POWER December 2013 - 52

OFFSHORE WIND
A Wind Energy Plan That Fits
America's Resources
The U.S. lags far behind Europe in offshore wind power production due in part to
deepwater challenges. Using floating vertical axis wind turbines and energy
storage could be the nation's best means to resolve technical and investment
issues and launch the fledgling sector.
Drew Devitt
A
merica is blessed with long coastlines
and relatively deep waters. But ironically,
this has been a disadvantage to
the U.S. offshore wind industry. Compared to
more than 1,000 turbines that are already operating
in the relatively shallow waters around
the British Isles, and the significant offshore
wind turbine generating capacity in many other
European countries, only one offshore turbine
is operational in the U.S. today. This is in no
small part because shallow U.S. coastal waters
are relatively close to the shoreline, which is
problematic because it means offshore installations,
limited by technical hurdles, must be
closer to people, migratory bird patterns, and
within state jurisdictions.
As for any new industry, it is a good idea
to get a big picture view of the sector's particular
circumstances and objectives. First,
it is a given that renewable wind generation
should be relatively close to demand, yet not
in someone's backyard. Several technical
trends are symbiotically conspiring to avoid
" not in my back yard " (NIMBY) issues and
dramatically change the offshore wind model
developed in Europe. These trends include
the development of floating wind turbines
as opposed to seafloor-supported designs,
the use of deep ocean water near U.S. coastlines
as an effective head for energy storage,
and the use of direct current (DC) deepwater
cables in energy transmission. This article
shows how these technologies could work
together in the context of America's natural
resources and political landscape.
The Case for Offshore Wind
Turbines
When used for offshore wind power production,
floating structures have the potential to
reach a much larger and significantly more
energetic wind resource than seafloor-mounted
turbines. At the same time, they increase
social acceptance because they allow turbines
to be installed far away from people.
Sandy Butterfield and his colleagues at
the National Renewable Energy Laboratory
52
(NREL) have published papers confirming
the huge potential advantages of floating wind
turbines, noting in 2010: " The NREL has estimated
the offshore wind resource to be greater
than the 1000 GW of the continental United
States. The wind blows faster and more uniformly
at sea than on land. A faster steadier
wind means less wear on turbine components
and more electricity generated per turbine.
The wind increases rapidly with distance from
the coast, so excellent wind sites exist within
reasonable distances from major urban load
centers reducing the onshore concern of long
distance power transmission. "
To emphasize Butterfield's point regarding
transmission, the best winds within the
continental U.S. are class 3 and 4 winds in
the Great Plains and Mountain States-
but typically 1,500 miles from major load
centers. Comparatively, areas just 30 miles
offshore from major metropolitan hubs see
class 6 winds. This is significant because an
estimated 70% of U.S. electricity demand is
close to its coastlines and the Great Lakes.
It is also important to add that the energy in
wind increases as a cube function of its velocity,
so wind of 6 meters per second (m/s) has
more than double the energy of wind at 4 m/s.
Also, wind velocity near the ocean surface is
higher than on land, as thermal boundary layers
created by the sun heating the land are eliminated
farther from shore. About 20 miles out to
sea, wind currents aloft sink and reattach to the
ocean surface, becoming trade winds. This reduces
the need to elevate the turbine into the air
and improves its capacity factor.
Types of Wind Turbines
Wind turbines can have either a horizontal or
a vertical axis of rotation. Another important
point of differentiation is that wind turbines
employ two basic principles to capture energy
from moving air: aerodynamic turbines
use low-pressure lift (like an airplane wing),
while impulse turbines use drag (like a water
wheel). The differentiating factor is that the
blade tip speed of aerodynamic turbines is a
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multiple of the wind speed, but an impulse
turbine will not spin faster than the wind.
Aerodynamic turbines can be more efficient
than impulse turbines (Figure 1).
An anemometer, a device for measuring
wind speed, is an example of an impulsetype
device with a vertical axis of rotation-
though vertical axis wind turbines (VAWTs)
may employ aerodynamic or impulse design.
Conventional horizontal axis wind turbines
(HAWTs) used widely at utility scale are
an example of aerodynamic turbines with
1. Turbine efficiency by type. This
chart describes the efficiency of different types
of utility-scale wind turbines. The chart's vertical
axis represents the turbine's efficiency as a
percentage of the total energy in the wind. The
horizontal axis represents the relationship between
wind speed and turbine tip speed. Aerodynamic
turbine types have tip speeds of four
to seven times the wind speed, and impulse
turbines have tip speeds on the order of the
wind speed. Aerodynamic turbines are favored
because they have roughly twice the efficiency
of impulse-type turbines, but impulse turbines
have historically been used whenever cost,
reliability, or capacity factor is more important
than efficiency. Source: Jean Lucmenet
2-blade HAWT
Betz max. efficiency
aerodynamic
2-Blade Darrieus VAWT aerodynamic
2-Blade Dutch HAWT aerodynamic
blade American HAWT hybrid
Savonius VAWT impulse
60
50
40
30
20
10
1
2 3 4 5
6 7
POWER | December 2013
3-Blade HAWT aerodynamic
MultiMulti-blade
Efficiency
(%)
http://www.powermag.com
POWER December 2013

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