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How to link an electric vehicle battery cooling system with the air conditioning system

Siemens Theorist Siemens Theorist
Siemens Theorist

Purpose

 

Battery-Electric-Vehicle.jpgVehicle electrification brings a lot of technical challenges related to the thermal regulation systems. The battery is one of the most important parts of the vehicle: it is massive, and its temperature needs to be regulated (most of the batteries should be kept around 25 degrees Celsius). The HVAC system is designed to ensure good battery cooling but also passenger comfort in the cabin.

 

When starting our car left in the sun in the summer, the cooling requirements are enormous. The control strategy needs to balance the cooling requirements for the battery and the cabin.

 

Description

 

This Simcenter Amesim demonstration example presents an electric vehicle for which we model the HVAC and electric systems to accurately capture the interactions between cabin cooling and battery cooling systems.

 

The complete model is shown in the picture below:

 

Simcenter-Amesim-Electric-Vehicle-Battery-Cabin-Cooling.pngSimcenter Amesim sketch: electric vehicle battery and cabin cooling

 

NB for Simcenter Amesim users: the model is further detailed in the Simcenter Amesim demo “Electric vehicle battery cooling”.

 

Vehicle subsystem

The driver model computes the braking pedal and accelerator pedal position signals to match the velocity profile. The control unit translates the acceleration demand into a motor torque demand and splits the braking demand into a regenerative braking generator torque demand and a mechanical braking demand if necessary. In this example, the driver accelerates from a null speed to a constant 90 km/h speed.

 

Electric subsystems

A high-voltage battery is connected to two electric motors: one to drive the air conditioning compressor and another one to drive the vehicle.

 

The low-voltage battery supplies the electric motor driving the coolant centrifugal pump, the radiator fan, the evaporator blower and other auxiliary consumers.

 

Refrigerant subsystem

The refrigerant loop cools down the battery (via the coolant and the chiller) and the car cabin (via the air and the evaporator). The chiller and the evaporator have a thermal expansion valve each for superheat control and are connected to the refrigerant loop in two parallel branches.

Simcenter-Amesim-Chiller-EvaporatorWithTheirThermalExpansionValves.pngChiller and evaporator with their thermal expansion valves

 

The air flow through the evaporator and the coolant flow through the chiller are linked to the controller.

 

Coolant subsystem

The battery cooling loop is composed of:

  • a valve used to select the cooling source (radiator or chiller),
  • a centrifugal pump which drives the coolant flow rate,
  • an expansion tank storing initially the coolant,
  • a radiator to cool down the coolant with the external air,
  • a chiller connected to the air conditioning loop,
  • a convective element connected to the casing to cool down the battery.

The valve brings the fluid to the radiator and to the chiller if necessary.

 

Simcenter-Amesim-Thermal-hydraulic-Valve.pngThermal-hydraulic valve

Battery pack subsystem

The battery cells are modeled with a thermal mass heated by the battery thermal losses. These cells are connected to the battery casing via a contact conduction. The battery casing is then connected to the coolant loop with a convective element. Since the casing is in direct contact with ambient air, convection and radiation are considered.

 

Simcenter-Amesim-Battery-Cells-Casing-Modeling.pngBattery cells and casing modeling

 

Simulation scenario

 

A cool-down scenario is simulated here. A car has spent a long time directly exposed to the sun. Therefore, two of its subsystems require cooling:

  • the car cabin: the passenger expects the air-conditioning loop to cool down the cabin air efficiently. The comfort zone is supposed to be [20 - 24] °C in this demo. The target is set to 22°C.
  • the battery: limiting the battery heating is necessary for its safety and its lifetime. The operating zone is supposed to be [20 - 32] °C in this demo. The target is set to 28°C.

Three strategies are tested:

  • Strategy 1: priority is set to cooling the car cabin, whatever the state of the battery.
  • Strategy 2: priority is set to cooling the battery, no matter the temperature in the car cabin.
  • Strategy 3: a compromise between the two previous strategies is set to sufficiently cool down both the car cabin and the battery.

Simcenter-Amesim-Statechart-of-the-control-strategy.pngStatechart of the control strategy

 

Each of the strategies is tested for three different ambient temperatures, 20, 30 and 40 degrees Celsius. Due to solar heat flux, the cabin initial temperature is assumed to be equal to 160% of the ambient temperature.

 

Results

 

For each strategy, we capture the evolution of the temperatures of the cabin and the battery over time. Here is an example corresponding to the first strategy.

Simcenter-Amesim-Strategy1-Car-Cabin-And-Battery-Temperatures.pngStrategy 1: Car cabin and battery temperatures

 

From this we can derive interesting criteria like the cumulative cabin discomfort duration and the battery cooling duration. Both these durations should be as low as possible.

 

Simcenter-Amesim-Cabin-Comfort-Battery-Cooling-Duration.pngAmbient temperature and strategy impact on cabin discomfort and battery cooling duration

 

And finally, it will be interesting to have a look at the remaining state of charge of the battery, once the cooling has done its job:

 

High-Voltage-Battery-State-Of-Charge.pngHigh voltage battery state of charge at the end of the driving mission

 

Conclusion

 

This model shows how to assess the impact of various cooling strategies regarding both passenger comfort and battery temperature, a statechart can be used to reach this target. It shows that a mixed strategy leads to a good compromise:

  • the battery cooling is close to what can be achieved when the cooling priority is only set to the battery,
  • the car cabin cooling is still reasonable.

This model also shows global system behavior depending on the operating conditions and integrated system global consumption. This demo highlights multi-phenomena contributions and the impact on a global system energy consumption. This perfectly illustrates the added value of system simulation.

 

For more information, please open the “Electric vehicle battery cooling” demonstrator in Simcenter Amesim and watch the how-to video:

 

To find out more about Simcenter simulation capabilities dedicated to battery design, watch this webinar.