High-temperature thermochemical energy storage via reversible fluid-phase based cycles: A review

Thermochemical energy storage (TCES) is a promising approach for long-duration, high-energy-density thermal storage in renewable energy systems, particularly for high-temperature (>400∘C) applications such as concentrated solar power (CSP) and industrial processes. This review critically evaluates three pivotal reversible chemical pathways: sulfur/H2SO4–, ammonia (NH3) cracking/synthesis–, and methane (CH4) reforming/methanation–based cycles. Unlike other TCES systems, these pathways offer a dual-purpose framework that integrates energy storage with the synthesis of high-value chemical commodities, thereby enhancing the overall economic viability of the plant. The CH4 and NH3 systems predominantly involve homogeneous gas-phase or gas–liquid processes, while sulfur-based cycles span gaseous, liquid, and fluid particle phases. The operating principles, reaction mechanisms, and catalytic requirements for each system are systematically analyzed, followed by a review of recent component and system-level developments. A rigorous comparative assessment of these cycles is provided based on key performance indicators, including operating temperature ranges, energy densities, cost and efficiencies. Additionally, this review explores environmental impacts and policy considerations. Finally, this paper underscores the technical challenges—such as multi-phase material handling and high-temperature corrosion—and proposes research directions to advance these technologies for sustainable energy and chemical co-production.