Stability analysis of wind sites
Student name: Ms S. Lakshmi
Guide: Dr V V N Kishore
Year of completion: 2011
Host Organisation: Centre for Wind Energy Technology (C-WET)
Supervisor (Host Organisation): Dr E. Sreevalsan
Abstract: Wind Atlas Analysis and Application Program (WAsP), the flow modeling and
prediction tool used for majority of the wind power projects around the world is conceptualized
with a basic assumption of neutral atmospheric stability. It is modeled for a logarithmic wind
profile, influenced by stability effects. Despite having an exceptional accuracy and reliability, the
default horizontal and vertical extrapolation of WAsP is still biased towards slightly stable
onshore conditions and slightly unstable offshore conditions. Consequently, the onshore
predictions are influenced by the higher wind shear that characterizes a stable atmosphere. In
this study, the stability behavior of two sites: Kayathar and Site A has been analyzed. Due to the
lack of data regarding the more acceptable measured heat flux values and Monin-Obukhov
length, this analysis is based on three other parameters: Wind shear exponent, turbulence
intensity and Laikhtman’s stability parameter. It gives a reasonable approximation of the
predominant stability condition as a percentage of time. On observation, these two sites are
found to have a higher incidence of unstable conditions. Hence the default predictions compared
to measured values are on a higher side.The WAsP best practices recommend cautiously
changing some of the parameters case specifically for improved results. A sensitivity analysis of
the two stability parameters: offset and RMS heat flux over land, followed by self-prediction
optimization has proved a pattern of variation and has resulted in certain corrections to fine-tune
the default values. On applying these corrections, the predicted values are found to decrease
generally and the annual energy production reduces by about 1-3% for a 600 kW machine in
Kayathar. Herewith, the effects of changing standard heights from default values are also studied
and the outcome analyzed for Site A. This study thus emphasizes the importance of a dedicated
research to study the atmospheric stability pattern throughout India.
Keywords: Atmospheric stability, WAsP, heat flux parameters, wind shear exponent, turbulence
intensity, Laikhtman’s stability parameter
