Novel triggerable phase change materials in supercooled and glassy state for thermal energy storage: synthesis, characterization, and performance analysis

Zhang, Xusheng (2024). Novel triggerable phase change materials in supercooled and glassy state for thermal energy storage: synthesis, characterization, and performance analysis. University of Birmingham. Ph.D.

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Abstract

This PhD research focuses on thermal energy storage (TES) technology, utilizing triggerable phase change materials (PCMs) for controllable energy release. Conventional PCMs typically release heat through spontaneous crystallization once the temperature falls below their freezing point, rendering them less effective for long-term storage applications. This research addresses these limitations by creating an energy barrier between liquids and crystals, forming supercooled liquids and glassy materials for long-term energy storage. Additionally, both stability and trigger sensitivity are enhanced through specific molecular design. This strategy facilitates simpler and more cost-effective methods for material design and preparation, enhancing the practicality and efficiency of PCMs for long duration storage use. The thesis comprises three distinct yet interconnected parts, and the key findings are summarized as follows:
I. The first part is concerned with the development of a long-term mechanically triggerable metal-organic supercooled liquid named MME, using manganese (II) chloride, methylurea, and erythritol. MME features a core-shell molecular structure, where the metal-organic octahedral coordination core from 1 Mn2+ and 4 methylurea molecules and 2 erythritol molecules enhances stability, and the hydrogen bond shell formed by residual erythritol molecules offers triggerability. Characterization techniques such as XRD, EXAFS, XANES, UV-Vis, FTIR, NMR, TEM, and DSC assessed core-shell interactions and the reversible supercooled liquid-to-crystalline transformation, reliant on ligand exchange between erythritol and Cl- ions. By altering the content of erythritol, the storage stability and trigger sensitivity can be controllably adjusted. For the MME series samples, they can release a maximum heat of approximately 215 kJ/kg and can be stably stored at room temperature for up to three months. The nucleation energy barrier for these samples is around 10-19 J, and the required triggering stress is less than 10 Pa. Thermal cycling showed less than 15% enthalpy loss after 1000 cycles.
II. The second part of this work is on the development of a new glassy PCMs, named G-DMI, made from d-mannitol and myo-inositol. G-DMI is cost-effective, scalable, eco-friendly, and non-flammable, featuring a significant cold-crystallization enthalpy. Its crystalline form, DMI, has a charging temperature of 160 °C, a high latent heat of 280 kJ/kg, and a glass transition temperature of 22 °C. DMI can retain its glassy state for over a year at room temperature and rapidly releases nearly 180 kJ/kg of heat at about 60 °C upon thermal triggering. Characterization techniques such as FTIR, NMR, XRD, TEM, and DSC analysed its hydrogen bonding and crystallization. Additionally, iron oxide nanoparticles were integrated into DMI to create Solar DMI, a volumetric solar absorber that absorbs over 95% of solar radiation and efficiently releases stored heat, showcasing its utility in solar-thermal applications.
III. The third part introduces a dual-layer thermosensitive energy storage smart window, combining transparent, triggerable PCMs with thermochromic technology to dynamically adjust light transmittance based on temperature. It also improves thermal comfort by heating indoor spaces during cold nights through triggered crystallization with light shielding. The window consists of two layers: an outer light-regulating layer made of poly (N-isopropylacrylamide) (PNIPAM) modified with tartaric acid (TA) and dimethylacrylamide (DMA), which can tune its lower critical solution temperature (LCST) from 10-70 °C for precise light control. The inner layer, made of calcium chloride hexahydrate (CCH) modified with ethanol and urea, serves as the thermal energy storage layer with a melting point adjustable between 10 and 30 °C and maintains stability in a supercooled state.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):