Study On Thermochemical Energy Storage Characteristics Of Direct Solar Radiation Absorbing Reactor

Efficient and effective solar energy conversion technologies are vital at this time as energy demand is increasing,as well as the need to reduce the use of fossil fuels and consequent carbon emissions.Pertinently,the development of high-temperature absorbing thermochemical energy storage metallic oxide materials and effective designs are essential to the creation of a solar energy storage reactor.Advanced power conversion units,such as gas turbines combined cycle power plants,are used as high-temperature steam or gas as the working medium.To heat the working fluid to a high temperature,an integrated energy storage reactor with Concentrated Solar Power(CSP)for power generation is needed.Several researches have been conducted on the traditional tubular or porous absorber and volumetric receivers which revealed poor thermal performance(less than 75%)and high thermal losses.Improving the design of the reactors using different inputs such as geometry parameter optimisation and adjustment of initial conditions are indispensable in the thermal analysis of thermochemical energy storage underpinning principles and mechanisms.A well-designed thermal energy storage reactor enhances the heat and mass transfer to the working fluid thereby lowering thermal losses and uniform temperature distribution inside the reactor.Thus,the direct solar radiation applied to energy storage and chemical conversion reactors with weakly nitrogen gas as both heat transfer and heat absorber causes the incoming solar radiation to be absorbed directly by the gas in the reactor and inner walls.The most difficult aspect in designing a high-temperature thermochemical energy storage reactor/receiver is maximising solar energy absorption and efficient heat and mass transfer in the heat storage medium,thanks to the effect of the working fluid,such as air,O₂,He,N₂,Ar,and other reactive gases.The cavity-type energy storage reactors are well-insulated enclosure designed to efficiently absorb incident solar radiation by allowing it to pass directly through an aperture.Thus,in this study,a combination of numerical simulation and experimental research was carried out.Various parameters were evaluated for thermal performance of energy storage reactors such as fluid inlet location along the frustum's edges,fluid flow rates and the direction of applied radiation intensity,which was applied either directly through the quartz glass or guided the incoming radiation intensity to the frustum's boundary,where it was reflected and concentrated about 0.043 m and the effect of the location of fluid inlets and outlets on the thermal performance of the proposed reactor was investigated.By taking the frustum base or the aperture as a reference,four fluid inlets along the edges of the frustum and two outlet locations at the base and side of the reactor were computed.Inlets were located at 4.81 cm from the base of the frustum and the outlet located at the side of the reactor was found to have better thermal performance with a short conveyer energy flow system.As fluid inlet locations approach the quartz glass,the heat flux and the velocity of the heat absorbed gas were found to be increased.However,as the heat absorbed gas passes the aperture,the velocity and flux were reduced and increased again to the gas outlet region.It was also deduced that radiation applied at the edges of the frustum had better thermal performance than that applied at the quartz edge.In the beginning,the gas flow rate was set at 0.36 l/h at an inlet temperature of 298.15 K.However,as the added gas flow rate increased to 3.6 l/h while keeping other conditions constant,it was found the outlet temperature being increasing by 400 K.Subsequently,the thermal efficiency of the energy storage reactor increased to 94%.Thus,increasing the laminar inflow rate from0.36 to 3.6 l/h increased the temperature distribution in the reactor.However,increasing the fluid flow rate beyond 3.6 l/h resulted in decreasing the efficiency as well as the average outlet temperature,and it was also observed that as more heat flux was absorbed in the reactor,and the surface average temperature increased.Furthermore,it was observed a high mass and heat transfer to the reactor such that maximum outlet average temperature and heat flux occurred in the system.From the results,it has been recognized that properly optimize input parameters during the design of the thermal energy storage reactor was the main important factor and had a significant role in the enhancement of mass and heat transfer to the energy storage reactor.The design elements considered during the construction of a horizontal fixed thermochemical reactor determine its thermal performance.This thesis investigated the effect of design elements such as boundary layer thickness,insulating materials for outlet tube design,and fluid inlet locations on the frustum on the thermal performance of a proposed chemical conversion reactor with incident radiation heat transfer through the quartz glass.The P1 approximation for the radiation model and fluid flow in the shallow path were integrated into a proposed radiation model.The result indicated that inlet mass flow rates from 5×10^-4 to 14×10^-4 kg/s increased the temperature in the cavity and the outlet.Insulating the reactor above the edge of quartz glass using 15 mm thick and 20 mm height material was very important for the prevention of radiation loss through quartz glass,and sedimentation of fluid-particles around the quartz glass edge,and the facilitation of fast heat transfer towards the internal part of the reactor.The thermal efficiency of this horizontal fixed thermochemical conversion reactor critically affected by outlet design as a result the outlet tube that was designed with aluminium oxide type with an insulator of 50 mm boundary layer thickness was found to increase the average outlet temperature of the reactor to 1200 K and efficiency>70%.The study also identified that the thickness of the wall making material of the reactor and its thermal conductivity were the most significant parameters to facilitate heat and mass transfer to the thermochemical energy conversion reactor and it was found that increasing the thermal conductivity of the wall-making material or using high thermal conductivity material reduced the heat flux,mass transfer and resulted in low outlet average temperature.On the other hand,increasing the thickness of the wall-making the material resulted in storing heat between the layers of the materials by inducing the heat transfer reduction,consequently,it affects the chemical conversion efficiency of the reactor.The sophisticated lab-scale measurement and analysis systems of thermogravimetric analysis(TGA),differential scanning calorimetry(DSC)and scanning electron microscopy(SEM)were developed to analyse the thermochemical energy storage characteristics of metallic oxide materials consisting of pure oxides,such as cobalt oxide(CoO),manganese oxide(MnO)and iron(II,III)oxide(Fe₃O₄)and their composite 25%MnO+75%CoO,75%MnO+25%CoO,and 50%MnO+50%CoO through a two-cycle redox reaction.To study the solar thermochemical reacting system for developing effective metallic oxides with high oxygen uptake ratio,parametric evaluation of energy storage,and a chemical energy flux conversion reactor were developed.Solar energy storage material-based Iron,cobalt oxide,and manganese metallic oxide were synthesized considering their weight proportional.The appropriate composition of the metallic oxides was determined for the development of a novel real energy storage material from the parent oxide materials.It was revealed that(50%MnO+50%CoO)and 100%Fe₃O₄ materials had the noteworthy oxygen uptake and oxygen release efficiency of 0.83 and 2.1,respectively in the first cycle,and had 0.99 and 0.996,respectively in the second cycle.However,the addition of 100%Fe₃O₄ to 50%MnO+50%CoO decreased the oxygen uptake of the material from 0.99 to 0.84 while increasing the cyclic stability of the material.The influence of gas flow rate in air and the oxygen flow during the thermal energy charging-discharging processes were investigated,which revealed that 30 ml/min of Ar and 20 ml/min of O₂ were found to be suitable for cyclic stability and increased the oxygen uptake ratio in the redox reaction.The thermochemical energy storage material composite of 50%MnO+50%CoO was found to be the most stable mixture during the redox reaction.The effect of heating rates of 10,20,and 30°C/min on the redox reaction was evaluated.It was deduced that30°C/min was the most appropriate heating rate for the redox reaction.Energy dispersion X-ray spectroscopy(EDS)and SEM tests have been performed on the mixed oxides of Fe₃O₄,CoO,and MnO and the result revealed that the EDS showed uniform elemental weight distribution before and after the reaction,while the SEM image results showed a morphology of smooth and hexagonal structures that indicates the stability and the durability of the mixed metallic oxides.The X-ray diffraction(XRD)result showed the same pattern and occasion as it has been observed in TGA for phase changes in the applied temperature.However,in the XRD result,the test showed the different components of(Mn,Co,Fe)Ox when it was exposed to250-1400 ^oC,particularly,these results were more sounded when the material was exposed to 600-1200 ^oC.All the test results indicated the noble(Mn,Co,Fe)Ox was the important material to enhance heat transfer and energy storage system as it has high oxygen uptake and oxygen-exchange capacity.A heat storage medium prepared by mixing 14 g of MnO+4 g of CoO+2 g of Fe₃O₄ doped with 45 g of Al₂O₃ has been manufactured in a thermochemistry laboratory analysis centre of Harbin Institute of Technology,China.Using three Xenon lamps that produce 4.5-7.5 k W power and 200 ml/min of Ar gas flow rates during heating,while a mixture of 100 ml/min of Ar and 100 ml/min of CO₂ during cooling,it was found that the heat storage medium had 80%thermal efficiency during charging.From the analysis,it was found the temperature gain and loss during charging-discharging for the heat storage medium was faster compared to the insulator.The insulator for the charging-discharging temperature-time curve was smooth compared to the heat storage medium.The oxygen release efficiency during discharging was higher and the energy release efficiency was found to be 81%.The proposed novel solar thermochemical energy storage reactor design and the synthesized novel metal oxide heat storage material will be beneficial in industries,research,and development sectors,as well as satisfy clean energy demand for society.From the analysis.Materials manufacture from Fe₃O₄,MnO and CoO were very important for energy storage efficiency.Yet,the underlying mechanism of mixing,proportional weight added to the mixture and the reactor that the material inserted for heat storage was the major factor for enhancement of heat and mass transfer in the reaction system.The research results of this thesis have important theoretical significance and practical application values for the solar thermochemical energy storage technology directly exploiting a solar energy source.The thesis also proposed a novel cavity-type solar receiver design and the engineered design of novel heat storage material-based metal oxide concerning industrial and R&D research development benefits,as well as satisfying clean energy demand for society.