Roundtrip Energy Efficiency Optimization of an Energy Storage System Using Products of Iron Combustion

Abstract

Abstract. Iron has attracted increasing attention as a cyclic energy carrier for large-scale, long-duration, and transportable energy storage. In this study, an integrated iron-based energy storage system is presented and thermodynamically analyzed by coupling an iron reduction cycle with a power generation cycle. The reduction cycle is designed as an energy-recovery process to convert Fe2O3 into iron fuel for power generation, where hydrogen serves as the reducing agent and thermal energy is recovered through a series of energy exchangers. The power generation cycle is configured as a combined cycle consisting of a closed air Brayton topping cycle and a reheat steam Rankine bottoming cycle, where the chemical energy stored in iron is released through combustion for electricity generation. Since hydrogen is used as the reducing agent and iron is combusted with oxygen rather than air, NOx formation in the main reactive processes is avoided. The reduction and combustion processes are modeled using Reaktoro/NASA-CEA thermodynamic data, while the air and steam properties in the power cycle are obtained from REFPROP. A comprehensive parametric study and optimization were performed to evaluate the effects of reactor hydrogen inlet temperature, air cycle pressure, steam cycle pressure, and air turbine inlet temperature on system performance. The results show that the minimum fresh hydrogen requirement is achieved at a reactor hydrogen inlet temperature of 1233 K, corresponding to approximately 4.36 mol H2 per mol Fe2O3. In the power cycle, parametric studies have been made to determine the air cycle high pressure and steam cycle high pressure. The results indicate that when air turbine inlet temperature is either 1500 K or 1600 K, the higher the steam cycle pressure and air cycle pressure, the higher the power cycle efficiency. However, when air turbine inlet temperature is 1400 K, while the power cycle efficiency still increases when steam cycle pressure increases, the air cycle pressure reaches the maximum efficiency at about 1750 kPa. The maximum power cycle efficiency reaches about 61–64% when air turbine inlet temperature ranges from 1400 K to 1600 K. For hydrogen production electricity consumption ranging from 42 kWh/kg-H2 to 55 kWh/kg-H2, the overall roundtrip energy efficiency of the integrated system ranges from about 26% to 34%, with the best performance obtained at the optimized reduction condition, higher turbine inlet temperatures, air cycle pressure, and steam cycle pressure. These results demonstrate the thermodynamic potential of iron-based energy storage systems with hydrogen and combined-cycle power generation.