Degrees of Freedom Effects on a Laboratory Scale WEC Point Absorber

Abstract

The number of rigid bodies and operational degrees of freedom of a wave energy converter (WEC) have significant impacts on power production magnitude and efficiency across wave periods, but this concept is not well studied. Investigating the effect of the number of bodies and degrees of freedom will help WEC developers design devices that most efficiently meet power and cost needs. This paper presents experimental and numerical results for a point absorber WEC across three modes of operation in regular waves. The three modes of operation are ‘one body heave only’, ‘two body heave only’, and ‘two body six degrees of freedom’.

The Laboratory Upgrade Point Absorber (LUPA) was used as the test article. This work builds on the 2021 EWTEC paper [1] by completing fabrication and undergoing wave testing. The LUPA experiment took place in the Large Wave Flume at the O.H. Hinsdale Wave Research Laboratory at Oregon State University in Corvallis, Oregon, USA; the installation and a wave test are shown in Figure 1. LUPA is constructed as a 1:20 scale two body point absorber with a buoyancy driven float, and a spar with a heave plate to act as a reactionary body. The spar is 3.7 meters tall and the float has a 1-meter diameter.

Figure 1. (Left) Mooring line installation. (Right) Wave tests in the Large Wave Flume.

The ‘one body heave only’ mode locks the LUPA spar so only the float is moving, and only in heave (vertical). The ‘two body heave only’ mode allows both the float and spar to move but both bodies are restricted to heaving. The ‘six degrees of freedom’ mode has taut mooring lines connected to the spar and LUPA can translate and rotate in all directions.

A damping optimization of the power take off was performed experimentally and numerically by sweeping damping values across a wide range of regular wave periods. The optimal damping value is used to compare the power generation of the three modes of operation.

The LUPA project is an open-source, lab-scale wave energy converter that acts as a testing platform for students and researchers. The data from this testing along with numerical models and engineering design files will be publicly available here: https://github.com/PMEC-OSU/LUPA. This paper presents how increasing complexity of WECs through the number of bodies and degrees of freedom affects power production. It also provides a baseline case for LUPA which other researchers can compare against when applying their research to the LUPA project.

References

[1] B. Bosma, C. Beringer, M. Leary, and B. Robertson, “Design and modeling of a laboratory scale WEC point absorber,” 14th Eur. Wave Tidal Energy Conf., pp. 1–9, 2021.