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What are different Cooling Methods for batteries in Electric Vehicles?

Introduction to Electric Vehicle

Conventional vehicles use fossil fuel and pollution due to combustion is a serious concern on the environment. The scope of Electric vehicles (EV) has been essential due to the adverse impact of fossil fuels on the environment
An electric vehicle does not use any fossil fuel for power generation and has zero emission
Many initiatives have been taken to reduce air pollution using non-conventional energy sources. Electric vehicles use the electric battery and help to reduce pollution. External electric supply charges the battery which supplies electric power to the motor. The electric motors transfer power to the front and back wheels.
Many automobile industries are shifting from internal combustion engine cars to electric vehicle

Automotive industries have invested hugely in the research and development of electric vehicles at an affordable rate
Electric vehicles have several advantages like improved energy efficiency, performance, environmentally friendly and are free from combustion generated pollution
Technologies in electric vehicles have been developed for thermal management for battery systems, power controllers,s and electric motor

Parts of Electric Vehicle

Electric Engine or Motor: It is a prime mover and provides power to rotate the wheels. The electric motor can be DC or AC type, however, for electric vehicles, AC motors are widely used
Inverter: it converts the electric current from DC into AC
Drivetrain: EVs use a single-speed transmission which transfers electric power from the motor to the wheels of vehicles
Batteries (Power Bank): Battery modules store electricity required to drive an electric vehicle. The power capacity of the battery is measured in kW.
Charging Point: EV charging point in the form of the plug to charge the battery modules

Challenges of Battery of Electric Vehicles

The following are major challenges faced in the development of battery

Long Charging time
Overheating battery
High Cost of battery
Safety against heating or short circuit
The limited life of the battery

Effect of temperature on battery life

Ensure battery operation under optimal temperature range (15 C to 35 C)
Monitor the state of the battery and detect the critical point of battery failures and deliver alarm messages
Suppress thermal runway propagation

Battery Thermal Management System (BTMS)

The electric vehicle has a battery management system (BMS) to provide essential information such as:
- Thermal Protection
- Over and Under voltage protection
- Over-current Protection
- Prolong battery life
- Cell Balancing
- SoC and SoH calculation
- Communication with all battery components
- Data acquisition and analysis

The high battery temperature leads to poor performance, short lifetime, and risk of blasting. Therefore, a BTMS is essential for all battery modules.
The main purpose of a BTMS is to maintain the battery system in the optimum temperature range and keep uniform temperature variation in the battery modules
Other factors for battery selection are weight, size, reliability, and the cost
The following figure shows the most used thermal management techniques for battery module

Different Battery Cooling Methods Used in BTMS

To adapt to EVs, BTMS must ensure features such as high performance, simplicity, low weight, reduced cost, low use of parasitic power and fast packaging, and easy maintenance
Two types of battery cooling systems (BCS) are common which are external or internal. Many researchers and industries have been developing BCSs
External Battery cooling systems (EBCS) are classified into several different ways
Battery cooling can be categorized based on the method or technique.
- Liquid or gas cooling: plate type or use of mini-channel)
- Heat pipe
- Phase Change Material (PCM)

Air Cooling

Air is easily available for cooling purposes and it is widely used in many industries. However, the air is not an effective coolant because of its lower heat capacity and low thermal conductivity
Due to low maintenance of costs, the following type of air cooling system is popular in some automobile industries like Toyota Prius and Nissan Leaf

The cooling is possible either by forced convection (active cooling) mode and natural convection (passive cooling) mode
Natural convection cooling is suitable only for low-density batteries, and typically blowers/fans can be used to enhance the convection heat transfer rate
When air is used for cooling of battery modules arranged in series, the middle and rear portion of batteries are at high temperature to the low heat capacity of air. The temperature of the battery pack near the outlet is very high and the temperature distribution is highly non-uniform. To cool effectively, we need to increase the flow rate and turbulence of airflow.
CFD study of different battery modules can be carried out for different configurations of airflow rates and electric vehicles

Air Cooling Battery Pack in EVs

The following are popular battery pacts with air cooling in electrical vehicles

Honda Insight
Honda FitEV
Hyundai IONIQ
Nissan e-NV 200
Nissan Leaf
Renault Zoe
Toyota Prius Prime

Liquid cooling of Battery

Liquid coolants have a high convective heat removal rate due to higher density and heat capacity compared to air
A liquid cooling system is more compact than an air system. We can save up to 40% of separate power compared to the fans required for air cooling.
Moreover, liquid cooling can reduce the noise level. However, the complexity of the liquid cooling system and its leakage potential can be defined into direct and indirect cooling.

Indirect liquid cooling

Water has been an effective coolant for several industrial applications But the main challenge is the short-circuited potential for direct cooling of batteries
To avoid the short-circuit issue, indirect methods are effective to avoid electrical conduction with cells while maintaining high thermal diffusion
The addition of electrical resistance may resist thermal diffusion. But its effects on cooling is not significant.
Some EV manufacturers such as General Motors, and Tesla are using indirect cooling in their cars. General Motor used cold plates between each prismatic cell. The cold plates have several micro-channels for convective heat removal from batteries.
Tesla has used wavy tubes placed between cylindrical cells. Thermally conductive and electrically insulated material is used to fill the gaps between the battery cells and cooling ducts.
The wavy tubes may not be effective for cooling because of the small heat transfer contact area, it is safer from the mechanical and electrical point of view. All the connections of coolants are kept outside of the enclosure to avoid the leakage of coolant.

Liquid cooling of batteries is shown below and heat transfer takes place from the cells to cooling channels

The design strategy of thermal management system for battery modules, controller, and electric motor

Chevrolet Cooling System

This consists of a large number of micro-channels embedded in the metal plates to carry out a coolant
The plate is placed between two battery sets to remove the heat

Direct liquid cooling (Immersion)

Direct cooling is also called immersion cooling, covers the entire surface of the cell and cools it uniformly
Immersion cooling removes hot or cold spots in the cell and significantly improves the performance of the cell.
Ideal coolant mush be dielectric with minimum viscosity and high thermal conductivity and thermal capacity
Immersion cooling is widely used for data center servers and high-power electronics devices
BTMS for this method is not suitable for most mass-produced EV due to high cost and safety concerns
Immersion cooling is useful for batteries of high-performing EVs and EV racing. The fluid has a boiling point between 60 – 80 °C to prevent overheating of the cells and avoid thermal runaway. Modular type containers are used for the cells where they can be submerged in the liquid

Liquid Cooling Battery Pack in EVs

The following are popular battery pacts with liquid cooling in electrical vehicles

Audi R8 e-Tron
Ford Focus
GM Chevrolet Bolt and Chevrolet Volt
Tesla Model X, Model S, Model 3
Toyota –iQ
Volvo XC90 T8

Phase change Material (PCM) Cooling

The phase change material absorbs heat significantly due to high latent heat during battery discharge
The PCMs have been used for thermal management have a melting point in the optimum performing range of lithium cells. The cell temperature needs to remain for a long time.

PCM as paraffin has a melting point in the range of 32-38°C which is mixed with graphite flakes to improve thermal conductivity
The graphite flakes will also create a semi-matrix block which will contain the paraffin particles. When the paraffin melts, it stays within this matrix and the whole composition remains as the solid form
The basic equation for phase change material (PCM): V*ρ*L = P*t, where

V = Volume of PCM in cell (m3)

ρ = Density of PCM in cell (kg/m3)

L = Latent heat of fusion of PCM (per kg)

P = Power absorbed (Watt)

t = Time of power absorption (seconds)

The advantage of solid PCM is that they also act as shields, in case one cell enters a thermal runaway
Classification of PCM materials for battery cooling is given below

Select the phase change material which can maintain the temperature of cell 25-35 °C
Multiphase modeling needs to be considered for phase change materials during the cooling of batteries

Heat Pipe

Heat pipes have been considered versatile in many industrial applications for their efficient cooling and
thermal management
Similar to the passive cooling method of PCM, applying the heat pipes to cool or remove heat energy from the battery provides efficient
heat transfer t low power consumption
In this mechanism, heat is transferred through latent heat of vaporization (phase change) from the evaporator to the condense
The working fluid can be passively transported back to the evaporator by capillary pressures it is developed
within a porous wick lining. Operating in this situation the heat is continuously absorbed and released.

Comparison of Cooling Methods

Commonly used cooling methods are compared in the following table

Selection of Parameters for Optimum Performance

Selection of battery cooling system comprises many parameters
Operating conditions of the battery
- Load on battery
- Current rate and voltage
Environmental conditions
Cells
- Chemistry of battery cells like Lithium-ion
- The geometry of cells: cylindrical or rectangular
- Reactions: cyclic or non-cyclic
- Properties of cell materials
Packing of cells
- Thermal interface
- Cooling medium
Mode of cooling: forced convective or nature draft cooling. CFD simulations are useful to decide the rate of cooling and thermal design of the cooling circuit
Thermal interface and fitting of battery packs

Concept design of battery cooling

Scope of CFD Modeling for Battery Thermal Management System

Computational Fluid Dynamic (CFD) modelings have more scopes for design optimization for different cooling methods
The following parameters we can optimize based on CFD simulations
- Geometry of cells
- Thermal interface
- Determining the heat transfer coefficient for air or water cooling
- Optimum spacing between two cells
- Desired level of turbulent level in the fluid
Read the post on modeling of battery with governing equation and numerical models
- Selection of Power for Electric vehicles
- Shape and Size of battery
- Electro-chemistry
- Cooling Mechanism
- Charging time
- Heat Transfer Rate

The following figure shows the temperature contours of battery units with PCM materials. High temperature is observed in the middle portion of the battery unit.

Temperature distribution over a cylindrical cell is shown below

Summary

Several methods for battery cooling have been developed in the last two decades for the effective cooling of batteries in electric vehicles
The air cooling system is simple but the heat removal rate is low. In contrast, immersion cooling is fast but it can lead to short circuit or corrosion issues
Operating and environment conditions, safety, battery back, coolant, mode of cooling (forced or natural) and thermal interface are key parameters for battery thermal management systems
CFD modeling will help for the optimum design of the battery cooling system. The data can be used for the development of an AI algorithm for battery management with a better life for cells