Week 10 Thermal Management

Prepare a report summarizing thermal management techniques for the electric car battery pack.

A lithium-ion battery consists of an anode, cathode and electrolyte as well as a separator, the anode, is the oxidised electrode which removes electrons to the external circuit during discharge. a Li-ion battery is a rechargeable which means its electrochemical reactions are reversible. Lithium ions disperse from the negative to the positive when discharging and go in reverse when charging.

Thermal energy is moving from one place to another, tending to change the temperatures of the source and destination. Heat generation that means thermal energy is being added to a system, tending to increase its temperature. Heat sink that means thermal energy is being removed from a system, tending to decrease its temperature. There are various mechanisms by which thermal energy can travel, conduction, convection, and radiation.

Lithium-ion cells performance depends on both the temperature and the operating voltage. Lithium-Ion cells work well when cells operate within limited voltage and temperature. Otherwise, the damage will occur to the cells and will be irreversible. Thermal runaway consists of several stages and each stage will give rise to more irreversible damage to cells.

Lithium-ion battery resistance increases with decreasing in temperature whereas cold temperature increases the internal resistance and lowers the capacity. The EV battery pack is affected by two ways in this condition, battery power decreases in temperature also its impact on power capability of electric motor and vehicle acceleration. That means a battery that provides 100 per cent capacity at 27°C will typically deliver only 50 per cent at –18°C.

Operating a battery at elevated temperatures improves performance but prolonged exposure will shorten life. Lithium-ion battery capacity decreases with decreasing temperature and in this case, the useful energy from the battery decreases with a decrease in temperature and its impact on the driving range as well as the performance of an electric vehicle.

Battery temperature affects vehicle performance, reliability, safety, and life-cycle cost. Temperature affects battery in the following ways:

• Operation of the electrochemical system
• Round trip efficiency
• Charge acceptance
• Power and energy availability
• Safety and reliability
• Life and life-cycle cost

If battery temperature increases at higher range then it affects in the following ways:

• Life
• Safety
• Non-uniform ageing due to thermal gradients

Cooling for such a battery must achieve some typical condition in a hot environment it requires up to 24 hr to cool down. Use of Phase change material can improve the battery heating because when the temperature reaches the melting point, heat is absorbed and stored as latent heat until the latent heat is up to the maximum without the temperature increasing. At the same time, PCM changes its phase from solid to liquid. After that, PCM becomes liquid and heat is absorbed by PCM and stored as sensible heat

Electric Vehicle Battery Thermal Management

A thermal management system for batteries is needed because of the best optimal temperatures of the batteries do not comply completely with the possible operating temperatures of the vehicle. Every electric vehicle battery pack needed a thermal management system. It is needed to operate in the desired temperature range for best performance and life of the battery. A battery should maintain its temperature range from 15 ° C to 35 ° C. It is also required to control the potential hazards related to uncontrolled temperatures and thermal runaway

Battery Thermal Management System Required For

• Compact
• Lightweight
• Easily packaged
• Reliable
• Serviceable
• Low-cost
• Low parasitic power
• Optimum temperature range
• Small temperature variation

The increase in resistance will result in a higher heat generation. Depending upon high temperature and low temperature a battery may get affected and the conditions are as given:

During Low temperature i.e. 0°C

• Lithium-ion battery Capacity drop
• Lithium-ion battery Internal resistance increase

During higher temperature i.e. above 40°C

• Internal resistance decrease
• Accelerated ageing phenomena
• Higher self-discharge
• Decomposition of electrolyte
• Thermal runaway, safety considerations
• Reduced life cycle

Battery thermal management system is depending upon the type of battery, cooling or heating so that it can operate in best temperature mode. For the cooling aspect, it is important to distinguish three major cooling methods: liquid, air and Phase Change Material (PCM) cooling.

The technology of battery thermal management system

Thermal Control by using Air

For example, the ambient temperature is 45°C-50°C at that time the temperature inside the battery pack will be 50°C-55°C, and it will create a thermal runaway by which the battery will not work properly because the temperature is beyond the operating temperature. So for that, we need a thermal management system.

The lowest cost method for EV battery cooling is with air. A passive air-cooling system uses outside air and the movement of the vehicle to cool the battery. Active air-cooling systems enhance this natural air with fans and blowers. The figure shows the air cooler battery thermal management system used in Toyota’s Prius.

Air systems use air as the thermal medium. The intake air is directly entering into the system it can be atmospheric air. Also, the cabin air can be used for cooling purpose after the use of heater or evaporator of an air conditioner. This form of the air supply system is known as a passive air system. A passive system required about 100 watts of cooling or heating power supply. A figure is as given below.

In an active cooling system, there is the addition of cooling or heating power. An active system requires power supply up to 1 kW and this is the limited power to operate an active system.

In both cases the air is supplied by a blower/fan, they are also called forced air systems and this type of system is used in Prius & Insight car.

The air supply system is used to operate for full functions of heating, cooling and ventilation. There is no requirement to build an extra ventilator. In this case, the exhaust air cannot be used or return back to the cabin again.

In some situation, a heat exchanger has mounted after the battery pack in order to recover the heat from the exhaust air. It can stop the mixture of exhaust air with intake air and at the same time provide an extra saving potential. A figure is showing the forced air system with heat recovery. .  The system is used for i-MiEV, fast charging vehicle.

Advantage of Battery Heating and Cooling by Using Air Thermal Management

• All waste heat eventually has to go to air
• No leakage concerns
• The low mass of air and distribution system
• No electrical short due to fluid concern
• Simple design, less cost and maintenance is easy

Disadvantages of Battery Heating and Cooling by Using Air Thermal Management

• Low heat transfer rate
• Temperature variation in battery pack
• High fan/blower noise

Thermal Control by Using Liquid

The liquid is another heat transfer fluid to transfer heat. There are generally two groups of liquids applied for thermal management systems. One is dielectric liquid i.e. direct-contact liquid which can contact the battery cells directly, such as mineral oil. The other is conducting liquid i.e. indirect-contact liquid which can only contact the battery cells indirectly, such as a mixture of ethylene glycol and water. Depending on the different liquids, different layouts are designed. For direct-contact liquid, the normal layout is to submerge modules in mineral oil.

A figure highlights passive liquid cooling system

A figure highlights an active liquid cooling system, used in the Volt and Tesla vehicle.

Piped liquid cooling systems provide better battery thermal management because they are better at conducting heat away from batteries than air-cooling systems. One downside is the limited supply of liquid in the system compared with the essentially limitless amount of air that can flow through a battery.

Tesla’s thermal management system (as well as GM’s) uses liquid glycol as a coolant. Both the GM and Tesla systems transfer heat via a refrigeration cycle. Glycol coolant is distributed throughout the battery pack to cool the cells. Considering that Tesla has 7,000 cells to cool, this is a challenge.

Advantage of Liquid Cooling System

• Pack temperature is more uniform - thermally stable
• Better heat transfer
• Better thermal control
• Lower Pumping power
• Compact design

The disadvantage of Liquid Cooling System

• Weight is maximum
• Leakage Potential
• Maintenance is high
• Higher viscosity at cold temperatures
• Cost is high

Heat pipe cooling

The established technology of heat pipes can be divided into three sections: evaporator section, isothermal section, and condenser section. The working process of the heat pipe is that Firstly, liquid cooling medium absorbs heat and evaporates at the evaporator section. Secondly, the liquid cooling medium moves from the isothermal section to the condenser section, releases its heat, and changes into a liquid. Finally, the liquid flows to the evaporator section and continues to endothermic like the former. Due to the special heat conduction performance, lightweight and compact size, it also can be used to remove heat from the internal battery pack to outer

Phase Change Material for Cooling

Phase change material (PCM) is commonly used in the study of battery thermal management system (BTMS). The heat generation rate of the lithium-ion battery increases with its increased discharge rate. During melting, heat is absorbed by PCM and is stored as latent heat until the latent heat is up to the maximum. The temperature is kept at the melting point for a period and the temperature increase is delayed. Therefore, PCM is used as a conductor and buffer in battery thermal management systems. Also, a PCM is always combined with air cooling system or liquid cooling system to manage the battery temperature.

 

PCM cooling systems perform well on thermal management. Even under high ambient temperatures from 45°C to 50°C, the temperature inside the cell pack is always below 55°C because of high thermal conductivity and latent heat.

Combined PCM with the liquid cooling system.

Another way is by using a combination of PCM with Liquid cooling system. Each and every cell is fitted in a PCM layer for better thermal performance and liquid cooling is applied some certain temperature when the temperature goes higher then PCM. The detail of PCM combined with liquid cooling is given in the figure.

The important component is PCM Layer, Heater, Pump, Valve, Radiator and Evaporator. The state of the PCM is controlled by the battery temperature and heat generated by the battery.

Reference:

1. Electric Vehicle Battery Thermal Issues and Thermal Management Techniques, John P. Rugh, Ahmad Pesaran, Kandler Smith – SAE International & NREl
2. A Review of Lithium-Ion Battery Thermal Management System Strategies and the Evaluate Criteria Shuting Yang, Chen Ling, Yuqian Fan Yange Yang, Xiaojun Tan4,  Hongyu Dong, Int. J. Electrochem. Sci., Vol. 14, 2019
3. Battery Thermal Management Systems of Electric Vehicles, JILING LI, ZHEN ZHU