Stable paraffin composites for latent heat thermal storage systems
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Phase change materials (PCMs) have the ability to store thermal energy as latent heat over a nearly isothermal temperature range. Compared to sensible heat storage, properly chosen PCMs can store an order of magnitude more energy when undergoing phase change. Organic PCMs present several advantages including their non-corrosive behavior and ability to melt congruently, which result in safe and reliable performance. Because of these qualities, organic PCMs have been proposed for use in latent heat thermal storage systems to increase the energy efficiency or performance of various systems such as cooling and heating in buildings, hot water heating, electronics cooling, and thermal comfort in vehicles. Current performance is hindered by the low thermal conductivity, which significantly limits the rate of charging and discharging. Solutions to this challenge include the insertion of high conductivity nanoparticles and foams to increase thermal transport. However, performance validation remains tied to thermal conductivity and latent heat measurements, instead of more practical metrics of thermal charging performance, stability of the composite, and energy storage cost. This thesis focuses on the use of graphite nanoplatelets and graphite foams to increase the thermal charging performance of organic PCMs. Stability of graphite nanoplatelets in liquid PCM is realized for the first time through the use of dispersants and control of the viscosity, particle distribution, and oxidation. Thermal charging response of stable graphite nanoplatelet composites is compared to graphite foam composites. This study includes a correlation of thermal conductivity and latent heat to material concentration, geometry, and energy storage cost. Additionally, a hybrid PCM storage system of metal foam combined with graphite nanoplatelet PCM is proposed and evaluated under cyclic thermal conditions.