Multipurpose breakwater: Hydrodynamic analysis of flap-type wave energy converter array integrated to a breakwater

Highlights

• PTO coefficients and the distance of the WEC from the breakwater are optimized to increase the CWR to 85%.

• 2-D simulation can estimate power by 14% accuracy in comparison to 3-D when flaps have trivial phase response differences.

• The standing waves in the distance between the flaps and the breakwater can change the response amplitude from 28-87%.

• Tuning the distance between flaps and the breakwater can be exploited for increasing the efficiency of WEC.

• The short or long variability of the wave resource should be considered in tuning the WEC design properties.

Abstract

Wave Energy Converter (WEC) development needs a thorough dynamic characterization of the device and tuning the design properties to harness the maximum power. This paper addresses this need by using experimentally validated numerical simulation for an array of flap-type WEC mounted on a surface of a breakwater as a coherent approach in sustainable coastal protection. The developed numerical model is combined with a parametric iteration procedure to find the optimized values of power take-off (PTO) coefficients, and the flap's distance from the breakwater. It is shown that tuning the design properties of flap-type WEC integrated into a coastal structure leads to high energy capture around 85 percent of the available power. It was found that the amplitude of oscillation is significantly affected by the presence of different frequencies resultant from standing waves, and the confined water between the flap and the breakwater. It turns out that by tuning the distance between the flap and the breakwater, the standing waves can be used for increasing the amplitude of oscillation and consequently power enhancement.

Introduction

There are plenty of Wave Energy Converter (WEC) concepts with unique functionality that are developed for specific environmental loads. WEC concepts are mainly classified based on location, working principle, and size (López et al., 2013). From the operational aspect, the WECs are further categorized as oscillating water columns, overtopping devices, and wave-activated bodies (Czech and Bauer, 2012). The later concept harnesses energy by wave-induced motion of the submerged or floating structures (Qiao et al., 2020).

Representing the realistic performance of WECs is an important step in increasing the efficiency and economic viability of the device that can be achieved by developing reliable simulation tools which in turn is a challenging process due to the diversity of the concepts (Karimirad, 2014a), and the consequent numerical requirements (Ruehl et al., 2020).

The focus of this paper is on flap-type WEC, a wave-activated body (Qiao et al., 2020), that is considered as an efficient device with larger frequency bandwidth (Babarit et al., 2012; Todalshaug, 2017) among various WEC concepts (Falcão, 2010; Uihlein and Magagna, 2016). Normally, devices with reasonable body size don't have large bandwidth, and wider frequency bandwidth can be achieved by tuning the device with the wave conditions to produce resonance (Falnes, 2007).

Tuning device characteristics can be combined with the idea of using multipurpose coastal structures to increase the frequency bandwidth and consequently enhancing the efficient operation (Zhao et al., 2019) which is the opportunity-focused mindset on the functionality of the WEC devices and utilizing the existent coastal structures. This strategy is aligned with the sustainable coastal protection and the mutual benefits of WEC devices and coastal areas (Mendoza et al., 2014; Zanuttigh and Angelelli, 2013). The concept of the multipurpose breakwater is presented for different WEC concepts from oscillating WECs (Sammarco et al., 2015) to oscillating water columns (Rosa-Santos et al., 2019).

Multipurpose structures not only decrease the shared costs but, in some cases, their presence increases the frequency bandwidth of the WEC device. However, the challenge of survivability and reliability to withstand huge storms as is expected from a defence structure remains. Another important part of commercialization is the proven survivability to decrease the risk of failure and expand the operational time (Greaves and Iglesias, 2018). This paper presents the first development stages of the flap-type WEC integrated into the breakwater according to the development stages (Pecher and Costello, 2017); therefore, the study is dedicated to increasing the hydrodynamic efficiency through the external geometry and the PTO coefficients.

To increase the efficiency of the flap-type WEC integrated into the breakwater, some linear semi-analytical studies for the enhancement of exciting torque and tunning wall distance with the natural modes are conducted (Michele et al., 2016). This topic was also investigated by experimental tests and it was shown that there is some kind of instability in the flap behaviour due to the formation of antinodes when the ratio of wall distance to wavelength approaches $0.5$ (Cho et al., 2020). For overtopping WECs, it is also reported that less distance increases the amount of wave overtopping (Di Lauro et al., 2020a) which is equivalent to the wave height rise.

The goal of this paper is to provide a thorough understanding of different wave-structure phenomena affecting the behaviour of flap-type WECs mounted on the horizontal surface of the multipurpose breakwater. The presence of the breakwater changes the surrounding hydrodynamic which not only affects the wave reflection and the wave force (Di Lauro et al., 2020b) but also influences the dynamic characteristics such as mass, stiffness, and damping defining the flap-type WEC's movements.

Through this paper, the complexity involves due to the confined water between the flaps and the breakwater, the formation of standing waves or group waves, and their effects on the dynamic response are theoretically and numerically investigated. It is also responded how the whole system should be tuned to increase the power absorbance by optimization of design properties such as PTO coefficients, and the location for mounting the flap WECs. Since computationally demanding tools cannot be always a good solution, especially during the operation procedure, this paper develops 2-D models for various phases of optimization with a detailed description of its capabilities and limitations for this specific device. Later, the 3-D simulation shows that 2-D can effectively represent the flap's behaviour due to the similarity of the dynamic characteristics and the flaps' tendency to move together with a negligible phase difference (Saeidtehrani, 2015).

The organization of this paper to provide an investigation on design properties of flap WEC integrated to a breakwater is as follows: brief characterization of the device and site specification is provided in Section 2. Development of the mathematical formulation needs some understanding of the wave-body interaction which is the subject of Section 3.1. The development of a fairly accurate numerical model (Section 3.2) relies on the sensitivity analyses and the verification of the numerical tool with experimental tests that are presented in Sections 3.3 and 3.4. Design properties of the flap WEC such as PTO coefficients and the distance of the flaps from the breakwater are parametrically studied and optimized with a 2-D numerical model in Sections 4.1 and 4.2. The results are used for further 3-D investigation in Section 4.3, which proves the validity of the 2-D simulation and shows that the 2-D results can be effectively used for providing a benchmark for the design and analysis procedure. Finally, it is shown that by using the optimized values for PTO and the distance from the wall, high energy around $85$ percent of the available power can be captured; other conclusions are drawn in Section 5.