Sea based wind farms seem to have 3W/m^2 limit. Also low average density is not that bad - it means less bigger turbines can be used to extract same energy: "

Sea based wind farms seem to have 3W/m^2 limit. Also low average density is not that bad - it means less bigger turbines can be used to extract same energy: "In the onshore region, with characteristics similar to that of the USA Great Plains, the farm power density is bounded to around 1 W m−2. This is in line with previous studies (Adams and Keith 2013, Miller et al 2015). Despite the relatively low farm power density, the wind farm efficiency remains relatively high under the condition that the turbine spacing remains wide (10.5 D0). Therefore, in these areas it would be more efficient to build one very large wind farm with a wide turbine spacing, instead of several separated smaller wind farms with a more narrow turbine spacing." http://iopscience.iop.org/article/10.1088/1748-9326/aa5d86 http://iopscience.iop.org/article/10.1088/1748-9326/aa5d86