SL Saeid Loghavi
Multi-physics co-simulation of battery thermal model
United States Created on 2017.06.08 1156 views
The analysis and proper design of the energy storage system is a critical concern in the product development phase of Hybrid Electric Vehicles (HEVs). Development of high fidelity models and comprehensive analysis of the battery pack are critical and can significantly improve performance and reduce cost. Incorporating high fidelity battery model into a vehicle model can significantly increase the model fidelity and improve the fidelity of the vehicle model. This project features a novel modeling approach developed in order to analyze the battery pack in high performance hybrid-electric vehicles using a multi-physics co-simulation approach. This modeling capability can be extended to other multi-physics systems in order to develop high fidelity models while significantly decreasing the computational costs. The modeling approach used in order to analyze the battery performance, includes three domains: electrical, structural and fluid. The high fidelity electrical domain model includes, an individual cell model (multiple cells can be arranged to form a battery module and/or pack), control unit, and power cycle in the form of a table. The structural heat transfer model is developed using Finite Element Modeling (FEM) software Abaqus 2016 and includes all the battery pack components. The fluid domain model is developed using Abaqus 2016 and includes the model of cooling fluid which removes heat from the cooling plate. The new modeling and simulation capability presented, provides a novel approach which builds upon the co-simulation capability of the SIMULIA package. The cell model was calibrated using experimental test results. Finally the project lists the next steps of the project and potential applications of the battery pack model. In order to develop a high fidelity model of the battery cell, Modelica language is used. Modelica is an open source, object oriented, and equation based modeling language which can be used to easily model complex physical systems containing, electrical, mechanical, hydraulic, thermal, electronic, control and process oriented subcomponents. A wide array of open source Modelica Libraries have been developed, providing rapid and scalable model development capability. Dymola is a commercial modeling and simulation environment based on Modelica modeling language. Dymola has a built in capability to simulate object oriented models as well as export models as Functional Mock-up Units (FMU). FMUs are executable function files, generated following the Functional Mock-up Interface (FMI), which is a tool independent standard to support model exchange and co-simulation of dynamic models. In order to streamline the development of a cell model, the open source Electric Energy Storage (EES) library developed jointly at Austrian Institute of Technology and Vienna University of Technology was used. The library contains sub-components of different complexity such as individual cells and stacks interacting with loads, battery management components and charging devices.The EES library components are designed as universal components allowing the end users to easily simulate specific scenarios by varying the parameterization. The use of Modelica language allows seamless modifications of the equations used in each subcomponent. For the purposes of this analysis, the built in LinearDynamicImpedance cell model was modified in order to properly define the cell model based on experimental data. The Batteries package includes models for individual cells as well as stacks of ns cells connected in series and np cells in parallel. A single cell component is selected for the purpose of this analysis. The single cell model features a positive (pin_p), a negative (pin_n) and an optional temperature connector (heatPort) which is activated as cell parameters are defined as a function of temperature. Model can consider the effects of aging by considering both calendaric aging and cycling. Calendaric aging is estimated from time which for the purposes of the current analysis is minimal and therefore removed from the model. Cycling is directly proportional to absolute transferred charge Qabs . Aging of the cell mainly influences the performance by decreasing capacity and increasing the internal impedance. The cell capacity is temperature dependent and decreases with increased transferred charge due to cycling. The single cell model features a single, temperature dependent, ohmic impedance which does not consider the increase in impedance due to aging. The EES library contains a battery management block (Cycling) which is used to implement a rule based control of the battery cell. It has three boolean outputs which control the operation of the charging unit, power cycle look up table and the cell. The battery management block can be used to cycle the cell a predefined voltage range, while limiting the maximum current draw during discharge and charging current. The EES also includes a constant current, constant voltage (CCCV) charging device which charges the cell with constant current until the constant voltage level is reached. The battery module under study contains twelve individual cells arranged in two series sets of six parallel cells. Subcomponent labeled load includes the charging unit, power cycle look up table, a switch and the cycling rule based control unit. Switch works by disconnecting the power cycle when the cell/module drops below Vmin and connecting the charger. Similarly as the voltage reaches the constant voltage level, the switch disconnects the charger and connects the power cycle to continue the power cycle. In order to study the effects of heating on battery performance and investigate the cooling power needed to ensure the safe operation of the battery pack the heat generated during cycling is calculated and the temperature of the cell is calculated by the heatCapacitor model. An additional thermal resistance is included in the model to represent the heat resistance of the skin of the cell. In order to create a high fidelity model of the battery module, a transient heat transfer model is developed. Each part of the module was meshed on part using standard linear tetrahedron elements (DC3D4) with a global size 4 and applying curvature control by allowing the maximum deviation factor to equal 0.6. Under the assembly module a dependent instance was created containing all components and a surface of type geometry, named INTERFACE, was created using the internal surfaces of the cooling plate. The surface was used to define a heat flux interaction with the fluid domain model. In order to define the Co-simulation interaction property between electrical, structural, and fluid domain models, an input file, named Standard.inp, was generated using the structural heat transfer model created. Abaqus CAE interface does not allow the definition of the co-simulation interaction, thus the input file was edited. In order to create a high fidelity model of the cooling fluid, a transient CFD model is developed. The thermal properties of the coolant which is a mixture of 50% water and 50% Ethylene glycol. The fluid model, which was generated to fill the void in the cooling plate part, was meshed on part using CFD linear tetrahedron elements (FC3D4) with a global size 4 and applying curvature control by allowing the maximum deviation factor to equal 0.6. Under the assembly module a dependent instance was created and three surfaces of type mesh, named INLET, OUTLET, and INTERFACE were created.The INTERFACE surface was used to define a heat flux interaction with the structural domain model. The INLET surface was used to define the fluid inlet boundary conditions. Similarly, the outlet boundary conditions were applied to the OUTLET surface. Similar to the structural heat transfer model, in order to define the Co-simulation interaction property between electrical, structural, and fluid domain models, an input file, named Fluid.inp, was generated using the CFD model created. Abaqus CAE interface does not allow the definition of the co-simulation interaction, thus the input file was edited. Co-simulation between Abaqus/Standard (standard heat transfer model) and Abaqus/CFD (fluid domain model) is governed by an additional process, the SIMULIA Co-Simulation Engine (CSE) director. Typically the CSE director is automatically invoked and the co-simulation parameters are stored in the co-simulation configuration file. In order to execute co-simulation between models generated using Abaqus products and co-simulation format FMU files, the Abaqus command line should be used to invoke the CSE director and the co-simulation configuration file should be manually generated. Proper execution of co-simulation requires:
- Proper definition of the interaction between the three models as outlined in the CSE User’s Guide and API reference Guide.
- Compatible and consistent definition of co-simulation time and step size.
- Definition of co-simulation parameters in the co-simulation configuration file as specified by the CSE director.
- Definition of the co-simulation master and slave.
- components: subsystem simulator programs used in the co-simulation,
- component instances: subsystem simulations performed using identified simulators,
- connectors: available input and output simulation results from each subsystem,
- connection sets: pairing of subsystem simulation results,
- connection categories: time in the co-simulation when these pairings are relevant or active, and
- model of computation: numerical method used in the co-simulation.
- XML well-formedness. The CSE director or any other XML authoring tool can confirm the well-formedness.
- Abiding by the CSE schema definition. The rules and constraints are described formally in the CSE API kit.
- Internal consistency of references.
- Topological and algorithmic consistency. The definition of co-simulation and model parameters within the configuration file should be consistent with the problem definition in each subcomponent.
- External consistency of references. The author of the configuration file should ensure consistency of names registered with the CSE director and each subcomponent.
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