Week 10 Thermal Management

- Aim: To prepare a report summarizing thermal management techniques for the electric car battery pack.

- Method:

- 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). 

- 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. 

- The flow of current causes heating in the battery cells and their interconnection systems proportional to the square of the current flowing multiplied by the internal resistance of the cells and the interconnect systems. The higher the current flow the more the heating effect will be.

- The performance of Lithium-Ion battery cells is greatly impacted by their temperature, they suffer from the Goldilocks effect, they do not perform well when too cold or too hot, which can lead to permanent and extreme damage of the cells or accelerated degradation. So in addition to cooling, heating of the cells may also be required at lower ambient temperatures to prevent damage during fast charging when the cells are too cold; this is because the internal resistance of the cells rises when they are cold. Most lithium battery cells cannot be fast-charged when they are less than 5^oC and cannot be charged at all when they are below 0^oC. Lithium cells also begin to degrade quickly when their temperature is above 45^oC.

- There are few options to cool an EV battery:

1. Phase change materials:

- Phase change material absorbs heat energy by changing the state from solid to liquid. While changing phase, the material can absorb large amounts of heat with little change in temperature.

- 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 the overall temperature.

- In order to maximize the amount of stored battery energy available for its primary function, a passive management
the solution is appealing. This can be accomplished by integrating a structure with the battery so that the PCM is in direct thermal contact with the battery cells. When a soldier draws power from the battery, usually during a constant current discharge, the added thermal mass of the PCM system helps to absorb the waste heat given off by the cells and slows their temperature rise. If the temperature continues to rise, especially when the battery operates at a high rate and in high ambient temperatures, the PCM melts and effectively limits peak temperature. In addition, the thermally conductive structure holding the PCM provides a fast heat transfer path through a battery module, minimizing the temperature difference between the cells in the module. 

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. Figure 3.6 shows the working mechanism of PCM on battery cells. Also, a PCM is always combined with an air cooling system or liquid cooling system to manage the battery temperature.

2. Fins:

- Cooling fins increase the surface area to increase the rate of heat transfer. Heat is transferred from the battery pack to the fin through conduction, and from the fin to the air through convection.

- Fins have high thermal conductivity and can achieve cooling goals, but they add a lot of additional weight to the pack. The use of fins has found a lot of success in electronics, and traditionally they have been used as an additional cooling system on internal combustion engine vehicles.

- Using fins to cool the electric car battery has fallen out of favour since the additional weight of the fins outweighs the cooling benefits.  

3. Air or liquid 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. 

- 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.

- It has 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. 

- Heat transfer fluid is circulated by the pump within a closed system. The circulating fluid absorbs heat from the battery pack and releases heat via a radiator. The cooling power depends strongly on the temperature between ambient air and battery. Fans behind the radiator can improve the cooling performance, but if the ambient air is higher than the battery temperature or the difference between them is too small, the passive liquid system becomes ineffective.

- There are two loops. The upper is called the primary loop and the lower the secondary loop. The primary loop is similar to the loop in a passive liquid system, where the heat transfer fluid is circulated by the pump. The secondary loop is actually an air conditioning loop (A/C loop). The upper heat exchanger instead of being a radiator works as an evaporator (EVAP) for cooling operation and connects both loops. During heating operation, the 4-way valve will be switched, and the upper heat exchanger works as a condenser (COND) and the lower heat exchanger works as an evaporator. The heating operation loop is also called a heat pump loop.

- Reference:

- https://www.dober.com/electric-vehicle-cooling-systems#future_of_ev_battery_cooling

- https://avidtp.com/what-is-the-best-cooling-system-for-electric-vehicle-battery-packs/