Abstract:
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