Preparation,Properties And Application In Solar Water Heating System Of Light-to-thermal Conversion Phase Change Materials

Solar energy is the most promising renewable energy resource owing to its abundant reserves,widespread distribution as well as cleanness.However,solar radiation is unstable,intermittent and largely influenced by the climate and weather condition.As a result,the energy storage is critical to taking advantages of solar energy.Phase change thermal storage based on phase change materials?PCMs?,characterized by high thermal storage density and narrow temperature variation during thermal release and absorption,is considered a desirable solar energy storage technology.PCMs usually suffer from low thermal conductivity and liquid leakage during solid-liquid phase transition,which greatly restrict their wide application in energy storage.Most importantly,PCMs cannot directly store solar energy because of its weak visible light absorption.Aimed at overcoming above shortcomings related to PCMs and storing solar energy more efficiently,this paper focused on the development of functionalized PCMs with excellent thermal properties and a novel solar thermal storage device.Firstly,in order to improve the thermal conductivity of sodium acetate trihydrate?SAT?,a thermal conductivity enhanced SAT/xanthan/copper foam employing copper foam as heat conduction enhancer,xanthan as thickening agent was prepared.The phase change properties,thermal stability and thermal conductivity of the composite PCM were measured using a differential scanning calorimeter,a thermal gravimetric analyzer and a thermal constant analyzer.The mechanisms of increasing viscosity of SAT by xanthan and thermal conductivity by copper foam were analyzed.Experimental results illustrated that when the content of xanthan was 2 wt%,the SAT/xanthan composite did not show mobility,and its phase change temperature and latent heat were 58.8oC and 255.5 J·g^-1,respectively.A SAT/xanthan/copper foam composite PCM was then fabricated by immersing copper foam?20 PPI and 98%porosity?into the SAT/xanthan composite under a vacuum condition.The phase change latent heat of SAT/xanthan/copper foam composite was evaluated to be 197.2J·g^-1,and the thermal conductivity was measured to be 2.1 W·?m·K?^-1,2.76 times as high as that of SAT/xanthan.Secondly,for the purpose of endowing PCMs with the ability of direct solar energy storage,two light-to-thermal conversion PCMs were prepared.One was a form-stable PCM,composed of SAT?main PCM?,Na₂HPO₄?nucleating agent?,expanded graphite?EG,adsorbing material?and Cu S?light absorbing agent?.The influences of nucleating agent on the crystallization property of SAT and EG content on the leakage phenomenon of SAT were investigated.A simulated solar source was employed to irradiate sample to observe the temperature variation and calculate the light-to-thermal conversion efficiency.The function of Cu S in light-to-thermal conversion was also analyzed.Experimental results showed that the supercooling temperature of pure SAT was very large,and 0.5 wt%Na₂HPO₄ can make it decrease to lower than 4oC.There was no SAT leakage for SAT/EG composite when the mass fraction of EG increased to 19%.The form-stable light-to-thermal conversion PCM containing 72.5%SAT,0.4 wt%Na₂HPO₄,17.1 wt%EG and 10 wt%Cu S gave a phase change temperature of 56.9oC,a latent heat of 194.8 J·g^-1 and a high light-to-thermal conversion efficiency up to 94.1%,much higher than that of PCM without Cu S?66.9%?.Another light-to-thermal conversion PCM was a hydrogel.Though form-stable PCM can mitigate leakage of SAT,it also faces with strong rigidity,brittle failure and poor surface contact with controlled devices.Therefore,a light-to-thermal phase change hydrogel?LTPCH?was prepared,in which SAT was used as the main PCM,acrylamide-sodium acrylate copolymer?80A-51?as gel material and Cu S as light absorbing agent.The prepared LTPCH not only was free from leakage during solid-liquid phase change,but possessed shape variation ability.That is,it can change its form according to the shape of the container.When the mass fraction of SAT,Cu S and 80A-51 were 87%,4.35%and 8.65%,the LTPCH gave a melting latent heat of 202.4 J·g^-1,and a light-to-thermal conversion efficiency up to 87.1%.Light-to-thermal conversion PCMs not only possesses thermal storage ability,but can directly transfer solar radiation into thermal energy.Thirdly,in order to investigate the effect of graphene oxide?GO?on the light-to-thermal conversion performance,SAT/EG and SAT/GO/EG were also prepared.Test results illustrated that trace of GO cannot exert obvious influence on the light-to-thermal conversion performance,but increased the adsorption capacity of SAT in EG.A novel solar water heating system was subsequently designed.In the novel system,there was no special solar collector,because the SAT/GO/EG had double functions:thermal collection and thermal storage.The system performance was evaluated under a real environment,and the calculated system efficiency was equal to 54.5%.Finally,a pentaglycerine?PG?-based electro/light driven solid-solid PCM?SSPCM?was prepared for the sake of avoiding evaporation of crystal water derived from hydrated salt.The solid-solid phase change temperature and latent heat of the SSPCM consist of 95 wt%PG,3wt%graphite paper and 2 wt%carbon nanotube were 80.5oC and 145.6 J·g^-1.Driven by irradiating with sunlight of 1 Sun or applying a low voltage of 10 V,such SSPCM can undergo solid-solid phase change with the absence of any liquid.Theoretically,the prepared SSPCM can be directly used without complex encapsulation process.