Could crop roughness impact the wind resource at agriculturally productive wind farm sites?

The Great Plains and Midwest regions of the United States feature some of the most productive farmland in the world. These same lands lay below the strongest and most consistent onshore wind resource in the United States. The colocation of these two resources encourages complex meteorological interactions that could affect both agricultural output and wind power production. The summer season is perhaps the most relevant in the context of these interactions; that time period features rapidly changing crop roughness, as farmers plant, cultivate, and harvest their crops. Meanwhile, the well-studied Great Plains Low-Level Jet exerts a dominant influence on the nocturnal wind resource aloft. These processes, through their dependence and impact on the state of the land surface, could potentially create unique opportunities to modify wind resources through crop management. In this study, we use the Weather Research and Forecasting (WRF) model to quantify the impact of crop roughness changes on the turbine-rotor-layer winds at a wind farm in central Iowa.

The choice of central Iowa as the location for the model investigation is based on the siting of the 2013 Crop Wind Energy Experiment (CWEX) within a wind farm in that region. The field campaign, a collaboration between the National Laboratory for Agriculture and the Environment, Iowa State University, the University of Colorado Boulder, and the National Renewable Energy Laboratory, was designed to investigate the interactions between cropland and a large wind farm through the use of remote sensing and in-situ meteorological measurements of the boundary layer. The suite of instruments included four lidars that were deployed throughout the month of August to measure the boundary-layer wind profile at the wind farm. Here, we use these observations to evaluate the skill of the WRF model, configured with various input data and physics options, in simulating turbine-rotor-layer winds. We also quantify the accuracy of each configuration's representation of the nocturnal low-level jet. These model sensitivity tests inform our decision on what configuration to use in the subsequent crop-wind interaction simulations.

To investigate the impact of the summer crop-cycle on the elevated wind resource, we run three simulations of the month of August in which we modify the crop forcing through changes to the land-surface aerodynamic roughness length. Roughness length values of 1 cm, 10 cm, and 25 cm, representing a fallow field, a mature soybean crop, and a mature maize (corn) crop respectively, are imposed on a region approximately 65 times larger than the footprint of the wind farm. The modified roughness patch is large, approximately the areal extent of the entire state of Iowa, but its size is justifiable for initial investigation given that most of Iowa is agricultural land and maize and soy are the two dominant crops grown in the area. In the center of the roughness patch in each simulation, we include elevated drag representation of a typical utility-scale wind farm using the WRF Wind Farm Parameterization available in the WRF public distribution. The model wind farm features 121 1.8 MW turbines organized in an evenly-spaced square-grid layout. Inclusion of the wind farm enables quantification of the impact of the roughness-length changes in the presence of a turbine array momentum sink, which prior studies have shown to be significant. Our simulations suggest that, even within this large wind farm, surface characteristics (i.e. crop height) can impact the wind resource at turbine hub height.

Using these results, we quantify the economic impact of managing the wind resource through crop management. We caution that calculated numbers are the result of simplified processes, and do not include other considerations that farmers must account for when managing their land. Finally, we discuss possible subsequent analyses that could further inform these important economic decisions.