Physical Properties And Thermal Storage Performance Of The Composite Ca(NO₃)₂-NaNO₃/Expanded Graphite Form-Stable Phase Change Materials

Solar energy is considered as one of the most promising renewable energy sources since it is free and inexhaustible,and it has been extensively utilized through photovoltaic(PV)or solar thermal technologies.Recently,eutectic salts of nitrate are widely used as mid-to high-temperature thermal energy storage(TES)materials because of the wide temperature range,low causticity,high thermal stability and reasonable commercial price.Eutectic salts of nitrate have also been investigated as PCMs for applications.However,nitrate salts as PCM candidates usually suffer from low thermal conductivity and liquid phase leakage in solid-liquid phase transition.Therefore,the low-cost binary eutectic nitrate Ca(NO3)2-NaNO3(molar ratio of 3:7)with the melting temperature of 220? was chosen as the PCM and expanded graphite(EG)was as the structural supporting material and thermal conductivity enhancer.Firstly,a novel Ca(NO3)2-NaNO3/EG composite form-stable phase change material(FS-PCM)for mid-to high-temperature TES was successfully synthesized by the developed impregnation,cold-pressing and sintering method.After the Ca(NO3)2-NaNO3/EG composite FS-PCM was fabricated,its chemical compatibility and micro structure were characterized by X-ray diffraction(XRD),Brunauer-Emmet-Teller(BET)analysis and scanning electron microscopy(SEM).The thermal performance of the FS-PCMs was investigated using differential scanning calorimeter(DSC),thermal gravity analysis(TGA)and Hot Disk.The results showed that the binary eutectic nitrate and EG have good chemical compatibility,and the composite with 7 wt%EG can overcome the problems of cracks or liquid leakage during the phase change.Benefiting from the established better conductive passageway within the composites,the thermal conductivity of the composite with 7 wt%EG can be significantly increased by about 7.3 times in comparison with the binary eutectic nitrate.However,the measured phase change latent heats of the form FS-PCMs are markedly lower than the theoretical ones,meaning some losses of the latent heat during the material fabrication process.After the 500 cycles,the melting and solidifying temperature changed less than 1.5%,and the latent heat changed less than 2%,indicating it has good thermal reliability for a long-term period.Compared with other PCMs for mid-to high-temperature applications,this novel cost-effective composite FS-PCM is very promising due to the wide operating temperature range and high thermal conductivity.Then,three kinds of PCM samples involving different fabrication steps(impregnation,cold-compression and sintering)were fabricated and characterized,to investigate the effects of the different fabrication steps and molar ratios of the Ca(NO3)2-NaNO3 binary salts on the microstructure and some thermal properties of the composite FS-PCMs.The results show that the cold-compression and sintering led to a quite compact arrangement of EG regions separated by salt particles,which constrained the Ca(NO3)2 crystal arrangement and orientation of nitrate molecular chains within the mesopores of EG.As a result,two overlapped or convoluted DSC peaks,instead of one peak,and a wider melting temperature range appeared during the melting process of the composite FS-PCMs when the Ca(NO3)2 molar ratio was no less than that of the eutectic binary salt.The cold-compression and sintering also caused evident increases of the melting temperature range,reductions of the phase change enthalpy by 21-23%,and significant increases of the thermal conductivities of the FS-PCMs by over 2.5 times from 1.56-1.9 W·m-1·K-1 to 5.02-6.27 W·m-1·K-1.The cold-compression and sintering are suggested to be employed in the fabrications of the composite FS-PCMs to establish a better conductive passageway and surge the energy storage density.However,the composite FS-PCMs could have an anisotropic structure due to the intercalation structure of EG,which may lead to anisotropic thermal properties of the composite.Thus,the anisotropic thermal properties of the cuboid-like Ca(NO3)2-NaNO3/EG composite were investigated.The effects of bulk density and temperature on the anisotropic thermal conductivities were studied,and suitable models for prediction of the anisotropic thermal conductivities were analyzed and developed.The results showed that a layer-by-layer structure assembled by EG microsheets with the deposition of the salt particles was formed.When the compressing pressure was 4 MPa,the composite showed isotropic thermal performance,while the composites became more and more anisotropic with the increase of the compressing pressure.At room temperature,the axial thermal conductivity increased from 4.016 W·m-1·K-1 to 7.694 W·m-1·K-1 and the radial thermal conductivity increased from 4.040 W·m-1·K-1 to 5.645 W·m-1·K-1 when the compressing pressure was increased from 4 MPa to 20 MPa.While,the working temperature showed little effect on the thermal conductivities.Finally,it was shown that the Maxwell-Eucken model and the modified Parallel model proposed in this work can be used to predict the axial and radial thermalconductivities of the Ca(NO3)2-NaNO3/EG composites,respectively.Finally,based on a two-dimensional numerical model established in this paper,transient simulations of the melting processes of the binary eutectic nitrate in the double-pipe phase change unit were performed.Meawhile,the effect of different operating parameters on the the dynamic heat storage process was studied.Besids,the numerical simulation method was carried out to study the impact of the thermal properties of the different PCMs.The results showed that the double-pipe phase change unit with FS-PCMs is a good choice for its larger heat storage capacity and higher rate of heat transfer.