All wind farm uncertainty is not the same: The economics of common versus independent causes Page: 1 of 12
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Proceedings, Windpower '95, AWEA, Washington DC, March 27-30, 1995.
ALL WIND FARM UNCERTAINTY IS NOT THE SAME:
THE ECONOMICS OF COMMON VERSUS INDEPENDENT CAUSES*
Paul S. Veers
Sandia National Laboratories
Wind Energy Technology Department
Albuquerque, NM 87185-0708
USA
ABSTRACT
There is uncertainty in the performance of wind energy installations due to unknowns in the local wind
environment, machine response to the environment, and the durability of materials. Some of the unknowns
are inherently independent from machine to machine while other uncertainties are common to the entire fleet
equally. The FAROW computer software for fatigue and reliability of wind turbines is used to calculate the
probability of component failure due to a combination of all sources of uncertainty. Although the total
probability of component failure due to all effects is sometimes interpreted as the percentage of components
likely to fail, this perception is often far from correct. Different amounts of common versus independent
uncertainty are reflected in economic risk due to either high probabilities that a small percentage of the fleet
will experience problems or low probabilities that the entire fleet will have problems. The average, or
expected cost is the same as would be calculated by combining all sources of uncertainty, but the risk to the
fleet may be quite different in nature. Present values of replacement costs are compared for two examples
reflecting different stages in the design and development process. Results emphasize that an engineering
effort to test and evaluate the design assumptions is necessary to advance a design from the high
uncertainty of the conceptual stages to the lower uncertainty of a well engineered and tested machine.
INTRODUCTION
The return on an initial capital investment in wind turbines is obtained by continuous operation of the
machines over several years. The financial risk, or expected costs, must be examined and quantified before
large investments can be made and large numbers of machines can be built. Certainly, investors expect
some estimate of the risk they are taking with their money for comparison with the projected returns and
other investment options. However, risk can be difficult to quantify with relatively new technologies or new
kinds of hardware. In the case of wind turbines, the risk is driven by uncertainty, especially in the durability
of the structure. A large part of the financial risk of operating wind turbines is in the replacement costs
(and ancillary loss of revenue) associated with broken components.
The fatigue life of many wind turbine components is susceptible to large uncertainties for two reasons.
First, the fatigue resistance of all materials has a large amount of inherently random scatter. That is, given
two nominally identical pieces of material repeatedly stressed under identical conditions, the two pieces
may fail at lifetimes different by factors of ten or even hundreds. Second, the nature of the fatigue process
is such that a small change in the loading experienced by the material will lead to a large change in the
material lifetime. This sensitivity exacerbates the problem of not knowing the loadings perfectly. Small
uncertainties in the loadings lead to large uncertainties in component lifetimes. The sum of these two effects
is to create wide range of possible lifetimes for fatigue-susceptible wind-turbine components.
This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000.
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Veers, P.S. All wind farm uncertainty is not the same: The economics of common versus independent causes, article, December 31, 1994; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc711879/m1/1/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.