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[kristopher]

Energies 2009, 2, 816-838; doi:10.3390/en20400816
Investigating the Effect of Large Wind Farms on Energy in the Atmosphere
Magdalena R.V. Sta. Maria * and Mark Z. Jacobson
Atmosphere/Energy Program, Civil and Environmental Engineering Department, Stanford University,
Stanford, CA 94035, USA; E-Mail: [email protected]
September 2009

Abstract: This study presents a parameterization of the interaction between wind turbines
and the atmosphere and estimates the global and regional atmospheric energy losses due to
such interactions. The parameterization is based on the Blade Element Momentum theory,
which calculates forces on turbine blades. Should wind supply the world’s energy needs, this
parameterization estimates energy loss in the lowest 1 km of the atmosphere to be ~0.007%.
This is an order of magnitude smaller than atmospheric energy loss from aerosol pollution
and urbanization, and orders of magnitude less than the energy added to the atmosphere
from doubling CO2. Also, the net heat added to the environment due to wind dissipation is
much less than that added by thermal plants that the turbines displace.


Conclusions
A BEM model was developed for the purpose of determining the forces exerted onto the atmosphere by turbine blades. This provides a more detailed parameterization for the modeling of wind turbine effects on the atmosphere. The model was evaluated against three turbines, where the power curves from the turbines were compared with model-generated power curves. The best agreement between the model and data power curves occurred at wind speeds between 5–15 m/s, which is the wind speed range that is most relevant to wind energy. When the power curves were weighted with a typical wind frequency distribution—where the majority of wind speeds that occur are between 5–15 m/s—the agreement between model and data increases significantly. Based on these results, the model was found to be sufficient for the purpose of simulating the interaction between the turbine blades and the atmosphere in the context of wind power generation. Because of the resolution of this model-it uses a number of data points along a turbine blade-it will be a good tool to use to couple with an atmospheric dynamics model in order to create a better parameterization for the presence of wind farms. The model was combined with efficiency data to estimate the energy lost from the atmosphere due to a large deployment of wind farms. The rough estimates from this model show that if the world’s energy needs were supplied by wind energy, the L1 layer over global land plus ocean would lose only 0.006%–0.008% of its energy. Even with the added energy consumption of putting hydrogen in the energy mix will only result in a loss of 0.010%–0.013%. If only the U.S. energy needs are supplied, the loss from L1 above U.S. land ranges from 0.19%–0.23%, and above global land plus ocean ranges from 0.0012%–0.0014%. Replacing U.S. onroad vehicles with wind-powered BEVs reduces energy in L1 over U.S. land by 0.04%–0.05% and over global land plus ocean by 0.00026%–0.00031%. Certainly less than 100% of the entire energy demand and vehicle energy demand will be satisfied by wind, so the actual percentages of energy loss in the L1 layer over the regions specified will likely be lower than those shown. Such losses are also estimated to be at least an order of magnitude less than energy losses due to other anthropogenic influences, such as by aerosol pollution and urbanization. Moreover, the maximum energy loss estimated in this study translates to a power density that is a few orders of magnitude less than the radiative forcing due to the doubling of CO2 in the atmosphere. Also, any heating effects of this energy loss is outweighed by the thermal pollution that it will avert when wind farms displace the thermal power plants driven by fossil fuels.

In sum, the energy losses due to wind turbines, while high immediately downwind of a turbine, are quite small when averaged over large geographic regions, even if the entire world were powered by wind. A complete evaluation of the effects of wind turbines on local meteorology though, requires three-dimensional simulations of turbines interacting with the environment when the turbines are resolved. The BEM module discussed here can be used in such a resolved model to calculate these feedbacks.