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This page describes the National Renewable Energy Laboratory's (NREL) development of power electronics interfaces for renewable electrolysis systems and the use of electrochemical impedance spectroscopy for the characterization and performance testing of electrochemical devices. Learn more about renewable electrolysis.
The renewable-electrolysis systems being studied today are small systems that incorporate a common direct current (DC) bus (electrical conductor) fixed with a battery bank connected to a wind turbine, photovoltaic array, and electrolyzer. Typically, the wind turbine is set up to charge batteries. This requires connection to a constant voltage DC bus and power electronics to convert wild alternating current (AC) to DC, and to regulate power output. In some systems, the electrolyzer can accept DC power input, but also includes its own power electronics to regulate power input and convert DC at one voltage level to another. This is necessary, for example, if the electrolyzer has been designed to accept a range of DC voltage from a photovoltaic panel.
PEM stack from Proton Energy electrolyzer. Barriers and Solutions
Essentially, the entire system and the link between the wind turbine and electrolyzer are designed around the constraints imposed by the power electronic interfaces of these components.
There are a number of weaknesses with this configuration. Its redundant power electronics lead to increased cost and failure potential. Its inability to match wind turbine power output to electrolyzer power requirements causes reduced energy capture.
One goal of testing an electrolyzer with a wind turbine is to determine the effect of the fluctuating power output of a wind turbine on electrolyzer operation.
Research Focus
Based on the characterization of an electrolyzer and wind turbine, NREL has completed a power electronics interface between a 10-kW wind turbine and an electrolyzer. Using knowledge of the performance characteristics of variable-speed wind turbines and power electronics, NREL developed a solution. This solution involved replacing two separate power electronics interfaces with a single one that takes AC directly from the variable-speed wind turbine generator output and provides DC power to the electrolyzer, thereby reducing the cost and increasing the robustness of the wind turbine-electrolyzer link. The single point of control allows matching of the wind turbine and electrolyzer electrical characteristics, thereby increasing the energy capture of the wind turbine.
NREL is developing and constructing a power electronics package for electrolyzers at the 50-kW size as part of the NREL-Xcel wind-to-hydrogen project. This power electronics interface will incorporate the ability to adjust the control algorithms to optimize performance between the 100-kW wind turbine and the 50-kW electrolyzer.
Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy (EIS) enables the detailed characterization and performance testing of electrochemical devices such as fuel cell and electrolysis equipment. EIS perturbs a fuel cell or electrolysis stack with high-frequency sinusoidal waveforms and performs a frequency analysis on the resulting stack or cell waveforms. Impedance analysis provides detailed stack characteristics that typical current-voltage polarization curves do not. Improving the efficiency of these electrochemical devices hinges on the ability to understand and reduce the "overpotentials" of the cells that make up these stacks.
EIS is becoming the standard in electrochemical characterization. Much of the recent research surrounding EIS focuses on proton exchange membrane (PEM) fuel cell applications. EIS work at NREL focuses on improving the performance of electrolytically-produced hydrogen.
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