Advanced & Functional Materials
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Salt-hydrate phase-change thermal storage
Phase-Change Materials
What fraction of latent heat do three salt hydrates retain after 500 cycles across 10–15 mg pans, 1 kg neat slabs and 1 kg porous-matrix composite slabs?
THE QUESTION
Which composition provides latent heat above [threshold] in a named temperature window with degradation below [threshold]% over [cycles] cycles? Verifier: DSC plus thermal cycling. [baseline: to verify]
WORK ORDER
Partition of 500-cycle latent-heat loss across composition, matrix and scale for three salt hydrates
BACKGROUND
What this Apodex Lab is
Building thermal storage relies on phase-change materials that store and release heat within the 0–65 °C window over thousands of thermal cycles. Inorganic salt hydrates such as Glauber’s salt (Na2SO4·10H2O), calcium chloride hexahydrate (CaCl2·6H2O), and sodium acetate trihydrate provide dense volumetric heat storage and thermal conductivity without the flammability of organic alternatives. However, their practical life is limited by phase separation during repeated melting, supercooling that prevents complete solidification, and water loss through containment seals.
Qualification in this field relies on differential scanning calorimetry of 10–15 mg samples subjected to hundreds of melt–freeze cycles. A formulation is typically accepted if it maintains latent heat above roughly 150–200 J/g with less than about 10 % enthalpy loss after 500–1,000 cycles. This standard test conceals critical failure mechanisms. In a sealed milligram pan, gravity-driven segregation cannot fully develop, water cannot escape, and bulk nucleation statistics are absent. Furthermore, inter-laboratory comparisons demonstrate wide scatter when different facilities measure the same materials. Reported degradation figures therefore confound the intrinsic chemical stability of the salt hydrate with its containment, matrix support, and sample scale.
The Phase-Change Materials Apodex Lab measures how 500 melt–freeze cycles degrade identical batches of three salt hydrates across three physical formats: 10–15 mg calorimetry pans, 1 kg neat salt sealed in 20 mm aluminium slabs, and 1 kg composites impregnated in a porous carbonate matrix. Combining heat-flux calorimetry, sectioned gravimetric water analysis, and X-ray diffraction, the measurement separates latent-heat loss caused by composition breakdown from losses driven by matrix interactions, seal leakage, and vertical segregation. This attribution gives engineers verified retention figures at kilogram scale, replacing unverified milligram screening data in thermal storage specifications.
WHY NOW
Why the field is spending on an assumption
Screening programmes select salt-hydrate phase-change materials using differential scanning calorimetry on milligram samples. This practice confounds chemical degradation with containment effects: sealed pans do not experience the phase segregation, water loss or nucleation statistics of kilogram modules. In addition, published inter-laboratory comparisons reveal wide scatter in measured enthalpies across different facilities. While porous carbonate matrices can now physically stabilise salt hydrates to isolate matrix effects from composition, existing rankings omit cycle degradation altogether. Integrators are therefore deploying kilogram modules based solely on pan cycling data, without measurements separating composition losses from containment and scale effects.
WHAT WE’D BUY
What we would ask a laboratory to run
We are not buying the answer. We are buying a measurement with an interval — and the bars it will be compared against.
One 8 kg homogenised batch each of Glauber’s salt, calcium chloride hexahydrate and sodium acetate trihydrate is split into three forms: 10–15 mg pans, 1 kg neat sealed 20 mm slabs, and 1 kg porous-carbonate composite slabs. All forms undergo 500 melt–freeze cycles in a shared liquid bath. Latent heat is measured across four checkpoints by calorimetry. The measurement returns the retained latent heat of fusion at cycle 500 with a 95% confidence-interval half-width no wider than 2 percentage points for pans and 4 percentage points for slabs. Mass logging, sectioned calorimetry and X-ray diffraction partition losses between water escape, segregation and matrix interaction.
SCOPE & LIMITS
The 20 mm slab geometry represents the conduction layer but under-represents gravity-driven segregation over a full-height module column, meaning the measured slab degradation provides a lower bound on segregation loss.
WHO EXECUTES
The kind of lab that can run this
A thermal-analysis and materials-testing laboratory (university core facility or CRO) equipped with calibrated DSC, T-history / bulk calorimetry rigs, programmable thermal-cycling chambers and gram-to-kilogram sample handling for salt hydrates and their porous-matrix composites.
EXECUTOR KIND
wet-lab
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