| The International Maritime Organization(IMO)pointed out in the Fourth Greenhouse Gas Study(GHG4)that the emission from shipping industries has amounted to about one third of that of the global greenhouse gases each year,implementation of"green ship"and"green shipping"is therefore the necessities at this time.The application of hydrogen energy on ships has been widely concerned,and great attention has already been paid on the research and development for efficient hydrogen storage methods for marine purpose.Recently,due to the advantages in high structural diversity,light weight,high specific surface area as well as adjustable pore size,metal organic frameworks(MOFs)has been being considered as promising medium for hydrogen storage.However,the results from the relevant researches carried out so far show that considerable amount of hydrogen on MOFs can only be obtained at such a low temperature as that of liquid nitrogen,and significant progresses should be made in terms of tackling such problems as high cost,safety and reliability of the storage system.It also notes that the development of the hydrogen storage system for practical applications calls for the highly efficient hydrogen storage mediums and their suitable thermal management measures.Based on the results from our previous studies,the following studies were to be conducted typically for evaluating the measures for heat conducting enhancement of the storage vessel packed with typical kinds of MOFs.Preparation and characterization of MIL-101(Cr)and its composite adsorbent(here denotes as MEI-01,formed by mixing the ENG with the MIL-101(Cr)in mass ratio 0.1:0.9).MIL-101(Cr)was synthesized by solvothermal method.TC3000 was used to measure the Thermal conductivity of the prepared sample.The mass ratio of the expanded natural graphite(ENG)added to mix with the as-prepared MIL-101(Cr)sample was determined based on the apparent thermal conductivity of the unit formed by the MIL-101(Cr)and honeycomb heat exchanging device.The adsorption and desorption isotherms of nitrogen on adsorbent samples at77.15K were measured by employing Micromeritics 3Flex.Horvath-Kawazoe equation and BET plot were then respectively applied to determine the pore and pore size distribution(PSD)and the specific surface area of the samples.Results show that the specific surface area of MIL-101(Cr)and MEI-01 are 2956 m2·g-1 and 1723 m2·g-1,and their apparent thermal conductivities are about 0.0623 W·m-1·K-1 and 0.2361 W·m-1·K-1,which reveals that the addition of the ENG in 10wt%brings about 380%increment in the thermal conductivity but41.71%decrement in the specific surface area.It suggests that comprehensive evaluation should be carried out to balance among the effect of enhancing in thermal conductivity and impairing the adsorption amount by adding the ENG.Analysis of adsorption equilibrium of hydrogen on MIL-101(Cr)and MEI-01.Adsorption isotherms of hydrogen on the samples were measured by employing Micromeritics 3Flex within the temperature-pressure range 77.15K-87.15K and 0-100k Pa,and Setaram PCT Pro E&E was used to measure an adsorption isotherm at 77.15K under the equilibrium pressure about 6MPa.Toth equation was used to calculate the absolute amount and Toth potential function was then applied to calculate the isosteric heat of hydrogen adsorption on the samples.It shows that the relation between the adsorption equilibrium data and the pressure conforms to Henry's law within a very low pressure region,and the mean value of the limit isosteric heat of hydrogen adsorption on MIL-101(Cr)and MEI-01 is respectively about 9.368 k J·mol-1and 9.090 k J·mol-1.Results also reveals that the isosteric heat of hydrogen adsorption on MIL-101(Cr)and MEI-01is respectively about 2.248-3.673k J·mol-1 and 2.240-3.673 k J·mol-1,and the maximum capacity of the absolute adsorption of hydrogen is 22.029 mmol·g-1 and 15.613 mmol·g-1.It suggests that there exists a strong interaction between hydrogen molecules and the adsorption wall of the two samples,but adding of ENG will impair the adsorption capacity of hydrogen on the samples.Tests of charging/discharging of hydrogen on the storage vessel packed with different adsorbent beds.For evaluating the effect of the measures for enhancing the thermal conductivity of the adsorbent bed,and providing valuable information for establishing the model for numerical simulation,a cylindrical vessel,which is in a net volume about 0.5L and packed with a Teflon liner as well as a 100W electric heating rod,was designed to be performed the charge and discharge tests of hydrogen at ambient temperature and 77.15K under the flow rate 20L·min-1.Comparison tests were conducted on the storage vessel respectively filled with MIL-101(Cr),MEI-01 and MIL-101(Cr)+honeycomb heat exchanging device about 148.4g,143.1g and 145.3g.It shows that,while conducting at ambient temperature,the average temperature rise of the adsorbent bed is respectively 5.8?,3.7?and 3.8?,which is correspondence with the cumulative amount of charge 1.002g,0.893g and 0.955g;the average temperature drop is respectively 6.2?,4.1?and 4.0?,and the cumulative amount of discharge is 0.874g,0.787g and 0.851g.Results also reveal that,while discharging at 77.15K within the effective discharging duration 306s,in comparing with those from the storage vessel packed with MEI-01,packing with honeycomb heat exchanging device brought about respectively 12%and0.6%increment in cumulative amount of discharge(3.669g)and the desorption ratio(DR),but a smaller usable capacity ratio(UCR)of the storage vessel.It suggests that both measures presented in this thesis can effectively alleviate the thermal effect of the adsorption bed in the process of charging and discharging,but the addition of ENG has a greater influence on the cumulative amount of charge and discharge.It can be concluded that equipping a honeycomb heat exchanging device is a better solution to enhancing the heat conducting and improving the adsorption and desorption performance of the adsorption bed.Numerical simulations of charge and discharge process of hydrogen on the storage vessel.Fluent was employed to develop a numerical simulation model of the storage vessel based on the three conservation equations,linear driving force model(LDF)and Toth equation.Experimental data from charging and discharging tests were used to determine the initial and boundary conditions of the storage system.Simulations were conducted at ambient temperature/77.15K with a flow rate 20L·min-1 of hydrogen charged or discharged on the vessel respectively packed with MIL-101,MEI-01 and MIL-101+honeycomb heat exchanging device.Results show that the relative errors between the results from simulations and experiments are all less than 4%.It also shows that,while discharging the vessel within the bath of liquid nitrogen at 77.15K,the average temperature rise of the storage vessel is respectively about 214.7?and 210.5?while the storage vessel is respectively equipped with honeycomb heat exchanging device and packed with MEI-01;within the effective discharging duration 310s,the cumulative amount of discharge from the adsorbent bed equipped with honeycomb heat exchanging device is 3.688g,which has12.3%increment in the discharged amount 3.284g and 2.16%increment in DR while packing the storage vessel with MEI-01.It suggests that the simulation results from the developed model can describe the main characteristics in charge and discharge processes,and equipping a honeycomb heat exchanging device with the adsorbent bed is a better solution to manage the thermal effect which is vital to meeting the fuel requirement of the fuel cell stack. |
PDF Full Text Request