Elsevier

Ocean Engineering

Volume 255, 1 July 2022, 111434
Ocean Engineering

Design and analysis of the optimal spinning top-shaped buoy for wave energy harvesting in low energy density seas for sustainable marine aquaculture

https://doi.org/10.1016/j.oceaneng.2022.111434Get rights and content

Highlights

  • Low energy density seas were considered for geometric variation analysis.

  • Derived five new geometries from cylinder buoy based on identical conditions.

  • Optimal and suboptimal PTO modes were employed to study the effects of shape change.

  • Cyl-AS buoy showed maximum power extraction ability among the other five shapes.

  • The harvested energy can supply power to the auxiliaries of the marine aquaculture.

Abstract

The floating buoys are an emerging source for renewable energy harvesting using ocean waves. Various point absorbers have been developed based on multi-freedom oscillating structures to capture wave energy. However, the choice of geometrical parameters for buoy design is still a critical problem. This paper presents a novel method to develop new geometries of floating buoy for wave energy harvester (WEH) in heave motion and aims to maximize WEH absorbed power and performance. Fives shapes (Cyl-Cone, Cyl-HS, Cyl-TCC, Cyl-AS and Cyl-SP) were derived from a cylinder buoy based on identical parameters (radius, draft, mass, water-plane area, volume and natural frequency). The Cyl-HS buoy was selected as a reference buoy for comparative analysis. The effects of shape change on the performance and output power were investigated for regular and irregular wave conditions in ANSYS AQWA at an incident wave frequency of 0.22 Hz (low energy density seas). The optimal resonant buoy was identified employing optimal and sub-optimal modes of the PTO system at external stiffness and damping parameters. Moreover, the performance of resonant buoys in optimal and suboptimal working modes was validated statistically using the F-test for variances of two samples in Origin Pro (2019). The energy conversion efficiencies of Cyl-AS buoy were examined as highest among others, with a maximum of 8.4% and 7.4% rise in optimal and suboptimal PTO modes for regular waves, respectively. For irregular waves, the Cyl-AS buoy resulted in maximum output power of 33.9% higher than the Cyl-HS buoy (reference buoy) with a maximum conversion efficiency of 90%. Finally, the simulation and experimental results for the scaled model were compared for RAO and average power and found a good agreement. The findings of this research agree with the UN-2030 Agenda for Sustainable Development Goals (in particular, SDG-7 and SDG-14 goals) while providing a path towards clean energy and blue economy developments.

Introduction

Non-renewable energy resources are the primary cause of environmental pollution and climate change, whereas the prices of these energy resources are too high. Environment protection agencies and research scholars worldwide are working on renewable, low-cost, and eco-friendly energy solutions to compete for energy demand (Lavidas and Blok, 2021; Zheng, 2021). In terms of renewable energy, ocean wave power is one of the most promising, with immense potential, and these technologies are continuously progressing toward commercialization (Curto et al., 2020). Specifically, marine turbines demonstrate their dependability in increasing applications. In 2020, ocean power generation surpassed 60 GWh, with deployments adding roughly 2 MW, increasing total operational installed capacity to approximately 527 MW by the end of 2021 (Murdock et al., 2021). Ocean power is remained underdeveloped due to a lack of policy support and technological innovation to minimize high installation costs. Efforts are being made to shift the temporary small-scale devices and pilot projects by large permanent structures with multiple array devices. On the other hand, Europe has set new deadlines for constructing naval power plants, with a capacity of 1 GW by 2030 and 40 GW by 2050 (Commission, 2020). However, consistent policy and revenue support can result in more rapid and increasing energy technologies.

Budar and Falnes designed a point absorber to harness the energy of ocean waves in 1975 (Budar and Falnes, 1975), and the idea soon gained popularity among many scientists. Their concept was followed by various concepts and designs about marine energy harvesters. Among these harvesters, the floating body wave energy harvesters are relatively small in size, complex in structure, and low in cost and can be employed in array arrangement to generate an enormous amount of power (Gomes et al., 2020). The working principle of floating body wave energy harvesters is based on incident waves interacting with a floating absorber and producing motion of absorbing body in six directions, including heave, pitch, roll, sway, yaw, and surge. The periodic up and down or heave motion is an excellent source for continuous power generation (Algarín and Bula, 2021). In general, there are three energy conversion steps in the wave energy conversion process. The primary stage, also considered the energy input module, collects the wave energy for further conversion into mechanical, pneumatic, or potential energy using the power take-off (PTO) system (Sun et al., 2021b). The buoy floats and gathers energy from incident waves and transfers energy to a specified energy conversion PTO system. Secondary energy conversion is accomplished using the PTO system to convert the available wave energy into useable mechanical energy. The popular PTOs include mechanical, electromagnetic and air and water turbines. In the tertiary energy conversion stage, the PTOs are connected to the generators and the valuable mechanical energy is converted into electricity (Pelc and Fujita, 2002; N. Zhang et al., 2021). The PTO is chosen based on the wave energy converter design, whereas PTO dampening has essential effects on the hydrodynamic performance of WEH (Jin et al., 2021; Liang et al., 2017).

Various strategies have been explored to improve the overall efficiency of floating buoy systems. Because of the dynamics of floating bodies, strategies for increasing efficiency are primarily concerned with optimizing the geometry (shape) of the floating buoy such that it operates around its resonance frequency (Aderinto and Li, 2019). The most used methodologies are the design of PTO and geometry optimization of floating buoys based on resonance frequency method for optimal power generation and the maximum achievable output power occurs in the resonance region when the natural frequency of the floating buoy corresponds to the frequency of the incoming waves (Berenjkoob et al., 2021). Other techniques include reducing the viscous damping when the buoy moves through the water (Liang and Zuo, 2017). Liu et al. (2016) studied the feasibility of 10 kW WEH using sea trials of the pilot heaving buoys to investigate the hydrodynamic parameters due to the influence of damping coefficients. Guo et al. (Guo and Ringwood, 2021) compiled nine geometric shapes for floating single body wave energy harvesters and concluded that the WEH shape optimization substantially affects conversion efficiency. In another study, Garcia-Teruel and Forehand (2021) reviewed different geometries (single body hovering devices) to compare and optimize geometries for WEHs with maximum output power. Gao and Xiao considered the effects of buoy shape and PTO parameters on wave energy extraction for energy conversion system and compared three shapes (Cylinder, Cyl-Cone and Cyl-hemisphere) and found that the diameter and draft have a direct impact on the natural frequency of buoy. Moreover, they concluded that the draft of buoy has little effect on efficiency, but the diameter of buoy has a strong relationship with extracted power and the Cyl-Cone showed the efficient extraction ability among these three shapes (Gao and Xiao, 2021).

For resonant buoy design, Ref (Tao et al., 2021). examined that any change in draft significantly affects WEH conversion efficiency, especially when it changes the bandwidth and resonance frequency of the floating buoy. At a suitable draft of the floating buoy, the conversion efficiency reaches its peak value and the natural frequency of the buoy is nearly equal to the frequency of the incident wave. In another study, the optimal and sub-optimal resonant buoy criteria were employed by Berenjkoob et al. (2021), focusing on varied shapes of single-body WEH systems at different hydrodynamic parameters. An algorithm was developed to create identical conditions for comparative study based on geometry optimization and remarked that the performance of the PTO system could be improved by using an appropriate design for the buoy shape. Many studies (Bachynski et al., 2012; Shadman et al., 2018) have been conducted by varying the geometrical parameters of the floating buoy, but few studies (Berenjkoob et al., 2021; Gao and Xiao, 2021; Gao and Yu, 2018) compared the effects of shape variation while considering limited identical parameters.

The current study develops an algorithm to select the appropriate shape for the wave energy harvesting buoy based on the characteristics of incident waves. Different geometries of WEH buoy were derived from cylinder buoy at identical conditions, including mass, diameter, draft, water-plane area, volume and natural frequency. Optimal and sub-optimal PTO modes are employed to design an optimal resonant buoy utilizing PTO parameters, including external damping and stiffness, in regular and irregular waves. Based on the computational and hydrodynamic analysis and experimental validation, the proposed method identifies the effects of changing geometric characteristics on the overall performance and conversion efficiencies of floating buoys. Finally, the findings of this research agree with UN-General Assembly Resolution 2015, known as the 2030 Agenda for Sustainable Development, while providing a path towards clean energy and blue economy development (Azam et al., 2021a, 2021b; Desa, 2016; Gissi et al., 2022).

The rest of this paper is organized in the following manner: significant wave height and peak period selections were made based on hourly wavefield data using probability analysis, spectral analysis and power level analysis and presented in section 2. The mathematical modeling for derived buoys is described in section 3. Section 4 considers parametric optimization such as novel buoy design, optimization methodology, simulation validation and PTO optimization modes. The findings of the hydrodynamic analysis for resonant buoys in regular and irregular wave conditions are given in section 5. Sections 6 Experimental validation of the results, 7 Impact of current research on blue economy debated the validation of results and the impact of current research on the blue economy, respectively. Finally, sections 8 Future recommendations, 9 Conclusions present the future recommendations and findings of the current study.

Section snippets

Data collection and analyses

For this study, the significant wave height and peak period were selected as key indicators to analyze the applicability and energy potential for ocean energy harvesting devices. The hourly wavefield data were obtained for the XiaoMaiDao wave station, Shandong Province, China (36°3′ N, 120°25′ E), as presented in Fig. 1(a). The wavefield data for one year was obtained from January 10, 2019 (00:00) to 09/31/2020 (23:00) using the online database (National Marine Science Data Centre China) on

Mathematical modeling

The basic principle of wave energy conversion is based on motion conversion (upward and downward movements of floating buoy), which drives the PTO system installed inside the floating buoy to generate power (Chen et al., 2019; Guo and Ringwood, 2021). The energy conversion system for the current study is based on the principle mentioned above, in which the floating buoy receives the kinetic energy of the incident waves and drives the PTO system. The WEH system comprises of wave energy input

Parametric optimization

The buoy geometry plays an essential role in power absorption and efficiency, so geometrical optimization is necessary to achieve the maximum absorbed power of floating buoy under wave excitation, particularly in heave motion. Previous studies suggested three criteria that need to be fulfilled for better optimization, as listed below:

  • (i)

    For enhancing efficiency and controlling floating buoy movement, the floating buoy diameter should be proportionate to the wavelength of incident waves (Chen et

Hydrodynamics of the resonant buoy

The dynamic response and performance of the resonant buoys are influenced by the hydrodynamic and hydrostatic coefficients of the WEH, such as diameter, draft, mass, submergence volume, hydrostatic stiffness, added mass, radiation damping, PTO stiffness and PTO damping. The parameters mentioned above were made identical to obtain natural frequencies of the buoy close to the frequency of incident waves. The parametric optimization aims to find the pure effects of shape variation on the

Experimental validation of the results

The simulation results of the scaled model of Cyl-AS buoy were validated with experimental analysis. The energy conversion efficiency of the scaled model of Cyl-AS buoy was compared to cylinder-hemisphere buoy used in published experimental results (Tampier and Grueter, 2017) under similar conditions (no viscous effects are taken into account). A deep water tank with a 45 m length 3 m wide and 2 m water height was employed for the experimental study. The specifications of the scaled and

Impact of current research on blue economy

The aquaculture sector is a sustainable source of world economic growth, and China has the world's largest aquaculture output (about 60% of the world's). China has constructed the world's largest entirely submerged net cage, which can raise 300,000 salmon over a growth cycle, with a projected yearly output of 1500 tonnes.2 The most coastal aquaculture ponds are in Guangdong province, followed by Shandong province, Jiangsu province,

Future recommendations

In the current study, various innovative buoy designs were developed and tested to determine the effects on the hydrodynamic response and conversion efficiency of the floating buoy. However, several research gaps were found that should be considered in future studies to obtain a more profound understanding of efficient WEH design and are suggested as:

  • a)

    The current study examines the influence of shape on performance at {M,D,L,V,ωn=constant}; however, more research can be carried out to assess

Conclusions

The current study enhances the knowledge of wave energy harvesting by introducing novel geometries for floating buoys in regular and irregular waves to harvest energy. The WEH model with novel floating buoys was analyzed through a simulation study using ANSYS AQWA considering heave motion. Various shapes have been derived from the cylinder buoy based on identical parameters, including diameter, draft, water-plane area, volume, mass and natural frequency. An algorithm was developed to design the

CRediT authorship contribution statement

Ali Azam: Conceptualization, Methodology, Numerical analysis, Investigation, Formal analysis, Validation, Data curation, Writing – original draft, Language editing, Revision, Visualization. Ammar Ahmed: Conceptualization, Methodology, Numerical analysis, Language editing, Revision, Data curation, Validation. Hai Li: Visualization. Alaeldin M. Tairab: Validation, and, Resources. Changyuan Jia: Validation, and, Resources. Ning Li: Validation, and, Resources. Zutao Zhang: Supervision, Project

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Foundation of China under Grants No. 51975490; and by the Science and Technology Projects of Sichuan under Grants Nos. Nos. 2021YFQ0055, 2021YFSY0059, 2021JDRC0118, and 2021JDRC0096; and by the Science and Technology Projects of Chengdu under Grant No. 2021YF0800138GX.

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