Battery Cooling Techniques in Electric Vehicle

What are different Cooling Methods for Battery 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 of 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 is 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

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

  • 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

Battery Thermal Management System (BTMS)

  • 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
  • BCS can be external or internal while limited internal BCS has been documented for LIBs which require further research. There has been a detailed discussion of BCS external to the battery
  • External BCS are categorized into several different ways. BCS are categorized based on the method or technique. It can be liquid cooling, gaseous cooling (plate type or use of mini-channel), heat pipe and PCM cooling.

Air Cooling

  • Air is easily available for cooling purposes and it is widely used in many industries. However, air is not an effective coolant because of its lower heat capacity and low thermal conductivity
  • Due to low maintenance of cost, air is still used by some automobile industries like Toyota Prius and Nissan Leaf for cooling of battery modules

  • 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

Liquid cooling of Battery

  • Liquid coolants have high convective heat removal rate due to higher density and heat capacity compared to air
  • Liquid cooling system is more compact than 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, complexity of 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-circuit 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
  • An addition of electrical resistance may resist thermal diffusion. But its effects on cooling is not significant
  • Some EV manufacturer such General Motor, and Tesla are using indirect cooling in their cars. General Motor used cold plates between each prismatic cell. The cold plates have several microchannels for convective heat removal from batteries.
  • Tesla has used wavy tubes placed between cylindrical cells. Thermally conductive and electrically insulated material used to fill the gaps between the battery cells and cooling ducts
  • The wavy tubes may not be effective for cooling because of small heat transfer contact area, it is safer from the mechanical and electrical point of view. All the connections of coolants are kept the outside of 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

  • Design strategy of thermal management system for battery modules, controller, and electric motor

Chevrolet Cooling System

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 of 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

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 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 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
    • 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

CFD and flow pattern for Battery cooling

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 optimum design of battery cooling system. The data can be used for AI algorithm of battery management with better life of cells

Reference

A.Chu , Y.Yuan, J. Zhu, X. Lu,C. Zhou, The Design and Investigation of a Cooling System for a High Power Ni-MH Battery Pack in Hybrid Electric Vehicle (2020), 10 (1660) Applied Sciences

 

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