Abstract
A new synthesis method is developed for preparing form-stable expanded graphite (EG)/stearic acid (SA) composite phase change material (CPCM) by using “impregnation of liquid PCM into EG matrix and then compressing into stable-shape block”. The effects of EG content and packed density on the thermal-physical property and thermal stability of form-stable EG/SA CPCM are firstly analyzed and its optimum parameters are investigated, and then an advanced form-stable modular thermal energy storage unit is manufactured and its thermal performance is evaluated. The SEM analysis shows that SA is well impregnated into graphite flakes and a regular laminar structure gradually forms with increasing packed density. The DSC analysis reveals that the CPCM nearly has no supercooling problem and the addition of EG and compressing operation have negligible effect on the phase change temperature and latent heat of SA. The form-stable CPCM has obvious anisotropic thermal conductivity and the largest difference between axial and radial thermal conductivity reaches up to 4 times. Both axial and radial thermal conductivities can be enhanced significantly by using the additive of EG, and the radial thermal conductivity is as high as 23.27W/mK. The TGA analysis indicates that the form-stable CPCM has good thermal stability within a wide range of temperature and those samples with low porosity are susceptible to liquid leakage. Experimental results show the form-stable EG/SA CPCM exhibits excellent overall thermal performance by employing the optimum parameters of 25wt.% EG content and packed density of 900kg/m3. An advanced design method for modular energy storage unit is proposed by integrating copper tubes into the form-stable CPCM, and it can be easily used to achieve different energy storage capacities by assembling different numbers of modular thermal energy storage units. It appears that the proposed new method is very effective to synthesis high-performance form-stable graphite-based CPCM in comparison with the conventional methods, and the thermal conductivity can be enhanced about 130 times higher than that of pure PCM.
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