1. Floating Solar Photovoltaic Farm
The worldwide energy demand is continuously rising, and finding alternative and more sustainable sources of energy is crucial to mitigate the negative environmental impacts associated with fossil fuel-based electricity generation [
1,
2,
3,
4]. Floating photovoltaic (FPV) systems, which involve installing solar panels supported on a floating platform and deployed on water bodies such as oceans, lakes, reservoirs, and canals, have emerged as an attractive option to overcome land constraints [
5,
6,
7]. Over the decades, the cumulative installed capacity of floating solar PV farms (FPV) has been increasing year over year, with installations expanding in oceans, lakes, estuaries, and natural basins [
8,
9]. The advantages of FPV systems include fewer obstacles to block sunlight, convenience in installation, increased energy efficiency, higher power generation efficiency, and reduced water evaporation [
10,
11]. However, there are challenges related to wind and wave loads as well as corrosion issues, which differ from onshore conditions [
6,
12], when deploying floating solar systems in marine environments, specifically in the open sea.
The cost of solar panels can vary by location, e.g., countries located in a tropical region receive higher annual solar irradiation as compared to the sub-tropical countries, thereby making solar energy more enticing for the former. Due to the advancement of solar PV technology over the decades, the cost of solar PV panels has dropped drastically [
13], where the price of solar panel installation has significantly decreased by 89% over the past decade [
14]. This drives the worldwide adoption of solar PV panels to convert solar irradiation into electricity as an attractive alternative to hydrocarbon. The monocrystalline solar panel, which is the most energy-efficient option, costs approximately from
$1 to
$1.50 per watt [
15], whereas the polycrystalline solar panel, which is less energy-efficient, costs around from
$0.90 to
$1 per watt [
16]. The solar PV panel price is predicted to continue its downward trend from 2023 onwards, as more polysilicon manufacturers come into operation [
17].
The steady reduction in the cost of solar PV panels has resulted in an increase in the solar panel footprint globally as an important initiative to reduce the global reliance on hydrocarbon. The International Energy Agency (IEA) reported that solar PV accounted for 4.5% of global electricity generation in 2022, making it the third largest renewable electricity technology, following hydropower and wind [
18]. Some of the world’s largest FPV systems deployed in lakes and reservoirs can be found in Asia. For example, the FPV system located in Sirindhorn Reservoir in Thailand covers an area equivalent to about 70 soccer fields and has the capacity to generate 45 MW of power [
19]. China is also home to three of the largest FPV systems in the world, i.e., Dingzhuang FPV system in Dezhou (320 MW) [
20], the FPV system in the Three Gorges (150 MW) [
21], and the CECEP FPV system in An hui (70 MW) [
22]. In 2021, Singapore unveiled one of the world’s largest FPV systems, spanning an area equivalent to 45 football fields [
23]. The solar farm located on a reservoir in western Singapore has a 60 megawatt-peak (MWp) capacity and aims to reduce carbon emissions by about 32 kilotonnes annually. Korea has planned to install the Saemangeum floating solar energy project, which will be the biggest operational floating solar power plant in the world, with a capacity of 2.1 gigawatts (GW), when completed in 2024 [
24].
With the promising electricity output from FPV panels deployed on water bodies, energy providers have looked into offshore FPV farms (OFPVs) to be deployed in the open sea, as the ocean offers an enticing option with a theoretical global PV capacity of around 4000 gigawatts [
25]. Research suggests that floating solar panels at sea perform nearly 13% better on average than land-based installations, with some months generating up to 18% more energy due to lower temperatures and reduced cloud cover [
26]. These OFPVs have increased in size over the years, in the order of hectares. Sunseap OFPV, another solar PV farm located in Singapore, has a 5 MWp and is considered one of the largest offshore solar developments in the world [
27]. The project occupies a five-hectare footprint and is estimated to produce up to six megawatt-hours (MWh) of energy per year. At the same time, European energy providers such as the Dutch-Norwegian company SolarDuck are working with the German energy company RWE to build a floating solar plant with a raising deck at the North Sea wind farm [
28]. Meanwhile, the Norway-based Ocean Sun has developed a floating rig where the solar panels rest on a base which flexes as the waves pass underneath [
29].
2. Hydroelastic Response
The increase in size of OFPVs means that the fluid–structure interaction (FSI) becomes prominent, especially under the wave action. The flexible deformation of the structure under hydrodynamic loading is termed the hydroelastic response. As OFPVs are constructed further out to sea, they are exposed to greater wave loadings; therefore, OFPVs such as the one by Ocean Sun, where the PV panels are supported on a floating base, allow the structure some flexibility as wave passes underneath. The allowance for some flexibility means that the structure could be constructed with less rigidity. This is important, especially when the structure is larger, as it could significantly reduce the cost of OFPVs.
Conventional solar PV panels are supported by multi-connected modular floating platforms where the structures are simply assumed to be rigid bodies and structure deformations under wave action are neglected. Song et al. [
30] studied the dynamic response of a multi-connected floating solar panel system by assuming that the supported structure was a rigid body. A hydrodynamic analysis to study the motion of floating offshore solar farms subjected to regular waves was also conducted by Al-Yacouby et al. [
31], who made the same assumption that the structure was a rigid body. Having said that, the hydroelastic response of OFPVs has been investigated by researchers such as Sree et al. [
32], who considered a multi-scale numerical approach to predict the responses of 6000 interconnected floating modular units where the FSI was solved using the arbitrary Langrangian–Eulerian method. As the solution to the fluid motion involves solving the Navier–Stokes equation, the computation time required to solve the FSI is costly. The computational time could be accelerated by modelling the fluid as a potential flow. This has been carried out by Xu and Wellens [
33], who considered a large-scale floating PV farm supported by a membrane. They investigated the fully nonlinear FSI, focusing on the performance of the modules under potential flow, which they solved analytically up to the third order. The interconnected floating modular units in their case were modelled using the Euler Bernoulli-von Kármán beam model.
This paper will study the hydroelastic response of multi-connected floating modular units that serve as a platform to support solar panels. The interconnected floating units are modelled using the Kirchhoff–Love thin plate theory [
34,
35], which better represents an OFPV which has a small depth relative to its length dimension. The linear potential flow theory is used to represent the fluid, where the fluid is assumed to be inviscid, incompressible, and flow irrotational. The vibration mode of the floating platform is obtained from the proposed numerical scheme and is first validated with that obtained from an established finite element model. The hydroelastic response of an OFPV subject to regular waves is then studied. Various configurations with different module dimensions and spacing of the floating solar platform are considered, and the optimal configuration that gives the best performance in terms of the overall minimum response, known as compliance, is then suggested.
7. Conclusions
A three-dimensional hydroelastic analysis of OFPVs was presented, where the floating structure was assumed to be a mat-like VLFS in grid configuration that could be modelled using the Kirchhoff–Love thin plate theory, whereas the water was assumed to be an ideal fluid modelled using the potential wave theory. The hybrid boundary element-finite element method was used to solve the fluid-structure interaction problem. The free vibration analysis was first carried out by solving the eigenvalue problem, where the natural frequencies (eigenvalue) and vibration modes (eigenvectors) were verified with their counterparts obtained from the finite element software ABAQUS. The verification showed good agreement in the natural frequencies and modes between the present model and those obtained from ABAQUS. This paper then proceeded to study the hydroelastic response of the OFPVs, where two case studies were carried out, i.e., OFPV-I (OFPV in one whole piece) and OFPV-II (OFPV in separate modules). It is important to note that the present method is limited to OFPVs with a small thickness-to-structural length ratio due to the limitation of the Kirchhoff–Love thin plate theory. A higher order plate theory such as the first order or third order shear deformation plate theory could be used to model OFPVs with a larger thickness. As a conclusion, the following findings were obtained:
The hydroelastic response and compliance for OFPV-I showed that the elastic deformation of the OFPV increases with a reduction in wave periods;
OFPVs with a smaller aspect ratio (long-ish structure) have greater elastic deformation but deflect in smaller magnitudes compared to the square OFPV-IA (aspect ratio ;
The hydroelastic response under headsea conditions could be reduced by increasing the longitudinal stiffness of the OFPV. This can be done by reducing the spacing between the longitudinal modules in the OFPV;
By splitting the OFPV into smaller OFPVs, i.e., OFPV-II, the compliance values implied that the hydroelastic response was smaller for OFPV-I when it wasconstructed in one piece;
The gap spacing between the separated farms in OFPV-II showed a profound difference in the hydroelastic response due to the interference effect between the separated farms, and OFPV-IIA, which comprises four separated farms, has a higher compared to OFPV-IIB and OFPV-IIC, both having two separated farms;
The plot contours of the hydroelastic deflection showed that the farms located on the leeward side have smaller values compared to their counterparts located on the windward side.
In conclusion, this paper presents a numerical framework for computing the hydroelasticity of OFPV farms and the results presented here provide insight into the preferable layout configuration for OFPVs subject to wave action.