A promising solution for decarbonizing space and water heating in winter is the use of supercooled phase change materials (PCMs) for seasonal thermal energy storage. Erythritol, a sustainable mid-temperature PCM with high latent heat, was combined with carrageenan (CG), a bio-derived food thickener, to stabilize its supercooling. This composite PCM can maintain an ultrastable supercooled state below -30°C, preventing accidental loss of latent heat in severe cold regions. The common ultrasonication method was used to trigger the release of latent heat stored in the CG-thickened erythritol, demonstrating its potential as a high-performance, eco-friendly PCM for long-term seasonal solar energy storage. Seasonal thermal energy storage (TES) is crucial for mitigating the temporal mismatch between abundant solar energy in summer and high heating demands in winter. However, significant heat loss during long-term storage poses a challenge. Supercooled PCMs offer a solution with less thermal insulation concern and higher thermal storage density. Erythritol, with a high degree of supercooling and latent heat of fusion, is a promising candidate. However, its supercooling is insufficient for extreme cold conditions. Adding CG significantly improves the supercooling behavior of erythritol, enabling it to maintain an ultrastable supercooled state even at -50°C. The addition of CG also enhances the interfacial energy and viscosity of erythritol, acting as a thermodynamic "dam" to prevent inadvertent loss of stored latent heat. The CG-thickened erythritol exhibits excellent cycling stability, maintaining its supercooled state over multiple charging and discharging cycles. It also demonstrates high sustainability due to its bio-derived components. The ultrasonication method was found to be effective in triggering the crystallization of CG-thickened erythritol, enabling controlled release of latent heat when needed. The CG-thickened erythritol has a high latent heat of fusion (>200 J·g⁻¹) and is highly sustainable, making it a promising eco-friendly PCM for seasonal solar energy storage. The addition of CG significantly improves the supercooling behavior of erythritol, making it suitable for long-term seasonal storage in severe cold regions. The CG-thickened erythritol is also highly sustainable, as both erythritol and CG are biomaterials used in food, pharmaceutical, and chemical industries. This simple yet effective thickening strategy can be extended to other types of PCM in practical TES scenarios with various temperature ranges. The overall high performance of the CG-thickened erythritol makes it a very promising eco-friendly, mid-temperature PCM for seasonal storage of solar thermal energy.A promising solution for decarbonizing space and water heating in winter is the use of supercooled phase change materials (PCMs) for seasonal thermal energy storage. Erythritol, a sustainable mid-temperature PCM with high latent heat, was combined with carrageenan (CG), a bio-derived food thickener, to stabilize its supercooling. This composite PCM can maintain an ultrastable supercooled state below -30°C, preventing accidental loss of latent heat in severe cold regions. The common ultrasonication method was used to trigger the release of latent heat stored in the CG-thickened erythritol, demonstrating its potential as a high-performance, eco-friendly PCM for long-term seasonal solar energy storage. Seasonal thermal energy storage (TES) is crucial for mitigating the temporal mismatch between abundant solar energy in summer and high heating demands in winter. However, significant heat loss during long-term storage poses a challenge. Supercooled PCMs offer a solution with less thermal insulation concern and higher thermal storage density. Erythritol, with a high degree of supercooling and latent heat of fusion, is a promising candidate. However, its supercooling is insufficient for extreme cold conditions. Adding CG significantly improves the supercooling behavior of erythritol, enabling it to maintain an ultrastable supercooled state even at -50°C. The addition of CG also enhances the interfacial energy and viscosity of erythritol, acting as a thermodynamic "dam" to prevent inadvertent loss of stored latent heat. The CG-thickened erythritol exhibits excellent cycling stability, maintaining its supercooled state over multiple charging and discharging cycles. It also demonstrates high sustainability due to its bio-derived components. The ultrasonication method was found to be effective in triggering the crystallization of CG-thickened erythritol, enabling controlled release of latent heat when needed. The CG-thickened erythritol has a high latent heat of fusion (>200 J·g⁻¹) and is highly sustainable, making it a promising eco-friendly PCM for seasonal solar energy storage. The addition of CG significantly improves the supercooling behavior of erythritol, making it suitable for long-term seasonal storage in severe cold regions. The CG-thickened erythritol is also highly sustainable, as both erythritol and CG are biomaterials used in food, pharmaceutical, and chemical industries. This simple yet effective thickening strategy can be extended to other types of PCM in practical TES scenarios with various temperature ranges. The overall high performance of the CG-thickened erythritol makes it a very promising eco-friendly, mid-temperature PCM for seasonal storage of solar thermal energy.