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Title: Encapsulated salt hydrate phase change materials for thermal energy storage
Author: Graham, M. J.
ISNI:       0000 0004 7428 4261
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2017
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Man-made climate change is the biggest threat to humanity and other species inhabiting planet Earth. As technological advancement becomes ubiquitous with modern life, energy demand has increased immensely. It is vital to develop sustainable sources for energy production. Around 80% of worldwide energy is currently produced by fossil fuels. Coal, natural gas and oil are huge contributors to the release of CO2 and methane into the atmosphere, which has led to a global average temperature increase of 0.8°C since the Industrial Revolution. Much research has been undertaken in developing renewable energies, with great recent advancements made on solar, wind and hydropower amongst others. Renewable energies garner much attention, both within the scientific community and mainstream media. However, renewable energies suffer from intermittency and cannot be used continuously. An often overlooked yet essential component of renewable energies is thermal energy storage, which will vastly improve their continued use and efficiency. Chapter 1 provides a literature overview of the current world energy problem and energy storage. Thermal energy storage is currently achieved using sensible heat storage materials, which store heat as they increase in temperature. They have low volumetric energy capacity, especially when compared to latent heat storage materials, which store and release energy as they change phase. These materials are known as phase change materials (PCMs), of which organic paraffin waxes and inorganic salt hydrates (also known as crystallohydrates) are the most promising candidates. PCMs not only have the potential to improve efficiency of renewable energy sources, they may also be used for applications such as passive thermal regulation. Thermal regulation can greatly reduce air conditioning demands of buildings, and increase lifetimes of photovoltaics and electronics. Unfortunately, PCMs are not available for use in their bulk state due to several drawbacks which lead to short lifetimes. Numerous approaches have been made to increase their lifespan. Encapsulation within a polymer shell is considered the best approach, as it gives numerous advantages whilst employing simple reaction methods, allowing for industrial scale-up. Reducing capsule diameter to the nanometre range hugely increases surface area to volume ratio, which can eliminate several inherent drawbacks of PCMs. Salt hydrates are the most promising PCM, due to their very high volumetric energy storage density. However, they are also the most problematic PCM to work with. They are corrosive, incongruently melt and are prone to supercooling. Due to their hydrophilicity, they are also difficult to encapsulate. Our initial approach to confine them on the nanoscale was to produce water-in-oil emulsions using surfactants. Chapter 2 details how we found that several surfactant combinations could be used to provide an initial shell with salt hydrates solubilised within them. However, a more robust shell was required for full analysis. To fully stabilise the salt hydrates, a polymer shell needs to form around the emulsion droplets. Chapter 3 details the use of poly(ethyl-2-cyanoacrylate) (PECA) for the formation of nanocapsules containing magnesium nitrate hexahydrate and sodium sulphate decahydrate. The salt hydrates showed vastly increased stability and thermal properties once encapsulated. Hydration level of the salts could be maintained by employing ultrasound to create miniemulsions, whilst supercooling was greatly reduced. The PECA nanocapsules were stable for at least 100 cycles, with results suggesting they would be stable for many more. This is in stark contrast to the bulk PCMs which were stable for less than 10 cycles. Developing several polymer shells for encapsulation is of benefit as capsules can then be tailored to suit different applications. We used polyurethane (PU), one of the most commonly used polymers in industry, which displays great versatility. Chapter 4 documents how micro- and nanocapsules can be synthesised with a PU shell. Initial results have been promising, with the materials stable for at least 10 cycles when encapsulated. PU also displays better thermal stability and chemical resistance compared to PECA. Our results demonstrate the potential of salt hydrates for use in thermal energy storage applications.
Supervisor: Shchukin, D. G. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral