Standardized evaluation protocols are crucial for assessing long-term stability of phase change materials (PCM). This study investigated the thermal stability, possible degradation mechanisms, and material compatibility of an organic-organic eutectic PCM, while evaluating the relevance of different stability assessment methodologies. The representative case study focused on a biobased eutectic composed of 1-octadecanol (OD) and palmitic acid (PA) (0.548:0.452 mass ratio of OD:PA) exhibiting a melting temperature of 46.9 ± 0.1 °C and a latent heat of 194 ± 1 J/g. This PCM was subjected to a series of stability assessments including benchmark testing, accelerated thermal cycling in a heating/cooling device and differential scanning calorimetry (DSC), thermal stress exposure, and compatibility tests with metals.The results showed severe degradation characterized by a progressive reduction in latent heat and significant modification of phase transition behavior for the 5 mL samples cycled in the refrigerated heating/cooling device compared to the 20 μL samples cycled in DSC. Thermal stress tests for large samples confirmed degradation. Corrosion compatibility tests indicated moderate corrosion of copper while stainless steel, carbon steel, and aluminum exhibited low or negligible corrosion. However, corrosion effects on PCM properties are secondary compared to thermally induced degradation. A regeneration test after 600 cycles showed no recovery, indicating chemical degradation, as further confirmed by 1H NMR spectroscopy. The findings highlight the limitations of relying solely on DSC-based cycling and emphasize the need for multi-scale stability assessments. Beyond the specific OD–PA PCM, this work provides methodological insights for the evaluation of long-term reliability in organic-organic phase change materials.
Flywheel energy storage systems (FESSs) offer significant advantages over other storage methods, such as high-power capability for short-duration charge/discharge and long cycle life. This study explores a fixed-speed FESS that enables charging and discharging by varying the moment of inertia of a rotating body, eliminating the need for a power electronic interface for frequency conversion, which is typically characterized by high cost and low overcurrent tolerance. A prototype incorporating a wire-rope-based inertia control mechanism was developed and experimentally validated. Electrical discharge to the grid was successfully demonstrated, although net electrical energy discharge remained limited owing to steady-state losses in the induction machine. Notably, the system achieved a mechanical discharge efficiency of 53% after subtraction of the steady-state mechanical power input. Although this value remains lower than those reported for conventional variable-speed FESSs, it represents a significant improvement in efficiency over existing fixed-speed frameworks. Operational analysis revealed that friction in the inertia control mechanism and power losses in the induction machine are the primary sources of inefficiency, suggesting that further optimization of mechanical design and the adoption of a synchronous machine could yield additional improvements. These findings highlight the potential applicability of fixed-speed FESSs for direct grid-connected operation and frequency-support applications. Moreover, the proposed framework can facilitate the integration of active power charge/discharge capability into synchronous condensers, enabling inertia support and active power balancing in modern power systems.
