Torsional galloping / torsional instability of single-axis tracking systems

Torsional galloping / torsional instability of single-axis tracking systems

In the past, galloping-like instabilities (a mix of vortex lock-in and torsional galloping) have been observed on single axis trackers in the field at tilt angles close to 0°. This happens due to a sudden excitation of the first mode of vibration. This mode is a helical twisting which increases with distance from the torque motor. The instability is a result of vortices forming on, and then shedding from, the leading edge as it twists up and down. The sudden release of torque as the vortex is shed lead to torsional galloping, but only for trackers initially positioned roughly parallel to the ground. The instability is quite robust with a strong dependence on torsional stiffness and a weak dependence on damping.

To suppress self-induced oscillations which may seriously damage the structure, stowing at a tilt angle above 20deg is recommended. Proof of torsional stability is possible by conducting simplified two-dimensional Computational Fluid Dynamics (CFD) simulations with varying wind speed neglecting any three-dimensional helical deformation of the torsional mode.

Another way to eliminate the torsional instability, when stowing for survival would be to lock the torque tube position. This could be realized by using a rotational stop (torsional safety valve, torsional relief system) or by undertaking other counteracting measures. It is also important to monitor the health of the dampers.

It is equally important to keep an order of installation of tracking system components which ensures that torsional instability cannot occur prior to commissioning of a solar plant. With regard to this, it is highly recommended that dampers be installed ahead of modules.