The Effect of Natural Ventilation through Roof Vents Following Hydrogen Leaks in Confined Spaces

Hydrogen energy is gaining global popularity as a green energy source, and its use is increasing. However, hydrogen has a rapid diffusion rate and a broad combustion range; thus, it is vital to take safety precautions during its storage. In this study, we examined the change of hydrogen concentration in a confined space exposed to a hydrogen leak according to the size of the leakage hole and the leakage flow rate, assuming an extreme situation. In addition, we investigated rectangular vents (that serve as explosion panels in the event of an explosion) to assess their ventilation performance according to the area of the vent when used for emergency natural ventilation. The vent areas tested represented 12%, 24%, and 36% of the floor area, and they were installed in the ceiling of the test enclosure. When exposed to a simulated hydrogen leak, the enclosure acquired a hydrogen concentration of 1%, which is 25% of the lower flammability limit (LFL), in less than 6 s across all test cases. The time to LFL varied from approximately 4–81 s. In an assessment of the emergency ventilation duration, the ventilation time required to reach safe hydrogen concentrations decreased and showed less deviation as the vent size was increased. For the largest vent size tested, the LFL was reached in <1 min; it took 145.6 s to acquire a 1 vol% of hydrogen, which is relatively fast. However, there were no significant differences between the performance of large and medium-sized vent areas. Therefore, through the results, we found that it is reasonable to apply the area Kv = 3.31 (24% of the floor area) or less when considering the design of a roof vent that can serve as both an emergency ventilation and an explosion vent. This suggests that it is difficult to expect an improvement in ventilation performance by simply increasing the area of the vent beyond a certain area. Through these results, this study proposes a practical and novel method for future design and parameters of safety functions that protect areas where hydrogen is present.