Energy-Maximising Control Philosophy for a Cyclorotor Wave Energy Device

Abstract

Recently, cyclorotors, utilizing lift rather than buoyancy forces for energy extraction, have been proposed and inherit many of the appealing features characteristic of modern wind turbines. In particular, the ability to spill energy though foil pitching allows the device to remain at rated power despite significant variations in input power level, while additional cyclorotor features, including variable submergence depth and rotor radius, also permit a significant degree of modulation of the device structure, and energy absorption characteristics, offering considerable flexibility. These configuration flexibilities, in addition to torque control of the rotor/generator shaft (also characteristic of wind turbines) offers the control engineer considerable freedom in adjusting the device characteristics to maximise the effectiveness of the device in capturing wave power, while maintaining structural integrity and minimizing harmful stresses on system components. However, such flexibility also provides a significant challenge in the form of a multivariable control problem for a system described by significantly nonlinear hydrodynamics. This paper describes a proposed hierarchical control system for a cyclorotor wave energy device, utilizing submergence depth, rotor radius, foil pitch angles, and shaft torque as control inputs. The hierarchy involves the separation of the control actuators into two classes: structural (or slow) control effectors, and wave-by-wave (or fast) control effectors. In particular, the paper will examine the interaction between the two levels of the control hierarchy and the need, if any, for simultaneous optimisation of the control parameters at both levels.

validated analytical model for a cyclorotor wave energy device

“…The newly derived exact analytical formulae for free surface elevation, and perturbation in fluid velocity, caused by a rotating foil can also help developers of existing, and new, cyclorotor concepts. The model is suitable for development of various control strategies [24]- [26] which target different performance metrics, such as the wave cancellation proposed by Siegel et al [4], or maximisation of the power coefficient (6) proposed by Scharmann [3].…”

Control Strategies for Power Enhancement and Fatigue Damage Mitigation of Wave Cycloidal Rotors

“…For the rotor, we consider adjustable submergence that changes with each sea state, such that 𝑦 0 = −1.5𝑅 − 0.5𝐻 𝑠 . This promotes efficient power capture by ensuring that the rotor remains in close proximity to the free surface but that the hydrofoils always remain submerged during operation [45].…”

Experimental evaluation of phase and velocity control for a cyclorotor wave energy converter

“…The 3D LiftWEC scaled prototype assessments include tests of prescribed rotational velocity control strategies. The tested rotational velocity changes are based on the analytical control solutions presented in [18]- [20], which show, that the variation of the rotational velocity θ(t), within the period of monochromatic wave T , permits an increase of the power production compared to a constant rotational velocity strategy (for which θ = ω). In the case of monochromatic waves, the control goal of maximisation of generated mechanical power can be presented in the following form:…”

“…where the coefficients a i and b i need to be determined from the optimisation problem in (19). The optimal angular velocity θ and acceleration θ can be obtained by differentiation of the series in (20) with respect to time t. Thus, the mechanical energy which can be generated in monochromatic waves can be evaluated by substitution of (20) into the functional (19). It was shown, in [18], that m=15 time series coefficients is sufficient to achieve convergence of the solution.…”

“…1b) at ∼ 140 €/MWh [10]. Such a relatively small LCoE value is However, such a promising LCoE value is estimated under an assumption of the implemented optimal control strategy [18]- [20]. Real time control for cyclorotor WECs is a challenging problem due to the high nonlinearity of even simplified mathematical models of interaction between the waves and hydrofoils [6], [18], [21].…”