Vinh Duy Cao

Time-dependent structural breakdown of microencapsulated phase change materials suspensions

Abstract Microencapsulated phase change materials (MPCM) suspensions are multi-phase heat transfer fluids which exploit the latent heat of phase change materials. The effect of MPCM on the rheological properties of suspensions of microcapsules in glycerol were investigated to explore the suitability of the suspensions as a pumpable heat transfer fluid. Three different rheological models were utilized to characterize the time-dependent structural breakdown of the suspensions, and the second-order structural kinetic model was found to give a better fit to the experimental data than the Weltman and Figoni-Shoemaker models. The MPCM form agglomerates, which are disrupted by shear forces. The breakdown of the agglomerated structures was most pronounced at high shear rates where the microcapsules are subjected to stronger disruptive forces. More agglomerates are present at higher concentrations, which causes a stronger breakdown of the agglomerated structures when the concentration is raised. The time-dependent structural breakdown of MPCM suspensions plays an important role for improving the efficiency of heat transfer liquids based on such materials.GRAPHICAL ABSTRACT

Rheological and thermal properties of suspensions of microcapsules containing phase change materials

The thermal and rheological properties of suspensions of microencapsulated phase change materials (MPCM) in glycerol were investigated. When the microcapsule concentration is raised, the heat storage capacity of the suspensions becomes higher and a slight decline in the thermal conductivity of the suspensions is observed. The temperature-dependent shear-thinning behaviour of the suspensions was found to be strongly affected by non-encapsulated phase change materials (PCM). Accordingly, the rheological properties of the MPCM suspensions could be described by the Cross model below the PCM melting point while a power law model best described the data above the PCM melting point. The MPCM suspensions are interesting for energy storage and heat transfer applications. However, the non-encapsulated PCM contributes to the agglomeration of the microcapsules, which can lead to higher pumping consumption and clogging of piping systems.

Salinity Gradient Energy from Expansion and Contraction of Poly (allylamine hydrochloride) Hydrogels

Salinity gradients exhibit a great potential for production of renewable energy. Several techniques such as pressure-retarded osmosis and reverse electrodialysis have been employed to extract this energy. Unfortunately, these techniques are restricted by the high costs of membranes and problems with membrane fouling. However, the expansion and contraction of hydrogels can be a new and cheaper way to harvest energy from salinity gradients since the hydrogels swell in freshwater and shrink in saltwater. We have examined the effect of cross-linker concentration and different external loads on the energy recovered for this type of energy-producing systems. Poly(allylamine hydrochloride) hydrogels were cross-linked with glutaraldehyde to produce hydrogels with excellent expansion and contraction properties. Increasing the cross-linker concentration markedly improved the energy that could be recovered from the hydrogels, especially at high external loads. A swollen hydrogel of 60 g could recover more than 1800 mJ when utilizing a high cross-linker concentration, and the maximum amount of energy produced per gram of polymer was 3.4 J/g. Although more energy is recovered at high cross-linking densities, the maximum amount of energy produced per gram of polymer is highest at an intermediate cross-linking concentration. Energy recovery was reduced when the salt concentration was increased for the low-concentration saline solution. The results illustrate that hydrogels are promising for salinity gradient energy recovery, and that optimizing the systems significantly increases the amount of energy that can be recovered.