A preliminary design methodology for fatigue life prediction of polymer composites for tidal turbine blades

Abstract

Tidal turbine blades experience significant fatigue cycles during operation and it is expected that fatigue strength will be a major consideration in their design. Glass fibre reinforced polymers (GFRP) are a candidate low-cost material for this application. This paper presents a methodology for preliminary fatigue design of GFRP tidal turbine blades. The methodology combines (i) a hydrodynamic model for calculation of local distributions of fluid-blade forces, (ii) a finite element structural model for prediction of blade strain distributions, (iii) a fatigue damage accumulation model, which incorporates mean stress effects, and (iv) uniaxial fatigue testing of two candidate GFRP materials (for illustrative purposes). The methodology is applied here to the preliminary design of a three-bladed tidal turbine concept, including tower shadow effects, and comparative assessment of pitchregulated and stall-regulated control with respect to fatigue performance. INTRODUCTIONThe emerging field of ocean energy is naturally turning to composite materials because of their perceived non-corrosive properties in the harsh saltwater environment as well as their high specific strength and stiffness. Tidal turbines are to the forefront due to their reliable and predictable power delivery to the grid and the absence of overload conditions. A number of different designs of tidal turbines are already at utility scale trials. A low-solidity, two-bladed turbine, similar to wind turbines, and a high solidity, multi-blade, ducted rotor are presently undergoing customer testing [1]. Wave energy converters are also under consideration with a number of devices at full size prototype stage [2]. Glass-fibre reinforced polymers (GFRP) are candidate low-cost materials for the blades of tidal turbines and the energy collection surfaces of wave devices. It is therefore important to understand the durability and performance of such materials for these applications. A 10 rpm tidal turbine would see approximately 4 million revolutions per year and wave devices will encounter approximately the same number of waves. Therefore fatigue failure will need to be considered in the design of these devices.The structural properties of GFRP materials depend on orientation of the fibres, polymer type and fibre/polymer volume fraction. One of the main advantages of fibre reinforced materials is the ability to align the strong, stiff fibres with the main loads and thereby use the material to its maximum advantage. There are situations, however, particularly in relation to emerging technologies e.g. ocean energy, where (i) the loads are not very well understood, or (ii) the loads are complex and multi-directional in nature, for which a class of fibre-reinforced laminates called quasi-isotropic (QI) can be used. The typical QI layup has equal numbers of fibres at 0, 45, 90, and 135 degrees although many other configurations are possible. Often QI laminates are used for an entire structure or they can be implemented locally on other laminate types by addin...

Design of composite tidal turbine blades

“…However, it is reasonable to assume that the maximum strain experienced by the blade should be significantly lower that the composite's ultimate strain due to the difficulties in replicating the complex multi-axial loads the blade is likely to experience when in operation [10]. The use of CFRP for the spar caps is shown to greatly improve the stiffness of the blade, with the maximum strain registered being approximately 4 times lower than for GFRP for a similar lay-up.…”

“…With this in mind, it is likely that fatigue loading will be the critical design criterion [10]. It should be noted that PreComp has a tendency to underestimate the maximum strains and a considerable factor of safety is required to account for possible degradation of the composite when exposed to seawater.…”

“…A report released by the European Commission [17] suggests that a full scale tidal turbine should occupy approximately half the depth of the channel where it is to be installed. Combining this recommendation with the maximum depth at which current support structures can be constructed limits the rotor to a maximum diameter of approximately 25 m. In addition, excessively long blades can generate prohibitively large bending moments at the root of the blade [10], meaning a shorter blade would be preferable.…”

“…The surface tractions were applied directly to the spar caps of the blade, which is based on the assumption that the resultant loads acting on the blade act at a point which is 25% of the chord length back from the leading edge of the blade [10], with the spar cap sector of the structure starting at 15% of the chord length.…”

“…With information about how the lift coefficient and the drag coefficient vary with the angle of attack of the hydrofoil, the forces on the blade for known axial and tangential flow induction factors can be determined. The hydrodynamic model used here is based on the work of Kennedy et al [10].…”