times as expensive), and assembly and transportation (each two times as expensive).
These early estimates have now been updated to relect current experience, costs, and
additional items required for offshore applications in relatively shallow water [Fingersh et
al. 2006]. Results from this study are summarized in Table 2-5. Cost data in this study were
calculated for a 51 MW wind power station, composed of 17 turbines rated at 3.0 MW each.
Overall, offshore application costs are now estimated to be approximately 80 percent higher
than equivalent onshore costs. Costs of the turbines themselves are expected to be roughly
equal. The major increase is in the offshore balance of station costs, which are estimated to
total about 2.2 times those of onshore wind power stations. As listed in Table 2-6, offshore
foundation costs are estimated to be approximately 17 times those for onshore foundations,
even for installations in relatively shallow water. Economical deep-water foundations are a
subject of current research [Butterield et al. 2007].
The additional items speciic to offshore applications listed in Table 2-5 add about
28 percent to the system cost. These items include a warranty premium (10 percent) and
turbine marinization (8 percent), which refers to the application of special materials and
coatings to turbine components to resist deterioration from moisture and salt in the marine
environment.
Potential for Increased Energy Output of Offshore Wind Turbines
In the long term, offshore winds are expected to be more advantageous for wind energy
conversion than onshore winds. Winds at sea tend to blow faster and more uniformly than over
land [Musial and Butterield 2004]. Winds over open water are characterized by signiicantly
lower wind shears because of the lower surface roughness of large expanses of water (see Table
8-3). In order to measure annual wind speed durations and wind shears undisturbed by land
roughness and temperature variations, special installations of high-elevation anemometers are
required miles from shore. Such wind monitoring sites are rare.
Figure 2-21 shows the installation by helicopter in 2005 of one such anemometer tower
on the water intake crib located in Lake Erie 3.5 miles north-northwest (NNW) of Cleveland,
Ohio . The Cleveland Water Crib is a large offshore structure 50 ft in diameter, containing
the primary inlet to the municipal water supply. Cup anemometers, wind direction vanes, and
thermometers are now mounted on this tower at elevations of 30, 40 and 50 m above water