Systematic study of TWES performance

Tornado wind energy system (TWES) models of spiral shape (R = Riexp (0.1σ), Ri = 0.08 m) and tower aspect ratios 1.25 ⩽ H 2R i ⩽ 3.5 were test ed by grid-simulated power measurements with turbine radii 0.19 ⩽ R T R i ⩽ 0.63 and total pressure p∞ at the turbine entrance to provide a basic data set. Maximum turbine flow occurs for H 1R i ≲ 75 and maximum power output for H 2R i = 2 . Power output decreases towards larger H 2R i . For a modified model ( H 2R i = 3.0 and reduced viscous interaction between vortex and free stream over the top), power output increased beyond the above value for H 2R i = 2 (basic configuration). The flow field, determined by the hot wire probe, shows only a mild internal vortex breakdown, induced by a superimposed positive axial pressure gradient within the core, while the outer gradient is negative. Severe vortex breakdown, however, with pronounced pressure losses, is likely for all basic configurations H 2R i t 2 . Optimal power output for the modified model is obtained for a turbine duct exit near the upper tower end. It is 1.6 times larger than for the basic configuration with H 2R i = 2 , although pressure loss from breakdown is significant. By use of a ram air supply at the turbine entrance, constant power coefficients based on tower frontal area ( 11% ⩽ C P. front ⩽ 14% ) are estimated within 1.25 ⩽ H 2R i ⩽ 3.0 , in contrast to previous investigations which reported only decreasing power coefficients vs H 2R i .