Measured power coefficients from the GHT and LST are plotted versus tip speed ratio in Fig. 6. Note that NSC23766 our testing method, namely the lack of a closed loop turbine speed control system, only allowed turbine operation at values of λ above those of maximum torque. This is similar to the comments by Kirke regarding the deficiencies of manually-actuated brakes for turbine loading . This is due to the fact that there are two tip speed ratios corresponding to each torque output value (except the maximum), and the turbine will settle at the higher of the two if the turbine is allowed to spin up to its no-load λ before being loaded.
Fig. 6. GHT (top) and LST (bottom) power coefficients versus tip speed ratio.Figure optionsDownload full-size imageDownload as PowerPoint slide
The GHT performs better and operates at slightly higher tip speed ratios compared to the LST. The LST's lower performance is expected considering crossopterygians is not designed for such a low-blockage flow condition, and the shift of the power curve toward lower λ is a consequence of the turbine's higher solidity.