Non-equilibrium Thermodynamics Of Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?is used as efficient and clean energy conversion equipment.The irreversible processes such as heat and mass transfer,micro flow,charge transport,electrochemical reaction and catalytic reforming are its internal intrinsic processes,which inevitably produce irreversible losses.Entropy generation is a function of various irreversible losses.Traditional research on SOFC performance is to maximize output power density.This is a short-term goal to improve the performance of fuel cell.However,from the perspective of entropy generation and exergy analysis the non-equilibrium thermodynamic properties of SOFC can reflect the energy savings and ultimate goal for a long time.To this end,in this paper,a three-dimensional multi-physical model of SOFC?H₂-SOFC and CH₄-SOFC?with hydrogen as fuel and methane as fuel is established,and the macroscopic phenomenological framework of the entropy generation of viscous friction(G_fric),heat conduction?G_h?,molecular diffusion mass transfer?G_m?,activation polarization(G_act),ohmic thermal(G_ohm)and methane reforming reaction(G_ch)is established by combining the entropy generation balance formula.Explore the influence of operating parameters and geometric structure on power density,global entropy generation,exergy destruction,fuel utilization,energy efficiency and exergy efficiency,as well as its changing rules during power generation process,which is conducive to locating the distribution of irreversible entropy generation on each component of SOFC,quantifying the degree of SOFC internal irreversibility in each process of SOFC,and is of great significance for optimizing the thermodynamic performance of the SOFC.There are few relevant reports on research in this area.Due to the calculation of the irreversibility of SOFC,it is necessary to obtain the driving force and flux parameters of each process first.To calculate the irreversibility of SOFC,the driving force and thermodynamic flux parameters of each process need to be obtained first.Meanwhile,due to the limitation of high operating temperature and small battery size.It will be difficult to obtain through experiments.A numerical simulation method is used to establish the relationship between parameter calculation-structure design-performance comparison.The following aspects of research are the main work of this article:?1?Construct a plate SOFC three-dimensional multi-physics fully coupled numerical model and the driving force and flux of the corresponding transport process are calculated by with COMSOL software.It is used to solve the irreversible entropy generation model caused by the effects of viscous friction,heat conduction,mass transfer,activation polarization,ohmic heat and methane reforming in SOFC.?2?A anode support plate SOFC single cell experimental test equipment was established to test the I-V-P performance curves of H₂-SOFC and CH₄-SOFC at different operating temperatures,verify the reliability of transportation model,indirectly prove that are used to calculate the accuracy of the driving force and flux required for entropy generation guarantees the effectiveness of system performance such as exergy efficiency and energy efficiency.?3?The distribution area and range of local entropy generation in SOFC,the variation law of global entropy generation,exergy destruction,exergy efficiency and energy efficiency in the process of power generation are simulation explored.The results showed that G_m predominates in H₂-SOFC,followed by G_act,while G_chh predominates in CH₄-SOFC,followed by G_act,then is G_m,G_fricric can be ignored.S_chh is concentrated on the top of the anode support layer inlet,S_m is distributed at the contact point between the battery inlet channel and the electrode layer,and both the anode S_actct and S_m are larger than those on the cathode.S_ohmhm is distributed throughout SOFC components,but concentrated in the electrolyte central surface.S_h is concentrated at the contact point between the inlet channel and the connector,and in CH₄-SOFC,it first decreases and then increases in the direction of battery length with the decrease of voltage.During the power generation process,as the output current density gradually increases,exergy destruction increases approximately linearly,and the power density and energy efficiency increase first and then decrease,exergy efficiency gradually decreases from about 92%reduced to 60%gradually in H₂-SOFC,while in CH₄-SOFC,it increases first and then decreases.The main reason is that the effect of the former is more affected by the power density than by the exergy destruction,and the latter is the opposite.The greater exergy efficiency means that the more work can be effectively utilized,which means that SOFC is more energy-saving.The difference between exergy efficiency and energy efficiency is obvious,indicating that the irreversible loss of SOFC is relatively large and the efficiency of heat energy utilization is not high.?4?The effects of operating temperature,fuel inlet flow rate,initial mole fraction of H₂,rib width/channel width?RCR?and channel height/channel width?CSR?on the thermodynamic performance and power generation performance of SOFC were investigated in H₂-SOFC system.The results showed that the lower exergy destruction,the higher the power density,fuel utilization,energy efficiency and exergy efficiency were with the increase of temperature and the decrease of H₂ mole fraction and the less the fuel inlet flow.By reducing fuel flow rate and air flow rate,the energy efficiency can be greatly improved within a certain range.Under the same current density,the smaller RCR is,the less irreversible the system will be,and obtain the greater the exergy efficiency and energy efficiency.Therefore,H₂-SOFC with wide channels and narrow ribs has better performance.Compared with the single channel,the multi-channel single-stack cell has greater exergy destruction,but at the same time its exergy efficiency is greater.At the same current density,increasing the number of channels can obtain higher power density and efficiency.?5?The effects of operating temperature,steam-to-carbon?S/C?,prere-forming rate,RCR and CSR on the thermodynamic performance and power generation performance of SOFC were investigated in the CH₄-SOFC system.Results show that:high temperature can help the SOFC performance enhancements,the lower the S/C and the pre-reforming rate,the greater the power density,and the higher the exergy efficiency and energy efficiency.RCR significantly affects battery performance.Reducing RCR can enhance battery performance,and CSR has little effect on battery performance.But the narrow-ribbed,wide and low-channel SOFC geometric model can get better thermodynamic performance of the second law of the battery.Under high temperature,the CH₄-SOFC system has strong irreversibility at high CSR,low RCR,low S/C and low pre-reforming rate.