The battery has emerged as the most prominent energy storage device to meet changing consumer needs in both the electric mobility and stationary energy storage industries. In fact, all major vehicle original equipment manufacturers (OEMs) have goals to aggressively electrify their fleets, adding electric vehicles (EVs) and hybrid-electric vehicles (HEVs) to their selections—some are even promising to go fully electric in just a few years. As a result, many engineers who have typically worked on conventional vehicles are now being asked to work on EVs and HEVs. As these engineers face the challenges of designing new technologies, the need for modeling and simulation tools for the creation of robust thermal management strategies is critical.
Thermal Management Strategies for EVs
Thermal management is critical for battery performance and service life. When deciding how to package and integrate a battery pack into a vehicle, it is important to make design choices with your thermal management strategy in mind. Batteries are like Goldilocks—they do not perform well when it is too hot or too cold. They need to maintain a temperature that is just right to achieve the performance, reliability, and safety that is desired by OEMs. Poor thermal management will affect the charging and discharging power, service life, cell balancing, capacity, and fast charging capability of the battery pack. A proper cooling strategy will ensure an even temperature distribution and eliminate potential hazards from un-regulated battery temperatures.
It is no easy task to balance thermal management design trade-offs when OEMs desire solutions that are lightweight, compact, reliable, serviceable, and low-cost. In this blog, we will take a look at current trends in battery packaging and thermal management, including passive cooling, active air cooling, and liquid cooling thermal management strategies.
The passive cooling strategy is the most straight forward approach, taking advantage of conduction through mounts and brackets, as well as natural convection with the air in the pack, to transfer the heat generated inside the pack to the environment with no additional hardware for increasing heat transfer. Passive cooling is low cost and "energy efficient," as it requires no energy from the car. However, even though it is the most prevalent cooling method seen today, it is not capable of keeping a battery pack within optimal cooling temperatures for high-performance applications and long-distance driving with multiple fast charges. This method is falling out of favor as companies seek thermal management strategies that keep warranty claims to a minimum and extend usability.
The active air-cooling strategy uses a fan with forced air passed over the batteries to remove heat. This strategy extends the lifetime of the battery pack compared to passive cooling as it keeps the batteries at a more consistent operating temperature. It is cheaper and lighter than liquid cooling and is more manageable to design because it does not have to interface with the other cooling networks in the vehicle. However, it can be challenging to develop battery packaging and mounting for airflow. Bracing and mounts can get in the way of airflow, and it can be challenging to distribute the airflow to maintain a uniform temperature. Using modeling techniques such as Computational Fluid Dynamics (CFD) or fluid nodal networks helps engineers solve these problems, but they can be tricky to overcome given the size and weight restrictions associated with designing a vehicle. Even though active air cooling is better than passive cooling at maintaining optimal operating temperatures, the most effective way to keep a uniform temperature within a pack and meet warranty requirements is liquid cooling.
Liquid cooling is the most effective way to remove heat from the battery pack. It is also better than active air cooling at keeping the battery pack within optimal operating temperatures. Designing a system that cools all of the batteries uniformly leads to better battery performance and lifetime. Liquid cooling also allows the battery pack to be operated with higher peak power loads because it dissipates more heat than other cooling methods.
There are three main approaches to liquid cooling: serpentine ribbon-shaped cooling tubes, cooling plates with cooling channels inside them, and direct/immersive cooling. The cooling tube approach is the most effective at maintaining uniform cell temperatures but is more challenging to manufacture and can result in higher pressure drops. The cooling plate approach is reasonably simple to implement but can lead to large temperature gradients across individual batteries. The direct cooling approach may present the most effective means of heat removal but is relatively new and requires expensive dielectric coolants instead of conventional cooling fluids.
It can be more challenging to design a system with liquid cooling because it has to be integrated with other electrical and fluid systems in the vehicle. The potential for a fluid leak must also be considered because it can cause an electrical short. Liquid cooling systems are generally heavier, more expensive, and more complicated to repair. However, the trade-offs can be worth it because liquid cooling systems provide extended lifetime and higher performance compared to air-cooled and passively cooled packs of the same size.
Key Indicators for Thermal Management Success
Despite the many benefits and drawbacks of each cooling strategy, one of them is generally best for a specific vehicle design and customer use case. Battery lifetime and performance are critical indicators of success for car owners, so regardless of the particular vehicle model and customer usage pattern, the right thermal management strategy will always be essential for success.
Using Simulation to Discover the Best Strategy
Finding the best thermal management strategy for a vehicle design can be taxing. As with conventional internal combustion engine vehicles, thermal analysis software helps design EVs and HEVs by allowing an engineer to look at design performance for many different environments and operational scenarios with relatively little effort to run additional conditions. The alternative is the "test-and-fix method," which is expensive and time-consuming. A prototype has to be made before a new design can be tested, and then a new prototype must be built for each successive design modification. The additional time it takes to complete the “test-and-fix” method can limit the ability of engineers to reach an optimal design. Simulation helps engineers stay ahead of the curve by analyzing designs numerically and then confirming them with physical testing after achieving confidence in the design.
Using Simulation for Full Vehicle Analysis
Using simulation allows a designer to see how the battery pack, its packaging, and its cooling method integrate with the entire vehicle. Fast and accurate simulation software enables a designer to evaluate the full vehicle in 3D. This means teams can understand how their changes affect other systems and work together to balance design priorities.
Using Simulation for Cabin and Drive Cycle Analysis
Adopting simulation can further improvement for the entire vehicle by using advanced modeling techniques. These techniques include analyzing cabin comfort and how it affects the range of the vehicle; examining different drive cycles various situations like city driving, race track driving, and key-off scenarios; or researching the lifetime predictions of a battery utilizing different duty cycles.
Using simulation opens doors for greater innovation and collaboration. The automotive industry continues to trend toward design initiatives that cost less, result in better designs, and provide more visibility for all of the teams and team members that make a vehicle design successful. Simulation is the best way to remain competitive in an environment where efficiency, cost, and superior design win the day.