Techno-economics of solids-based thermochemical energy storage systems for large scale, high-temperature applications

This work evaluates the techno-economic feasibility of the three most promising solids cycling systems (carbonates, thermally-reduced and chemically-reduced metal oxides) for thermochemical energy storage when deployed for large-scale applications with high-temperature operation. For each system, a specific material is selected (Ca, Co and Fe, respectively) and two process layouts are formulated: one focused on energy storage and one co-generating additional high-value byproducts (to offer CO2 capture to nearby processes, production of pure O-2 and production of H-2 respectively). The study compares deployments sized to absorb 100 MW of solar power with an intermittency period of 12 h and dispatch it continuously as a constant high-temperature energy discharge. For each process layout, mass, energy and exergy balances are solved and the cost structure is calculated using a bottom-up approach. The technical assessment shows the Co-based system to have the best performance (electrical efficiency up to 45 %) for sole energy storage due to its higher extent of reaction and energy density. The results show that the generation of valuable byproducts (to offer CO2 capture and pure O-2, respectively) is done at the expense of electrical efficiency and dispatchability, although the Fe-based process with co-production of an energy carrier (H-2) reaches high energy efficiencies (>80 %). Nevertheless, the generation of valuable byproducts tends to improve the economic performance (reduction in breakeven electricity selling price - BESP) in all three systems. The results indicate that the deployment of chemically-charged (Fe-based) layouts involves up to 10 times larger costs than thermally-charged ones (Ca and Co) due to the cost of the reducing agent. Despite this, when involving production of H-2, the Fe-based process offers a significantly lower BESP than all other layouts. If high plant costs represent a financial barrier, Ca-based layout with CCS at the largest size investigated (250 MW) presents the lowest BESP among the rest of layouts investigated. Additionally, a parametric study reveals that the cost structure of the Ca-based process is the most robust to the variations considered in the study (costs of solid material, cooling water and heat input, as well as product selling prices).