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.