Low-temperature phase-change materials for energy-storage applications
Creators: name: Logan, Hannah Thomson
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: thermal-energy storage
phase-change materials
tetrahydrofuran-hydrate
THF-hydrate system
zeolite
sub-cooling
renewable energy

description: The increasing use of renewable energy sources across the world requires new technologies for energy storage, to address the intermittency of these energy sources. Thermal energy storage (TES) can be used to store energy as heat. In particular, latent-heat storage, which uses phase-change materials (PCMs), is a promising TES method that can also be used in low-temperature applications. Low-temperature PCMs of interest include clathrate hydrates and salt-water eutectics. Although these materials have been identified as potential PCMs, there are a number of challenges associated with their use, e.g. sub-cooling. Hence, there is a requirement to understand and better the behaviour and characteristics of these materials in order to reduce subcooling and enable their use in TES applications. Tetrahydrofuran-hydrate is a clathrate system with a melting point of +4.4 °C and is therefore of interest for air-conditioning and refrigeration applications. In this research, the THF-hydrate system has been investigated in order to reduce its tendency to sub-cool. Heterogeneous additives were tested as potential nucleators for the THF-hydrate system. Molecular sieves based on the zeolites 13X, 4A, 3A and 5A were found to reduce the extent of sub-cooling for the THF-hydrate system during vial-scale thermal cycling. THF-clathrate doped with 4A molecular sieves was investigated further in order to gain better understanding of this system. This culminated in the development of a formulated PCM based on THF-hydrate that has the potential to be incorporated into a commercial TES system. The mechanism of nucleation of the 4A molecular sieves was also investigated. This was performed by conducting compositional analysis of the molecular sieves, modifying their surface chemistry through silylation, and comparing the crystal structures of the THF-hydrate and the 4A zeolite. It was concluded that the likely mechanism is related to similarities of the crystal structure between the THF-hydrate and the 4A zeolite. It was also found that the formation of structures within the cages of the zeolite may play a role in the nucleation mechanism. Furthermore, the role of temperature on the ability of the zeolite to act as a nucleator was investigated. It was found that deactivation of the nucleator occurred when the system was exposed to elevated temperatures and that successful extended cycling was only achieved when cycled to a maximum temperature of 10 °C. In understanding the nucleation and deactivation mechanisms involved in the 4A nucleated THF-hydrate material, nucleators for further clathrate PCMs can be better selected. In addition to this, commercial TES systems, based on the 4A nucleated THF-hydrate can be tested and implemented with documented material boundary conditions. A further system studied in this work was a PCM based on the magnesium nitrate/water eutectic, which has a melting point of -30 °C. This system is currently of commercial interest as a TES for applications in low-temperature food refrigeration but is known to suffer from sub-cooling. The composition and structure of the eutectic is poorly understood – the low-temperature phase is believed to comprise Mg(NO3)2·9H2O and ice (Ih), but the nonahydrate has not been structurally characterised. Using powder X-ray diffraction (PXRD), the conditions required for the reproducible crystallisation of the eutectic composition were identified and a subsequent single crystal X-ray diffraction study identified the low-temperature hydrate as the previously unknown octahydrate, Mg(NO3)2·8H2O. An additional metastable phase was discovered during the variable temperature PXRD studies, but it has not yet been possible to index this powder pattern. The characterisation of the octahydrate’s crystal structure will provide the structural information required to allow the discovery of new nucleators for this system, facilitating its implementation into commercial TES for cold-storage applications.