How accurately can we measure the power produced by a wave energy device in the laboratory?
For an Oscillating Water Column (OWC), estimating power is more difficult than it may first appear. The device generates energy through the interaction between the moving water surface inside its chamber and the air trapped above it. In laboratory models, directly measuring the rapidly changing airflow is challenging, so researchers often estimate it indirectly from pressure measurements or from a small number of wave gauges.
The problem is that the water surface inside the chamber does not always move uniformly. A measurement taken at only one or two locations may therefore miss important spatial variations and introduce error into the estimated power output.
In this study, we developed an alternative approach based on video recording and digital image processing. Instead of observing the water surface at only a few points, the method tracks the complete free surface across the chamber and follows how its shape changes with time. This provides a much richer description of the water motion and allows the airflow, and ultimately the available pneumatic power, to be estimated more accurately.
The image-processing procedure transforms each video frame through a sequence of calibration, contrast enhancement and edge-detection steps until the air–water boundary can be identified and converted from image pixels into real physical measurements. The resulting method achieved a water-surface resolution of approximately 0.25 mm and reduced the estimation error substantially compared with conventional image edge-detection techniques.
From video to measurement. Successive stages of the image-processing method used to track the water surface inside the Oscillating Water Column. The recorded image is calibrated and processed to identify the air–water interface, which is then converted from pixels into a continuous physical measurement of the water surface.
Source: Almalki, Y.R., Cao, R. & Karmpadakis, I. (2025), Applied Ocean Research, 165, 104806, Fig. 7. Licensed under CC BY-NC 4.0.
More importantly, when this more complete measurement was used to assess commonly adopted experimental approaches, the study showed that the choice of measurement method can materially affect the predicted performance of the device. Under some wave conditions, errors in estimated OWC efficiency ranged from approximately 16% to 33%.
The broader message is simple: improving a wave energy device is only meaningful if we can measure its performance with sufficient confidence. Better experimental measurements therefore play a direct role in making laboratory results more reliable before technologies progress toward larger-scale testing and deployment.
Citation
Y.R. Almalki, R. Cao, I. Karmpadakis (2025). Power output estimation for oscillating water columns: An assessment of experimental methods. Applied Ocean Research, 165, 104806.
Commentary
How accurately can we measure the power produced by a wave energy device in the laboratory?
For an Oscillating Water Column (OWC), estimating power is more difficult than it may first appear. The device generates energy through the interaction between the moving water surface inside its chamber and the air trapped above it. In laboratory models, directly measuring the rapidly changing airflow is challenging, so researchers often estimate it indirectly from pressure measurements or from a small number of wave gauges.
The problem is that the water surface inside the chamber does not always move uniformly. A measurement taken at only one or two locations may therefore miss important spatial variations and introduce error into the estimated power output.
In this study, we developed an alternative approach based on video recording and digital image processing. Instead of observing the water surface at only a few points, the method tracks the complete free surface across the chamber and follows how its shape changes with time. This provides a much richer description of the water motion and allows the airflow, and ultimately the available pneumatic power, to be estimated more accurately.
The image-processing procedure transforms each video frame through a sequence of calibration, contrast enhancement and edge-detection steps until the air–water boundary can be identified and converted from image pixels into real physical measurements. The resulting method achieved a water-surface resolution of approximately 0.25 mm and reduced the estimation error substantially compared with conventional image edge-detection techniques.
Source: Almalki, Y.R., Cao, R. & Karmpadakis, I. (2025), Applied Ocean Research, 165, 104806, Fig. 7. Licensed under CC BY-NC 4.0.
More importantly, when this more complete measurement was used to assess commonly adopted experimental approaches, the study showed that the choice of measurement method can materially affect the predicted performance of the device. Under some wave conditions, errors in estimated OWC efficiency ranged from approximately 16% to 33%.
The broader message is simple: improving a wave energy device is only meaningful if we can measure its performance with sufficient confidence. Better experimental measurements therefore play a direct role in making laboratory results more reliable before technologies progress toward larger-scale testing and deployment.