Wind turbine reliability : understanding and minimizing wind turbine operation and maintenance costs. Page: 2 of 15
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Current Industry Status
A wind turbine's reliability is dependent largely on the particular machine model, how well it is
designed, and the quality of manufacture. Reliability also varies with operating environment, as
it is the machine's reaction to the wind environment that determines the loading imposed on the
components. The variety of potential component failures - gearbox bearings, generator bearings
and windings, power electronics, gearbox torque arms, pitch drive electronics - indicate that the
operating conditions and load conditions for a large wind turbine and not completely understood.
The number of wind turbine models and the wide range of operating wind regimes make it
difficult to distill useful reliability numbers that can be globally applied. However, attempts have
been made by several researchers using selected historical data. These efforts are summarized in
the following sections.
Reliability and the Cost of Energy
In the wind energy research community, the accepted COE calculation for a wind turbine system
is as follows [1]:
COE = ICC * FCR + LRC + O & M
AEPNET
AEPNET = AEPGROSS * Availability * (1- Loss)
COE Cost of Energy ($/kWh)
ICC Initial Capital Cost ($)
FCR Fixed Charge Rate (%/year)
LRC Levelized Replacement Cost ($/year)
O&M Operations and Maintenance Costs ($/kWh)
AEP Annual Energy Production (kWh/year)
This calculation method has been adopted by the Department of Energy in the Low Speed Wind
Turbine (LWST) program. It provides a reasonable approximation of the COE that would be
estimated by a potential investor and takes equipment reliability into account when determining
the AEP, O&M, and LRC terms. AEP is affected by equipment reliability through turbine
downtime associated with both scheduled and unscheduled maintenance. O&M consists of both
scheduled (preventive) and unscheduled (repair) maintenance costs, including expenditures for
replacement parts, consumables, manpower and equipment.
LRC costs are associated with major overhauls and component replacements over the life of a
wind turbine. Usually this category includes only major components and is based on components
whose expected life is less than the wind turbine's design life. Although the replacement
frequency will vary over the equipment life, especially in the case of campaign rebuilds, the total
assumed cost is spread over the machine lifetime.
Equipment reliability directly affects the LRC in that the LRC figure is only as accurate as the
component life estimates. Wind turbines are commonly designed so that the major component
design lives are equal to the turbine's design life. However, there are numerous examples where
the design life for major components is not realized in practice [2]. The reasons for thisPage 2 of 15
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Wind turbine reliability : understanding and minimizing wind turbine operation and maintenance costs., report, November 1, 2004; United States. (https://digital.library.unt.edu/ark:/67531/metadc930422/m1/2/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.