Basical Research On Ionic Liquids Used As Phase Change Materials

The base and core of phase change energy storage technology is Phase Change Materials (PCMs), of which the quality determines the level of energy efficiency. PCMs, mainly indicating solid-liquid PCMs, include inorganic PCMs and organic PCMs. Inorganic PCMs possess high fusion heat, high thermal conductivity and small volume change. While they are prone to get supercooling and phase separation, which can decrease their thermal storage capacity severely. At the same time inorganic PCMs are usually corrosive. Organic PCMs have not the disadvantage of phase separation and are stable. While organic PCMs are highly volatile and flammable, this represents a considerable safety concern for many applications. Also their thermal conductivities are generally low. Therefore new PCMs overcoming the defects of inorganic and organic PCMs are urgently needed. In this work, ionic liquids (ILs) were supposed to be used as PCMs. Quantum chemistry calculation methods were used to recognize the structure effects of ionic liquids to their properties and used for directing the experimental preparation of novel ionic liquids with good performances. Thermodynamic properties of the synthesized ILs were characterized, and their applications in seasonal heat storage, industrial waste heat, and heat pump were investigated.QSPR methods were firstly adopted to study the relationship between structures and heat of fusion of ionic liquids. The geometries of ILs were optimized by PM3semi-empirical calculation and Density Function Theory (DFT) separately. Then, Quantum chemistry parameters were obtained from the output file:cation-anion interaction energy (Ei), Dipole moment (μ), energy molecular orbital (ELUMO and EHOMO), electric charge of front atom (Ccation and Canion), molecular volum (Vm), surface area (S) and shortest hydrogen bond distance (LH).6-parameters model for44ILs,4-parameters model for30ILs,5-parameters model for22imidazolium ILs,3-parameters model for10halide ILs and2-parameters model for9halide imidazolium ILs were constructed. The correlation coefficients were greater than0.93which showed satisfactory internal consistency. The predictabilities of QSPR models were evaluated by external datasets. It was concluded that the validity of the correlation models were greatly depending on the consistency of cation and anion structures of ILs. Besides, the most effective descriptor for heat of fusion was molecular volume. The other descriptors, such as MW,μ and ELUMO, were also important factors. The influencing rules of alkyl side chain length, methyl substitution at C2, and variations of anion on the thermodynamic properties of ILs were obtained and were used for directing the preparation of ILs. Eight kinds of imidazolium halide ILs and six kinds of caprolactam ILs were prepared by changing the length of alkyl side chain, methyl substitution at C2, and selecting small volum anion (ie, Cl-and Br-).Thermodynamic properties of ILs prepared in this work were characterized. The results indicated that the heat of fusion of [C16MIM]Br and [C16MMIM]Br were144.37J·g-1and123.68J·g-1, respectively.[C16MIM]Br and [C16MMIM]Br performed good sensible heat storage. The heat capacities of two ILs at solid state were higher than1.25J·g-1·℃-1and increased with increasing temperature. The heat capacities of two ILs at liquid state were about2.3J·g-1·℃-1. The initial decomposition temperatures for two ILs are230℃and250℃. When temperature ranges from10℃to50℃, thermal conductivities of imidazolium ILs are between0.15～0.35W·m-1·℃-1which are higher than that of Therminol(?) VP-1heat transfer fluid.Melting points, heat of fusions and heat capacities6kinds of caprolactam halide ILs were characterized. They performed comparatively good latent heat and sensitive heat storage properties. The thermodynamic properties of caprolactam halide ILs are below:melting points are in the range of60～75℃, heat of fusions are above120J·g-1, heat capacities at solid state and liquid state are above1.33J·g-1·℃-1and2.0J·g-1℃, respectively.The active use of supercooling of ionic liquids for seasonal solar energy storage was proposed, which means that the thermal energy is stored in a supercooled liquid state and released by nucleating agents when needed.[C16MIM]Br and [C16MMIM]Br are selected in this work, copper powder, graphite powder and1-octadecanol as nucleating agents are added with different mass ratios (0-25wt%) and their effects on the thermodynamic properties of two ionic liquids are investigated. The result indicates that the [C16MIM]Br+copper powder behaves proper melting points and stable supercooling state that are more suitable for seasonal thermal energy storage.In order to solve the problems of IL-PCMs, such as the volume changes during phase change, loss during the using process and combination with the surroundings, ILs were supposed to be packaged into microcapsule. Using melamine-formaldehyde resin (MF) as wall materials and [C4MIM]PF6or [C16MIM]PF6as inner materials, the microencapsulated ILs were prepared. The effects of preparation method and the molar ratio of melamine/formaldehyde were studied. The microencapsulated [C4MIM]PF6were successfully prepared by one step method when the molar ratio of melamine/formaldehyde was1:4.16.