Open Sea Trials on Floating Wave Energy Device Backward Bent Ducted Buoy and Its Performance Optimization

Abstract

A floating wave power device called backward bent ducted buoy (BBDB) which was developed by National Institute of Ocean Technology works on oscillating water column (OWC) principle. The power module on BBDB consists of a unidirectional impulse (UDI) turbine and a permanent magnet direct current generator (PMDC). It underwent open sea trials several times for studying performance characteristics and improvising the power module from year 2011 to 2015. These trials were carried out off the Chennai coast near the south breakwater of Kamarajar Port Limited (KPL). BBDB was deployed and monitored over several weeks and it generated electric power. During the trials the buoy also withstood the strong winds and large waves during Cyclone Thane in December 2011. Though BBDB produced power, it was felt it needed improvement on wave to wire energy conversion. For any OWC-based wave energy device, it is necessary that the power module provides appropriate damping to oscillating water column in order to achieve efficient wave to wire energy conversion. Damping provided by the turbine depends on the relation between pressure drop across a turbine and volumetric flow rate. For a geometric similar construction, turbines with different diameters provide different levels of damping and it became necessary to study the performance of turbines with different diameters. Thus, two major areas were earmarked for optimization studies, namely, wave to pneumatic energy conversion (energy capture by buoy) and pneumatic to mechanical power conversion (turbine performance). In situ pneumatic performance of BBDB was evaluated using orifices with different pressure drop—flow characteristics. The orifice providing optimum damping was chosen and a turbine with similar pressure drop—flow rate characteristics were fabricated. This turbine with polycarbonate stator guide vanes and rotor made using an additive manufacturing technique was extensively tested in oscillating airflow test rig. After this, an open sea trial of BBDB with this new turbine and four-point mooring was carried out in 2015 and it was observed during the trials that there was a significant improvement in overall performance of BBDB. This article gives a detailed account of turbine-OWC matching exercise, including all sea trials. For the entire duration of this long exercise, a comprehensive instrumentation plan was devised and implemented in order to record performance data for assessing the BBDB’s performance. This article gives details of the instrumentation and data acquisition system and its transmission to the shore station inside port premises. This exercise has paved way for developing OWC-based BBDB for producing larger power outputs.