Large-Eddy-Simulation Studies and In-situ Observations of Land Atmosphere Exchanges in Large Wind Farms

Johns Hopkins University, Baltimore MD

Investigators

Abstract

The study will develop and apply computational tools for predicting and understanding fluxes of scalars such as heat and moisture in very large wind farms. These fluxes play a crucial role in the land-atmosphere couplings and in possible perturbations stemming from modifications of the land-atmosphere interface with the anticipated global growth of wind energy. At present, effects of wind farms are parameterized in regional scale and global scale models using effective roughness lengths and, sometimes, increased turbulence kinetic energy due to wakes. Such approaches continue to be based on classical similarity theory of the atmospheric boundary layer (ABL). The theory assumes a uniform land surface yet has also often been found acceptable in flows over heterogeneous features of the land surface such as heterogeneities induced by wakes and other effects from wind turbines. This is due to the turbulent flow in the ABL, which efficiently blends the various sources and inhomogeneities across the landscape. However, the appropriateness of such parameterizations, and the values of parameters to be used, are highly uncertain especially in the case of wind farms under general atmospheric conditions (convective, stable, neutral). Intellectual merit: The relevant high-resolution data will be generated via a series of suitably chosen parametric Large-Eddy-Simulations (LES) that quantify accurately the land-atmosphere exchanges of sensible heat and moisture to be expected at the ground surface, underneath wind turbine arrays. These LES resolve significant portions of the individual wakes behind wind turbines and the concomitant modifications to mixing and entrainment. The simulations will cover a wide range of atmospheric conditions (neutral, convective, stable) and wind farm arrangements (turbine spacings, ground properties, loading factors). The computational results obtained under fairly idealized, and thus manageable, conditions will be complemented with in-situ observations in a wind farm. Field studies will take place through an international collaboration with researchers in Switzerland who have access to the La Muela wind farm near Zaragoza in Spain. The results of the validated simulations will be analyzed with the specific purpose of deriving new Monin-Obukhov-type relationships for atmospheric boundary layers including wind turbine arrays. For instance, modified stability corrections and modified effective scalar roughness lengths will be derived as function of relevant parameters. The results of this study will enable more accurate prediction of possible feedback mechanisms of extensive wind farms with local and regional meteorological conditions, regional scale evaporation, etc. Broader impacts: Regional scale surface fluxes of momentum, sensible heat and water vapor play a crucial role in quantifying and understanding the water and energy cycles at various spatial and temporal scales. With the advent of computational modeling, there has been much increased understanding of the effects of manmade modifications to the land surface on land-atmosphere interactions. The growth of wind energy as an important contributor to the renewable energy portfolio suggests the possibility that non-negligible portions of the land surface of the U.S. and the world may ultimately be used for large wind farms. Predicting and better understanding the physical processes coupling the land and atmosphere under such conditions is a very timely and critical area of research. Graduate education and training will stress the interplay between simulation, parameterization, and in-situ field experimental campaigns. Recruiting and educational outreach will leverage an Integrative Graduate Education and Research Traineeship (IGERT) on modeling complex systems and the PI's ongoing efforts to recruit U.S. Hispanic graduate students through contacts in Puerto Rico. The PI's ongoing outreach to a local Baltimore high-school will be actively continued by providing research experiences for junior or senior high-school students.