CROSSWINDS

THE FUTURE OF WIND

FUNDAMENTALS
OF WIND
TURBINES

Wind turbines are the fastest-growing renewable energy source,

and wind energy is now cost-competitive with nonrenewable
resources. (Courtesy: ©Can Stock Photo/ssuaphoto)

36  OCTOBER 2019
Global capacity has grown continuously since 2001, including a 9-percent
increase in 2018.
By JANE MARIE ANDREW

T
he rising concerns over climate change, environmen-
tal pollution, and energy security have increased in-
terest in developing renewable energy. We are seeing
an unparalleled enthusiasm, demand, and growth
in renewable energy production, wind energy being at the
forefront. Wind energy is expanding both onshore and off-
shore with bigger, more powerful turbines, creating new
demands and markets.
The global capacity for generating power from wind en-
ergy has grown continuously since 2001, reaching 591 GW
in 2018 (9-percent growth compared to 2017), according to
the Global Wind Energy Council [1].
Figure 1: Plot of the frequency of occurrence of different wind speeds
over a period of a year. (Courtesy: Sentient Science Corp.)
WIND-PHYSICS FUNDAMENTALS
Wind arises from processes driven by solar energy. The
sun’s energy creates temperature differences that drive air
circulation. Hot air rises, reducing the local atmospheric
pressure; nearby cooler air flows into this region of lower V — Wind speed at height H above ground level.
pressure; this air flow is wind. Vref — Reference speed.
Wind is shaped by both global and local forces. Global Href — Reference height.
patterns are in part the result of the Coriolis force, which H — Height above ground level for the desired velocity, V.
arises from the Earth’s rotation. As cool air flows from H0 — Roughness length in the current wind direction.
higher to lower pressure areas, it is deflected by the Coriolis
force; the direction of deflection depends on latitude. As a EQUATIONS FOR WIND TURBINES:
result, different regions of Earth have different prevailing TURBINE POWER
wind directions. The energy contained in a mass, m, of moving air with ve-
At the other end of the spectrum, local geographical fea- locity v is:
tures can have specific effects. One such effect, familiar to
anyone living near the ocean, is the land breeze. At night,
the water is warm relative to the land, so air is warmed over
the water and rises; the resulting low pressure draws cool The mass flow rate of moving air with a density r through
air from land out to sea: the land breeze. a cross-section area A is:
Although there may be a prevailing wind direction, it is
not the only wind direction. Both direction and speed are
highly variable with geographical location, season, height
above the surface, and time of day. Understanding this vari- The power contained in a flowing mass of air through
ability is key to siting wind-power generation, because high- area A is:
er wind speeds mean higher duty cycles (i.e., longer periods
of active power generation). It is necessary to measure the
characteristics of the wind in great detail, including how
often winds of certain speeds occur (see Figure 1) and how The power extracted by blades of diameter d is:
the surrounding terrain affects the stability of air flow.
A stable flow with a consistent speed is important for
both generating efficiency and structural integrity. Vari-
ability leads to wind shear and wake forces. Wind shear is a where the power coefficient cp has a theoretical limit
function of wind speed, which increases with height above of approximately 0.6; this is referred to as the Betz limit,
the surface. Thus, the shear forces on the rotor blade are which defines the maximum amount of wind kinetic ener-
greater when it is in the top position. gy that can be converted to kinetic energy.
Wake forces are created because the wind slows down
EQUATIONS FOR WIND TURBINES: WIND SHEAR and becomes turbulent as it passes the turbine blades. This
An important consideration for turbine siting and operation is why turbines are widely spaced, usually five to nine rotor
is wind shear when the blade is at the top position. Wind diameters in the direction of the prevailing wind and three
shear is calculated as: to five rotor diameters in the perpendicular direction.

windsystemsmag.com   37
CROSSWINDS THE FUTURE OF WIND

Wind speed also changes as a result of turbulence, which

can be caused by nearby rough terrain, including trees and
buildings; these can cause wind speed to vary greatly even
within several hundred yards or meters. This effect, called
turbulence, decreases efficiency and causes fatigue loading.

WIND POWER FUNDAMENTALS

Energy is captured from wind through the phenomenon of
lift — the same phenomenon that allows birds and airplanes
to fly. (Turbine blades are, in essence, captive wings.) The lift
generated as wind passes over the blade causes it to move,
thereby rotating the main shaft. The rotation is transmitted
through a gearbox to a generator, which converts it into
electricity. The magnitudes of the lift and drag on the tur-
bine blade are dependent on the angle of attack between
the apparent wind direction and the chord line of the blade.
Several different factors influence the power output of
a wind turbine. Among other factors, wind speed and rotor
diameter are the two primary parameters (see Equations
for wind turbines). Figure 2: Profile of power output from a wind turbine over a year.
] Turbine power increases with the square of blade (Courtesy: Sentient Science Corp.)
length. For example, increasing the rotor diameter from
262 feet (80 meters) to 394 feet (120 meters) allows power
to increase from 2 MW to 5 MW (a factor of 2.5).
] Turbine power increases with the cube of wind veloc-
ity. For example, a turbine at a site with an average wind
speed of 16 mph would produce 50 percent more electricity
than the same turbine at a site with average wind speeds of
14 mph.
These two fundamental physical relationships are be-
hind the drive to scale up the physical size of turbines. A
larger rotor diameter allows a single turbine to generate
more electricity, providing better return on installation
cost. And because wind speed and consistency both increase
with height, taller turbines produce a higher and more con- Figure 3. Simplified view of components of an upwind-facing,
sistent supply of electricity. horizontal-axis wind turbine with a gearbox drive. An animation is
A given design operates with a range of wind speeds. available. [2]. (Courtesy: Union of Concerned Scientists, www.ucsusa.
Below the cut-in wind speed, the turbine cannot produce org)
power because the wind does not transmit enough energy
to overcome the friction in the drivetrain. At the rated out- WIND-TURBINE TECHNOLOGY
put wind speed, the turbine produces its peak power (its Turbines come in several general categories based on orien-
rated power). At the cut-out wind speed, the turbine must tation and drivetrain type.
be stopped to prevent damage. A typical power profile for The turbine blades can be oriented around either a ver-
wind speed is shown in Figure 2. tical or horizontal axis. An advantage of the vertical axis is
In addition to an operating range, an installed turbine that blades do not have to be mechanically reoriented when
has a capacity factor that reflects its actual power gener- the wind direction changes. Horizontal-axis turbines also
ation. The capacity factor is the annual average of power come in two general designs. In a downwind design, the
generated divided by the rated peak power. For example, if blades face away from the incoming wind; in an upwind
a turbine rated at 5 MW produces power at an average of design, the blades face into the wind (see Figure 3). More
2 MW, then its capacity factor is 40 percent. In general, a than 90 percent of currently installed turbines are of the
higher capacity factor is preferred, although it may not be upwind type, as this design does not create wind shade be-
advantageous economically. For instance, in a windy loca- hind the tower.
tion, it will be advantageous to use a large-size generator For the drivetrain, in a gearbox-drive design, a gearbox
with the same rotor diameter. This would tend to lower is used to increase the speed transmitted from the rotors to
the capacity factor, but it will lead to substantially larger the generator. In a direct-drive design, the speed is transmit-
annual production. ted directly to an annular generator. Aside from the gear-

38  OCTOBER 2019
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CROSSWINDS THE FUTURE OF WIND

box, the components are generally similar; however, in a

direct-drive turbine, the generator is much bigger because
it must rotate at the same speed as the turbine blades.
The wind-turbine components that experience friction
and wear and require lubrication are the following:
] Pitch bearing (grease).
] Main shaft bearing (grease).
] Gearbox if any (oil).
] Yaw drive (grease).
] Generator bearing (grease).
The pitch drive is used to adjust the angle of the blades.
This adjustment is made for two reasons: 1) to capture
maximum power from winds below the rated output wind Figure 4: Power flow diagram of a typical three-stage wind turbine
gearbox. The low-speed input from the rotors (far left) is converted
speed or 2) to slow the blades for safe operation at winds
into high-speed torque at the output shaft (HSS) to feed the generator
above the rated speed. The yaw drive moves the blade and (top right). (Courtesy: Sentient Science Corp.)
housing assembly (the nacelle) to the optimum direction in
relation to the wind. An animation prepared by the Union
of Concerned Scientists is helpful in visualizing the action SUMMARY
of these drives [2]. Wind turbines are the fastest-growing renewable energy
Figure 4 shows a typical three-stage wind turbine gear- source, and wind energy is now cost-competitive with non-
box. A planetary stage (bottom left) transfers the torque first renewable resources. Growth in generating capacity is con-
to a low-speed intermediate stage (bottom right) and then centrated in five to 10 states, notably Texas. Five companies
to a high-speed intermediate stage (middle), which drives lead in the installation market. The field of turbine manu-
a high-speed stage (top) that feeds the generator. Such a facturers is crowded, but GE Renewable Energy and Vestas
design might, for example, convert 14 rpm input from the are clear leaders. Increasingly, capacity is being purchased
rotors into 1,500 rpm to the generator; the exact conversion by entities other than utilities, and offshore installations
of course depends on the gear ratio. Different bearing types are becoming more attractive and viable.
are used in these various components. In terms of technology, turbine design focuses on opti-
Some technical differences should be noted between mizing power output by focusing on two key parameters:
land-based and offshore turbines. As noted previously, blade length and average wind speed. The latter is affected
offshore installations account for more than 3 percent of by surface terrain and varies spatially, directionally and
global capacity. Offshore construction presents different seasonally. The effectiveness of a particular installation is
challenges, the most obvious being how the structure is an- quantified by a capacity factor: the ratio of actual annu-
chored. The strategy differs depending on the water depth. al energy output to the theoretical maximum output. A
For depths less than about 100 feet (30 meters), monopile number of basic designs are in use, but most commercial
construction is used. For transitional waters (100-200 feet installations use a horizontal axis, upwind-facing design.
or 30-60 meters), a cross-braced “jacket” foundation is used. Turbines are becoming ever larger, in both physical size and
For deeper waters, prototype floating platforms are being generating capacity, in order to capture more stable winds
tested. The transformer design also is different for differ- and to maximize return on installation costs.
ent water depths, and in general, offshore installations are
moving from gearbox to direct-drive designs. REFERENCES
Another significant difference is size. Without the need [1] Global Wind Report 2018, by Global Wind Energy Council. Available
at https://gwec.net/global-wind-report-2018/.
to limit noise or accommodate terrain-induced turbulence,
designers can pursue truly giant scales. GE has built an off- [ 2 ] “How Wind Energy Works,” by the Union of Concerned Scientists.
shore design rated at 12 MW, significantly higher than the Available at www.ucsusa.org/clean-energy/renewable-energy/
how-wind-energy-works.
2017 average of about 2.3 MW. It is indeed a giant: The rotor
diameter is on the scale of the towers of the Golden Gate
Bridge, and the surface area of the blade sweep is equivalent ABOUT THE AUTHOR
to seven American football fields. In this design, the torque Jane Marie Andrew is a free-lance science writer and edi-
is transmitted directly to the generator. Why build such tor based in the Chicago area. She can be reached at jane@
giants? In addition to raising power output, large turbine janemarieandrew.com. Reprinted with permission from the
reduce installation cost. Installing one 12-MW turbine is August 2019 issue of Tribology & Lubrication Technology
cheaper than installing six 2-MW ones; thus the final cost (TLT), the monthly magazine of the Society of Tribologists
per megawatt is lower. For these reasons, and because of and Lubrication Engineers (STLE), an international not-for-
the abundance of offshore wind resources, the industry is profit professional society headquartered in Park Ridge, Il-
moving to an emphasis on offshore wind power. linois, www.stle.org.

40  OCTOBER 2019