GL Garrad Hassan
Wind-farm designs and energy assessments are complex processes that vary greatly depending upon the constraints and challenges of each site. Stakeholders such as developers, utilities, and financiers must technically evaluate a project to determine its profitability and feasibility. Software that can model the performance of all types of farms is essential in the wind industry. Some key features and models needed in such wind-farm design software are highlighted here, using the latest WindFarmer 4.2 software developed by GL Garrad Hassan as a platform for discussion. A number of features in the software come from years of development and hence deserve attention. For example:
Energy calculations: Energy-production values calculated for a wind farm require several inputs to properly evaluate a site’s estimated energy generation. The topography for a wind farm and its site boundaries can be loaded into the software as ESRI shape files. The wind flow should be characterized from on-location measurements and extrapolated to the rest of the site. Its historical air density must also be evaluated.
Efficiencies and loss factors specific to the site should be understood, and a turbine layout and power curve must be selected. Once this basic site information is entered, the type of analysis method chosen to calculate energy depends on the characteristics of the wind farm being assessed.
A wind farm’s energy production greatly depends on the wake effects from upwind turbines. Hence, an accurate wake model is key to an accurate estimate. WindFarmer software has several wake models that can consider specific challenges found in the broad range of wind farms in development and operation.
Wind-farm wake modeling: The Eddy Viscosity Wake Model is a CFD model developed, refined, and validated using operational wind-farm data for many years. The model provides a successful wake-modeling algorithm for a wide variety of common situations. The software also includes several modifications to this base model for wind farms that fall outside a normal operational envelope of the model. With these adjustments, the exceptional characteristics of regions with such observed wind regimes can be taken into account.
Wakes in complex terrain: Environmental parameters such as air density and turbulence levels as well as development of the wake are a function of the underlying topography. Based on the turbine’s position in the terrain, the software adjusts turbine-wake development and the energy calculated.
Closely spaced wind farm layouts: A closely spaced wake model allows completing wake modeling on wind farms that are built for one or two-directional wind regimes. This type of wind farm has unique challenges. Such regimes lend themselves to turbine layouts that are positioned close together in one direction to maximize electricity generation per given area. Turbine wakes in such a layout do not propagate independently, and so must be modeled differently. The software’s Eddy Viscosity Model for Closely Spaced Turbines effectively models wakes propagated for this type of wind farm by altering the classic Eddy Viscosity model and letting velocity deficits caused by the wake effects add cumulatively.
Large wind-farm model: As the wind industry has developed, the size of wind farms has increased on and offshore to include some that are greater than 100 MW in size and many rows deep. A Large Wind Farm Wake Model simulates increased wakes generated by large wind farms. The model works by considering several characteristics specific to large wind farms, the most important being alterations to the boundary layer caused by the presence of many turbines extracting momentum from the atmosphere. As a result, the wind profile varies across the farm, an effect analogous to an increase in surface roughness length. Within WindFarmer, this technical and crucial feature is engaged with a check box where the software performs all analysis of upstream turbines to derive added wake losses due to the large wind-farm wake effect. The Large Wind Farm Wake Model has been tested and validated with operating offshore and onshore data. Furthermore, the speed of the energy yield calculation – typically 10 to 15 minutes for a 100-turbine array – lets wind farm designers explore many different layout options within a reasonable timeframe.
Uncertainty analysis: Once a site’s estimated energy production calculations complete, the uncertainty associated with the results must be quantified. Uncertainty calculations are essential to quantify and assess the asset’s viability and profitability to raise project financing. As such, the software includes uncertainty analyses into its results to evaluate the many sources of uncertainty present within the development process, such as data collection sensors, topographic modeling, and yearly variations of the on-site wind regime. These uncertainties derive the relevant exceedance levels so stress cases can run and debt-service coverage ratios converted into currency for pro forma preparations.
Efficient wind farm design: Software for designing wind farms offer a range of tools to support the engineer designing a productive wind farm. These tools include aids for automatic generation of both random and symmetric layouts with high-energy yield. Using the advanced tools to design a wind farm can help mitigate project deficiencies found later in the detailed energy assessment. As wind farms become larger and more complex, scientific research is increasing understanding of how wakes behave within such a complex environment. Software such as WindFarmer is essential to leverage this knowledge and produce reliable estimates of electrical-power production.
Filed Under: Software, Turbines