Preparation And Building Energy Conservation Application Of Form-Stable Phase Change Material

Using the latent heat of phase change material (PCM) for decreasing the indoor temperature fluctuations,recovering the waste heat, air-conditioning and ventilation and storing solar energy, etc.is considered as one of the most effective and attractive methods for building energy conservation. In this paper, a series of form-stable PCMs are prepared by encapsulation of some fatty acids and their binary eutectic mixtures as the thermal energy storage PCM into organic polymer and inorganic nano-particals which act as supporting material, respectively. The graphite powders are chosen as the thermal conductivity modifier to enhance the heat transfer performance of fatty acid/polymer composite PCM.The building energy conservation effect of the form-stable PCMs is predicted by both the mumerical and experimental simulation methods.By the method of bulk polymerization, the form-stable PCMs composed of different fatty acid as core and polymethl methacrylate (PMMA) as supporting matrix are prepared. The structure, phase change behavior, characterization and service performance of the composites are investigated by Fourier-transform infrared, polarizing optical microscopy, differential scanning calorimetry, thermogravimetric and mechanical properties test, respectively. The results show that fatty acid has been enwrapped by PMMA so as to be immobilized even heated above its melting point. These two components are compounded physically and exhibit good compatibility with each other. Phase change temperature of the prepared composite PCMs ranges from 20.26℃to 53.78℃and the enthalpy from 59.29 J/g to 86.35 J/g, which are equivalent with the thermal properties of the corresponding fatty acid. In virture of the PMMA, thermal degradation temperature range of the form-stable PCMs obviously increases about 30℃.It is indicated that PMMA effectively protects the thermal properties of the fatty acid. Meanwhile, the phase change temrperature and enthalpy of the form-stable PCMs change slightly after subjected to repeated accelerated thermal cycling tests. Compared with pure fatty acid, the form-stable PCMs have samller volume expansibility and higher thermal conductivity, nearly 0.27W·m-1·K-1.The mechanical properties at R.T. and heat storage/release performance of the form-stable PCMs are feasible for thermal energy usage.A series of LA-MA/PMMA composite PCMs containing varied weitht percentage of LA-MA eutectic are taken as an example to investigate its optimal composition ratio.The latent heat of the form-stable PCM increases with the increase of the LA-MA, while the phase change temperature keeps near the melting point of LA-MA eutectic.The volume expansibility and mechanical properties decrease with increasing the addition of LA-MA. The mass fraction of fatty acid in the composite PCM is suggested to be no more than 70%.For improving the thermal conductivity of the fatty acid/PMMA composite PCM, the graphite powder which is modified by titanate coupling agent is blended into the composite. The modified graphite can dispered well in the form-stable PCM.The chemical characterization of fatty acid/PMMA composite is not changed and the three components in the composite have good chemical compatibility. Phase change temperature of the composite PCMs increases a little resulting from the adsorption of fatty acid molecular by the graphite particle pores. The densities of the composites are relitivily low when the mass fraction of graphite is less than 5% due to the pores inside the composite and increase with inceasing the graphite content. With the increase of graphite, the thermal expansibility and compressive strength increase, while the flexural strength slightly inceases firstly and then decreases gradually.Thermal conductivity of the composite PCM increases slightly with increasing the mass fraction of graphite from 1% to 5%.When the addition of the graphite is more than 10wt%, thermal conductivity of the composite increases significantly. The thermal conductivity reaches to 0.509W·m-1·K-1 with the graphfite mass fraction of 15%, being 195.9% to the fatty acid/PMMA form-stable PCM.Thermal conductivities of the graphite/form-stable PCMs are predicted based on the four classic theoretical models.Owing to the absorption of fatty acid by graphite, the volume percentage of the graphite in the composite remains an error, the thermal conductivity of the composite can not be precisely described by the models. Nidsen-Lewis model is relatively well concident with the measured values.Water glass with the advantages of low price and easy accessibility as nano-silica source is coumponded with CA-LA eutectic to be the form-stable PCM by using the sol-gel method in the weak acid reaction environment. The optimal reaction condition for the composite preparation is determined to be silicate:CA-LA:silane coupling:EtOH (wt)=1:1:0.1:0.075 and the pH value is 4.8.The encapsulation weight of CA-LA in the prepared composite PCM is 46 wt%.Phase change temperature and enthalpy of the composite is 20℃and 71.02 J/g, respectively.TGA and termal cycling test results show that the CA-LA/nano-SiO2 composite PCM has no weight lose below 100℃and good thermal stability. Thermal conductiviy of the composite PCM is measured to be 0.178 W·m-1·K-1 which is coincided with the thermal insulation cretior in bulidng energy conservation.Phase change thermal storage coefficient is formulated to evaluate the service efficiency, and the calculation formula is derived base on the heat transfer theory to beφ=2λΔT/ρL2.The form-stable PCM plates are supposed to be applied in the floor heating system and a simplified heat transfer mathematical model is settled to describe the heat transfer process in the room.The building thermal energy conservation effects using the form-stable PCM floor heating system in varied areas are simulated and the results show that the system can adjust the room temperature to the range satisfying with the living comfortability. Moreover, the influences of enthalpy, thickness and phase change temperature range of the form-stable PCM plate are predicted through mumerical simulation. By coordinating themo-physical properties, constitute mode and usage method of the plate, the building thermal energy conservation effect would be improved.