Wind energy offers many
advantages, which explains why it's the fastest-growing energy source
in the world. Research efforts are aimed at addressing the challenges
to greater use of wind energy.
Advantages
Wind energy is fueled by
the wind, so it's a clean fuel source. Wind energy doesn't pollute the
air like power plants that rely on combustion of fossil fuels, such as
coal or natural gas. Wind turbines don't produce atmospheric emissions
that cause acid rain or greenhouse gasses.
Wind energy is a domestic
source of energy, produced in the United States. The nation's wind
supply is abundant.
Wind energy relies on the
renewable power of the wind, which can't be used up. Wind is actually
a form of solar energy; winds are caused by the heating of the
atmosphere by the sun, the rotation of the earth, and the earth's
surface irregularities.
Wind energy is one of the
lowest-priced renewable energy technologies available today, costing
between 4 and 6 cents per kilowatt-hour, depending upon the wind
resource and project financing of the particular project.
Wind turbines can be built
on farms or ranches, thus benefiting the economy in rural areas, where
most of the best wind sites are found. Farmers and ranchers can
continue to work the land because the wind turbines use only a
fraction of the land. Wind power plant owners make rent payments to
the farmer or rancher for the use of the land.
Disadvantages
Wind power must compete
with conventional generation sources on a cost basis. Depending on how
energetic a wind site is, the wind farm may or may not be cost
competitive. Even though the cost of wind power has decreased
dramatically in the past 10 years, the technology requires a higher
initial investment than fossil-fueled generators.
The major challenge to
using wind as a source of power is that the wind is intermittent and
it does not always blow when electricity is needed. Wind energy cannot
be stored (unless batteries are used); and not all winds can be
harnessed to meet the timing of electricity demands.
Good wind sites are often
located in remote locations, far from cities where the electricity is
needed.
Wind resource development
may compete with other uses for the land and those alternative uses
may be more highly valued than electricity generation.
Although wind power plants
have relatively little impact on the environment compared to other
conventional power plants, there is some concern over the noise
produced by the rotor blades, aesthetic (visual) impacts, and
sometimes birds have been killed by flying into the rotors. Most of
these problems have been resolved or greatly reduced through
technological development or by properly siteing wind plants.
How Wind
Turbines Work
Wind is a form of solar
energy. Winds are caused by the uneven heating of the atmosphere by
the sun, the irregularities of the earth's surface, and rotation of
the earth. Wind flow patterns are modified by the earth's terrain,
bodies of water, and vegetation. Humans use this wind flow, or motion
energy, for many purposes: sailing, flying a kite, and even generating
electricity.
The terms wind energy or
wind power describe the process by which the wind is used to generate
mechanical power or electricity. Wind turbines convert the kinetic
energy in the wind into mechanical power. This mechanical power can be
used for specific tasks (such as grinding grain or pumping water) or a
generator can convert this mechanical power into electricity.
So how do wind turbines
make electricity? Simply stated, a wind turbine works the opposite of
a fan. Instead of using electricity to make wind, like a fan, wind
turbines use wind to make electricity. The wind turns the blades,
which spin a shaft, which connects to a generator and makes
electricity.
This aerial view of a wind
power plant shows how a group of wind turbines can make electricity
for the utility grid. The electricity is sent through transmission and
distribution lines to homes, businesses, schools, and so on.
Sizes of Wind Turbines
Utility-scale turbines
range in size from 50 kilowatts to as large as several megawatts.
Larger turbines are grouped together into wind farms, which provide
bulk power to the electrical grid.
Single small turbines,
below 50 kilowatts, are used for homes, telecommunications dishes, or
water pumping. Small turbines are sometimes used in connection with
diesel generators, batteries, and photovoltaic systems. These systems
are called hybrid wind systems and are typically used in remote,
off-grid locations, where a connection to the utility grid is not
available.
Wind Energy
Enabling Research
Taking turbine designs to
the "limit" of cost and performance will require advances in several
research disciplines. While some of the near-term cost of energy
reductions may be possible based on current levels of technology
(e.g., tall towers) others will require investment in fundamental
research in order to be successful.
Enabling research activities to support low wind speed technology
and distributed wind technology program goals fall within four major
topic areas:
-
Advanced Rotor Development
-
Site-Specific Design
-
Generator, Drive Train and Power
Electronics Efficiency Improvements
-
Systems and Controls
Advanced Rotor Development
The wind turbine rotor is
the only component unique to wind turbines in the system. The rotor's
blades control all the energy capture and almost all the loads, and
are therefore a primary target of research efforts. The challenge for
researchers is to create the knowledge and engineering tools to enable
blade designers to squeeze the most performance out of the lowest
possible cost, using new materials, improved manufacturing processes,
and enhanced design tools. This work will assist the industry in
meeting the low wind speed technology and distributed wind technology
goals, by stretching rotors longer to produce electricity at wind
sites that previously were not cost effective.
Advanced rotor development work can be segmented into three areas:
-
Blade development
-
Aerodynamic code development and validation
-
Aeroacoustics research and testing
Blade Development
A significant step toward
the wind turbine cost goals will be achieved with blades that are
stiffer, stronger and span a greater area, while lighter and adaptive,
to reduce system loads. Beyond that, design details need to be
evaluated so that the entire industry is led in the direction of
efficient material usage. Finally, substantial testing, both in the
laboratory and in the field, is required to validate the tools, loads,
and designs, and to make sure they can be linked to the site
characteristics.
Aerodynamic Code Development and Validation
The current generation of
aerodynamic performance codes don't accurately predict performance in
turbulent winds. To gain a better understanding of wind turbine
aerodynamics, researchers at the NWTC and Sandia are combining
two-dimensional field and wind tunnel test data to predict aerodynamic
loads on turbines under varied inflow conditions. To improve the
predictive power of these analyses, researchers are adapting a more
accurate approach used for helicopter and aircraft design. Their
evaluation will be based on analyses of three-dimensional computations
of fluid dynamics.
Aeroacoustic Research and Testing
Turbine noise can be caused
by rotor speed, blade shape, tower shadow, and other factors. The
program is sponsoring both wind tunnel and field tests to develop a
noise prediction code that turbine manufacturers can use to ensure
that new rotor designs and full systems aren't too noisy. This is
especially true for high-growth U.S. markets
for small wind turbines that will demand quieter rotors, especially
when turbines are sited in residential neighborhoods. Small turbines
operate at high rotational speeds and tend to spin even if they are
furled (pointed out of the wind). Aeroacoustics research activities
will be conducted to explore how to reduce noise produced by
distributed wind turbines in a variety of wind regimes and to develop
a noise standard with industry participants that can be used for the
growing domestic distributed wind turbine market. This research will
support the program's public-private partnerships, both directly in
working with industry and indirectly in providing necessary underlying
research.
In the longer term, program
researchers will work to develop physics-based aeroacoustics codes for
both design and problem solving applications. These will enable more
slender blades and higher tip speeds, enhancing both cost and
performance of future designs.
Site-Specific Design
For wind use to continue to
grow, future wind energy installations will need to be in areas with
more challenging winds. Wind farms will need to move into areas with
less wind, using taller towers and longer blades to harvest the more
rarified energy. Wind turbines designed to be strong enough to survive
high wind areas will drive up the cost unnecessarily for less windy
sites and could limit the area into which wind energy is able to
provide cost effective energy. The benefits of designing significant
installations (100 MW or more) for specific site conditions are
substantial. The nature of the atmospheric loading at increasing
heights must be assessed and documented. The types of blades,
including aerodynamic geometry, controls, and structural details need
to be tuned to the energy capture requirements and durability suitable
for low energy and lightly loaded sites. Every structural strength
requirement throughout the system is sized based on the expected
maximum event and turbulence at the site.
Site specific design will
also be crucial for the development of
offshore wind farms.
This area of work is
therefore two-fold. One is to create systematic methods of specifying
specific site energy and load conditions. The other area will be to
conduct the field measurements that validate the procedure, and to
work in public-private partnerships that fill in the site-specific
information at interesting regions of the country, both onshore and
offshore.
To address research needs
specific to passively and actively controlled distributed wind
turbines, improvements in current design model capabilities are
needed. These models are implemented in computer codes used by
industry to improve turbine designs as they go through the engineering
design process. Many small turbines use a passive overspeed control
such as furling. In furling, the force of the wind turns the rotor
sideways, just as farm water-pumping windmills have done for 100
years. So far, no computer codes have been able to reliably predict
the performance or assist in the design of furling mechanisms. This
means such designs need to be performed empirically, raising
development costs.
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