Week 10 Thermal Management
AIM - To prepare a report summarizing thermal management techniques for electric car battery pack.
Why Electric Vehicle's require Thermal Management System?
While advancements have been made in electric vehicle batteries that allow them to deliver more power and require less frequent charges, one of the biggest challenges that remain for battery safety is the ability to design an effective cooling system. In electric cars, discharging the battery generates heat; the more rapidly you discharge a battery, the more heat it generates. Batteries work based on the principle of a voltage differential, and at high temperatures, the electrons inside become excited which decreases the difference in voltage between the two sides of the battery. Because batteries are only manufactured to work between certain temperature extremes, they will stop working if there is no cooling system to keep it in a working range. Cooling systems need to be able to keep the battery pack in the temperature range of about 20-40 degrees Celsius, as well as keep the temperature difference within the battery pack to a minimum (no more than 5 degrees Celsius).
If there is a large internal temperature difference, it can lead to different charge and discharge rates for each cell and deteriorate the battery pack performance. Potential thermal stability issues, such as capacity degradation, thermal runaway, and fire explosion, could occur if the battery overheats or if there is non-uniform temperature distribution in the battery pack. In the face of life-threatening safety issues, innovation is continually happening in the electric vehicle industry to improve the battery cooling system.
Thermal Management Techniques for Electric Vehicle's :- Similar to conventional ICE vehicles, a battery pack in EVs or HEVs also needs a cooling system. An EV battery pack comprises cell modules, interconnects power management system and thermal management system.Different types of cooling system will influence the performance and cost of the battery pack thermal management system.
There are three common battery thermal management method used today-
1. Air Cooling
2. Liquid Cooling
3. Phase change material(PCM), Heat pipe or combination of them.
1. Air Cooling - Air cooling uses the principle of convection to transfer heat away from the battery pack. As air runs over the surface, it will carry away the heat emitted by the pack. Air cooling is simple and easy, but not very efficient and relatively crude compared to liquid cooling. Air cooling is used in earlier versions of electric cars, such as the Nissan Leaf. As electric cars are now being used more commonly, safety issues have arisen with purely air-cooled battery packs, particularly in hot climates. Other car manufacturers, such as Tesla, insist that liquid cooling is the safest method.
Air cooling is not suitable for most new high-performance applications due to the power density required and the inability to cope with a wide range of ambient temperatures. It is simply not possible to remove sufficient heat from within the battery with an air cooling system alone, some air cooling will generally occur on a battery that is positioned on the underside of the vehicle due to airflow during driving but this is insufficient to meet the full cooling needs of the battery.
2. Liquid Cooling - Liquid Cooling technique can be implemented with two types - direct contact with cells and indirect contact with cells. At present no one using direct cooling but some developers are trying to use non conducting coolants. Indirect cooling is most common technique and it uses a mixture of glycol and water mostly. This technique is used by EV manufacturer like Tesla, Jaguar, BMW, others.Liquid coolants have higher heat conductivity and heat capacity (ability to store heat in the form of energy in its bonds) than air, and therefore performs very effectively and owns advantages like compact structure and ease of arrangement. Out of these options, liquid coolants will deliver the best performance for maintaining a battery pack in the correct temperature range and uniformity. Liquid cooling systems have their own share of safety issues related to leaking and disposal, as glycol can be dangerous for the environment if handled improperly
a. Direct cooling systems place the battery cells in direct contact with the coolant liquid. These thermal management schemes are currently in the research and development stage, with no cars on the market using this system. Direct cooling is more difficult to achieve, due to the fact that a new type of coolant is required. Because the battery is in contact with the liquid, the coolant needs to have low to no conductivity.
b. Indirect cooling systems are similar to ICE cooling systems in that both circulate liquid coolant through a series of metal pipes. However, the construction of the cooling system will look much different in electric vehicles. The structure of the cooling system that achieves maximum temperature uniformity is dependent on the shape of the battery pack and will look different for each car manufacturer.
3. Phase Change Material(PCM) – In this technique material changes the phase from solid to liquid after absorbing heat from battery pack. PCM stores absorbed heat as latent heat up to its maximum limit. The thermally conductive structure holding the PCM provides a fast heat transfer path through battery pack, minimizing temperature difference between the cells. This technique can meets the cooling requirement but volume change during phase change limits its application and it gathers heat not transfers it away, so it is not useful for electrical vehicle. Phase change material cooling systems can meet the cooling requirements of the battery pack, however the volume change that occurs during a phase change restricts its application. Also, phase change material can only absorb heat generated, not transfer it away, which means that it won’t be able to reduce overall temperature as well as other systems. Although not favorable for use in vehicles, phase change materials can be useful for improving thermal perfomance in buildings by reducing internal temperature fluctuations and reducing peak cooling loads.
