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. With the simulation package Simulink developed by MathWorks, a model is developed for the WEC system utilizing sea-state data and an 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. Hence, 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 at room temperature and also at rated temperature utilizing a thermal chamber and equipment interconnected by the general purpose interface bus (GPIB) and programmed by the MathWorks developed computing environment Matlab. 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 at ambient and rated temperature, and application testing at ambient temperature, have undergone approximately 4 000 000, 700 000, and 500 000 cycles, respectively. The results demonstrate cycle lifetimes in excess of manufacturer specifications.