Wind is the movement of air and varies for location with height and time. Wind speeds are primarily dependent on the global flow patterns but can be locally influenced by the uneven heating of large bodies of land and water, as well as local geographical irregularities such as mountains and valleys. The energy contained within the wind available for energy extraction is due to its motion (kinetic energy) and is proportional to the cube of its speed. Subsequently, a slight difference in the average speed can significantly influence the power output.

Flow within the urban environment is complex. The influences of surrounding buildings, trees and other obstacles significantly alter the flow patterns throughout the environment. Increased turbulence levels yield higher fluctuations in the wind speed and wind direction in the urban environment, wind characteristics typical deterred from for traditional turbine locations. The urban environment does offer numerous distinct benefits including local energy production, reduced capital investment in towers and transmission lines, as well as increased wind speeds over and around buildings producing a higher energy potential.

Contrary to the traditional horizontal axis wind turbines (HAWT), a vertical axis wind turbine (VAWT) rotates about the shafts vertical axis. A number of critical advantages are generated for this configuration in the urban environment. VAWTs can capture energy from the wind independent of the direction, not requiring any yaw mechanism. This is significant within the urban area since the wind is characteristically veering in direction. VAWTs also typically operate at lower rotational speeds, reducing / eliminating the turbine vibrations and generated noise.


Numerical model of a building with wind flow normal to the building’s face.



The nominal operating principle of a lift type VAWT is shown in the figure below. For a rotating turbine, the blades will observe a rotating wind velocity and an incoming wind velocity. Combining these wind velocities yields a total apparent wind at an incoming angle of attack. An incoming flow not parallel to the blade yields high and low pressure regions on the blade, yielding overall lift and drag forces. These forces can be decomposed into the turbines frame of reference, yielding radial and thrust forces. Summing the forces over the rotation of the turbine yields an overall positive thrust which can be extracted as electrical power through the generator.


Schematic depiction of the operating principles of a VAWT.