(481c) Hydrogen and Ammonia for Renewable Energy Storage: The Economics of Location, Scale, and Demand Type

Generation of electrical power from intermittent sources of renewable energy such as the wind or the sun is a promising approach to improving energy sustainability. Energy storage is necessary to balance this intermittent renewable generation with electrical power demands. Batteries are the most commonly used energy storage technology, but their capital cost and relatively low energy density makes them unsuitable for longer-term, high-capacity energy storage. Hydrogen, produced via water electrolysis and later converted back to electrical energy using a fuel cell, is another potential energy vector. The main limitation of hydrogen-based energy storage is the storage infrastructure itself; high-pressure gaseous storage necessitates expensive types and amounts of material, and hydrogen liquefaction requires significant additional energy. Ammonia production using electrolysis-derived hydrogen and nitrogen obtained via electrically driven air separation has recently received worldwide attention [1-3] as a potential route to decrease these storage costs, as ammonia is liquid at ambient temperature and moderate pressures. Ammonia can be used to generate electrical power through traditionally fossil-fueled technologies (e.g. internal combustion gensets, fuel cells) with minor modifications. Potential limitations of ammonia-based energy storage are (i) the capital investment needed for additional process units (e.g. nitrogen production, ammonia production), (ii) the additional energy needed to transform the hydrogen to ammonia, and (iii) the potentially lower power generation efficiency of ammonia as compared to hydrogen which along with (ii) can lower overall power-to-power efficiency.

In this work, we elucidate this trade-off between storage cost, chemical production cost, and energy efficiency to determine the best chemical-based energy storage systems for a variety of locations, power demand scales, and power demand types. We perform this investigation through a mixed integer linear programming (MILP) optimal combined capacity planning and scheduling model, which minimizes the levelized cost of energy supply (LCOE) for islanded (not grid connected) renewable energy systems. The model optimally selects and sizes the units in the system, while simultaneously scheduling the operation of these units (i.e. on/off, chemical production rates, storage inventories, power generation rates) during each distinct period of a scheduling horizon which captures both diurnal and seasonal variation in intermittent renewable generation and power demand. The modeling framework generates variable length scheduling periods by clustering consecutive hours of full year time series data for wind capacity factor, solar capacity factor and power demand. This temporal aggregation makes the model computationally tractable, allowing for high throughput computational studies. In our previous work [4], we used our capacity planning and scheduling model to determine the minimum LCOE of using hydrogen and/or ammonia as energy storage for 1 MW residential renewable energy supply systems in 15 American cities which represent climate-demand regions throughout the United States. We found that ammonia is generally more economical than hydrogen as a single method of energy storage and that in every location, cost savings can be achieved by using both synergistically. In the present study, we expand the scope to optimally size and schedule hydrogen and ammonia renewable energy storage systems in each location for power demand scales up to 100 MW, with various combinations of commercial and residential demand profiles at each scale. The results provide a more comprehensive understanding of the economics of chemical-based energy storage and reveal the most promising applications for ammonia energy storage.

References

[1] Klerke, A., Christensen, C. H., Nørskov, J. K., & Vegge, T. (2008). Ammonia for hydrogen storage: challenges and opportunities. J. Mat. Chem., 18(20), 2304-2310.

[2] Zamfirescu, C., & Dincer, I. (2008). Using ammonia as a sustainable fuel. J. Power Sources, 185(1), 459-465.

[3] Lan, R., Irvine, J. T., & Tao, S. (2012). Ammonia and related chemicals as potential indirect hydrogen storage materials. Int. J. Hydrogen Energy, 37(2), 1482-1494.

[4] Palys, M. J., & Daoutidis, P. (2020). Using hydrogen and ammonia for renewable energy storage: A geographically comprehensive techno-economic study. Comput. Chem. Eng, 127, 106785.