Impact of Separation Distance on the Performance and Annual Energy Production of a Dual-Flap Oscillating Surge Wave Energy Converter

Among the different concepts for wave energy conversion, oscillating surge wave energy converters have been shown to have a high capture width ratio. The primary wave capture structure consists of a flap hinged at the seabed or to a floating platform. Different flap configurations, including single and dual-flap, have been investigated. The separation distance between the oscillating surge wave energy converters can have an impact on their response when deployed in arrays. We consider the case of a dual-flap oscillating surge wave energy converter and investigate the impact of the separation distance between them on the performance of each flap. We estimate the absorbed wave energy and the annual energy production by the two flaps when deployed at the PacWave South site. Inviscid numerical simulations were conducted to predict the response of the oscillating surge wave energy converters. The simulations are validated with experimental measurements of a 1:10 scaled model in a wave tank. The results show that for a short separation distance, the interaction between the oscillating surge wave energy converters has a destructive and constructive effect depending on the wave frequency. However, these effects tend to balance each other out when considering the broad range of wave excitations. For longer separation distances, the interaction always results in a constructive effect. The results reveal that the separation distance has an insignificant impact on annual energy production when considering all wave frequencies and amplitudes.

💡 Research Summary

This paper investigates how the separation distance between the two flaps of a dual‑flap oscillating surge wave energy converter (OSWEC) influences the dynamic response of each flap and the resulting annual energy production (AEP). The study builds on a full‑scale dual‑flap OSWEC design previously presented by Ahmed et al. (2024) and examines separation distances ranging from 10 m to 86 m, which correspond to fractions of the dominant wave lengths at the target deployment site (PacWave South).

A 1:10 scale model of the device was fabricated and tested in the Davidson Laboratory wave tank. Experimental data were used to validate an inviscid numerical model that solves the Euler equations with the FLUENT CFD package. Validation showed good agreement, confirming that the medium‑fidelity, potential‑flow‑based approach can reliably capture the hydrodynamic interaction between the flaps while remaining computationally affordable.

Two complementary simulation strategies were employed. First, torque‑forced simulations isolated the contributions of added mass, radiation damping, and coupling terms in the equations of motion. Five excitation cases were explored: (i) only the right flap receives a harmonic torque while the left flap is fixed, (ii) the right flap is forced but the left flap is free to rotate, (iii‑v) both flaps are forced with phase differences of 0°, 180°, and an arbitrary phase equal to d/λ·2π (where d is the separation distance and λ the wave length). These cases allowed the authors to separate the effects of wave diffraction/reflection from the direct coupling between the flaps.

Second, wave‑forced simulations applied regular incident waves with periods between 7.5 s and 10.5 s and heights from 1.25 m to 5.25 m—representative of the most energetic portion of the PacWave South spectrum. Seven separation distances were examined (10 m, 15 m, 33 m, 45 m, 55 m, 70 m, 86 m). For each case a power matrix was generated, and a partial power matrix focusing on the highest‑frequency, highest‑energy waves was used to estimate AEP.

The results reveal distinct interaction regimes. At short separations (≤ 45 m) the response is highly frequency‑dependent: when the incident wave period is close to the flap’s natural period, the radiated wave from the upstream flap arrives at the downstream flap out of phase, causing destructive interference and a reduction in absorbed power. For other periods the same geometry yields constructive interference and a power boost. Because real sea states contain a broad distribution of periods, these opposing effects largely cancel when integrated over the full spectrum.

At larger separations (≥ 70 m) the radiated wave has attenuated sufficiently that the interaction is consistently constructive, leading to a modest increase in power capture for individual wave conditions. However, when the full wave spectrum is considered, the AEP differences among all tested separations are less than 2 % of the total production. Consequently, the separation distance has an insignificant impact on the long‑term energy yield, even though it does affect instantaneous power for specific wave periods.

From a design perspective, this finding is important. Reducing the flap spacing can lower the mass of the floating platform, decrease mooring line loads, and reduce manufacturing costs without sacrificing annual energy output. The study also demonstrates that medium‑fidelity inviscid simulations, validated against scaled experiments, provide a practical tool for array layout optimization, avoiding the prohibitive computational cost of full RANS simulations.

In summary, the paper concludes that while flap‑to‑flap spacing influences the instantaneous hydrodynamic interaction of a dual‑flap OSWEC, the effect on annual energy production is negligible across realistic sea‑state spectra. Designers can therefore prioritize structural and economic considerations when selecting separation distances for OSWEC arrays.