Supercooled erythritol for high-performance seasonal thermal energy storage

The study addresses the challenge of decarbonizing space and water heating, which constitutes a large fraction of building energy use, especially in regions with hot summers and very cold winters. Solar thermal energy is efficient but suffers from a seasonal mismatch between summer availability and winter demand. Conventional seasonal thermal energy storage (TES) systems (sensible heat in water or soil) exhibit substantial heat losses over long durations, analogous to battery self-discharge, reducing effective efficiency. Supercooled PCMs offer a way to "lock" latent heat and mitigate heat loss requirements, making insulation less critical. A suitable PCM for domestic applications should melt within 80–200 °C. Erythritol (Tm ≈ 118 °C) is promising due to its high latent heat and large inherent supercooling compared to other candidates (e.g., MgCl2·6H2O, adipic acid, dulcitol). However, existing supercooling stability is insufficient for extreme climates where winter temperatures can drop to -30 °C or lower. The research question is whether a simple, sustainable strategy can enhance erythritol’s supercooling to an ultrastable level that enables long-duration seasonal storage without accidental crystallization, and whether the release of stored heat can be controllably triggered on demand.

Sugar alcohols such as erythritol are known for high viscosity and significant supercooling, as viscosity reduces molecular mobility and crystal growth. Prior efforts to stabilize erythritol’s supercooling include dispersing it in sodium polyacrylate matrices (degree of supercooling ~110 °C, stability up to 97 days) and adding alkali hydroxides to increase solidification activation barriers (stabilized supercooling at room temperature, Tsup ~ -100 °C for 30 days). Nevertheless, achieving stability at very low temperatures (e.g., below -30 °C ambient) effectively requires degrees of supercooling >150 °C, which prior approaches have not reached. Other candidate PCMs in the 80–200 °C range include MgCl2·6H2O, adipic acid, and dulcitol, but erythritol stands out with the highest latent heat and sustainability. Theoretical frameworks relate supercooling stability to nucleation barriers governed by viscosity and interfacial energy; however, practical strategies to substantially raise the nucleation barrier for erythritol had been limited.

Materials: Erythritol (Macklin Ltd.) and thickeners carrageenan gum (CG), guar gum (GG), xanthan gum (XG), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), and sodium CMC (CMCNa) (Aladdin Ltd.). Preparation: Thickeners and erythritol were ground/mixed using a planetary ball mill (QM-3SP04), then melted on a hot plate at 160 °C (IKA C-MAG HS 7). Molten mixtures were cooled to 25 °C to crystallize. CG loadings up to 15 wt.% were prepared. Comparative data for other thickeners are in Supporting Information. Characterization: Structural and chemical analyses via SEM (HITACHI S-3700N), XRD (Bruker APEXII), and FTIR (Bruker Vertex 70). Thermal stability by TGA (Mettler Toledo TGA/DSC3+). Phase change behavior by DSC under non-isothermal cycles (-100 to 150 °C, 10 K min⁻¹, sealed alumina crucible, N2 purge 20 ml·min⁻¹; 3 cycles) and isothermal temperature-history tests (samples of 10 g in quartz tubes at controlled environments of 90, 50, 10 °C; ultralow temperatures with liquid nitrogen to ~ -140 °C). Rheology measured with a modular compact rheometer (Anton Paar MCR102) using a cone-plate geometry, temperatures 150–120 °C, shear rates 0.001–10,000 s⁻¹. Interfacial energy measurements: Solid-liquid contact angles by sessile drop at 80 °C on polished solid samples; liquid-gas surface tension by pendant drop method (computing S and H shape factors) to obtain γlg; for highly viscous alternatives, an automatic surface tensiometer (AFES FST300M) at 80 °C was used. Solid-liquid interfacial energy and nucleation parameters were inferred to assess the nucleation barrier. Ultrasonication triggering: For samples unable to crystallize spontaneously (e.g., 15 wt.% CG), crystallization was actively triggered with an ultrasonic probe at varied powers (162.5, 325, 487.5, 650 W) and durations (10, 20, 30 min), both during cooling and after holding at -30 °C. Mass sensitivity tests (20–200 g) were also conducted.

- CG is the most effective thickener among tested options (CG, GG, XG, PVA, CMC, CMCNa) for stabilizing erythritol’s supercooled state. - Structural/chemical integrity: XRD peaks of erythritol remain unchanged with CG, indicating no new compounds. FTIR shows intact hydroxyl groups and strengthened hydrogen bonding peaks at higher CG loading (e.g., 15 wt.% shows strong bands at ~1053.9 and 1080.9 cm⁻¹ and broad OH stretch 3000–3300 cm⁻¹). SEM reveals increasing porosity/fragmentation with CG ≥10 wt.% consistent with gelling/thickening effects. - Thermal stability: TGA shows onset of mass loss near 180 °C; up to ~7.8% mass loss at 220 °C. CG addition up to 15 wt.% does not notably degrade thermal stability relative to pure erythritol, supporting compatibility with solar thermal collector temperatures. - Supercooling performance (DSC, non-isothermal): Pure erythritol crystallizes upon cooling, releasing -198 J g⁻¹ at ~33 °C. With 5 wt.% CG, crystallization starts at ~16 °C with ~140 J g⁻¹ released. At 10 and 15 wt.% CG, samples maintain supercooled state down to about -50 °C (vitrification observed), preventing latent heat release during cooling. - Cold crystallization: At higher CG loadings, little to no crystallization occurs during cooling down to -100 °C; latent heat of crystallization is released upon reheating (cold crystallization). For 5 wt.% CG, ~38.9 J g⁻¹ is released on the second heating. For ≥10 wt.% CG, ΔHc appears mainly via cold crystallization. - Latent heat values: Pure erythritol ΔHm = 327.3±2.0 J g⁻¹, ΔHc = 207.0±4.5 J g⁻¹. With 15 wt.% CG, ΔHm = 259.3±15.5 J g⁻¹ and ΔHc = 150.1±31.3 J g⁻¹; with 10 wt.% CG, ΔHc ≈ 153.6±26.5 J g⁻¹. ΔHm and ΔHc decrease roughly linearly with CG loading, though ΔHc shows nonlinear cycling trends. - Cycling stability: Over 15 cycles between -50 and 150 °C, supercooling is maintained during cooling. ΔHm decreases modestly by 10.4% (10 wt.%) and 11.0% (15 wt.%), suggesting longevity (>15 years at one cycle per year). - Specific heat capacity effect: In the ultrastable supercooled range, Cp(liquid) ~2.6 J g⁻¹ K⁻¹ vs Cp(solid) ~1.6 J g⁻¹ K⁻¹; higher cold crystallization temperature can yield more released heat due to Cp differences. - Isothermal behavior: Pure erythritol crystallizes at ~94.7 °C isothermally; cannot maintain supercooling at 60 °C. CG-thickened samples (10–15 wt.%) remain supercooled at 90 °C and down to below -100 °C in isothermal tests. Visual tests show pure and 5 wt.% CG samples crystallize within a day at -30 °C, while 10 and 15 wt.% CG remain supercooled for >60 days. - Rheology: CG substantially increases viscosity and induces non-Newtonian shear-thinning at higher loadings. At 120 °C and 10,000 s⁻¹, shear stress rises to ~828 Pa (+~70%) for 10 wt.% CG and ~1277 Pa (+>160%) for 15 wt.% CG versus pure erythritol. Viscosity increase factors at 1 s⁻¹: 0.5 wt.% CG ~3.2×, 5 wt.% ~20.6×, 15 wt.% ~314.7×. - Interfacial energy and nucleation barrier: Measured liquid-gas interfacial energy at 80 °C increases with CG. By sessile+pendant analyses, γlg increases from ~33.98±2.02 to ~49.39±0.506 mJ m⁻² (≈+45%) for 15 wt.% CG; pendant-drop computations also report ~34.31±2.0 to ~52.68±0.54 mJ m⁻². Increased γ raises Gibbs free energy barrier ΔG* and critical nucleus size Rc, and reduces nucleation rate Ihomo dramatically (reported decreases >10^160-fold at certain conditions), making nucleation far less probable. Theoretical analysis indicates the minimum spontaneous crystallization temperature is lowered from ~60 °C for pure erythritol to about -70 °C with CG. - Ultrasonication triggering: Crystallization of 15 wt.% CG-thickened erythritol requires exceeding thresholds in power and duration. Powers <325 W (≤30 min) do not trigger crystallization; powers ≥487.5 W with durations ≥10 min successfully trigger. At -30 °C, 487.5 W for 30 min triggers crystallization; after triggering, exotherm continues to complete discharge. Triggering power not sensitive to sample mass from 20–200 g. Ultrasonication serves as a controllable, energy-efficient key to unlock stored latent heat on demand. - Overall: CG at 15 wt.% yields ultrastable supercooling (>200 °C degree of supercooling noted), high latent heat (>200 J g⁻¹), strong cycling stability, and eco-friendly constituents, offering reduced insulation requirements versus conventional seasonal TES.

The findings demonstrate a simple, bio-based thickening strategy that substantially enhances supercooling stability of erythritol, addressing the seasonal storage challenge where long-duration heat retention is needed without catastrophic latent heat release in cold environments. By simultaneously increasing viscosity (slowing crystal growth) and elevating solid–liquid interfacial energy (raising nucleation energy barriers and critical nucleus size), CG produces a strong thermodynamic barrier—a “dam”—against spontaneous crystallization. This stabilizes the supercooled state over months at low temperatures (e.g., below -30 °C), greatly mitigating heat-loss concerns and potentially reducing insulation needs for seasonal TES systems. The controlled, on-demand discharge enabled by ultrasonication provides a practical activation mechanism compatible with severe cold conditions and varying sample masses. Although latent heat is modestly reduced with CG addition, the composite retains high storage density and exhibits good cycling durability, with minimal degradation over repeated cycles. The approach provides a pathway for reliable, sustainable, and corrosion-benign PCM-based seasonal TES within the 80–200 °C operating window.

Incorporating 15 wt.% carrageenan into erythritol yields an eco-friendly composite PCM with an unprecedented, ultrastable supercooling capability (>200 °C degree of supercooling) while maintaining high latent heat (>200 J g⁻¹). The mechanism combines increased viscosity and a ~45% rise in interfacial energy to significantly suppress nucleation and crystal growth, effectively locking latent heat through extended cold periods. Ultrasonication serves as a robust, controllable trigger to release the stored energy on demand, including at -30 °C. The material exhibits good thermal stability and cycling performance, suggesting longevity for seasonal TES applications and reduced insulation requirements. Future research could optimize thickener types and loadings for balancing latent heat and stability, quantify precise ultrasonic triggering thresholds across scales and geometries, integrate the PCM into full TES prototypes for field validation, and extend the thickening strategy to other PCM systems across different temperature ranges.

- External activation required: High-load CG-thickened erythritol (e.g., 15 wt.%) does not crystallize spontaneously; discharging requires seeding or mechanical means. The study adopts ultrasonication but does not pinpoint exact threshold conditions beyond demonstrating powers ≥487.5 W and durations ≥10 min as effective. - Reduced latent heat: CG addition decreases ΔHm and ΔHc relative to pure erythritol, though values remain high. - Measurement constraints: Low shear-rate rheology involved challenges and potential errors; pendant-drop measurements for some highly viscous samples were difficult, requiring alternative instrumentation. - Material stability considerations: Although overall thermal stability up to relevant temperatures is maintained, CG decomposes at elevated temperatures (>60 °C) as an isolated component; long-term high-temperature exposure in practical systems was not field-tested. - Scale-up and system integration: Demonstrations were at lab scale with masses up to 200 g; full-scale TES integration, long-duration field trials, and long-term cycling beyond 15 cycles remain to be validated.