Preparation And Heat Storage Performance Of Na₂HPO₄·12H₂O/sepiolite Composite Phase Change Material

Phase change energy storage is a new energy conserving technology which can coordinate the imbalance between energy supply and demand in time and space and improve energy efficiency.The application of phase change material(PCM)with heat storage characteristics in building heating system can improve the indoor thermal comfort and transfer the electrical load to off-peak period,so as to achieve the purpose of"peak shifting and valley filling"of electric power.As one inorganic salt hydrate phase change materials,disodium hydrogen phosphate dodecahydrate(Na₂HPO₄·12H₂O)has the advantages of high latent heat and low cost.With the phase change temperature about 36?,it is suitable for building heating system and has a broad application prospect,but its application is limited due to serious undercooling and leakage problems.In this paper,nucleating agent and thickening agent were applied to avoid the supercooling phenomenon of Na₂HPO₄·12H₂O.Raw sepiolite was modified as the adsorption carrier to prepare form-stable composite phase change material(CPCM),which was integrated into the electric radiant heating system.The influence of different operating parameters on the thermal performance and energy efficiency of the test room was explored.Firstly,the effect of nanometer alumina(?-Al₂O₃)as the nucleating agent on phase change performance of Na₂HPO₄·12H₂O were discussed.At the same time,in order to reduce the agglomeration phenomenon of nanoparticles,the thickener sodium alginate was added to explore its effect on supercooling degree of Na₂HPO₄·12H₂O.The results indicated that adding 4%?-Al₂O₃ and1.5%sodium alginate can reduce the agglomeration phenomenon of nanoparticles.The supercooling degree of Na₂HPO₄·12H₂O decreases from 18.3?to 1.0?due to the crystallization process is accelerated,while the PCM owns a phase transition temperature of 35.1?and a melting enthalpy of209.5 J/g.Secondly,the raw sepiolite was modified with hydrochloric acid and calcination to remove its internal impurities,and the modified sepiolite with rich fibrous structure was obtained.Using the modified sepiolite as adsorption carrier,The Na₂HPO₄·12H₂O/sepiolite composite phase change material was fabricated by melt impregnation method.The thermal properties,supercooling degree of the CPCM with different PCM contents were investigated.The morphology,chemical composition and thermal stability of CPCM were characterized by SEM,FT-IR,XRD and thermogravimetric analysis.Besides,the thermal conductivity and cycling reliability of CPCM were tested.The results show that the maximum content of PCM without leakage is 60%.In this case,the supercooling degree of CPCM decrease to 1.7?,while the CPCM owns a phase transition temperature of 35.7?and melting enthalpy of 127.8 J/g.The test of thermal conductivity,thermal stability and cycling reliability suggested that the CPCM is suitable for building heating system.Finally,the Na₂HPO₄·12H₂O/sepiolite composite phase change material was pressed and encapsulated to make a PCM board,which was combined with electric radiant heating system and applied in a small test room.By evaluating the effects of the PCM board thickness and heating power on thermal performance of test room,it was found that the electricity consumption is decreased with the increase of the PCM thickness,while the frequency of thermal comfort(FTC)is increased.On the other hand,the electricity consumption and FTC are both increased first and then decreased as the heating power decreases.The electricity consumption is minimized when the PCM board thickness is10 mm and heating power set to 230 W/m².The experiment under the Peak-Valley time-of-use electricity price shows that the electricity consumption of the test room is increases by 5.2%compared with the reference room without PCM board due to the center temperature is maintained at a higher range,but the electricity fee is reduced by 16.5%compared with the reference room.In conclusion,the Na₂HPO₄·12H₂O/sepiolite composite phase change material with simple preparation method and low cost is suitable for electric heating and radiation heating system,and has broad application prospects in the field of building energy conservation.