The Stable Atmospheric Boundary Layer: A Challenge for Wind Turbine Operations

The growth in the installation of very large wind farms has been increasing exponentially. It is not uncommon for such installations to have an aggregate nameplate capacity of 500 MW or more. Currently there are individual wind plants being planned with capacities exceeding 1 GW. While the latest wind turbine designs now provide individual capacities approaching and sometimes exceeding 3 MW, large numbers of such machines will need to be installed and operated in juxtaposition to one another. The challenge for the turbine manufacturers and wind plant designers is to provide an intersection of optimum designs that provides for reliable and efficient wind plant operation while at the same time minimizing the costs of maintenance and repair. Current experience in the operation of large wind plants has shown that a combination of under production and higher maintenance and operation costs are commonplace. The National Wind Technology Center has been involved with developing an understanding of the interaction of atmospheric boundary layer turbulence with operating wind turbines and its operational consequences for almost twenty years. Field measurement campaigns have been employed to acquire very detailed measurements of the turbulent inflow synchronized with the corresponding dynamic responses of operating wind turbines and many of their individual components. The results of this work have identified turbulent conditions associated with the nocturnal or stable atmospheric boundary layer as a being a major source of the structural loads responsible for fatigue accumulation in many wind turbine components. The repetitive nature of various nocturnal turbulence generating processes provides the environment to deliver relatively low levels of loading over a diurnal period. However over longer periods these loads contribute to increased wear and result in shortened component lifetimes. We will provide a brief overview of the atmospheric processes in the stable boundary layer that contribute to this accelerated wear and occasionally are of sufficient severity to cause turbines into fault conditions. We will also discuss what avenues are being investigated to minimize the impact of such conditions in order to improve the operational reliability and lifetime of modern wind turbines.