Exploring the Reliability and Potential Climatic Impacts of Large-Scale Deployments of Offshore Wind Turbines

Wind power is widely available renewable energy source, but a large-scale deployment of several million wind turbines would be required to meet the estimated global energy demand in 2100 of 44 TW. Wang and Prinn sought to examine the environmental impacts and reliability of such an extensive use of offshore wind power (2011). The researchers had previously conducted a study to examine the effects of a similar large-scale deployment of wind turbines over land to meet about 10% of the predicted global energy needs in 2100. Their model suggested that such a significant use of wind turbines over land could lead to a significant temperature increase in the lower atmosphere over the installed regions. This model also predicted a significant disturbance in air circulation patterns as well as cloud and precipitation distribution. Unlike the previously modeled land-based wind turbine installations, the offshore wind turbine installations were found to cause a surface cooling over the installed regions. The disturbance to the global climate caused by offshore wind installations was calculated to be relatively small when compared to land-based installations. Yet there are significant concerns about the intermittent nature of power generation from offshore wind turbines caused by seasonal wind variations, so the operation of such a substantial offshore wind would demand significant measures taken on the part of system operators to cope with this variability. —Meredith Reisfield
Wang, C., Prinn, R. G., 2011. Potential climatic impacts and reliability of large-scale offshore wind farms. Environmental Research Letters. doi: 10.1088/1748-9326/6/2/025101.

Wang and Prinn, working at the Center for Global Change Science and the Joint Program of the Science and Policy of Global Change at the Massachusetts Institute of Technology, sought to compare the effects of deploying wind turbines over semi-arid grasslands to the effects of large-scale offshore wind installations. The land-based model found that using wind turbines on this scale could cause global surface warming exceeding 1 K over designated surface areas and alter global distributions of rainfall and clouds. To model the effects of large-scale deployment of offshore wind turbines, the researchers used the Community Atmospheric Model of the Community Climate System Model developed by the US National Center for Atmospheric Research. This model was coupled with a slab ocean model and the Community Land Model to simulate the long-term climate responses to large-scale offshore wind turbine use.
This three-dimensional climate model presented several advantages including a high spatial resolution of 2° by 2.5° along the latitudinal and longitudinal directions respectively, 26 vertical layers of atmospherics modeling, coverage of a large range of geographic areas, and consideration of multiple assumed strengths of wind turbine effects. To simulate the climate effect of offshore wind farms, Wang and Prinn modified the surface drag coefficient to represent the turbine-induced change to sea surface roughness.
The researchers conducted six simulations of offshore wind turbine effects. The wind turbines in each simulation were installed over regions between 60°S and 74°N in latitude, at depths of 200, 400, or 600 m and an assumed sea surface drag coefficient of either 0.007 or 0.001. The turbines were simulated to be in five regions free from sea ice that are likely to become actual offshore turbine installation sites, including the Southeast and East Asian coasts, the North American coast, the West European coast, the South American coast and Oceania. The higher drag coefficient was based on reported measurements over mesoscale wind farms, while the lower coefficient approximately doubles the average sea surface drag coeffient in areas without wind turbines. Each model was set up to run 60 years and take about 40 years to reach an annually repeating climatic steady state. These simulations utilized present day greenhouse gas levels to isolate the climatic effects of wind turbines from effects due to greenhouse gasses. The results compared the mean parameters of the last 20 years of each model used in analysis. Power gain from mean flow kinetic energy due to wind turbines was calculated by using the models with and without wind turbines, then subtracting. Raw wind power consumption increases proportionately with installation area and the sea surface drag coefficient assumed in the model. The researchers assumed a 25% conversion rate of raw wind power converted to electric power by wind turbines. The estimated output of electric power ranged from 6.8 to 11.9 TW with the higher drag coefficient, and from 1.7 to 3.1 TW with the lower drag coefficient. At most, these numbers would account for 25% of the predicted 44 TW of future global energy needs.
The climatic impacts of the simulated installations were significant. The surface air temperature over tropical and mid-latitude sites were reduced by nearly 1 K, with even greater cooling observed in the Arctic region and a slight warming in Antarctica. The cooling was due principally to enhanced latent heat flux from the sea surface to the lower atmosphere, driven by an increased turbulent mixing caused by the wind turbines, and extended vertically into the lower and middle troposphere through mixing. The annual averaged surface air temperature ranged from 0.4 to 0.6 K in models with a high surface drag coefficient and was about 0.2 K in the low drag coefficient cases. Cooling was also shown to be greater as a smaller fraction of the turbines were installed in tropical latitudes. Due to changes in the patterns of geographical locations of installed regions as turbines were modeled to be in depths of 200, 400, and 600 m. Though these turbines also create impacts on clouds, temperatures, precipitation and air circulation beyond the installation sites, the impacts were less significant then those observed with a similar deployment of turbines over land.
The large deployment of offshore wind turbines also presents concerns about intermittency and reliability, as well as the need to lower the high current unit wind power costs. Intermittency is especially serious over European coastal sites, where the potentially harvested with power could vary by a factor of 3 seasonally, and was generally found to be greater than a factor of 2 . The inconsistency of offshore wind as a power source would require solutions such as on-site energy storage, backup generation and long-distance power transmission for an electrical system dominated by offshore wind power.

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