Influence of Restoring Force Absence and Unidirectional Power Flow on the Power Absorption Performance of WavePiston

The WavePiston wave energy converter (WEC) is a novel device which consists of multiple energy collectors coupled on a string. Each collector includes a sail designed to harness the surge excitation of incident waves and a hydraulic power take-off (PTO). This recently developed WEC possesses a variety of distinctive features. First, since the sails are constrained to move horizontally and are assumed not to deform in this study, and because there is no external spring mechanism on the energy collectors, no restoring force acts on the WEC. This distinguishes the WavePiston from conventional wave energy converters that function as resonators and possess inherent self-centering capabilities. Second, in its current version, the PTO is passive, and is only capable of absorbing power, meaning that power transmission from the PTO to the energy collector is not feasible. Considering (i) the lack of resonating capabilities of the WavePiston baseline configuration, and (ii) the inherent passivity of its PTO, this study aims to evaluate how these properties impact the device power absorption performance. To achieve this, four different WEC configurations are assessed: Two of them introducing an external spring mechanism—one featuring a passive PTO and the other a non-passive PTO—and two configurations without the spring mechanism, featuring passive and non-passive PTOs, correspondingly. The external spring mechanism may operate as a complement to the PTO control action, in addition to ensuring self-centering capabilities. For each scenario, the optimal direct transcription-based spectral controller is evaluated. In cases where non-passive PTO is assumed, both positive and negative power transmission is allowed on the control side (reactive control); however, with passive PTO, only positive power transfer (passive control) is permitted. In all cases, position and force constraints are considered, and the spectral optimal solution is evaluated in terms of the energy absorption across varying sea states. For the sake of simplicity, this work focuses on a device consisting of a single energy collector. Finally, an analysis is conducted by comparing the results for the baseline WavePiston configuration, with no inherent restoring force and a passive PTO, with the other three proposed scenarios. The discussion highlights the implications for power absorption performance arising from the limitation of passive PTO operation, and the unique design characteristics of the WavePiston that differentiate it from conventional resonator WECs. Furthermore, the findings identify critical factors that could influence key design aspects of the WavePiston WEC, posing a control co-design challenge to address. This study also provides a foundation on the control side for future research on how reliability and power absorption performance may collectively impact the levelized cost of energy (LCOE), marking an initial step toward this essential analysis.