Hydrogen generation systems using powdery sodium borohydride (NaBH4) have been attracting attention owing to their higher energy density and lower infrastructure costs than current mainstream high-pressure hydrogen tank systems. In order for a NaBH4 hydrogen generation system to produce the desired amount of hydrogen, the concentration of the fuel solution must be properly controlled. However, it is difficult to directly measure the mass of water and NaBH4, the values of which are needed to derive the fuel concentration. This paper proposes a fuel concentration estimation method using a state observer from measurable parameters such as hydrogen pressure and discharged hydrogen flow rate. In this paper, it is shown that a NaBH4 hydrogen generation system with an output delay can be represented with a state-space model. A state observer for estimating the concentration of the aqueous solution is designed based on the state-space model. In order to verify the effectiveness of the proposed estimation method, experiments are conducted to estimate the fuel concentration when the fuel input rate is changed and when the fuel concentration is changed during the process of hydrogen generation. As a result, the maximum relative error of the concentration is less than 8.70%, and the validity of the estimation method is clarified when the fuel input rate and the fuel concentration are changed.
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