Fundamental Research On Mechanisms Of Heat Generation And Irreversibilities For Supercapacitor Energy Storage Process

Supercapacitors have shown advantages in terms of ultrafast charge/discharge rates,high power density,and wide operational temperature window.These beneficial features make supercapacitors a prime candidate for high-power demanding applications,such as uninterruptable power supply,braking energy regeneration,emergency power support and aerospace systems.In electrostatic adsorption process,the input electric energy,on the one hand,is converted into electrostatic potential energy by ion adsorption for storage,on the other hand,it is wasted by heat dissipation.To achieve the energy storage efficiency improvement,long cycle life and reliability of supercapacitors,it is necessary to deeply understand the microscopic mechanisms of heat and mass transfer,and to reduce heat loss in the process of energy storage procedure.Therefore,an in-depth understanding of the heat and mass transfer mechanism in the process of supercapacitor energy storage,an accurate description of the thermodynamic properties of solid-liquid electrostatic adsorption and desorption process,and the acquisition of directional regulation mechanism and regulation method are the theoretical basis for enhancing the solid-liquid electrostatic adsorption energy storage performance.It has significant academic value and practical prospects.However,the current solid-liquid electrostatic adsorption thermodynamics theory is based on the continuum hypothesis and Debye-Hückel approximation,which is unable to accurately describe the thermodynamic phenomena of solid-liquid electrostatic adsorption in nano pores.For a long time,there is a lack of electrostatic adsorption thermodynamics theory that can accurately describe the thermal behaviors of supercapacitors.Considering the above background,this thesis carries out an in-depth analysis on the thermodynamic properties of the supercapacitor energy storage system.The heat loss is accurately described,which is mainly determined by the electrostatic adsorption entropy change caused by the change of ion degrees of freedom as well as the entropy generation as a result of ion transport collision.The predictions of heat generation,regulation and optimization mechanism are further proposed,providing theoretical support and implementation scheme for the improvement of the thermodynamic performance of solid-liquid electrostatic adsorption energy storage.This thesis includes seven chapters in total.Specially,in the third chapter,the entropy change and heat generation mechanisms of electrostatic adsorption during the solid-liquid electrostatic adsorption process are studied,and the multi-scale coupled electrochemical-thermal model is for the first time conducted,and a long short-term memory（LSTM）neural network is trained to build a temperature database for practical supercapacitor modules under different operating conditions.The fourth chapter explores the thermodynamic irreversibility mechanism in the process of solid-liquid electrostatic adsorption energy storage,and focuses on the heat transfer entropy generation,mass transfer entropy generation and ohmic loss entropy generation within the system.The fifth chapter explains the mechanisms of thermodynamic irreversibilities in the thermal management process of supercapacitor modules,and focuses on the entropy generations due to heat transfer and fluid friction.The sixth chapter reveals the internal correlation mechanisms between thermodynamic properties and energy storage properties,and investigates the relationship between electrochemical properties and thermodynamic irreversibilities.Firstly,the mechanisms of electrostatic adsorption entropy change and heat generation of supercapacitors are revealed.And a multi-scale research method combining molecular dynamics simulation and finite element simulation is first proposed to analyze the heat generation behaviors of supercapacitors via probing the atomic-level information.In addition,significant deviations between experimental observations and traditional predictions are always observed,because most of the conventional electro-thermal models are based on the assumption of the 1 nm effective thickness of the interface,and the atomic-level information is usually neglected.In this paper,a multi-scale coupled electrochemical-thermal model of the supercapacitor is constructed for the first time to the accurate description of the heat generation and heat transfer characteristics of the supercapacitor.As a consequence,the deviations between our improved model and experimental results are substantially reduced from 11.73%to3.98%in the adiabatic conditions and are reduced from 6.04%to 4.27%in the natural convection condition.Furthermore,deep neural network based on the LSTM approach is trained for predicting the temperatures of supercapacitor modules,building a temperature database for practical supercapacitor modules under different operating conditions,which opens up a new path for solving complex engineering problems.Secondly,the mechanisms of irreversibilities of supercapacitors are expounded.An entropy generation analysis is implemented for the first time in the supercapacitor cell,satisfying direct identification of the inefficiency mechanisms that cannot be achieved by the conventional energy analysis.Entropy generation analysis accurately quantifies the irreversibilities due to heat transfer,mass transfer and ohmic loss of the supercapacitor cell,indicating that the main contribution to the irreversibilities is due to ohmic loss.In addition,the effects of the electrolyte,porosity,and charge/discharge current on the thermodynamic irreversibilities and heat transfer characteristics are investigated.The optimal design scheme of the supercapacitor cell is obtained using the optimization approaches based on the combined energy and entropy generation analyses.Results indicate that the TEMABF₄/ACN electrolyte with the porosity of 0.3is found to be the optimal choice.Furthermore,based on the linear phenomenological laws,the microscopic relation between the entropy generation due to mass transfer and ohmic loss and the pore size is first proposed.As a result,the optimal configuration of the nano channel under the minimum entropy generation is obtained.The proper nano pore size is directionally determined by the size of the dominant ion.When the nano pore size is equivalent to the hydration diameter of the dominant ion,the minimum irreversibilities of the system are obtained.The outcomes provide a fundamental and computational framework for the micro regulation and optimization of supercapacitor systems with reduced irreversibilities.Then,this paper reveals the mechanisms of irreversibilities in the supercapacitor thermal management system based on linear non-equilibrium thermodynamics.The entropy generation analysis is conducted for the first time to accurately quantify the irreversibilities caused by the heat transfer and fluid friction,and the entropy generation due to fluid friction is the dominant contribution.In addition,based on energy analysis and entropy generation analysis,four evaluation criteria,namely heat transfer entropy generation,fluid friction entropy generation,Bejan number,Nusselt number and Ecological Coefficient of Performance（ECOP）are proposed to evaluate and optimize the thermodynamic performance of the supercapacitor thermal management systems.The effects of physical parameters such as module structure,cooling fluids,inlet temperatures of cooling fluids,inlet velocities of cooling fluids and charge-discharge current on thermodynamic irreversibilities and heat transfer characteristics of supercapacitor modules are studied.The optimal thermal management scheme of supercapacitor modules is obtained through multi-parameter optimization,opening a new possibility for macro-control of supercapacitor energy storage thermal management systems.Finally,the internal correlation mechanisms between the energy storage characteristics and thermodynamic characteristics are firstly explored.And the quantitative functional relations between electrostatic potential energy and the entropy change well as the ohmic loss entropy generation are obtained.The relationships between thermodynamic irreversibilities and macroscopic electrochemical characteristics such as power density,energy density,specific capacitance and Coulomb efficiency are analyzed and studied.As a result,the bridge and link between the thermodynamic behaviors and electrochemical characteristics of supercapacitors are established.This work lays a foundation for guiding the performance improvement of energy storage equipment more comprehensively and effectively.