Hybrid thermal energy storage systems for heat pumps and refrigeration: materials, system integration, performance evaluation, and future perspectives

The rising demand for flexible, low-carbon heating and cooling is driving the development of Advanced Thermal Energy Storage (TES). While traditional TES methods exist, they often struggle with slow performance, high costs, and material wear. To address these drawbacks, hybrid TES systems that smartly combine Phase Change Materials (PCMs) with sensible media, thermochemical sorption systems, or microencapsulated PCMs have been proposed as a viable option, especially for Heat Pump and Refrigeration (HP&R) applications. This review classifies hybrid TES concepts and assesses their material-design integration streams (cascaded, embedded, composite, slurry-based). It evaluates performance impacts using metrics such as COP enhancement, load-shifting potential, capacity stabilization, defrost-cycle elimination, and power/energy density ratios. PCM–sorption systems can improve solar heat-pump performance while providing high-density, long-term storage. Adding MPCMs accelerates heat transfer, enabling more compact and powerful thermal storage designs. Representative studies reported a 33.9% increase in COP for PCM-assisted heat-pump operation, a 0.4 increase in seasonal COP from 3.4 to 3.8, and an 8.8% reduction in energy consumption for PCM–thermochemical integration in a seasonal heating and cooling application. These results indicate that hybrid TES can improve HP&R performance under suitable conditions, although the magnitude of improvement remains system- and application-dependent. However, significant research gaps in lifecycle durability, cost modeling, exergy optimization, and standardized testing remain and must be addressed to facilitate commercial adoption and the use of hybrid TES in net-zero thermal infrastructures.