Control-Informed Geometric Optimization of Wave Energy Converters: The Impact of Device Motion and Force Constraints

Abstract

Abstract:The energy cost for producing electricity via wave energy converters (WECs) is still not competitive with other renewable energy sources, especially wind energy. It is well known that energy maximising control plays an important role to improve the performance of WECs, allowing the energy conversion to be performed as economically as possible. The control strategies are usually subsequently employed on a device that was designed and optimized in the absence of control for the prevailing sea conditions in a particular location. If an optimal unconstrained control strategy, such as pseudo-spectral optimal control (PSOC), is adopted, an overall optimized system can be obtained no matter whether the control design is incorporated at the geometry optimization stage or not. Nonetheless, strategies, such as latching control (LC), must be incorporated at the optimization design stage of the WEC geometry if an overall optimized system is to be realised. In this paper, the impact of device motion and force constraints in the design of control-informed optimized WEC geometries is addressed. The aim is to verify to what extent the constraints modify the connection between the control and the optimal device design. Intuitively, one might expect that if the constraints are very tight, the optimal device shape is the same regardless of incorporating or not the constrained control at the geometry optimization stage. However, this paper tests the hypothesis that the imposition of constraints will limit the control influence on the optimal device shape. PSOC, LC and passive control (PC) are considered in this study. In addition, constrained versions of LC and PC are presented.

The Influence of Sizing of Wave Energy Converters on the Techno-Economic Performance

“…However, the displacement limit is considered in this paper, which could ease this problem [11]. Secondly, frequency domain modeling does not allow the implementation of real-time control strategies by which PTO parameters can be adjusted instantaneously with the PTO force saturation and buoy displacement constraints [19,28]. Although there are limitations in frequency domain modeling, it is considered reasonable given the purpose of this paper to get insight into the influence of sizing on the technoeconomic performance.…”

“…Recently, the influence of PTO sizes has started to be considered in a few of studies regarding buoy geometry optimization. In Reference [19], the geometry optimization of a cylindrical WEC with only a few of different force constraints was performed. The paper indicates that the PTO force constraint has a notable influence on the optimization results, and this finding is highly valuable for the design optimization of WECs.…”

“…The paper indicates that the PTO force constraint has a notable influence on the optimization results, and this finding is highly valuable for the design optimization of WECs. However, there are some limitations in Reference [19]. Firstly, the force constraint is not treated as an optimization variable and only a limited number of force constraints were included.…”

“…Thus, the PTO damping should be selected from the range expressed in (19) to satisfy the constraints. According to Reference [28], the optimal PTO damping for maximizing the absorbed power without any constraint is expressed as…”

Initial conceptual demonstration of control co-design for WEC optimization

“…The upper bound of power absorption for a WEC is represented by the well-known "complex conjugate control," (CC) in which perfect impedance matching allows for maximum power absorption (see, e.g., Falnes 2002). The intrinsic impedance of a WEC is defined as:…”