Systems Integration and Component Development
Through systems integration and component development, NREL develops power electronics interfaces for renewable electrolysis systems for the characterization and performance testing of electrochemical devices.
The renewable-electrolysis systems that NREL studies are small and incorporate a common direct current (DC) bus (electrical conductor) fixed with a battery bank connected to a wind turbine, photovoltaic array, and an electrolyzer. Typically, small wind turbines are set up to charge batteries and require connection to a constant voltage DC bus and power electronics to regulate power output and to convert wild alternating current (AC) to DC.
In commercially available electrolyzer systems, the electrolyzer stack accepts DC power input from its own onboard power converter. The electrolyzer regulates power to the stack and operates at a fixed stack current. Characterization testing surrounds this idea of operating the hydrogen-producing stacks at variable current without a fixed power supply.
Learn more about renewable electrolysis.
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.
A number of weaknesses exist 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. Performance of these systems is quantified based on stack and electrolyzer system efficiency and their ability to accommodate renewable electricity sources.
Based on characterization of an electrolyzer and wind turbine, NREL completed a power electronics interface between a 10-kW wind turbine and an electrolyzer stack. Using knowledge of the performance characteristics of variable-speed wind turbines and power electronics, NREL developed a solution that replaces two separate power electronics interfaces with one interface that takes AC directly from the variable-speed wind turbine generator output and provides varying DC power to the electrolyzer stack. This solution reduces the cost and increases the robustness of the wind turbine-electrolyzer link. The single point of control allows matching of the wind turbine and electrical characteristics of the electrolyzer and increases the energy capture of the wind turbine.
The NREL–Xcel Energy wind-to-hydrogen project provides valuable data as wind turbines and photovoltaic (PV) electricity are closely coupled with three electrolyzer systems and stacks. NREL designed, built, and is testing a DC-to-DC conversion between a 10-kW PV array and a 6-kW electrolyzer stack. Testing includes directly wiring the PV array to the stack and comparing the overall energy capture with the DC-to-DC power converter that utilizes a maximum power point tracking algorithm.
The first tests showed the power converter delivered 10%–20% more energy to the electrolyzer stack than the direct-connect (no-loss) configuration. The improved energy capture is highlighted by the fact that the DC-to-DC power converted has losses between 10% and 30% based on the input voltage of the PV array.