Preparation And Photothermal Conversion Performance Of Magnetic Nanofluids For Direct Absorption Solar Collectors

Solar energy,as a clean and inexhaustible renewable energy,has been widely used in photovoltaics,photocatalysis,artificial photosynthesis and photothermal conversion.Among them,the solar thermal utilization is one of the effective and promising method.The key factor affecting its photothermal efficiency is the selection of collector and working fluids.Nowadays,direct absorption solar collectors with nanofluids are considered as the important alternative for improving the efficiency of solar radiation capture and widely studied.However,there are still plenty of shortcomings in the actual applications,which obstructs their large-scale application.The shortcomings can simply be divided into three sections:1)nanofluids are thermodynamically unstable systems due to their high surface energy and high specific surface area.Therefore,nanofluids can only be used as working mediums and cannot be used directly due to the presence of nanoparticles.The absorbed energy of nanofluids must be transferred to target fluids through the heat ex-changer in the practical application.There is inevitable heat loss produced by the heat ex-changer and cause some serious problems such as blockage of heat ex-changer in practical application;2)the temperature distribution of the receiver is extremely uneven when the concentration of nanofluids is high or the receiver is deep.This makes the temperature of upper layer much higher than that of the lower layer,resulting in much heat loss to the surrounding by convection;3)Notably,almost all the researches focus on the middle low temperature type（<80℃）,which has been widely used in domestic hot water.Nevertheless,the literature about the medium-temperature（80-120℃）is relatively little.This temperature type can be used in a wide variety of applications ranging from building heating,seawater desalination to some industrial hea,etc.This is also one of the most important factors to further restrict its large-scale applications.Based on the above description,the main content are summarized as follows:（1）An available magnetic photothermal nanofluids for direct application.In this study,the magnetic photothermal nanofluids not only can avoid the heat transfer process of heat ex-changer,but also can direct use the base liquid of nanofluids.The magnetic cobalt nanoparticles（NPs）embedded in nanoporous carbon（Co@NC）are prepared via carbonization of zeolitic imidazolate framework-67（ZIF-67）.The obtained Co@NC retains the morphology of original ZIF-67 and the magnetic Co nanoparticles distribute evenly in the nanoporous carbon.Co@NC has an excellent magnetic property and broad absorption spectrum in the visible and infrared region.The maximum photothermal conversion efficiency has been achieved by 100ppm Co@NC/EG nanofluids at carbonization temperature of 900℃,which is 99.6%at 60s.This magnetic photothermal nanofluilds can be reused at least 60successive cycles without significant loss of photothermal conversion efficiency.（2）Enhanced photothermal conversion properties of magnetic nanofluids through roting magnetic field for direct absorption solar collector.In this study,reduced graphene oxide（RGO）–EG nanofluids with variable RGO content are selected as examples to analyze the effects of concentration and receiver depth on the photothermal conversion performance of nanofluids.To overcome the shortcomings of high receiver height,a forced convection nanofluid absorption system is designed,where theα-Fe₂O₃magnetic nanoparticles are prepared and used as the nano-rotor.The power for rotation is provided by an external rotating magnetic field.The photothermal conversion efficiency of the binary nanofluids（α-Fe₂O₃–graphene oxide/ethanol glycol nanofluids）in the new system reaches 56.8%,which is 14.5%higher than non-external rotating magnetic field nanofluids for an RGO content of0.007 wt%.This enhancement is ascribed to two effects ofα-Fe₂O₃:improving the optical absorption capacity of binary nanofluids and acting as a nano-rotor.The optimum photothermal conversion efficiency is achieved at an RGO/α-Fe₂O₃ratio of 3 in the current system.（3）Medium and high temperature solar energy absorption and collection system based on magnetic nanofluids.We prepare the magnetic Fe₃O₄@C-oil nanofluids and use its excellent optical absorption properties to obtain high-quality energy under high power sunlight irradiation at external rotating magnetic field.Fe₃O₄@C nanoparticles can be separated from nanofluids by magnetic separation technology.The obtained pure base liquids with high temperature will flow to heat exchanger.In addition,the viscosity and photothermal conversion properties of Fe₃O₄@C-oil nanofluids are investigated in detail.This system not only reduces the flow resistance of fluid,but also effectively solves these problems such as blockage and corrosion in heat exchanger.Meanwhil,the maximum temperature can reach98℃under 3 solar irradiations.