Design of Battery Thermal Management System Based on Dymola One-Dimensional Simulation Software

Summary

The main functions of battery thermal management include: accurate measurement and monitoring of battery temperature; effective heat dissipation when battery temperature is too high; rapid heating under low temperature conditions; uniform distribution of temperature field of battery pack; battery cooling system and other heat dissipation units match.

Figure 1 Battery Thermal Management Diagram

The cooling of the battery pack is air-cooled and liquid-cooled. Studies have shown that the air-cooling method is easy to implement, but the battery pack temperature gradient changes greatly, which is not conducive to stable battery operation. The liquid cooling method in which heat is exchanged between the coolant and the refrigerant of the air conditioning system has gradually become mainstream. For the scientific management of the heat problem of new energy vehicle batteries, it is necessary to consider the interaction of multiple systems. The relationship between the systems is shown in Figure 1. The battery pack cooling has different degrees of coupling with multiple systems such as automotive air conditioning systems, motor cooling systems, and engine cooling systems. In this way, when doing battery system temperature control strategy and thermal management, it is necessary to analyze the influence relationship with other systems at the same time.

solution

In order to solve the complex coupling system between the fluid systems in the thermal management of the battery, a one-dimensional simulation model can be built using the evaporation cycle library, liquid cooling library and battery library of Dymola software. To simulate the entire model system and analyze the coupling relationship between different systems, so as to realize the control of complex systems.

Figure 2 Dymola model library

Dymola software has a rich model library, which can be easily built with a base library and a commercial library. The Evaporation Cycle Library covers almost all major refrigerants on the market, with accurate two-phase flow models and heat exchanger models modeled by structure; resistors, diodes, thyristors, which take into account component heat and temperature effects on the electrical performance of the component. Basic component model such as motor; heat transfer component model with heat capacity, heat transfer, convection, radiation, temperature, heat flow boundary conditions, etc.

The liquid cooling library that can be used for battery flow pipeline modeling, component selection, and system performance research includes pipelines, control valves, thermostatic valves, pumps, fans, heat exchangers, expansion tanks, etc.; The Modelon battery library, which affects battery capacity and external characteristics, can be used to analyze the battery's electrical, thermal, and life characteristics.

Control systems are essential for battery thermal management. The Dymola base library contains a library of models for control and logic modeling that can be used to build control systems. In addition, the FMU corresponding to the FMI interface can be used to construct the control law model through Simulink, and the model can be converted into FMU and imported into Dymola, which can be co-simulated with the battery system model and the heating/cooling system model.

Figure 3 control system model

The air conditioning system model of the thermal management system can be built by using the evaporation cycle library provided by Dymola software; the battery cooling cycle, engine cooling cycle and power electronic component cooling cycle can be built by using the liquid cooling library in Dymola software; the battery library in Dymola software is used. It is possible to build a battery drive system consisting of a motor and a battery. Evaporation cycle library, liquid cooling library and other model libraries can be seamlessly connected to form a large system, which is convenient for thermal management model system simulation analysis.

Dymola can also build a control algorithm. At the same time, it can also use the Simulink interface to call the control algorithm of Matlab/Simulink software to realize the joint simulation between the thermal management system control model and the simulation physical model, which is used to design and verify the control strategy. Engineers better design thermal management system models.

Applications

Figure 4 shows a one-dimensional simulation model of battery thermal management using Dymola software. The red dotted line on the left is the evaporation cycle of the air conditioning system built by the evaporation cycle library; the middle red dotted line area is the battery cooling cycle built by the liquid cooling library; the black solid line between the evaporation cycle and the cooling cycle is the coolant and The heat exchange unit between the refrigerants; the rightmost red solid line area is a drive system composed of components such as a motor battery.

Figure 4 Battery Management System One-Dimensional Simulation

The battery motor supplies power, the motor drives the load, and the heat generated by the battery exchanges heat between the coolant and the refrigerant through a heat exchanger between the liquid cooling cycle and the air conditioning system, and then passes through the air conditioning system to the engine compartment, and finally the heat is taken up by the air. take away. Figure 5 shows the battery pack temperature curve for different pump speeds. Changing the speed of the pump in the cooling cycle maintains the temperature of the coolant flowing through the battery pack within the desired temperature range.

Fig. 5 Temperature curve of battery pack at different speeds of liquid cooling pump

Figure 6 shows the addition of a simple control system model based on the built model system. The model can be used to view the evaporation cycle pressure map in real time and monitor the system operation status. The blue zone detects the evaporator outlet temperature and ensures that the evaporator outlet temperature is constant by controlling the displacement of the variable displacement compressor. The black zone maintains the battery temperature within the desired temperature range by adjusting the speed of the pump in the cooling cycle and the air side air flow of the condenser in the evaporation cycle.

Figure 6 simple control model

Figure 7 Temperature curve of battery pack under different heat dissipation power

Figure 7 shows the relationship between the set temperature of the battery pack (red) and the actual temperature (green). At 100s, the battery pack's heating power suddenly decreases, and the battery pack temperature also changes, but by adjusting the cooling pump speed and the condenser side. The fan speed quickly adjusts the heat dissipation of the system, so that the battery pack temperature is stable within a reasonable range.

to sum up

The simulation model system construction and simulation analysis can be completed by using Dymola one-dimensional simulation software. The model can be used for model matching design, component selection, and system simulation for model system energy allocation analysis. It can also be used as a simulation model to improve engineers' understanding of system performance, as the controlled object is used to control strategy design, verify the accuracy and control effect of the control model.