Solar thermal energy storage: global challenges, innovations, and future directions for renewable energy systems

Solar thermal energy storage is considered one of the key technologies for overcoming the intermittency of solar energy and expanding its applications to power generation, district heating and cooling, and industrial heat supply. The thermal energy storage market is growing in value from $4.28 billion in 2019 to $8.56 billion in 2026, with installed capacity increasing from 234 GWh to over 800 GWh by 2030. Various thermal energy storage technologies have been developed, including molten salt, phase change materials, hydrogen storage, and thermochemical storage; however, unaddressed technological, economic, and environmental challenges have hindered their widespread adoption. This review highlights key issues in solar thermal energy storage, such as technological, financial, and environmental challenges. It identifies gaps in current literature regarding high-temperature materials and underground storage impacts. The review fills these gaps by critically evaluating recent advances and outlining future research directions to improve thermal energy storage systems. Emerging solutions now come with the means to surmount these barriers: coupled thermal energy storage technologies, cascade modular systems, and next-generation materials such as lithium hydroxide and encapsulated phase change materials. For instance, lithium hydroxide exhibits six fold higher volumetric energy density when compared to traditional materials, while the operating temperature for coupled thermal energy storage systems is beyond 1400 °C. Recent advancements have also explored integrating thermal energy storage technologies for hydrogen production and storage, particularly utilizing high-temperature thermochemical processes to improve system efficiency and decarbonization potential. Policy-driven funding also marks the global momentum in thermal energy storage development, with Europe expected to add 275 MWh of thermal energy storage capacity by 2025. This review has provided a roadmap toward the advancements of thermal energy storage technologies by synthesizing fragmented research into actionable recommendations toward material innovation, standardized system designs, and integration with smart energy grids. Acceleration in renewable energy use is foreseen, helping transition toward a sustainable energy future with sensible and latent thermal energy storage systems at efficiencies of over 90 % and thermochemical systems at up to 50 %.