Salt Hydration in Pores for Heat Storage

Due to climate change and the depletion of fossil fuels, a transition towards renewable energy sources is ongoing like solar collectors and wind turbines. However, there is often a mismatch between production and major consumption times. For residential homes, the most energy is used for heating purposes. Therefore, thermal energy storage forms a good support for the energy transition to compensate for the discrepancy between production and consumption. For thermal energy storage, there are three types available: sensible, latent, and thermo-chemical. The thermo-chemical materials have the highest energy density between 1-3 GJ/m3. Additionally, thermochemical materials can store heat loss-free and are thus best suited for long-term and seasonal storage.Salt hydrates are the favored thermo-chemical materials for residential application due to their temperature release in the desired range. However, these salts can have stability issues such as expansion, shrinkage, conglomeration, and deliquescence. To compensate for these drawbacks, salt hydrates can be stabilized inside porous materials. This offers the use of deliquescence as an additional phase transition to store thermal energy. By impregnating salt hydrates into porous systems, called composite, the properties of the salt could be changed due to the limited space, the interface with the pore walls, and capillary condensation. In this thesis, we focus on the changes to the phase transitions of salt hydrate when put under confinement inside porous materials and how these changes can be utilized for thermal energy storage applications in residential homes. Therefore, salt hydrates with a single hydration transition and salt hydrates with multiple hydration transitions inside different porous materials were investigated with TGA, PXRD in situ, and DVS measurements on their phase transition properties compared to the behavior of the bulk salt. Understanding these changes in the salt hydrate transition can help to choose and combine the right material for the conditions of the application and can improve the uses of composites for the use as storage materials. In Chapter 3, the effect of the confinement inside porous materials on the hydration and deliquescence transition of salt hydrates was tested on the single-hydrate CuCl2 inside mesoporous silica gels with different pore sizes. It was found that the hydration and dehydration onsets remained unaffected by the confinement and pore size. Contrary to that, the deliquescence transition was shifted to lower water vapor pressures with decreasing pore size to the point where the two transitions were not distinguishable anymore. This shows that the kinetics of the solid-solid phase transition is not influenced by smaller crystal size and interface with a host matrix. However, the deliquescence transition is shifted due to the increased surface area in smaller crystals and the advantageous shape of the meniscus inside a pore compared to a salt particle. This shift in deliquescence increases the output temperature of this phase transition and makes more salt hydrates in composites usable for thermal energy storage. In Chapter 4, the complex hydration and dehydration steps of CaCl2 were investigated. The path-dependent hydrate steps from the anhydrate via the tritohydrate to dihydrate and from the dihydrate via the monohydrate to the anhydrate are created due to kinetic hindrances between the phase transitions towards or from the trito- or monohydrate. These kinetic hindrances could be connected to the newly found crystal structures of the trito- and monohydrate compared to the structures of the anhydrate and dihydrate.In Chapter 5, the hydrate steps found in the former chapter for CaCl2 were examined in the confinement of different porous matrices. It was found that the pore size of the composites changes the kinetic hindrance between the hydrate steps. Evidently, the kinetic hindrances between the equilibrium line and (de-)hydration onsets are influenced by different parameters than the kinetic hindrances between phase transitions for hydrates. This made the hydration and dehydration simpler for the smallest pore sizes since either the tritohydrate or both intermediate hydrates are not formed anymore eliminating the path-dependency of CaCl2. The simplified pathways can help the use in applications because the output and charging temperature remain constant every cycle and even for partial charging or discharging making the material more predictable. In Chapter 6, The sister-salt of CaCl2, CaBr2, was investigated as another multihydrate salt for its hydrate steps and its behavior inside mesoporous silica gels. Even though the anhydrate and hexahydrate of CaBr2 and CaCl2 have the same crystal structure, CaBr2 has no path-dependent hydration-dehydration steps. Additionally, the pathways of CaBr2 are the same in composites as in the bulk salt due to no change in kinetic hindrance between hydrate steps being possible. This supports the findings in Chapter 3 for single-hydrate composites being also applicable to multihydrate composites. Additionally, the fact, that CaCl2 is a special case under the multi-hydrate salt hydrates with its path-dependent (de-)hydration, was further cemented. In Chapter 7, the findings of the former Chapters were used to find good composites for low-temperature thermochemical energy storage. Therefore, the clays, Sepiolite and Halloysite, were tested for their host matrix properties with the test salt hydrates, LiCl and LiBr. It was shown that for salt content below the thresholds the clay composites were stable for over 35 charging-discharging cycles without the salt leaking or creeping out. Additionally, the energy densities of the composites in this study were comparable with values in the literature. This makes the clays good host matrices for heat storage when combined with a more favorable salt hydrate than the test salts, LiCl and LiBr, like CaCl2, SrCl2 or SrBr2.