Optimizing energy conversion efficiency of a proton exchange membrane green hydrogen generation system while incorporating balance of plant modeling
Date
2023
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Abstract
Hydrogen has the potential to decarbonize several difficult to decarbonize sectors of the U.S. energy economy such as medium- and heavy-duty transportation, energy storage, and industrial processes such as steel making. Currently most of the hydrogen produced globally is produced with steam methane reforming and has a carbon intensity associated with partially burning natural gas. An alternative way of producing hydrogen is using electrolysis and renewable energy to split water into hydrogen and oxygen. Hydrogen produced in this way is called "green" hydrogen. The devices that are used to produce green hydrogen are electrolyzers and the most prominent type of electrolyzer today is the proton exchange membrane (PEM) electrolyzer. Most PEM systems are designed for continuous operation with a constant input of electricity. When PEM electrolyzers are coupled with renewable energy such as wind turbines and solar photovoltaics, the input electricity to the electrolyzer may follow the same variable and intermittent profile as renewable energy generation. System modeling while including balance of plant components can be used to optimize the green hydrogen generation system for the highest energy conversion efficiency across the range of possible operating conditions with renewable energy input. This work is focused on creating a system model of a PEM green hydrogen generation system including the balance of plant components such as power electronics, electrolyzer stack, hydrogen purification, hydrogen compression/storage, and system cooling. Literature primarily focuses on modeling the electrolyzer stack and ignores the balance of plant components. Some recent publications create system models with the balance of plant included but are unnecessarily complex. The model created in this work includes the balance of plant and reduces the complexity of recently published balance of plant models while maintaining the model's functionality in system optimization studies. Limited experimental data available in literature is used to verify and validate the model. The model is scaled to represent a utility scale system which would include multiple electrolyzer stacks and power electronics. A case study of wind and solar generation in Texas is used to demonstrate the model's capability in optimization studies. The model results show the effects of varying operating conditions such as electrolyzer cathode pressure and electrolyzer current density on the overall system efficiency for a single 120-kW electrolyzer green hydrogen generation system. At low electrolyzer power, the system energy conversion efficiency drops off significantly which is mainly driven by the increase in specific hydrogen loss in the balance of plant. Increasing the electrolyzer cathode pressure decreases the system efficiency and operating range but may provide benefit by allowing the hydrogen compressor to be removed from the system. Two different electrolyzer "loading" strategies were imposed on the multi-electrolyzer stack model with the Texas case study and show that there is a slight benefit in efficiency if the strategy maximizes the electrolyzer power and minimizes the amount of electricity that is wasted within the system. Other tradeoffs such as average electrolyzer power and the number of electrolyzer shutdowns are evaluated between the two loading strategies. If a minimum electrolyzer power is selected at 50% of the rated power, the parallel loading scheme produces 9,000 kg more hydrogen than the series loading scheme with the same input power profile. The model developed in this work is a valuable tool to optimize the production of green hydrogen by identifying and optimizing the interactions of different components within the system to maximize the energy conversion efficiency. Optimizing the green hydrogen generation system will improve the economic feasibility and accelerate the adoption of green hydrogen at a large scale.
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Subject
efficiency optimization
proton exchange membrane electrolyzers
green hydrogen
balance of plant modeling