Energy storage properties of mans nitro phase transition materials of multiwalled carbon nano tubes of greenhouse

It is significant to study low temperature phase change materials(PCM) widely used in hothouses.The performance of low temperature sodium sulfate decahydrate based PCM composite affects hothouse temperature directly.Sodium sulfate decahydrate based PCM composite has suitable phase change temperature, but presents supercooling and phase stratification phenomenon.These problems restrict its practical application.The result of related study indicated that the heat conductive material had a certain influence on supercooling and phase stratification of PCM.We studied that the effect of nano powders of sodium sulfate decahydrate on subcooling and phase stratification, of which the nano carbon had the better effects on sodium sulfate decahydrate′s supercooling and phase stratification.This paper explored the effect of multi walled carbon nano tubes (MWCNTs) with high thermal conductivity on 2 kinds of sodium sulfate decahydrate based PCM composites, named PCM A and PCM B.PCM A consisted of 90wt% Na2SO4·10H2O and 10wt% KCl.PCM B consisted of 86.4wt% Na2SO4·10H2O, 9.6wt% Na2CO3·10H2O, and 4wt% NaCl.Firstly, MWCNTs was modified on surface by acid treatment at 90, 120 and 140　℃ respectively, in order to obtain good compatibility with PCM A and B.Fourier transform infrared spectroscopy was used to detect the acid treated MWCNTs and the surface functional groups on the MWCNTs were analyzed.Low vacuum scanning electron microscope was used to observe the surface morphology of acid treatment MWCNTs.The acid treated MWCNTs were added into PCM A and B by physical dispersion method.The time dependence of the temperature of PCM A and B containing acid treated MWCNTs or not was recorded by date acquisition instrument to explore the effects of acid treatment temperature and mass fraction of MWCNTs on the degree of supercooling of PCM.The phase stratification phenomenon was photographed and investigated.The specific heat and thermal conductivity were measured by thermal constant analyzer.And the thermal conductivity and specific heat of thermally conductive material added into the PCM A and B were analyzed.The latent heat of PCM was analyzed by different scanning calorimetry measurements.Latent heat value directly affects the size of the heat storage capacity of the material.The results indicated that carboxyl group was formed in MWCNTs after acid treatment, which could improve the compatibility between MWCNTs and PCM A or B.By the scanning electron microscope, the surface topography change of the MWCNTs without oxidization and the MWCNTs with the surface modified by acid treatment at 90 ℃ was not obvious.And the MWCNTs with the surface modified by acid treatment at 120 ℃ had slight agglomeration.The MWCNTs with the surface modified by acid treatment at 140 ℃ had serious agglomeration and shear break.When 1wt% acid treated MWCNTs at 120 ℃ were added, both of PCM A and B showed low degree of supercooling.Their specific heat and thermal conductivity reached the maximum, which were 5.095 mm2/s, 0.932 5 w/mk and 4.235 6 mm2/s, 0.941 3 w/mk respectively in the vicinity of the phase transition temperature.Abrupt change of specific heat of the phase change material at this point showed that mutations occurred for latent heat material itself.The increase of thermal conductivity showed that the heat transfer effect of phase change energy storage material was enhanced in the process of heat absorption and heat transfer.Besides, PCM B containing 1wt% MWCNTs acid treated at 120 ℃ had higher latent heat than PCM A.They were 143.6 and 97.42 J/g respectively.These experiment results indicate that the PCM B containing 1wt% MWCNTs is more suitable for hothouse application.