Experimental evaluation of the heat transfer through small PCM-based thermal energy storage units for building applications

•The heat transfer through rectangular cavities filled with PCMs is experimentally evaluated.•The results show the influence of natural convection and subcooling during charging and discharging.•The thermal performance of the TES unit is very influenced by the aspect ratio of the cavities, sort of PCM and imposed boundary conditions.•The results allow discussing which arrangements/PCMs are better for specific building applications. This paper evaluates the heat transfer through small thermal energy storage (TES) units filled with different phase change materials (PCMs): free-form and microencapsulated PCMs. The experimental results are very useful for benchmarking and validation of numerical models to be used in the design and optimization of new TES systems for buildings. They also allow discussing which arrangement/PCM is better for specific building applications considering the thermal regulation effect during charging, the influence of subcooling during discharging, and the influence of natural convection during both processes. During charging, the influence of the aspect ratio of the cavities on three parameters is investigated: control-temperature on the hot surface; thermal-regulation period; time required for melting the PCM in the mid-plane. During discharging, five parameters are evaluated: time for solidifying the PCM in the mid-plane; time for starting crystallization; subcooling period; phase-change temperature after subcooling; difference between the solidifying temperature and the cooled temperature due to subcooling. It was concluded that natural convection in the free-form PCM must be considered in any simulation to well describe the charging process. During discharging, subcooling must also be considered. The effects of natural convection and subcooling can be neglected when modelling cavities filled with the microencapsulated PCM.