NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Experimental Investigation of the 

Power Generation Performance of 

Floating-Point Absorber Wave 

Energy Systems

Preprint

Y. Li and Y. Yu

National Renewable Energy Laboratory

J. Epler and M. Previsic

Re Vision LLC

To be presented at the 27th International Workshop on Water 

Waves and Floating Bodies

Copenhagen, Denmark

April 22-25, 2012

Conference Paper

NREL/CP-5000-54572

April 2012
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1

Experimental Investigation of the power generation performance of 

floating-point absorber wave energy systems

Ye Li1, Yi-Hsiang Yu1, Jeff Epler2, and Mirko Previsic2

1 National Renewable Energy Laboratory, Golden, CO

2 Re Vision LLC, Sacramento, California

1 Introduction

The extraction of energy from ocean waves has gained interest in recent years (Thorpe 1999). The

floating-point absorber (FPA) is one of the most promising devices among a wide variety of wave energy 

conversion technologies (Falcão 2010). Early theoretical studies mainly focused on understanding the 

hydrodynamics of the system and on predicting the maximum power that could be extracted by a heaving 

body (Budal and Falnes 1975, Evans 1976, Mei 1976, and Newman 1976). These studies evolve from the 

investigation of floating-body interactions in offshore engineering and naval architecture disciplines. For 

more information on this topic, see the comprehensive reviews provided by Falnes (2002) and Falnes

(2007). To our best knowledge, no systematic study has been reported about the investigation of the 

power generation performance of an FPA with a close-to-commercial design. A series of experimental 

tests was conducted to investigate the power extraction performance of an FPA system. Herein, we

present a preview of our experimental study.

SBFPA TBFPA Component Mass in 

kilograms (Kg)

A. Float 3.220

B. Central column 0.745

C. Damping plate 1.108

D. Top roller 0.115

E. Bottom roller 0.117

F. Cylinder and clamps 0.205

G. Heave guides 0.116

H. Load cell 0.012

I. Needle valve 0.015

J. Linear potentiometer (short) 0.077

K. Linear potentiometer (long) 0.114

Figure 1. Design of the floating-point absorber: (left) geometry sketch and (right) properties in model scale

2 Device Design

Typically, an FPA is a single buoy that reacts against the seabed or a multi-body system that generates 

energy from the relative motion between the oscillating bodies. The latter is generally designed for deeper 

water deployment, with a depth between 40 meters and 100 meters, while the single buoy system operates 
2

in shallower locations. Here, we consider a single-body FPA (SBFPA) system and a two-body FPA 

(TBFPA) system. A 1:33 scale model was built and the details of the model geometry and properties are 

shown in Figure 1. Both systems contain a float and a central column, and the TBFPA system includes an 

additional damping plate. The float and the central column are connected with a miniature hydraulic 

cylinder in closed circuits with a needle valve to provide damping to the relative motion to represent the 

power take-off (PTO) mechanism. The PTO damping is controlled by turning the needle valve.

3 Experimental Setup

The experimental test was conducted at the wave tank in Scripps Institution of Oceanography at the 

University of California, San Diego, in late summer 2011. Figure 2 shows the dimension of the wave tank 

and the experimental settings. The wave tank is 44.5 meters long and 2.44 meters wide, and the water 

depth is 1.46 meters (Figure 2a). The test apparatus for both the SBFPA and the TBFPA was designed to 

only allow heave motion. Specifically, the float was allowed to move along the central column and the 

system generates energy based on the relative motion between the float and the fixed carriage (for the 

SBFPA) or the float and the reaction plate (for the TBFPA).

(a)
3

(b) (c)

(d) (e)

Clockwise NREL/PIX 0980, 0981, 1002, 004 

Figure 2. Experimental setup: (a) configuration sketch, (b) load cell, (c) linear potentiometer,

(d) camera system, and (e) a snap shot

Throughout the test, a linear potentiometer (Figure 2c) was used to record the relative motion between the 

float and the central column. In order to ensure the linear potentiometer measurement quality, a camera 

tracking system was implemented. Four cameras were arranged in a semi-circle around the model (Figure 

2d) to ensure the camera system was able to fully capture the dynamic response of the device. To evaluate 

the power generation performance of the system, PTO force measurements were needed, and these values 

were obtained using a load cell (Figure 2b). In addition, a National Instruments USB 6009 data 

acquisition system was used with LabVIEW software to record, calibrate, and process the test 

measurements. The data for each run was sampled at a rate of 100 hertz for 60 seconds.

4 Results Discussion

Due to the page limitation, only a few preliminary results are presented here. The relative velocity of the 

motion between the float and the central column is calculated by differentiating the motion with respect to 

time. The power is calculated by multiplying the load cell force with the relative velocity obtained above.

4.1 SBFPA Results

Figure 3 shows the power generation performance of the SBFPA system in waves with a height of 2 

meters. The averaged power that can be extracted by the system is plotted against the wave period. Note 

that the natural period of the float is around 4.3 seconds, which was obtained from the decay test, and that 

the SBFPA system did not perform close to resonance in the study. Therefore, the power extraction 

efficiency, or Power/EL, where E is the wave energy flux per unit crest length and L is the capture width,

of this SBFPA is only around 10% for a typical wave period of T=10 seconds. The test of the SBFPA 

system was conducted to analyze the effect of the PTO damping, and the results can be useful for 

validating our computational fluid dynamics (CFD) simulation in the future.
4

Figure 3. Power generation performance of the SBFPA system (full scale)

4.2 TBFPA Results

The damping plate in the TBFPA provides extra damping compared to the SBFPA system, which

increases the natural period to 8 seconds. The TBFPA system is designed to have a natural frequency in 

heave that is close to the typical peak frequency of real seas in order to maximize the power generation 

performance. To test this, we conducted a series of experimental studies on the power extraction 

performance of the TBFPA system under waves with a height of 2.5 meters.

Figure 4 shows an example of the time history data from measurements, including the damping (reaction) 

plate motion, the relative velocity between the float and the central column, the force measurement from 

the load cell, and the power prediction. To evaluate the power generation performance of the TBFPA 

system, the averaged power is plotted against the incident wave period (Figure 5). The power is scaled by 

the square of wave height in order to compare the experimental results with existing CFD simulations. 

The results show that the maximum power extraction efficiency for this TBFPA is around 30%, when the 

system is close to the resonance. It is noticed that the experimental and CFD results show a similar trend, 

and both have maximum peak power generation performance that is close to the natural period of the 

system. However, the two results have different maximum values. This is expected and may be caused by 

(1) different incident wave conditions, where nonlinear waves and FPA motion effects are essential in 

waves with larger wave heights (Yu and Li 2011), and (2) the differences in the FPA geometries. Note 

that model used in the experimental test has a larger float height to reduce the chance of wave 

overtopping and a rounded reaction plate, which helps reduce the viscous damping effect due to flow 

separation.

4.3 Summary and Future work

In this work, we find that 1) the viscous effect significantly affect the power output of the FPA, 

particularly the viscous effect caused by the reaction plate design; and 2) within acceptable engineering 

range, the larger the device, the more cost-effectiveness. However, this is rather a speculation than a solid 

conclusion; it needs further investigation of the PTO mechanism and other components of the device, e.g., 

mooring lines and anchors.

In future work, more details on the results and discussions will be discussed in a journal paper shortly. 

Additionally, if possible, the geometry of the FPA in the CFD model will be made the same as in the 

experiment. In the next step, we intend to conduct the following: (1) to optimize device size and geometry 

to increase the maximum power extraction efficiency, and (2) to develop optimal control strategies, which 

can provide a broader range of wave frequencies that are more efficient at absorbing energy.
5

Figure 4. An example of the time history measurement data (T=1.39 seconds model scale) 

Figure 5. Power generation performance of the TBFPA system (full scale)
6

Acknowledgment

This work was performed by NREL in support of the U.S. Department of Energy's Water Power Program.

References

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