Decarbonizing space and water heating is crucial for a carbon-neutral future, particularly given that it accounts for a significant portion (~60%) of building energy consumption. Solar thermal energy offers a sustainable alternative, boasting higher conversion efficiency than photovoltaics. However, the temporal mismatch between summer solar abundance and winter heating demands presents a challenge, especially in regions with extreme seasonal temperature variations (e.g., northwestern China and eastern Turkey). Seasonal thermal energy storage (TES) addresses this by storing excess summer solar energy for winter use. Existing sensible heat storage (using water or soil) suffers from significant heat losses (up to 60%), while expensive ultra-insulation only mitigates this issue. Supercooled PCMs offer a solution, storing heat at a supercooled liquid state well below their melting point (*T*m), releasing it only upon triggered crystallization. The ideal PCM should melt within the operational temperature range of domestic heating systems (-80°C) and solar thermal collectors (<200°C). Erythritol (*T*m=118°C) emerges as a promising candidate due to its high supercooling degree (*T*sup=-60°C), high latent heat of fusion (*H*m=-340 J·g-1), and sustainability. However, its supercooling is insufficient for severe winter conditions (-30°C or lower). Existing methods for improving erythritol's supercooling haven't achieved the required ultrastable, low-temperature stability. This study explores enhancing erythritol's supercooling by increasing its viscosity through the addition of a thickener and using ultrasonication to controllably release the stored energy.

Several studies have investigated methods to stabilize erythritol's supercooling. Puupponen and Turunen et al. dispersed erythritol in a sodium polyacrylate matrix, achieving a supercooling degree of about 110°C for up to 97 days. Li et al. used alkali hydroxides to increase the solidification activation energy barrier, reaching a stable supercooled state at room temperature for 30 days. However, these methods haven't achieved the ultra-high supercooling degree (>150°C) needed for seasonal TES in extremely cold environments. The high viscosity of sugar alcohols like erythritol is linked to their supercooling ability, as high viscosity reduces molecular mobility and crystal growth rate. This study aims to improve erythritol's supercooling by increasing its viscosity using a suitable thickener, offering a simpler and potentially more effective solution than previous approaches.

The researchers tested various common and cost-effective thickeners, including bio-derived food thickeners (carrageenan gum (CG), guar gum (GG), xanthan gum (XG)) and other thickeners (polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), and sodium carboxymethyl cellulose (CMCNa)). Composite PCM samples were prepared by grinding and melting the thickeners with erythritol. CG was identified as the most effective thickener for improving erythritol's supercooling. The structure and thermal stability of the CG-thickened erythritol were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and thermal gravimetric analysis (TGA). Differential scanning calorimetry (DSC) under both isothermal and non-isothermal conditions was employed to investigate the supercooling behavior, latent heat, and cycling stability of the composite PCMs. Rheological measurements using a rotational rheometer determined the viscosity and shear behavior of the samples. The solid-liquid interfacial energy was determined using sessile drop and pendant drop methods to understand the mechanism behind the improved supercooling. Finally, ultrasonication was investigated as a method for triggering the controlled release of latent heat from the supercooled composite PCM.

Adding CG to erythritol led to structural changes, creating fragmentation and porosity in the crystal structure observed through SEM. XRD confirmed that the addition of CG didn't affect the erythritol crystal structure. FTIR analysis showed the presence of strong hydrogen bonds among erythritol molecules, which were not disrupted by CG, even at high loadings (15 wt%). TGA showed excellent thermal stability of CG-thickened erythritol similar to pure erythritol, allowing it to operate within the typical temperature range of solar thermal collectors. DSC non-isothermal tests revealed that increasing CG loading significantly enhanced the supercooling ability of erythritol. At 10 wt% and 15 wt% CG, the composite PCM maintained an ultrastable supercooled state down to approximately -50°C. A cold crystallization phenomenon was observed during subsequent charging cycles, with higher CG loadings intensifying this. The latent heat values (ΔHm and ΔHc) decreased linearly with increasing CG loading, but even at 15 wt% CG, the latent heat remained relatively high. Cycling tests over 15 cycles demonstrated good stability with only a minor reduction in ΔHm (around 10-11%). Isothermal DSC tests confirmed the ultrastable supercooling behavior at very low temperatures, even down to below -100°C. Visual observations confirmed the significantly improved supercooling of the CG-thickened erythritol compared to pure erythritol. Rheological analysis showed that the addition of CG dramatically increased the viscosity of erythritol, exhibiting shear-thinning behavior at higher CG loadings. A minute amount of CG (0.5 wt%) increased the viscosity by 3.2 times, while 15 wt% CG resulted in a 314.7-fold increase. The improved supercooling was attributed to an increase in both viscosity and solid-liquid interfacial energy (γsl). The addition of CG increased γsl by about 45%, which raised the energy barrier for nucleation and significantly reduced the rate of nucleus formation. Ultrasonication was found to be an effective method for triggering the crystallization and heat release from the supercooled CG-thickened erythritol, even at -30°C, demonstrating a controllable method for on-demand heat release. The power and duration of ultrasonication were shown to be critical parameters influencing triggering efficacy.

This study successfully demonstrated a simple yet highly effective method for achieving ultrastable supercooling in erythritol, a sustainable PCM, for seasonal thermal energy storage applications. The addition of CG, a bio-derived food thickener, significantly enhanced both the viscosity and the solid-liquid interfacial energy of erythritol, creating a high thermodynamic barrier against spontaneous crystallization and hence preventing accidental heat loss in extremely cold environments. The findings directly address the key challenge of long-term heat storage by achieving unprecedented ultra-high supercooling degrees. The use of ultrasonication to trigger controlled heat release further enhances the practicality of this approach. This work demonstrates a significant advancement in the development of high-performance, eco-friendly PCMs for seasonal solar energy storage. The strategy of enhancing supercooling by thickening could be extended to other PCMs, potentially broadening its applications in various temperature ranges.

This research presents a novel approach to enhancing the supercooling behavior of erythritol for seasonal thermal energy storage. By incorporating carrageenan, a sustainable thickener, the study achieved an ultrastable supercooled state below -30°C, suitable for extremely cold environments. The use of ultrasonication provides a controllable mechanism for heat release. This strategy offers a promising, eco-friendly solution for high-performance seasonal solar thermal energy storage. Future research could explore the optimization of CG loading, investigating other sustainable thickeners, and scaling up the system for practical applications.

While the study demonstrated significant improvement in supercooling, the latent heat of fusion decreased with increasing CG loading. Future research could focus on optimizing the CG concentration to balance supercooling and latent heat capacity. The study primarily focused on lab-scale experiments; further research is needed to evaluate the long-term performance and scalability of this approach for real-world TES systems. The exact threshold power and duration for ultrasonication triggering may need further refinement for optimized energy efficiency in large-scale applications.