Abstract
Marine and hydrokinetic (MHK) energy has the potential to contribute to the global energy mix as a clean and renewable energy source. However, large scale deployment of MHK turbines is limited by the capital and operating costs. This is in part due to the uncertainty associated with the loads at a given tidal site, and the uncertainty in how the turbine blades and structure react to these loads. For example, added-mass effects on turbine blades have the potential to impact the dynamic response of a MHK turbine, but are often not considered in the design process.
Added mass is the additional mass of fluid that has to be accelerated when a body is accelerating or decelerating through a fluid [1]. This causes an effective increase in the blade mass. For wind turbines, added-mass effects are minimal due to the relatively low density of the surrounding fluid in comparison with the density of the blade. However, because of the higher density of water, added mass may be important to consider for MHK turbine blades.
Previous work has shown that added-mass effects in seawater can significantly decrease the natural frequencies compared to the frequencies in air, making it important to consider for turbine systems as a whole [2]. Faudot [1] and Maniaci and Yi [3] showed that the implementation of added mass in a non-uniform inflow did not have a significant effect on the rotor loads for infinitely stiff blades. However, MHK turbine blades are typically composite material construction and hence the infinitely stiff assumption is not valid in most cases [1]. The focus of this work is therefore on investigating added-mass effects due to hydroelastic blade deformations for composite MHK blades.
The National Renewable Energy Laboratory’s (NREL) open source design code FAST v8 (Fatigue, Aerodynamics, Structures, and Turbulence) [4] was updated to consider the effect of added mass on MHK turbine blades. FAST v8 is an aeroelastic simulator capable of predicting both extreme and fatigue loads on two- and three-bladed horizontalaxis wind and MHK turbines. FAST v8 couples several physics-based modules including an aerodynamic simulator, AeroDyn V15.04 module [5], and a structural-dynamics code, ElastoDyn module. AeroDyn and ElastoDyn, along with active controls (ServoDyn module) are the main modules of FAST that were used in this work. ElastoDyn computes accelerations and motions of the blades and tower based on equations of motion for each active degree of freedom (DOF), and AeroDyn computes the aerodynamic loads on the turbine blades and tower based on blade element momentum (BEM) theory with several correction factors implemented to improve accuracy. More detail on the development of the FAST v8 code can be found in [6].
This work investigates the effects of added mass on MHK turbine loads and blade natural frequencies for the Verdant Power Generation 5b three-bladed, horizontal-axis MHK turbine.