Analysis of equivalent heat capacity forms and their influence on energy storage in PCMs

Thermal energy storage plays an important role in improving energy efficiency and managing energy demand. Phase-change materials (PCMs) are widely used for this purpose due to their ability to store and release large amounts of energy through latent heat during phase transitions. However, accurately modeling the thermal behavior of PCMs remains challenging, particularly when using the equivalent heat capacity method in numerical simulations.

This study investigates the thermal behavior of a bio-based phase-change material (PCM) derived from vegetable oils and beeswax. This material has a phase-change temperature range of 27 °C –38.3 °C, a latent heat of 63.85 kJ/kg and thermal conductivities of 0.212 and 0.165 W/(m·K) in solid and liquid phases, respectively. Experimental heat capacity measurements were obtained using differential scanning calorimetry (DSC). Numerical simulations were then performed using COMSOL Multiphysics 5.4a, based on the finite element method, to analyze heat transfer in a multilayer building wall incorporating a 3 cm PCM layer. Several equivalent heat capacity formulations were evaluated and compared with experimental Cp data.

The results show that the equivalent heat capacity model based solely on the function D(T)×L produces significant errors, leading to unrealistic heat capacity and enthalpy values. Dividing the term D(T)×L by the phase transition interval (T_endset−T_onset) provides results much closer to experimental measurements. The correlation model derived from experimental Cp data offers the most accurate representation of PCM thermal behavior, while the COMSOL integrated model introduces noticeable variations in enthalpy values. Overall, the results demonstrate that the accuracy of PCM energy storage modeling strongly depends on the formulation of the equivalent heat capacity. The improved formulation and experimentally based correlation model provide reliable predictions, whereas the COMSOL built-in model requires validation for each specific PCM to ensure accurate simulations.