Preparation And Properties Of Novel Heat Transfer And Thermal Energy Storage Materials

Energy storage technologies can effectively solve the mismatch in time and space between energy supply and demand,which is a useful means for improving energy utilization efficiency.Among these energy storage technologies,phase change thermal energy storage technology has a broad range of application prospects due to its high energy storage density.In the first chapter,thermal energy storage materials and systems are introduced.The second chapter presents challenges and research works in thermal energy storage field.The synthesis and properties of microencapsulated fatty acid eutectic phase change material are investigated in the third chapter.In the fourth chapter,the polymer-ceramic composites are prepared using solution-blending technique,which are used as backside encapsulation of photo-active cell in order to enhance heat dissipation in solar panels.In the fifth chapter in order to prevent leakage of phase change material over melting temperature,the phase change material(PCM)was chemically grafted to either itself or a polymer like poly(vinyl alcohol)using a cross-linking agent like di-phenylmethane di-isocyanate(MDI).The properties of PCM-graft-polymer material were tested and analyzed.In the last chapter,the conclusions of this thesis are summarized and future research projects of thermal energy storage field are pointed out.1.Synthesis and properties of microencapsulated phase change materialIn this work,silica shell material was used for encapsulation of phase change material(PCM)using sol-gel method.Eutectic mixture of myristic acid-palmitic acid was prepared as PCM.PCM was dispersed in the aqueous solution as micron sized droplets using emulsification technique.Oil-water emulsification was successfully carried out with the aid of surfactant SDS.Sol solution was prepared by hydrolysis of methyl triethoxysilane(MTES).Hydrolysis of methyl triethoxysilane(MTES)was carried out in a strongly acidic condition(pH value of the solution between 2 and 3)and a sol-solution with suspended solid particles of methyl silanol is formed.The sol solution is gradually mixed in droplets with PCM-water emulsion which is maintained at a pH of around 6.Methyl silanol molecules polymerize on the surface of PCM droplets through polycondensation reaction forming a micron sized capsule around PCM droplets.Polycondensation reaction of methyl silanol is triggered by sudden change in pH value.The microcapsules encapsulating PCM showed satisfactory thermo-physical properties and they showed good prospect for thermal energy storage.2.Preparation and properties of polymer/ceramic composite heat transfer enhancement materialsPolymer-ceramic composites were prepared using solution-blending technique.Using polymer-ceramic composites for backside encapsulation of photo-active cell,heat dissipation in solar panels can be enhanced.This helps bring down the temperature of the solar cell.Solar cell's electrical conversion efficiency is high at lower temperatures and therefore polymer-ceramic composite helps to improve the electrical yield.In PV/T panels,overall thermal efficiency can be improved.While polymer-ceramic composites have better thermal conductivity than original polymer,they still retain high electrical resistance like the original polymer.Thermal conductivity is measured using hot-wire method.Volume resistivity is measured using two-point method.Poly(vinyl butyral)was used as the polymer matrix.A comparison of relative performance of different nano-material fillers like boron nitride,zinc oxide and silicon carbide was done.The results indicated that boron nitride showed the best performance improvement among three ceramic nanomaterial fillers.Boron nitride also showed a very high electrical resistance exceeding even the poly(vinyl butyral)polymer's electrical resistance.3.Synthesis and properties of shape-stabilized phase change material based on polymerFor thermal energy storage,solid-liquid phase change is the most suitable phase change type.This is due to the fact that solid-liquid phase change latent heat is high compared to solid-solid phase change.Although liquid-gas phase change has higher latent heat than solid-liquid phase change,it has a problem of very large volume change during phase change process.However,the liquid phase of PCM creates a problem of leakage.This necessitates additional container or supporting matrix to hold the liquid.The PCM can be chemically grafted to either itself or a polymer like poly(vinyl alcohol)using a cross-linking agent like di-phenylmethane di-isocyanate(MDI).The isocyanate and hydroxyl functional groups form a urethane link.The modified copolymer molecules have both polymer as hard segment and PCM as soft segment.Therefore,although PCM segment undergoes a solid-liquid phase change near the phase change temperature of the PCM,the intermolecular forces between the hard polymer segments of adjacent molecules are still strong enough to hold them together.Instead of a solid-liquid phase change,a solid-solid phase change is observed.During the phase change process,the copolymer becomes soft and plastic,but it still remains in a solid phase.The latent heat of new solid-solid phase change material is good enough to be suitable for thermal energy storage purpose.