Research On Light-to-Thermal-to-Electric System Based On Shaped Composite Phase Change Materials With Light-to-Thermal Conversion And Its Properties

Solar energy possessing characteristic of abundant reserves and wide distribution is one of clean and sustainable energies.However,due to the limits of region,climate and rotation of the earth,solar energy has the shortcomings of the intermittent and unstable supply,which reduces the utilization efficiency of solar energy.With phase change material(PCM)as the core,latent heat energy storage,which has the characteristics of high energy storage density,stable and narrow operating temperature,is a promising technology for utilizing solar energy efficiently.However,PCMs have disadvantages of low thermal conductivity and leakage during phase change,weakness of light absorption,and lack of light-to-thermal conversion property,which limit the application of PCMs in solar energy storage.The shaped composite PCMs of paraffin/grapheme aerogel(GA)/copper foam with ability of light-to-thermal conversion were prepared.The copper foam offers the heat transfer channels to enhance the thermal conductivity of paraffin.As a porous material with ability of light-to-thermal conversion,GA can effectively prevent the leakage of paraffin during phase transition,and plays the role of photon trapping agent and light-to-thermal conversion agent to endow paraffin with light-to-thermal conversion function.The results show that when the mass fraction of paraffin is 65 wt%,the composite PCMs has no leakage when heated.The GA and copper foam have no effect on the structure and crystal form of the paraffin.The thermal conductivity of the composite PCMs can reach 3.34 W/(m·K),which is more than 10times the thermal conductivity of paraffin.The light-to-thermal conversion efficiencies of the composite PCMs were calculated by recording the temperature-time curves of the composite PCMs under the illumination of the simulated solar light source.The results showed that under the light intensity of 0.1 W/cm~2 for 150 min,the paraffin with phase change temperature of 60℃did not complete the phase change,while the composite PCMs completed the phase change and the efficiency of light-to-thermal conversion was as high as97%.Considering the economic cost and the outdoor working environment of the solar energy storage system,a novel composite PCM for light-to-thermal conversion was prepared.The novel PCM is composed of paraffin as PCM,styrene-ethylene-butylene-styrene block copolymer(SEBS)and hydrogenated epoxy resin(EP)with strong oxygen resistance and light resistance as supporting materials.Meanwhile,nano titanium nitride(Ti N)was introduced as photon harvester and light-to-thermal conversion agent.Results of differential scanning calorimeter test showed that when the mass fractions of paraffin,SEBS and EP are75 wt%,24 wt%,and 1 wt%,respectively,the phase transition temperature of the composite PCM is 62.7°C,and the latent heat of melting is 148.4 J/g.The leakage test indicated that there was no leakage of paraffin during the process of phase change.Experimental resules showed that nano-Ti N can effectively improve the light-to-thermal conversion performance of composite PCM.When the mass fraction of nano-Ti N is 0.1 wt%,the light-to-thermal conversion efficiency of composite PCM is about 80.7%.Multiple cycles of heating did not have much effect on the phase transition performance and light-to-thermal conversion performance of the composite PCM.The study also found that the increase the mass fraction of EP can inhibit the thermal expansion phenomenon of composite PCM.A novel heat collector combined of double-layer composite PCMs was designed,which can realize the integration of light-to-thermal conversion and energy storage,so that the system of light-to-thermal-to-electric for thermoelectric power generation(TEG)driven by solar energy can be greatly simplify.The experimental results show that the heat collector composed of PA/SEBS-Ti N composite PCMs with low thermal conductivity in the upper layer and PA/EG composite PCMs with high thermal conductivity in the bottom layer has an energy storage efficiency of 28.5%after 120 minutes of irradiation under the light intensity of0.1 W/cm~2(Thermal energy stored includes latent heat and sensible heat).It means that the novel heat collector can not only provide stable heat energy for the hot side of TEG under illumination,but also store the energy in the form of latent heat.After the illumination,the latent heat stored in the composite PCMs can still provide thermal energy for TEG to continuously generate electricity.It is also found that the collector can maintain the temperature difference of TEG within a certain range and stabilize the output voltage and current of TEG.The effect of the density and thickness ratio of the composite PCMSs on the energy storage efficiency of the novel collector was analyzed by numerical simulation.The smaller the density of the composite PCMs,the greater the thickness of the composite PCMs with low thermal conductivity which providing the light absorption surface,and the faster the energy storage rate of the heater collector.