# A Floating Ocean Energy Conversion Device and Numerical Study on Buoy Shape and Performance

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## Abstract

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## 1. Introduction

## 2. Design of FOECD

- The total length, width and height of the floating platform are 14 m, 13 m and 5.5 m, respectively. The platform body has three layers: the bottom cabins are water tanks to adjust the draught of the catamaran platform; the middle layer is the displaced oil circle and water circle; the upper layer is the control room.
- The floating platform can rise and sink with a range of 3 m by pumping water from the water tank, which is also convenient for checking oscillating buoys and current turbine blades.
- Considering the turbine blades’ need to face the direction of water flow, single-anchor mooring is adopted in the FOECD. When the current direction changes, the platform can adjust its position automatically under the current force.
- The floating platform can be towed to bay or other safe places to avoid typhoons or other severe sea conditions. It can also be conveniently dragged into port for maintenance.
- The platform can be towed to areas with abundant ocean energy considering the wave energy and current energy is changeable over a year. It can also be towed to areas near an island to supply electrical power for people who live in the island.

## 3. Theoretical Model of FOECD

- The fluid is an incompressible ideal fluid.

## 4. Proposition of Buoy Shape

## 5. Suitability and Power Harvesting Capability Performance Simulation for Turbinate Buoy

#### 5.1. Suitability Analysis for Turbinate Buoy

- (1)
- The current flow is constant and the flow velocity is 2 m/s.
- (2)
- To simplify the calculation, the floating platform is fixed and the fluid direction is horizontal or vertical.
- (3)
- The body of the buoy is semi-submerged in sea water with a draught of 1.8 m.

#### 5.2. Analysis of Power Harvesting Capabilities and Efficiency

## 6. Establishment of FOECD

- The total length, width and height of the floating platform are 14 m, 13 m and 5.5 m, respectively.
- The rated electrical generator is 25 kW, which includes current energy generation (15 kW) and wave energy generation (10 kW).
- The wave energy capture part is composed of four turbinate buoys, each with a diameter of 2.2 m. The current capture part has three blades with a horizontal shaft which has a diameter of 4 m. The FOECD starts to work when the wave amplitude is higher than 1.2 m or the current velocity is higher than 0.8 m/s.
- To ensure safe mooring, an iron chain is used. The total length of the chain is 80 m and the anchor weight is 1.5 t. The power of the electric motor used in Capstan is 5 kW.

## 7. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## Abbreviations

FOECD | Floating Ocean Energy Conversion Device |

PTO | Power Take Off |

NIT | Ningbo Institute of Technology, Zhejiang University |

SIMPLE | Semi-Implicit Method for Pressure-Linked Equations |

N-S | Navier-Stokes |

F-K | Froude-Krylov |

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**Figure 1.**Schematic of floating platform of offshore ocean energy converter. 1. Hydraulic energy storage and generation parts; 2. Floating buoy and link parts; 3. Current energy capture part; 4. Movable catamaran platform.

**Figure 2.**The system schematic of coupling of wave energy and current energy generation with hydraulic energy storage and pressure control. 1. Piston pump; 2. One-way valve manifold; 3. Accumulator; 4. Pressure sensor; 5. Two-way electro-hydraulic proportional flow control valve; 6. Solenoid valve; 7. Flow sensor; 8. Hydraulic motor; 9. Electrical generator; 10. Load; 11. Oil filter; 12. Oil tank; 13. Oscillating buoy; 14. Current capture turbine blade; 15. Hydraulic pump.

**Figure 4.**Von Mises stress and deformation of four types of buoys. (

**a**) cylindrical buoy; (

**b**) cylinder-hemisphere combined buoy; (

**c**) turbinate buoy with point tip; (

**d**) turbinate buoy with tip radius at 0.2 m.

Buoy Shape | Added Mass m_{ω}/kg | Water Plane Area A_{ωp}/m^{2} |
---|---|---|

Cuboid buoy (semi-submersible) | K_{m}πρLB^{2}/4 | LB |

Vertical cylindrical buoy (semi-submersible) | ρD^{3}/6 | πR^{2} |

Cone buoy (Full submersible) | ρD^{3}/9 | / |

^{1}${K}_{m}$ is the coefficient of added mass [20]. L and B are the length and width of the cuboid-shaped buoy, respectively. D and R are the diameter and radius of the cylinder buoy, respectively.

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**MDPI and ACS Style**

Song, R.; Zhang, M.; Qian, X.; Wang, X.; Dai, Y.M.; Chen, J. A Floating Ocean Energy Conversion Device and Numerical Study on Buoy Shape and Performance. *J. Mar. Sci. Eng.* **2016**, *4*, 35.
https://doi.org/10.3390/jmse4020035

**AMA Style**

Song R, Zhang M, Qian X, Wang X, Dai YM, Chen J. A Floating Ocean Energy Conversion Device and Numerical Study on Buoy Shape and Performance. *Journal of Marine Science and Engineering*. 2016; 4(2):35.
https://doi.org/10.3390/jmse4020035

**Chicago/Turabian Style**

Song, Ruiyin, Meiqin Zhang, Xiaohua Qian, Xiancheng Wang, Yong Ming Dai, and Junhua Chen. 2016. "A Floating Ocean Energy Conversion Device and Numerical Study on Buoy Shape and Performance" *Journal of Marine Science and Engineering* 4, no. 2: 35.
https://doi.org/10.3390/jmse4020035