Study On Organic Phase Change Energy-storage Materials For Air Energy Water Heaters

Phase change energy-storage materials are promising to be applied for air energy water heaters,due to their characteristics of high energy-storage density and low residual heat.Compared with traditional air energy water heaters,the new-type ones with phase change energy-storage materials involved show two thirds smaller size,while their overall efficiency can be increased by 20%.Therefore,it is of important significance to develop phase change materials for air energy water heaters.In the thesis,the material system possibly used as phase change energy storage materials for air energy water heaters was selected by physically combining various organic compounds.It is required to not only possess stable thermal cycling performance,but also show 46-50°C of phase transition temperature,and 200 J/g of phase change latent heat.The appropriate nucleating agent was added to the selected material system in order to alleviate the supercooling phenomenon of the phase change material,and the expanded graphite with high thermal conductivity was added to improve its thermal conductivity.The phase compositions and micro structures of the prepared composite materials were characterized by the technologies of XRD,SEM and FT-IR,and their thermal properties were analyzed by the means of differential scanning calorimetry?DSC?and thermal conductivity measurement.The main research contents and results are as follows:?1?Two material systems,composed of ethylene glycol monostearate and myristic acid with mass ratio of 2:8 and 3:7,were obtained by physically combining alcohols,acids,esters and alkanes,etc.The XRD and FT-IR results showed that the obtained material systems were the mechanical mixture of ethylene glycol monostearate and myristic acid.The DSC results showed that the latent heat of fusion and latent heat of solidification of the3:7 composite system were 194.30 J/g and 203.92 J/g,respectively.Its melting temperature and solidification temperature were 48.6°C and 42.0°C,respectively.The latent heat of fusion and the latent heat of solidification of the 2:8 composite system were 229.72 J/g and225.48 J/g,and its melting temperature and solidification temperature were 49.3°C and42.5°C,respectively.Based on the results of melting-solidification cycle test,it can be seen that the phase change latent heat and phase transition temperature of 3:7 composite system were not changed significantly after 1000 melting-solidification cycles.Therefore,the composite system of myristic acid and ethylene glycol monostearic exhibited excellent thermal stability.?2?The 2:8 and 3:7 composite systems have higher supercooling degree of 6.8°C and6.6°C,respectively.High supercooling degree was unfavorable for the practical application of phase change materials.In this thesis,SiO₂ modified with silane coupling agent was chosen as the nucleating agent.Finally,0.2 wt.%micron-sized SiO₂ modified by KH-602?N-?-aminoethyl-?-aminopropyl methyl dimethoxysilane?reduced the supercooling degree of 3:7 composite system from 6.6°C to 3.1°C,and decreased the supercooling degree of 2:8 composite system from 6.8°C to 2.8°C.The composite system of ethylene glycol monostearate and myristic acid with mass ratio of 2:8+0.2 wt.%SiO₂was used as the subsequent optimized material system.?3?Expanded graphite with high thermal conductivity was added to the composite system of ethylene glycol monostearate/myristic acid?2:8?+0.2 wt.%SiO₂,and the effects of the added expanded graphite was investigated on its thermal conductivity and cycle stability.The results showed that its thermal conductivity increased sharply from 0.2667W/m·K to 2.0014-3.6104 W/m·K for the composite system added 5-8 wt.%expanded graphite 80EG200,and from 0.2667 W/m·K to 1.7672-2.6589 W/m·K for the one added5-8 wt.%expanded graphite 80EG300,respectively.Their phase transition temperature was not changed a lot,while the phase change latent heat decreased obviously;when the two composite systems underwent 1000 melting-solidification cycles,their thermal conductivity decreased remarkably compared with the pre-cycle,decreasing to1.3711-1.8989 W/m·K and 1.1791-1.5128 W/m·K,respectively.However,the phase transition temperature of the two composite systems was not apparently changed,and the latent heat was changed by less than 6%.When the two systems experienced another 1000melting-solidification cycles,their thermal conductivity decreased less evidently,compared to the previous 1000 cycles,and their thermal conductivity was decreased to1.3000-1.5624 W/m·K and 1.0098-1.3215 W/m·K,respectively.Moreover,the phase transition temperature was not changed obviously,and the change of latent heat was also within 6%.The thermal conductivity of the composite system of ethylene glycol monostearate/myristic acid?2:8?+0.2 wt.%SiO₂ could be increased to 1.5087-3.0381W/m·K and 1.9772-2.4159 W/m·K by adding 4-7 wt.%expanded graphite 50EG300 and4-6 wt.%expanded graphite 50EG600.Their phase transition temperature was not changed much,while the phase change latent heat decreased.But the change was obviously smaller than that of adding 80 mesh expanded graphite.After 1000 melting-solidification cycles,the thermal conductivity of two composite systems obviously was decreased to1.2452-1.6269 W/m·K and 1.4250-1.6853 W/m·K,respectively.The phase change latent heat change was less than 5%and 3%,respectively.When the two composite systems underwent 2000 melting-solidification cycles,their thermal conductivity was decreased obviously smaller than the 1000 melting-solidification cycles,falling to 1.1923-1.5694W/m·K and 1.2735-1.6029 W/m·K,respectively.The phase transition temperature was not changed much,and the phase change latent heat change was less than 9%and 4%,respectively.Therefore,the phase change latent heat of 4 wt.%50EG600 composite system is higher than 210 J/g,and the phase transition temperature was also between 47°C and48°C,the supercooling degree is less than 1°C;After 2000 cycles,there was no obvious change of the phase transition temperature and phase changelatent heat,and the thermal conductivity was still higher than 1 W/m·K.The results showed that the composite system of ethylene glycol monostearate/myristic acid?2:8?+0.2 wt.%SiO₂+4 wt.%50EG600could be used as the optimal system which meet the requirements of air energy water heaters for phase change energy storage materials.