Novel Working Fluids Based On Ionic Liquids For Direct Absorption Solar Collectors

Solar energy utilization is important in modern society since the solar energy is the most abundant,easily available,renewable resource and can be supplied without any environmental pollution.Among all ways of harnessing the limitless power of the sun,solar thermal utilization is the most common and convenient one.In solar thermal utilization systems,the whole efficiency is highly depended on the receiver efficiency of solar collectors.As compared to the indirect absorption solar collectors,the direct absorption solar collectors(DASCs)have lower thermal losses,hence higher receiver efficiency.In DASCs,the heat transfer fluids(HTFs)absorb the incident solar intensity and store as thermal energy,hence the receiver efficiency of DASCs is determined by HTFs.Therefore,developing a new HTF with high photo-thermal performance in DASCs is pivotal.Ionic liquids,composed of organic cations and organic or inorganic anions,have been demonstrated to have a wide range temperature of liquid,good chemical stability and low vapor pressure,which are of great potential to be used as high temperature HTFs.In this work,the ionic liquids based HTFs were prepared to be used in DASCs.First,the graphene/ionic liquid nanofluids were prepared.To improve the dispersion stability of the nanofluids,the graphene was modified with the reagents for using in the synthesis of [HMIM]BF4 by the same process,and the modified graphene was dispersed in [HMIM]BF4 to prepare modified graphene /[HMIM]BF4 nanofluids.To further improve the thermal storage capacity of ionic liquids,the ionic liquid based latent functional thermal fluids(LFTFs)were prepared.The graphene was dispersed into the ionic liquid([HMIM]BF4)to form graphene /[HMIM]BF4 nanofluids and the thermodynamic properties,radiative properties and photo-thermal performance of nanofluids were investigated.A combined analytical and experimental study has been conducted on high temperature DASCs using graphene /[HMIM]BF4 nanofluids as the HTFs.A one-dimensional transient heat transfer model has been used to predict the receiver temperature and efficiency with varying parameters such as solar concentration,graphene concentration and receiver height.The results show that the thermal conductivity of the nanofluids increases by 22.9% as compared with that of [HMIM]BF4.The viscosity of nanofluids dramatically decrease with the increasing temperature.The specific heat and density of the nanofluids exhibit a slight decrease as compared with those of [HMIM]BF4.The transmittance of the nanofluids decrease significantly in the visible region and the corresponding extinction coefficient increase by 1000-fold.The experimental temperature is in good agreement with numerical results under the same conditions.The receiver efficiency increases with the solar concentration and receiver height,while decreases with the graphene concentration.The receiver efficiency could be maintained 85% with the conditions(20 × 1000 W·m-2,0.0005% of graphene,receiver height 5 cm).The surface of graphene was modified by the reagents with chemical method,and the modified graphene was characterized and dispersed in [HMIM]BF4 to prepare modified graphene /[HMIM]BF4 nanofluids.The thermodynamic properties,radiative properties and photo-thermal performance of the modified graphene /[HMIM]BF4 nanofluids were also investigated.Then,a combined analytical and experimental study was conducted on the DASCs using modified graphene /[HMIM]BF4 nanofluids as the absorbers.Further,the transient model was used to predict the temperature of modified graphene nanofluids under high solar concentration.Finally,the modified graphene nanofluids based DASCs were optimized varying solar concentration,modified graphene concentration,nanofluid height and exposure time.The modified graphene concentration and nanofluid height were coupled to the optical thickness,and the optical thickness and the DASCs were optimized.The results show that the graphene was modified by organic groups with similar structure of [HMIM]BF4.The modified graphene /[HMIM]BF4 nanofluids have much better dispersion stability than that of the one containing the unmodified graphene,even after being heated.The experimental temperature of the modified graphene nanofluid is higher than the unmodified graphene nanofluid.The experimental temperature of the modified graphene nanofluid is in good agreement with numerical results,while has large deviation of the nanofluid containing the unmodified graphene.The modified graphene nanofluid shows enhanced receiver efficiency due to their excellent dispersion stability,implying that the photo-thermal conversion performance of nanofluids greatly depends on its dispersion stability at elevated temperatures.The receiver temperature of modified graphene /[HMIM]BF4 nanofluids increases with the solar concentration and modified graphene concentration,while decreases with the receiver height.The relative thermal storage capacity of modified graphene /[HMIM]BF4 nanofluids increases with the modified graphene concentration,while decreases with the solar concentration and receiver height.The optical thickness was optimized to 1.75 unther solar concentration C=10 and the relative thermal storage capacity was optimized to 292%.Based on the optimal optical thickness,the solar concentration and exposure time were further optimized,hence the whole DASCs were systematically optimized.This work shows that the modified graphene /[HMIM]BF4 nanofluids have great potential as a novel HTF in DASCs.The microencapsulated phase change composite,in which paraffin as the core and graphite nanoparticles embedded melamine–formaldehyde(MF)as the shell,was prepared in situ polymerization and characterized.The paraffin @MF/graphite composite was dispersed into the ionic liquid to form a novel LFTF.The thermodynamic properties and photo-thermal performance of the LFTF were systematically investigated.The results show that the paraffin @MF/graphite composite is composed of spherical particles with diameters ranging from hundreds of nanometers to several micrometers.The DSC results indicate that the melting temperature and latent heat of the paraffin @MF/graphite composite are 50.5 °C and 90.8 J·g-1,respectively,in which the mass ratio of paraffin is calculated to be 51.1%.It is found that the temperature of the LFTF with 10% of paraffin @MF/graphite composite can increase from 30 to 113 °C under irradiation,indicating its remarkable photo-thermal conversion performance.The thermal storage capacity of this new kind of HTF is 4800 J,while the thermal storage capacity of ionic liquid is 3000 J.The thermal storage capacity improvement of ionic liquid based LFTF is 60% as compared to ionic liquid.It is indicated that the ionic liquid based LFTF have great potential to be used as a new HTF in DASCs.