This study formulates the uncertainty associated with future deployment levels of variable renewable energy (VRE) as a stochastic programming problem. The uncertainty is incorporated into a capacity and operation planning model, which is then solved to evaluate the energy mix required to achieve carbon neutrality in 2050, taking into account the utilization of imported hydrogen and synthetic methane. The results indicate that, regardless of the level of VRE deployment, carbon neutrality can be achieved by positioning VRE as the primary energy source in combination with various decarbonization technologies. Among these options, synthetic methane is found to be more economically viable than hydrogen, primarily due to its compatibility with existing infrastructure, which enables the continued use of current assets. Furthermore, distributed energy systems such as SOFC-CGS, which operate on synthetic methane and feature short lead times for deployment, are identified as promising technological options under uncertain conditions. These results underscore the importance of incorporating uncertainty into long-term energy planning and suggest that flexible, infrastructure-compatible technologies play a crucial role in developing a resilient and cost-effective energy system that supports the realization of carbon neutrality by 2050.

To ensure the safe transportation and handling of ammonia borane (AB) as a hydrogen source, its fundamental safety-related properties, which include toxicity, corrosivity, flammability, and storage stability, were evaluated in both aqueous solution and powder. Toxicity tests on an AB aqueous solution indicated carcinogenicity, highlighting the need for precautions to avoid ingestion, penetration into the skin, and contact with the eyes. Immersion tests with AB aqueous solution showed corrosion and surface deposition on several metal plates. The flash and ignition points of an 8 mol/L AB aqueous solution were 56.5 and 129 °C, respectively, while the flash point of the AB powder was 84.0 °C. AB powder retained its purity after four years of storage at room temperature with air exposure. AB powder absorbed moisture at >80 RH% at 25 °C and did not degrade when the absolute humidity was ≤4.4 g/m3 at temperatures below 50 °C. AB powder in the container with low humidity maintained its purity for one week outdoors during summer (below 50 °C). These findings provide a comprehensive dataset to support safety design, risk management, and regulatory guidelines for the practical use of AB in hydrogen systems.

Methane gas recovery from a methane plume on the seafloor in the Sea of Japan was conducted to demonstrate the highly efficient continuous recovery system. The demonstration site was located northeast offshore Sado Island at a water depth of approximately 150 m. The recovery system consisted of a bubble collector, a riser tube, a submersible pump, and a gas-liquid separator. Results showed that the recovery system was operated continuously while recovering gas seeping from the seafloor as well as entrained seawater. It was found that the gas composition analysis confirmed that the gas collected at the outlet of the gas-liquid separator on the research vessel contained methane at a measurable concentration. The methane was considered to have originated from the gas plume seeping on the seafloor.