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Experimental investigation of a multi-buoy cooperative point-absorber wave energy converter

Herman Martens Meyer, Leif Arne Tønnessen, Olav Gundersen, Atle Jensen

Abstract

This study presents a proof-of-concept experimental investigation of a multi-buoy cooperative point-absorber wave energy converter (WEC). The proposed concept consists of an array of surface-penetrating buoys connected through a shared closed-loop hydraulic power take-off (PTO) system. Energy is extracted through the collective motion of the buoy array, where pressurised flow generated by individual buoys drives a turbine within the hydraulic circuit. A 1:40 scale model was tested in the wave tank facilities at the University of Oslo. Experiments with regular and irregular long-crested waves at two different incident angles were conducted to assess power absorption, wave period response, and interaction effects. Two array configurations were investigated: an eight-buoy array with an axis-to-axis spacing of 1.5 buoy diameters, and a four-buoy array with a 3.0 diameter spacing. Although piston head leakage affected the power measurements, our results demonstrate that the WEC absorbs incoming wave energy and produces measurable power. The eight-buoy configuration achieved the highest power output per buoy compared to the four-buoy configuration, but exhibited increased sensitivity to wave period and wave heading, due to buoy-buoy interactions, such as collisions. This study highlights buoy count and internal buoy spacing as key design parameters for cooperative point-absorber wave energy systems. The results indicate that higher buoy counts enhance hydraulic cooperation, and increased buoy spacing improves robustness to wave heading and reduces destructive interaction effects. We also suggest that a lower system inertia can improve responsiveness to shorter waves. These insights provide a foundation for further optimisation and future full-scale development.

Experimental investigation of a multi-buoy cooperative point-absorber wave energy converter

Abstract

This study presents a proof-of-concept experimental investigation of a multi-buoy cooperative point-absorber wave energy converter (WEC). The proposed concept consists of an array of surface-penetrating buoys connected through a shared closed-loop hydraulic power take-off (PTO) system. Energy is extracted through the collective motion of the buoy array, where pressurised flow generated by individual buoys drives a turbine within the hydraulic circuit. A 1:40 scale model was tested in the wave tank facilities at the University of Oslo. Experiments with regular and irregular long-crested waves at two different incident angles were conducted to assess power absorption, wave period response, and interaction effects. Two array configurations were investigated: an eight-buoy array with an axis-to-axis spacing of 1.5 buoy diameters, and a four-buoy array with a 3.0 diameter spacing. Although piston head leakage affected the power measurements, our results demonstrate that the WEC absorbs incoming wave energy and produces measurable power. The eight-buoy configuration achieved the highest power output per buoy compared to the four-buoy configuration, but exhibited increased sensitivity to wave period and wave heading, due to buoy-buoy interactions, such as collisions. This study highlights buoy count and internal buoy spacing as key design parameters for cooperative point-absorber wave energy systems. The results indicate that higher buoy counts enhance hydraulic cooperation, and increased buoy spacing improves robustness to wave heading and reduces destructive interaction effects. We also suggest that a lower system inertia can improve responsiveness to shorter waves. These insights provide a foundation for further optimisation and future full-scale development.
Paper Structure (10 sections, 7 equations, 17 figures, 2 tables)

This paper contains 10 sections, 7 equations, 17 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Theory and performance measures
  3. WEC concept and model
  4. Experimental model
  5. Tank validation and experimental set up
  6. Results and discussion
  7. Regular waves
  8. Irregular waves
  9. Addressing leakage problem
  10. Conclusions

Figures (17)

  • Figure 1: Schematic illustrating the working principle of the Concrest Energy WEC closed-loop hydraulic circuit. Water displaced by the upward motion of one buoy enters the high-pressure line, driving a turbine and simultaneously filling the actuator of a buoy in a wave trough, thereby assisting its descent.
  • Figure 2: Dimensions of model-scale buoy and the corresponding full-scale version.
  • Figure 3: The model-scale Concrest Energy WEC employed in the experimental campaign, showing the placement of the pressure and flow measurement sensors as well as the choke valve. The length represents the distance between the outer edges of the first and last buoys.
  • Figure 4: Schematic representation of the experimental setup used in the campaign, illustrating the WEC position in relation to the tank geometry and the location of the wave gauges. Not to scale.
  • Figure 5: Comparison of the WEC response to two similar wavelength cases, highlighting the reflection effect. Incoming wave amplitude is $a=3.3$ cm for both graphs. The gray shaded region represents the analysis window for regular waves in this project.
  • ...and 12 more figures