| Due to the intermittency and instability of solar power generation,as well as the imbalance between energy supply and demand,the application of energy storage system becomes necessary.Thermal energy storage is one of the main energy storage methods,and can be roughly divided into three categories,namely sensible heat storage,latent heat storage and thermochemical energy storage.Compared with the former two,the thermochemical energy storage system has the advantages of high energy storage density,long distance and long-time storage,and almost no heat loss in the storage process,and so on.According to the different reaction temperature,the energy storage system can be divided into low temperature,medium temperature and high temperature systems.The thermochemical energy storage system with calcium oxide as the carrier(including Ca(OH)2/CaO/H2O and CaCO3/CaO/CO2 systems)is a typical medium and high temperature system,which has a wide application prospect in the fields of solar photovoltaic power generation,the recovery and utilization of the industrial waste heat.Some key problems existing in these two energy storage systems at the present stage,and obtains some important conclusions with theoretical guiding significance have been studied in this dissertation.The main research conclusions are as follows:In this paper,the thermochemical energy storage performance of calcium hydroxide with different structures was studied,and the reasons for the decline of the cycling stability of the materials were analyzed,and the structure-activity relationship between the energy storage characteristics of the materials and their structures was obtained.Firstly,calcium hydroxide nanomaterials with spindle-shaped and hexagon flake-like structure were synthesized by hydrothermal synthesis in constant temperature water bath.Then,combined with three kinds of commercial calcium hydroxide materials,the thermochemical energy storage performance of five kinds of calcium hydroxide materials with different structures was evaluated.The results show that the self-made spindle-shaped structure of calcium hydroxide showed excellent thermal chemical energy storage density and cyclic stability,the first three cycles of energy storage density is still up to?1300 kJ/kgCa(OH)2,the conversion of hydration reaction still can reach above 70%after ten cycles(corresponding to the energy storage density of 987 kJ/kgCa(OH)2),which is higher than that of other structures.The results of specific surface area test(BET)showed that the produced spindle-shaped calcium hydroxide had the highest specific surface area and pore volume among the five materials,which was much higher than that of the flake-like sample.Due to the tight adhesion between particles,the material with flake structure will lead to small pores between particles and increase the gas diffusion resistance.On the other hand,such a stacked structure with small particle spacing is more likely to agglomerate and sinter at high temperature,thus reducing the conversion ratio of dehydration/hydration reaction and resulting in reduced cycling stability.In view of the obvious sintering and agglomeration of calcium-based energy storage materials with the increase of cycling number,thus reducing their energy storage performance,hexagonal boron nitride doped(HBN)calcium hydroxide was synsiezed by ultrasonic and mechanical agitation.Thermogravimetric analysis,differential scanning calorimetry,thermal constant analyzer and scanning electron microscope were employed to characterize the thermodynamic,kinetic and cyclic stability of HBN-doped composites and pure calcium hydroxide.The results show that the addition of HBN increases the thermal conductivity of calcium hydroxide and the enthalpy of dehydration reaction of HBN-doped composite is higher than that of pure calcium hydroxide.The thermochemical energy storage performance of the samples with mass doping content of 5 wt%,10 wt%,15 wt%and 20 wt%was studied by the thermogravimetric analyzer.It was found that when the doping content was 15 wt%,the samples had higher conversion rate and better reaction kinetics performance.Based on Coats-Redfern(C-R)integral method and ArchAR-Brindly sharp-Wendworth(ABSW)differential method,the kinetic equation of dehydration reaction between pure calcium hydroxide and the best mass doped 15 wt%HBN doped composite was deduced.The error between the theoretical value and the experimental value is small,which indicates that the reaction dynamics equation derived in this paper has a certain guiding value for the future experimental design.The cyclic stability test results show that HBN-doped composite has better cyclic stability than the pure compound.After 10 dehydration/hydration cycles,the hydration conversion rate of the 15 wt%HBN doped composite was still maintained at 67%,and the corresponding energy storage density was 945 kJ/kgCa(OH)2,which had obvious advantages compared with pure calcium hydroxide(807 kJ/kgCa(OH)2).Aiming at CaCO3/CaO/CO2 high temperature thermochemical energy storage system,this paper further designs and renovates the spindle-shaped calcium hydroxide with structural advantages,and obtains a calcium oxide-based energy storage nanomaterial with hollow structure.The material has the advantages of low carbon,no long time high temperature hydrothermal synthesis and no template.At first,under the condition of stirring at room temperature,Tetraethyl orthosilicate releases silica in alkaline environment,and silica forms a coating layer on the surface of spindle body calcium hydroxide particles.The obtained sample Ca(OH)2@SiO2 has obvious core-shell structure.Then the hollow material CaO@Ca2SiO4 is calcined at a high temperature.BET test results show that the specific surface area and pore volume of the calcined sample greatly increase,which can not only increase the gas-solid reaction contact area,but also greatly reduce the diffusion resistance of the gas in the chemical reaction.Through thermogravimetric analysis,it is found that the coated materials show good thermochemical energy storage reaction characteristics and have great potential in thermochemical energy storage.By studying the carbon dioxide energy storage characteristics of the samples with different coating ratios,it was found that the rate of the coated samples was greatly improved compared with that of pure calcium oxide,and the reaction was mainly controlled by the reaction kinetics,and hardly affected by the diffusion resistance.Comparing the performance of carbon dioxide energy storage with different cladding ratios,it was found that when the cladding ratio was 10%mol ratio,the thermochemical energy storage and cycling stability were the best,and the thermochemical energy storage density remained?2334 kJ/kgcomposite after 20 cycles.In addition,calcium carbonate and calcium hydroxide were sprayed with aluminum oxide by atomic layer deposition in order to improve their thermochemical energy storage cycle stability.After the deposition of alumina,the cyclic stability of the samples was indeed improved.By comparing different deposition times,it was found that when the deposition times were 20,both calcium carbonate and calcium hydroxide showed better cyclic stability.By comparing calcium carbonate with calcium hydroxide materials,it was found that the energy storage density of calcium hydroxide and calcium carbonate remained at 1850 kJ/kgcomposite and 1005 kJ/kgcomposite after 15 cycles,respectively.In addition to the design and optimization of calcium-based thermochemical energy storage materials,numerical simulation of a fixed bed reactor with calcium hydroxide/calcium oxide thermochemical energy storage system was carried out in this work.The influence of the heat and mass transfer in the reactor on the reaction performance was analyzed in detail,and the entropy production of the reaction process was analyzed by using the second law of thermodynamics.The results of entropy production caused by heat transfer,mass transfer and chemical reaction in dehydration and hydration show that the entropy increase caused by heat transfer plays an important role in both dehydration and hydration,so improving the thermal conductivity of the bed can significantly improve the reaction performance.By studying the influence of porosity,wall temperature and reaction kinetics on the reactor performance,the results showed that the lower porosity and the higher reaction rate had a significant promoting effect on the dehydration reaction,while the increase of boundary heating temperature also had a significant positive effect on the dehydration reaction.The change of pre-exponential factor has little effect on the increase of reaction entropy and mass transfer entropy.The results can provide reference for the design and selection of thermochemical energy storage reactor. |
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