Supercapacitor testing for power smoothing in a variable speed offshore Wave Energy Converter

This paper investigates power smoothing in a full-scale offshore Oscillating Water Column (OWC) Wave Energy Converter (WEC) by integrating supercapacitors (SCs) with the inertia of a Wells turbine controlled at variable speed. A Simulink model is developed for the WEC system utilising sea state data and a SC system is sized to smooth and reduce the grid peak power for a 570 kW (peak) system. Long component lifetime is a requirement for offshore WECs, so a computer-controlled test rig has been built to validate SC lifetimes to manufacturer's specifications and determine application lifetime. Cycle testing is carried out on individual SCs utilising GPIB controlled test equipment programmed by Matlab. The application testing is carried out using time-compressed scaled-power profiles from the model to allow a comparison of lifetime degradation. The SCs under standard testing and application testing have to date undergone over 750,000 cycles and 85,000 cycles respectively corresponding to almost six months of continuous operation.