1. Introduction
Carbon emission from the use of fossil fuels is increasingly recognized as a worldwide concern [
1]. Marine renewable energy was deemed a possible source of clean and sustainable energy, and had been widely studied over the past decades. Wave energy has unique advantages such as massive reserves, wide distribution, high energy density, and easy to exploit. The principle of wave energy utilization is to convert the kinetic energy and the potential energy of waves into the electrical energy through wave energy converters (WECs). In terms of the energy capture mode, the WEC devices can be divided into three types: oscillating water column devices, overtopping devices, and point-absorption wave energy converters (PAWEC) [
2]. PAWEC uses the reciprocating motion in the six degrees of freedom to drive power take-off system and achieves the conversion process from wave energy to electrical energy. PAWECs have demonstrated some advantages over other WECs, such as a smaller volume and high wave energy conversion per unit volume [
3]; PAWECs are also easy to realize in modules (and therefore easy to scale up), and to combine with floating structures [
4]. The combination PAWEC systems with other ocean platforms (such as, for example, floating wind turbines) in hybrid platforms is starting to gain attraction [
5]. One pioneer work is the Spar-Torus Combination (STC) (
Figure 1a) concept developed by Muliawan [
6]. The working principle of the STC device is to have a floating torus component, at waterline level, around the SPAR support platform of a wind turbine, and to exploit the relative motion between the wind turbine and the torus to generate electricity. Similar wind-wave hybrid devices are the Wind Lens [
7] in Japan, the W2Power [
8] (
Figure 1b) in Norway, and the wind-wave hybrid platform [
9] (
Figure 1c). In addition to offshore wind turbines, semi-submersible platforms can also host a series of PAWECs, such as the Manchester Bobber [
10] (
Figure 1d), wave star [
11]. There is a fully packed spheroidal smart buoy hybrid generator (SB-HG) composed of triboelectric nanogenerator (TENG) and an electromagnetic generator (EMG) [
12,
13]. Recently, a research group from Harbin Engineering University (Liu et al. [
14]) filed a patent for the “Wave energy converter with funnel-shape moonpool structure” (
Figure 1e), which has been granted (authorization code: ZL201610293268.8).
Compared with the research on single PAWECs, for multiple PAWECs integrated with other energy devices, the coupling effect between PAWEC and the platform needs to be considered. The numerical methods used usually adopt both frequency and time domain approaches [
15]: the linear hydrodynamic properties are obtained using a potential flow solver in the frequency domain, and then the hydromechanics coefficients are used in the time domain through time delay function. Taghipour and Moan [
16] researched a multiple WEC configuration, in the frequency domain, using a mode expansion method, to evaluate the performance of the WEC devices in converting the wave energy and its dynamic characteristic in ocean waves. Lee et al. [
17] numerically simulated on the multi-body hydrodynamic interaction between a hybrid floating platform and a multi-wave energy converter, in the frequency domain. Since linear potential theory cannot capture viscous effects, usually empirical methods are adopted to include this effect, be it numerical or experimental. However, recent trends [
18] in the development of CFD methods have shown increasing interests in modelling WECs, where viscous effects are non-ignorable. Lo et al. [
19] used a CFD approach to analyze the performance of an air-blower wave energy converter, and to calculate the power output of two buoys. Jin et al. [
20] took the nonlinear viscosity into account to model the WEC hydrodynamics near resonance conditions. Nonlinear PAWEC system’s hydrodynamics, in conditions close to resonance (i.e., incident wave frequency near the systems’ natural frequency) or in high wave heights conditions, can be realistically carried out. In order to reduce the computational costs linked to CFD methodologies, Liu et al. [
21] introduced a nonlinear viscous dissipative term in the modelling the moonpool structure, and derived the relationship between the nonlinear dissipation coefficient and the resonant frequency of the moonpool.
Experimental methods are a useful way of carrying out feasibility studies of newly developed WECs. However, full-scale model tests at sea may be expensive and technically challenging, especially as a first step to validate a relatively new concept. A small-scale model test offers an alternative but effective way to tackle this problem. Zheng et al. [
22] investigated the motion and energy conversion of a WEC with two bodies relayed on tank experiment, deriving that the maximum efficiency is obtained when the wave period, the PTO damping coefficient, and the mass ratio are optimally tuned. In the case of hybrid platform, for example, Ren et al. [
23] analyzed a combined monopile hybrid floating platform and a multi-wave energy converter, and derived the optimum tuning of the PTO damping through the use of coordinated numerical and experimental analyses. Gao et al. [
24] studied three floating wind-wave hybrid concepts (STC, SFC, and OWC), comparing their energy efficiency and economic feasibility. Wan et al. [
25,
26,
27] studied green water phenomena of STC, and the model test was used to simulate these nonlinear phenomena as well as the survivability of the device in extreme sea conditions. Unlike for the STC [
28], Chen et al. [
29] introduced a high-power integrated generation unit for offshore wind power and ocean wave energy (W2P), and tested output power of energy conversion devices. There are several papers about some similar works on numerical and experimental analyses of different wave energy converter published in Energies. Chybowski et al. [
30] used the ANSYS AQWAWB and AQWA method to simulate the behavior of the device’s performance, and the experimental studies recorded the performance of the prototype device. Thomas et al. [
31] applied a shallow artificial neural network (ANN) which is a kind of machine learning language to obtain optimal working times. Wu et al. [
32] put forward a new computational fluid dynamic method to predict the hydrodynamic characteristic of the Duck WEC, the results of which agree well with the experimental results. Kong et al. [
33] adopted a semi-analytical approach based on the potential flow to assess the wave energy efficiency of the moonpool platform WEC in the journal of Energies.
Kong studied the WEC by the potential flow approach and ignored the viscous effect, so the viscous dissipation coefficient and experimental model were applied in the paper. This paper aims at investigating a new ocean platform-WEC device, which combines a moonpool platform and a WEC (MPWEC) system. Basically, an external cylindrical shell houses an internal moonpool, and an axysymmetrical point-absorber buoy is placed in this moonpool: the relative displacement between the cylindrical shell and the device is used to converter power. The displacement and power of the MPWEC in regular waves have been researched by a series of experiments in the wave tank at Harbin Engineering University. As mentioned, the experimental results are then used to benchmark the potential flow method and the CFD method, which are developed to numerically simulate the dynamics of the MPWEC device in the frequency and time domains, respectively. A satisfactory agreement is obtained between the numerical and experimental results.
5. Conclusions
The research object is the moonpool float, and we developed moonpool float wave energy conversion device model design and test research, compared with the potential method and CFD method of the calculation results with the experimental measurements of the pool. We observed that wave period and PTO damping are important factors influencing the dynamic characteristic of WEC. Further research on moonpool float device of wave energy conversion and resonance characteristics is necessary in order to improve the moonpool float and the performance of the system as a whole on a technical and scientific basis.
The MPWEC with PTO system is presented in the article. Comparing all the above numerical and experimental results, the conclusions can be drawn as follows.
(1) The moonpool wave energy converter including moonpool and cylindrical buoy was designed. The experiment of a single buoy and moonpool buoy has resonant frequency with one and two resonant points, respectively. We observed that wave period and PTO damping are important factors that influence the wave energy converter’s motion and energy extraction capability, according to the same experimental results.
(2) Either in the frequency domain or in the time domain, the experiment results and numerical results which were calculated by the potential method and CFD method has great agreement no more than 18.9%. Results showed that the efficiency of a single WEC reached the peak when the wave height was 0.12 m, wave period was 1.6 s, and the PTO damping corresponded to the resistance of 20 Ω. Results showed that the efficiency of MPWEC reached the peak when the wave height was 0.12 m, wave period was 1.8 and 2.2 s, and the PTO damping corresponded to the resistance.
(3) Compared with the single buoy and moonpool buoy, the moonpool can enhance the wave energy conversion in the frequency of 1.7–2.5 rad/s. On the contrary, when the wave period is short, the moonpool will hinder the motion of the cylinder buoy. It can be seen that the moonpool has wave elimination and wave gather.
In the future, the nonlinear PTO damping of wave energy device will be studied specially, and the optimal PTO damping coefficient will be determined. On this basis, the adaptive optimization algorithm of wave energy device can be further researched, and the control algorithm of PTO system can be explored in the time domain. The nonlinear wave theory will be applied to solve the complex hydrodynamic problems.