Prevention of supercooling and stabilization of inorganic salt hydrates as latent heat storage materials

Various inorganic salt hydrates have been studied as a latent heat storage medium. A super-absorbent polymer (SAP) made from an acrylic acid copolymer is proposed as an effective thickener to prevent undesirable phase separation of the high hydrate inorganic salts (Na₂SO₄· 10H₂O, Na₂HPO₄·12H₂O, Na₂CO₃·10H₂O). Most of these materials can be stabilize by the addition of 3 to 5 wt% SAP as a thickener. For the low hydrate inorganic salts (CH₃COONa· 3H₂O, Na₂S₂O₃·5H₂O), carboxymethyl cellulose (CMC) is found to be an effective thickener. Similarly, the phase separation of the low hydrate salts can be prevented by the addition of 2 to 4 wt% thickener. To overcome the supercooling of the thickened phase change materials, various potential nucleators have been evaluated. For the thickened Glauber's salt, borax reduces supercooling of the salt from 15 to 3–4°C. Three different powders of carbon (1.5–6.7 μm), copper (1.5–2.5 μm) and titanum oxide (2–200 μm) are found to reduce the supercooling of thickened Na₂HPO₄·12H₂O. Also, the supercooling of thickened CH₃COONa·3H₂O is reduced from 20 to 2–3°C by adding 2 wt% potassium sulfate. New compositions for preventing supercooling and phase separation of PCMs are developed in the temperature range 30–60°C: Glauber's salt/SAP/borax (94/3/3 wt%, T_m = 35°C), Na₂CO₃·10H₂/SAP/Sr(OH)₂ (93/3/4 wt%, T_m = 32°C), Na₂HPO₄·12H₂O (92.8/3.5/3.7 wt%, T_m = 35°C), Na₂S₂O₃·5H₂O/CMC/ SrSO₄ (92/3/5 wt%, T_m = 48°C), CH₃COONa·3H₂O/CMC/K₂SO₄ (95/3/2 wt%, T_m = 58°C).