Constant rotational speed vertical axis wind turbines (VAWTs) with fixed blades can have a limited performance envelope as the wind speed changes. Unlike horizontal axis wind turbines that can change the blade pitch to compensate for wind speed changes, vertical axis wind turbines are currently unable to achieve a blade pitch change reliably. A different method of changing the blade's lift characteristics is needed by vertical axis wind turbines to increase its power envelope.;Circulation control (CC) can change the characteristics of an aerofoil by expelling air, moving at multiple times the free stream velocity, out of a thin slot along a rounded trailing edge. This jet of air bends around the rounded edge and entrains the free stream air, which ultimately increases lift. The use of an upper slot increases lift for positive angles of attack, while another slot on the lower surface can be used for negative angles of attack. The opposite may also be applied for a resultant decrease in lift. These slots are controllable by a valve inside the blade and can be turned on and off many times during rotation. The amount of lift increase due to circulation control is roughly proportional to the mass flow and velocity of the jet, within blade stall.;The ability to control lift, depending upon the amount of blowing, was applied to the blade of a vertical axis wind turbine to control lift generation and thus affect the power envelope. The solidity, or the ratio of the number of blades times the chord divided by the radius, greatly affects the performance of a turbine. A low solidity out-performs a high solidity rotor at low tip speed ratios and the opposite for mid-to-high tip speed ratios. Analyzing simulations of a VAWT with the same radius but different solidities, it was found that the solidity and tip speed ratio are the determining factors in performance. Circulation control can, for a VAWT, change the virtual chord and in effect both increase and decrease the virtual solidity. To mimic the solidity from a given initial solidity, circulation control was used to match the performance during rotation. The result is a VAWT that can change its apparent solidity depending upon the tip speed ratio.;An analytical model of a vertical axis wind turbine with valved circulation controlled dual slots was created and used to develop a control scheme to increase a VAWT's performance envelope. A control scheme was then developed and simulated which changed the apparent solidity of the CC-VAWT depending upon the wind speed which resulted in an increase of about 22% in overall power capture.
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