Cooling the Powerhouse: Unleashing Performance and Safety in Electric Vehicles with Computational Fluid Dynamics

This article is contributed by Jai Makhija

Image source: Unsplash/Hyundai Motor Group

• A battery thermal management system is required to allow the next generation of electric vehicles (EVs) to compete with internal combustion engine (ICE) powered vehicles in terms of range, lifetime and fast charge.
• Computational Fluid Dynamics (CFD) can be used to rapidly iterate and optimize the thermal management system of the vehicle.
• Various cooling strategies are used in EV’s: air cooling, phase change material (PCM) cooling, liquid cooling, and immersion cooling.
• The use of CFD can be extended to simulate other scenarios such as battery heating, battery insulation and propagation of thermal runaway.

Introduction

As the internal combustion engine (ICE) is the powerhouse of a gasoline car, the battery pack is the equivalent for an electric vehicle (EV). With the exponential surge in demand for EVs all around the world comes a rise in customer expectations for higher payload capacity, increased battery range, prolonged battery life, and reduced charging time. If the battery pack is to support these features, it needs a proper thermal management system. This is where Computational Fluid Dynamics (CFD) comes into play!

What is Computational Fluid Dynamics (CFD)?

CFD is a methodology used to predict how fluids move in a given environment with the help of the Navier-Stokes equations. The Navier-Stokes equations use the principles of conservation of mass, momentum and energy to describe fluid behavior, and CFD uses these equations to analyze that behavior within a fluid domain. In the case of flow around a car, the fluid domain refers to the air surrounding the car. This domain is decomposed into small cells in which the Navier-Stokes equations are solved iteratively. CFD drives the design process of vehicles for all modes of transportation, including aircraft, ships, high-speed cars (such as Formula 1), and even rockets. It effectively serves as a virtual wind tunnel to help improve the performance and efficiency of these vehicles. For EVs, the use of CFD is not just restricted to controlling the external aerodynamics of the vehicle, but it can also be used to optimize the cooling strategy of the battery packs.

Flow across a Formula 1 Car (Simulation image generated by the author)

Along with cell configuration and mechanical design, thermal management is a crucial part of designing battery packs for EVs. After creating virtual prototypes of our battery pack using 3D modeling software, we can use CFD to test out various cooling strategies. This process helps us come up with an optimal design where we achieve performance without incurring significant prototyping costs during the testing and validation phase. It also helps lower material costs for the final product.

Why do the batteries need cooling?

A commonality between humans and lithium-ion batteries is that both operate best at ambient temperatures (20–30 degrees Celsius). But, given the amount of heat generated by these battery packs, especially during fast charging (a feature provided by almost every EV), the cells can only be maintained at these temperatures with the aid of a robust cooling strategy.

At the same time, it is important to ensure that the temperatures across the pack are as close as possible while cooling the pack, since different temperatures result in varying levels of degradation across cells in different parts of the battery pack. As the performance of the battery pack is limited by the performance of its worst cell, it is imperative that we ensure a homogeneous temperature distribution across the pack.

How are battery packs cooled?

Immersion cooling for lithium-ion batteries — Roe, Charlotte et. al. (2022).

These are the 4 common strategies used to cool battery packs:

• Air cooling: Air is used as the medium for managing the thermal system of the battery pack. The system can be passive or active: A passive system recirculates ambient air from outside the vehicle to cool the battery pack, whereas an active system uses additional components, such as a compressor and heat exchanger, which can drop the air temperature below ambient to cool the battery pack. While air cooling offers a relatively cheap system, especially when using passive cooling, the low specific heat value makes it less suitable for cooling battery packs with a heat generation of greater than 1 kW. This strategy was adopted by older versions of both the Nissan Leaf and the Toyota Prius Prime.
• Phase Change Material (PCM) cooling: This method involves using the latent heat of the PCM to absorb heat from the battery. Latent heat is the energy absorbed or released by the material as it undergoes a phase change process, such as freezing or melting. Because a PCM can absorb a large amount of heat at a fixed temperature during the phase transition, it has the potential to keep the temperature of the battery relatively constant. PCMs being used in EV’s include paraffin wax (melting point of 48 to 66°C) and capric acid (melting point of 31.4°C). PCM-based cooling systems are able to maintain a homogeneous temperature distribution at low cost, but these materials have low thermal conductivity, the materials’ thermal properties degrade with use, and auxiliary systems, such as air cooling, are still needed to regenerate the original solid phase of the PCM once the cells have cooled. This cooling strategy is suitable for electric 2-wheelers with lower heat generation, such as the Ather 450X.
• Liquid cooling: This system uses a liquid (water-glycol or a refrigerant) to cool the battery pack. Similar to air cooling, there are both passive and active systems. Due to the fact that the specific heat of the liquid is 3–4 times that of air, this system has a higher cooling capacity of around 3–4 kW. Heat is transferred from the cells through the cooling channel into the fluid, and the cooling channel is designed to maintain contact based on the form factor of the cells. Tesla has come up with a unique serpentine channel for keeping in contact with the curved sides of cylindrical cells, whereas the channel is typically in contact with the bottom or faces of prismatic cells, depending on the thermal conductivity and contact area offered by each cell. This system is currently used by all models of Teslas, the Chevy Volt, and the Jaguar iPace.
• Immersion cooling: Although active liquid cooling systems have been adopted by many companies, the fact that the cooling is still taking place indirectly introduces some inefficiencies to the process. That is, the coolant is not in direct contact with the cells but is instead separated by the cooling channel, and the thermal resistance of the materials between the coolant and cells contributes to some loss in cooling capacity. Hence, the concept of immersion cooling was developed, in which a thermally conductive, electrically insulative fluid absorbs heat via direct contact with the cells. Not only is this dielectric fluid more expensive, but material compatibility with the rest of a pack can be an issue for mass production. As a result, immersion cooling has not yet been applied to passenger vehicles and is currently only implemented on high-performance vehicles, like the Audi Dakar RS and the Koenigsegg Regera.

The cooling strategy is selected based on the form factor of the cell, heat generated during charge-discharge cycles, and bill of materials (BOM) costs.

Future prospects of CFD for batteries

Although this article focuses on the cooling of lithium-ion cells, CFD is not limited to this application. When ambient temperatures drop below 0 degrees Celsius in the winter, for example, some chemistries require the battery pack to be heated, and the design of these heating systems also uses CFD. In addition, CFD can be used for designing ventilation systems to remove hazardous gasses during thermal runaway as well as battery pack insulation. The difference between the pack temperature and ambient temperature is significant.

Summary

Battery packs in electric vehicles require proper thermal management systems to support large payload capacities, extended range, battery life, and fast charging. In order to design a proper battery thermal management system, Computational Fluid Dynamics (CFD) is used to optimize the cooling strategy of battery packs, leading to improved performance and cost savings. Different cooling strategies for battery packs include air cooling, phase change material (PCM), cooling, liquid cooling, and immersion cooling, and the cooling strategy is chosen based on the form factor of cells, heat generated during charge and discharge cycles, and BOM cost. The use of CFD can also be extended beyond cooling, including designing heating systems, ventilation systems for hazardous gasses, and insulation to reduce power consumption, as a robust thermal management system is critical for EV performance.

Jai Makhija has a Bachelor’s in Mechanical Engineering and is passionate about building and optimizing products with the help of CFD simulations. Jai has been a part of deep tech startups in the aerospace and automobile sectors where he has worked on solving issues such as aerodynamic heating of launch vehicles, sloshing in propellant tanks, and cooling of lithium-ion battery packs. To build on his skills, he is currently pursuing his Master’s in Race Car Aerodynamics at the University of Southampton.

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Disclaimer: The views expressed in this article are those of the author and do not necessarily reflect those at University of Southampton.