Simulating wave attenuation by an offshore floating solar system

Abstract

Because the demand for green energy increases rapidly, it is expected that solar and wind energy will dominate future power generation. New in this field is the development of offshore floating solar systems, consisting of multiple interconnected platforms that are covered with PV panels. Besides satisfying the demand for electricity, these floating solar systems may have a beneficial impact on their wave environment. If waves are attenuated, down-wave shadow zones could be useful for other marine activities such as aquaculture or for the protection of a harbour, creating opportunities for offshore multi-use. The overall research objective of this study is to research the effect of an interconnected multi-body floating solar structure on the transmission of wave height. This was done using a scale model test and a numerical model, in a two-dimensional setting.

From the scale model tests, wave transmission showed to be highly dependent on the wave frequency, and thus the wavelength, of the incident waves. The floating solar system acts as a low pass filter: it lets the low-frequency waves pass through, whereas the waves with higher frequency are attenuated by the structure. This dependence is governed by the ratio between the incident wavelength and the length of the rigid platforms that the solar structure consists of, measured in parallel to the wave propagation direction. When the wavelengths were smaller than twice the length of a platform, waves were almost fully attenuated. Apart from the dependence on frequency, the basin test results also showed that the amount of wave attenuation increases linearly with the total length of the system, but only with a small slope. Furthermore, analysis of the reflected wave signal indicated that higher-order dissipative effects are likely needed to describe the decrease in transmission accurately, because the amount of wave energy that was reflected by the structure was limited.

A proof-of-concept for applying a linear numerical boundary element diffraction model to a fixed shallow solar structure was carried out. The wave surface elevation could be obtained, and these solutions were in accordance with the physical expectations. The results showed similar trends to the basin test results, but did not match quantitatively. This difference could lie in the model approach, where a two-dimensional transmission coefficient is determined from three-dimensional simulations. It could also be that the scattering effects alone cannot approximate the transmission behaviour of the system, because linear radiation effects or higher-order dissipation effects might be of similar importance and are therefore required for an accurate simulation. Radiation could be included in a linear diffraction model, but dissipation requires higher-order wave theory.

Regarding the multi-use context, a down-wave shadow zone can form if the incident waves are short enough. For the North Sea location considered, waves are typically longer than twice the length of the rigid platforms, thus the floating solar structure might not have the breakwater performance that is desired to protect other offshore activities.