Salt hydrates for thermochemical energy storage
Thermal energy storage is required to address the growing energy demands worldwide. Energy storage improves performance and reliability, increases efficiency and reduces the mismatch between supply and demand of renewable energy sources. In particular, thermochemical energy storage has a high energy storage density, extended storage period and minimal heat losses. Anhydrous salts when reacted with water vapour in air are hydrated and generate heat. Salt hydrates have often been chosen for space heating application due to their high energy density, suitable dehydration temperatures and water vapour being a safe and cheap reaction partner. However, this method of thermal energy storage is yet to be fully developed or commercialized. The overarching aim of this work is to investigate salt hydrates for thermochemical energy storage for use in space heating applications. In particular, this thesis discusses material development, reaction kinetics and system design with a focus on the salt hydrate, SrCl2·6H2O. A screening study was completed to determine the most suitable salt hydrates to store intermediate temperature source of energy. A literature screening found that the salt hydrates SrCl2, MgSO4, Na3PO4, MgCl2 and SrBr2 were most promising. Hydration, dehydration and cycling studies were conducted, which revealed that SrCl2 and SrBr2 are the most suitable salts for residential heating applications. As a result, a composite material of SrCl2·6H2O (50 wt. %) and cement was developed and compared to the well-researched zeolite 13X in a labscale reactor. The materials were studied over several cycles with different dehydration temperatures of up to 150 °C. The cement- SrCl2·6H2O (50 wt. %) material proved promising for thermochemical energy storage with volumetric energy density of 136 kWh m-3. The hydration kinetics of the salt, SrCl2, and its composite with cement were experimentally investigated. Firstly, a reaction kinetics model was developed for the hydration of SrCl2 to SrCl2·6H2O. Following that, a shrinking-core model for salt in a cement porous host matrix was developed. This model combines both chemical reaction and moisture diffusion to define the overall reaction rate which can then be used to predict the performance of thermochemical energy storage reactors. In order to improve thermal and exergy efficiencies a cascade thermochemical energy storage system was experimentally investigated using the SrCl2-cement composite and zeolite 13X. The two materials were chosen based on their respective hydration and dehydration requirements. The volumetric energy density ranged from 108-138 kWh m-3 with dehydration temperatures of 50-130 °C. The cascaded system improved exergy efficiency by 6-38% when compared to traditional salt based system. Lastly, a system using an open reactor employing the salt SrCl2 was mathematically investigated to determine the feasibility under different atmospheric conditions. It was found that both hydration and dehydration are possible to operate within New Zealand winter weather condition. However, it was found that in drier climates, the hydration reaction may not operate efficiently and could cause significant undesirable drop in indoor air humidity. Overall, based on the results of this work, salt hydrates for thermochemical energy storage is still in its initial stages. Whilst there are many promising results, significant work is required at both a material and reactor scale in order for this technology to be implemented in society
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