Integrated Storage Solar Collectors for Continuous Thermal Energy Beyond Sunshine Hours

ABSTRACT The limited continuity of thermal energy production during periods without solar radiation remains one of the major challenges facing conventional solar collector systems. This has stimulated growing interest in Integrated Storage Solar Collectors (ISSCs), which combine solar energy collection and thermal storage within a single unit to enhance thermal stability and enable extended operation beyond sunshine hours. This review critically examines recent developments in ISSC technologies, with particular emphasis on thermal storage strategies for prolonged off‐sunshine operation. The review covers sensible, latent, thermochemical, and hybrid thermal storage systems, together with recent advances involving phase‐change materials (PCMs), porous media, nanomaterials, and intelligent thermal management techniques. Representative studies have reported that PCM‐based and hybrid storage systems can extend useful heat retention from < 2 h in conventional collectors to ~12 h or more under optimized operating conditions. At the same time, thermal efficiency improvements ranging from 15% to 40% have been achieved depending on the storage configuration, operating conditions, and enhancement techniques employed. The review also discusses heat‐release strategies and current technical challenges, including low thermal conductivity, PCM leakage, long‐term material stability, and commercialization barriers. Furthermore, it proposes a new classification framework based on heat retention duration and off‐sunshine operating capability, providing standardized categories for comparing ISSCs according to their operational performance. Finally, key research gaps and future directions are identified, highlighting the potential of intelligent thermal management, hybrid storage technologies, and AI‐assisted control systems for the development of next‐generation ISSCs capable of delivering reliable thermal energy beyond sunshine hours.