The U.S. Navy Wave Energy Test Site (WETS) in Hawaii, the United States' first grid-connected test site of its kind, has been operational since 2015. The Hawaii Natural Energy Institute (HNEI) at the University of Hawaii is providing key research support for this facility. In this paper, we present some of the numerical modeling research on Wave Energy Converters (WECs) undertaken as part of this effort. In the first part, we describe numerical modeling results and comparison with trial data for the Northwest Energy Innovations (NWEI) Azura WEC device. In the second part, the power performance of a WEC array currently being developed by a Honolulu based company -National Power Concepts (NPC)- is described.
The baseline Azura device - which consists of a float and a spar - was deployed at WETS during 2015–2016 at the 30m berth. The spar provides a reaction point against which the float rotates. A Power Take Off (PTO), located at the hinge generates power from the relative rotation of the spar and float. The mass of the spar is roughly ten times that of the float. An advanced version of the device -that consists of a modified float, and spar augmented with a heave plate- was deployed in 2018, also at the 30m berth. See Fig. 1. The modifications were expected to improve the performance of the device.
Modifications consist of addition of a heave plate to the spar and replacement of the original float with a new one that has an entirely new geometry.
The baseline and advanced device are considered half scale by the NWEI design team who are currently developing a full scale device. Briefly, the PTO unit of the Azura device consists of a generator and a hydraulic system, that drives this generator. In this paper, we focus on the experimental data obtained from the motion sensors located on the advanced device (Fig. 1(b)) when it was deployed at WETS during 2018 (February to August). Only data when the PTO was disengaged -and not generating any power- is covered in this paper. When the PTO is disengaged, there is no electric output from the generator, but the hydraulic system that is connected to the hinge is active and continues to rotate the generator. The experimental data consist of the spar motions - heave, pitch, roll and heading- and the float pitch. Response Amplitude Operators (RAOs) are estimated from the experimental data for those motions that primarily contribute to power generation when the PTO is engaged.