THESIS
2021
1 online resource (xii, 82 pages) : illustrations (some color)
Abstract
Gusty wind harnessing has promising possibilities for distributed wind energy conversion systems in
urban and sub-urban environments. However, the continuous wind speed tracking nature of variable-speed
wind turbines has been ineffective in such wind conditions due to the delayed response
components corresponding to short-duration gust, leading to futile attempts in adjusting their rotor
speed. Aiming to improve the adaptiveness of variable speed wind turbines in gusty wind environments,
the objective of this study is to develop theoretical models to investigate the behavior of wind turbines
in the frequency dimension where a finite wind speed tracking frequency can effectively filter out the
undesirable delayed response components, and develop an approach to obtain the critical wind sp...[
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Gusty wind harnessing has promising possibilities for distributed wind energy conversion systems in
urban and sub-urban environments. However, the continuous wind speed tracking nature of variable-speed
wind turbines has been ineffective in such wind conditions due to the delayed response
components corresponding to short-duration gust, leading to futile attempts in adjusting their rotor
speed. Aiming to improve the adaptiveness of variable speed wind turbines in gusty wind environments,
the objective of this study is to develop theoretical models to investigate the behavior of wind turbines
in the frequency dimension where a finite wind speed tracking frequency can effectively filter out the
undesirable delayed response components, and develop an approach to obtain the critical wind speed
tracking frequency for better responsiveness of wind turbines in gusty wind conditions.
This thesis combines the statistical properties of gusty wind and the IEC Kaimal turbulence model to
estimate the potential excess energy content (EEC) harnessing ability of wind turbines from gusty wind.
By applying filtering techniques to the turbulence spectrum, the statistical properties of the window
mean wind speed signal as well as the uncaptured fluctuating components are derived. These properties
build up a statistical model to predict the expected power of wind turbines. The model quantifies the
potential EEC harnessed as long as the power characteristics of the wind turbine and the local wind
parameters are given. The results shows that the potential EEC harnessing ability of wind turbines
monotonically increases with the wind speed tracking frequency in conditions that the response time
of wind turbines is negligible.
By incorporating the IEC Kaimal turbulence spectra into the control theories, the response time of
wind turbines is predicted under gusty wind conditions. The cross-correlation spectra between the
oncoming wind speed and the rotor speed response were evaluated to extensively explain the
mechanisms of time-averaging filtering at various tracking frequencies. The results show that a critical
wind speed tracking frequency exists for the rotor speed control systems due to the low-pass
characteristics of the physical system. By having a wind speed tracking frequency lower than the
critical value, the average response time of control systems can be greatly shortened compared with
control systems that continuously track the oncoming wind speed. The results are supplemented with
numerically simulations with several mixed wind speed signals as inputs, showing the practicality of
estimating the critical wind speed tracking frequency. With the time-domain numerical simulations
over a real wind speed series using computational dynamic fluid (CFD) method, it is found that
applying the critical wind speed tracking frequency in the control input decreases the response time of
the wind turbine by 60% which results in 25% more gusty wind energy harnessed, as compared with
attempting to track wind speed continuously The results provide strong evidences that applying the
critical wind speed tracking frequency is an effective measure to harness wind energy in gusty wind
environments.
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