Load alleviation technology for extending life in tidal turbines

Abstract

The two main obstacles to the commercial deployment of tidal turbines are cost and reliability. Behind both of these lies the ability of a turbine to withstand the hostile marine environment for long periods without failure or costly maintenance. Unsteadiness in the incoming flow seen by a tidal turbine causes large fluctuations in thrust and torque, which generate fatigue loads on the blade and drivetrain respectively and thus significantly reduce turbine lifespan. This paper will present the development of flow control devices to keep thrust and torque constant despite changes in flow speed. In particular, it will be shown that torque can be controlled using a small trip near the leading edge of the blade, and that thrust can be controlled via a flap at the trailing edge. Data from a small-scale turbine will be used to show the effectiveness of both devices, and a reduction in unsteady loading of up to 75% will be demonstrated. This could translate into an order of magnitude increase in fatigue life.

Unsteady hydrodynamics of a full-scale tidal turbine operating in large wave conditions

“…Moreover, detailed knowledge of the unsteady loads will enable the development of novel technology to mitigate the fatigue loadings and enhance the durability of tidal turbines [15,16].…”

Unsteady load mitigation through a passive trailing-edge flap

“…Recalling that M k s = −M h , this implies that 2π δα x = 2π δθ x, (7) and thus δα = δθ, and the load on the foil is unchanged (although the direction of the force has changed by δα). This shows that maximum unsteady load cancellation is achieved by having a torsional spring which provides an approximately constant moment and when the entire foil pitches around an axis through the leading edge, load fluctuations can be entirely cancelled.…”

“…Active pitch control is another possible option, but this has an upper frequency limit due to the inertial effects of the blade [6]. Trailing-edge flaps, by contrast, can respond to higher frequencies better than the whole blade due to their smaller inertia [7,8,9,10,11].…”

The Effect of 3D Geometry on Unsteady Gust Response, Using a Vortex Lattice Model

“…Figure 2c)ii shows a turbine with realistic camber, chord and twist distribution, based on a model tidal turbine which has been tested in steady and unsteady conditions. 23 This turbine design gives an indication of the combined effect of various 3D geometry features on the unsteady response of a real turbine.…”