13-06-2012, 04:48 PM
Wind Energy Conversion Systems
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Introduction
differential heating of the earth's surface by the sun causes the
movement of large air masses on the surface of the earth,
i.e., the wind. Wind energy conversion systems convert the
kinetic energy of the wind into electricity or other forms of
energy. Wind power generation has experienced a tremendous
growth in the past decade, and has been recognized as an
environmentally friendly and economically competitive means of electric
power generation.
More and more countries are ratifying the 1997 Kyoto Protocol, and
wind power has become one of the most effective ways to reach its
goals. The Kyoto Protocol sets targets for participating countries to
reduce greenhouse gas emissions to at least 5% below the 1990 level in
the commitment period of 2008 to 2012. According to the U.S. Energy
Information Administration, world electricity consumption will increase
from 12,833 TWh in 1999 to 22,230 TWh in 2020, mainly driven by
developing countries, where two billion people are still without access
to electricity [1]. The fuel mix for the world's electricity generation in
1999, as presented in Figure 1, indicates that fossil fuels accounted for
62% while renewables including hydropower, wind and solar etc.
accounted for 20.2% [2].
Recent Development in Wind Industry
Installed Capacity
Worldwide development of wind energy expanded rapidly starting in
the early 1990s. As shown in Figure 2, the average annual growth rate
from 1994 to 2001 of the world installed capacity of wind power is 31%
[6], making the wind industry one of the fastest growing. Unlike the last
surge in wind power development during 1970s and early 1980s which
was due mainly to the oil embargo of the OPEC countries, the current
wave of wind energy development is driven by many forces that make it
favorable. These include its tremendous environmental, social and economic
benefits, its technological maturity, the deregulation of electricity
markets throughout the world, public support and government incentives.
In Denmark wind power accounted for 18% of electricity
consumption in 2001, and this share will be increased to 50% by 2030.
Canada has a total of 198 MW of installed wind power capacity as of
2001, with additional wind plants planned in Ontario, PEI and Alberta.
Structure of Wind Energy Conversion Systems
The major components of a typical wind energy conversion system
include a wind turbine, generator, interconnection apparatus and control
systems, as shown in Figure 3. Wind turbines can be classified into the
vertical axis type and the horizontal axis type. Most modern wind turbines
use a horizontal axis configuration with two or three blades,
operating either down-wind or up-wind. The major components in the
nacelle of a typical wind turbine are illustrated in Figure 4. A wind turbine
can be designed for a constant speed or variable speed operation.
Variable speed wind turbines can produce 8% to 15% more energy output
as compared to their constant speed counterparts, however, they
necessitate power electronic converters to provide a fixed frequency and
fixed voltage power to their loads. Most turbine manufacturers have
opted for reduction gears between the low speed turbine rotor and the
high speed three-phase generators. Direct drive configuration, where a
generator is coupled to the rotor of a wind turbine directly, offers high
reliability, low maintenance, and possibly low cost for certain turbines.
Several manufacturers have opted for the direct drive configuration in
the recent turbine designs.
Interconnection with Electric Power Systems
Thirty six states in US have adopted, and several Canadian provinces
are considering adopting net metering programs, under which a utility
customer can install a small on-site renewable power generator and sell
electricity to the utility at the same rate at which he purchases it from
the utility. Net metering programs have substantially improved the economy
of small distributed resources (DR), including wind power.
Although standards exist for large power plants connected to electric
power systems, they fail to address special requirements for distributed
resources. To provide guidelines for all stakeholders including utilities,
independent power producers, users and equipment manufacturers,
efforts are being made, both in Canada and internationally, to develop
interconnection standards. Supported by Natural Resources Canada and
Industry Canada, Electro-Federation Canada is developing Canadian
guidelines for connecting small distributed resources to grids [9].