Geologically Constrained Underground Hydrogen Storage in Long-term Energy Balancing Models of Isolated Energy Systems

For relatively isolated energy systems, such as for island nations like New Zealand, energy balancing is an important consideration for ensuring system reliability. Underground hydrogen storage (UHS) is one possible technology for addressing seasonal fluctuations of solar, wind and hydropower generation on month to year timescales. Although prior power system modelling has considered UHS, it has typically represented subsurface storage with simplified tank models that neglect expected geological complexity and the operational constraints of managing a subsurface reservoir.

Here, we present an energy-balance model of a national power system that incorporates (1) seasonal generation fluctuations derived from New Zealand’s historical records, (2) a UHS facility based on geological characteristics of the Ahuroa gas field (a natural gas storage site in Taranaki, New Zealand), including structure, storage volume, and well configuration, and (3) operational constraints, including reservoir pressure limits, and co-production and treatment of formation water. The model is operated under future multi-year scenarios that incorporate expected growth in renewable generation as well as demand.

Our study finds that under typical meteorological conditions, a single UHS site with capacity of 5.6 PJ can buffer median annual electricity fluctuations of 2.6 PJ. This result is robust under a range of future scenarios including variation in electricity mixes and climatic conditions. However, as wind and solar increase to replace fossil fuels, the seasonal balancing requirement exceeds UHS capacity. Due to round-trip conversion losses – power to hydrogen to power – renewable overbuild that provides an additional 3 PJ annually is required to maintain sufficient hydrogen inventory for stable multi-year operation.

During meteorological dry years, when hydropower generation is well below average, the UHS is called upon to deliver gas at higher than ordinary rates. This causes low-pressure transients in the reservoir that lead to the gas-water interface moving upward, increased water co-production that exceeds treatment capacity, and hence inability of the UHS to meet the energy shortfall.