I just came across an interesting study [source: Gizmag] on the total efficiency of wind turbine clusters that compared vertical-axis wind turbines with the more popular horizontal-axis wind turbines. The study compared the efficiency of a cluster of wind turbines rather than individual machines. The measure of efficiency was related to the area occupied by the clusters.

Wind Farm

Surprisingly the vertical-axis wind turbine cluster was found to be more efficient per unit area than the horizontal-axis wind turbine cluster. The spacing between turbines in both cases was based on physical experiments to minimize aerodynamic interference. While not claiming definitive results, the researchers said their findings warrant further investigation into total cluster performance per unit area.