The integration of large-scale renewable energy sources requires robust energy storage and flexibility mechanisms. Power-to-Gas technologies, specifically green hydrogen injection into existing high-pressure natural gas transmission networks, offer a strategic solution. However, utilizing the pipeline's storage capacity (linepack) as a dynamic “battery” introduces severe operational challenges. Fluctuating hydrogen concentrations affect the Higher Heating Value and Wobbe Index, requiring coordination between production, grid injection, and auxiliary storage. Consequently, understanding the transient behavior of hydrogen-natural gas blends is critical for modern energy system management.

This study presents an original analysis based on topological field data from the Portuguese National Natural Gas Transmission Network, specifically the high-pressure Sines-Setubal backbone. To capture the complex physics of gas transport, a fully dynamic, one-dimensional computational fluid dynamics network solver was developed. The numerical model employs an implicit Backward Differentiation Formula (BDF) solver to integrate the coupled fundamental equations of fluid mechanics and real-gas equations of state. This approach enables precise tracking of the hydrogen-mixture front, transport delays, and pressure transients under variable demand profiles over a 24-hour cycle.

The simulation results characterize the temporal and spatial evolution of the hydrogen in the network, emphasizing the critical interplay between pipeline linepack and gas quality stability. Results indicate that when the dynamic injection capacity is constrained by gas quality standards, the model effectively quantifies the surplus hydrogen that must be diverted to alternative storage solutions, such as salt caverns or high-pressure buffer tanks. Unlike static models, which fail to account for time-dependent accumulation and transport lag, this transient approach identifies the exact windows of opportunity for grid injection versus external storage. This capability is essential for sizing storage infrastructure and ensuring that electrolyzer operations are not curtailed by the instantaneous limits of the gas grid.

The primary conclusion is that effective hydrogen integration requires a hybrid management strategy that balances grid blending with auxiliary storage assets. While the gas network provides significant buffering through its linepack, it cannot always absorb total renewable production while maintaining localized gas quality. The proposed transient framework serves as a decision-support tool for transmission system operators to coordinate hydrogen production, grid injection, and the deployment of external storage facilities. This ensures the full utilization of renewable resources and provides a technically sound pathway for the decarbonization of the national energy system.