Retrofit Winglets for Wind Turbines

Retrofit Winglets for Wind Turbines

Vijay Matheswaran1 and L Scott Miller2

Wichita State University, Wichita, KS 67260

Patrick J Moriarty3

National Renewable Energy Laboratory, Golden, CO 80401

The benefits of using winglets on wind turbines has been well documented. However, adding winglets to wind turbine blades leads to significant increases in blade root bending moments, requiring expensive structural reinforcement with cost and weight drawbacks. A unique design philosophy for retrofitting winglets on existing wind turbines is presented. These retrofit winglets offer an increase in power produced without the need for structural reinforcement. Predicted performance and cost benefits are illustrated via a study using the NREL 5MW reference wind turbine. The addition of winglets resulted in a 2.45% increase in Coefficient of Power (Cp) and 1.69% increase in Annual Energy Production (AEP).

Nomenclature

Cp = coefficient of power

V¥ = freestream velocity

𝑟i = blade section radius

𝜃t = blade section twist

𝐼$ = Initial Cost per year

𝑀$ = Annual Operating Expense

Et = Annual Energy Output

I. Introduction

The idea of winglets on wind turbines is one that has been periodically explored in the past few decades. The earliest studies incorporating blade tip devices on wind turbines were done by Gyatt and Lissamann1. Drawing from advanced tip shapes that were being applied to fixed wing aircraft to reduce drag, the authors tested four tip designs on a 25kW Carter Wind Turbine in San Gorgonio Pass, California. Further studies were carried out in subsequent decades. Van Bussel2 developed a simple momentum theory for blade winglet configurations. Imamura et al.3 analyzed the effects on winglets on wind turbines using a free-wake vortex lattice method. Guanna and Johansen4 developed a free wake lifting line model to compute the effects of winglets, comparing it with CFD results obtained using EllipSys3D. Johansen and Sorenson5 did further studies on increasing power coefficient with the use of winglets, showing that adding winglets definitely changes the downwash distribution, leading to an increase in the power produced by a wind turbine.

While the benefit of adding winglets has been well documented, there are drawbacks to adopting the traditional method of doing so. The addition of large, heavy winglets to maximize aerodynamic benefit leads to significant increases in root bending moments. Imamura et al.6 analyzed the effects of winglets on wind turbine blades using a free-wake vortex lattice method. Their study showed that a winglet at an 80°cant angle and height of 10% of the rotor radius resulted in a 10% increase in the blade root flapwise bending moment. This situation may require blade structural reinforcement, making winglets an expensive and often infeasible proposition. In order to address this, a novel design philosophy has been developed, allowing the use of retrofit winglets that offer an increase in power produced, but without the need to structurally reinforce the blade. This paper outlines the design philosophy, tools

used and results from initial simulations.

II. Design Philosophy for Retrofit Winglets

The key differentiator between this study and prio winglet studies is the design philosophy: designing a lightweight winglet at minimum cost that, while providing an improvement in the turbine’s Coefficient of Power (Cp), does not require blade structural reinforcement. Such a winglet does not seek to maximize Cp, but rather minimize blade bending moments with an acceptable increase in Cp. This is accomplished by balancing the centrifugal force and aerodynamic normal force generated by the winglet. Balancing forces minimizes increases in blade root bending moment, negating the need for an exceptionally strong winglet and allowing it to be light, and requiring noreinforcement of the main blade. Savings in weight are strongly related to cost, so a lighter winglet implies a cheaper, more cost effective one. Accordingly, the best winglet is not one that offers the maximum increase in Cp, but rather offers an increase in Cp while ensuring forces are balanced within a threshold. Figure 1 presents a freebody diagram of the retrofit winglet. A qualitative plot highlighting the design philosophy and the optimal design space is presented in Figure 2. To be able to guage the effects of winglets developed using the mentioned design philosophy, it was decided to use the NREL 5MW wind turbine7 as a reference turbine, and implement a vortex lattice method and cost function to evaluate aerodynamic efficacy and feasibility. The NREL 5MW reference wind turbine is a conceptual three-bladed upwind turbine that was primarily designed to support concept studies. It is heavily based on the Repower 5MW wind turbine; however, in cases where detailed information is not available, data from publicly available conceptual studies is used.

1 PhD Candidate, Department of Aerospace Engineering, AIAA Student Member

2Professor and Chair, Department of Aerospace Engineering, AIAA Associate Fellow

3Team Lead, Wind Plant Aerodynamics, AIAA Member