Improving Battery Design for eVTOL Aircraft With Simulation

August 1, 2024

Batteries for eVTOL aircraft need to deliver high power for efficient takeoff and landing, as well as high energy for the cruise period. To meet these demands, designers must consider the power–energy tradeoff of batteries and integrate a reliable battery management system into the overall design. Multiphysics simulation can be used to evaluate this tradeoff and consider all design requirements in a way that is comprehensive and saves time.

In recent years, more and more organizations have announced their development of electric vertical take-off and landing (eVTOL) systems and, in some cases, are even showing previews of systems that are intended to hit the market in just a few years. As new design ideas emerge, there is one important question that needs to be asked: To keep up with the developments in eVTOL aircraft, what design requirements need to be considered for the batteries that power them?

Batteries used in transport technology need to be safe and durable and have high performance and quick charging time. Although electric batteries that meet these needs already exist on the market due to the rise in electric vehicles (EVs), there are notable differences in the performance needs for EV and eVTOL batteries. For instance, with eVTOL systems, the stages of flight need to be considered, as the battery cannot be so heavy as to hinder takeoff yet needs enough power to support vertical takeoff and landing as well as (horizontal) cruising. (Ref. 1) In addition, an eVTOL battery needs to have a long cycle life and rapid charging capabilities so that it can be quickly recharged in the time between the aircraft landing and taking off again. Creating a safe battery that meets these demands requires evaluating the power–energy tradeoff, designing an optimal battery management system, and reducing the risk of battery degradation. Multiphysics simulation offers an efficient way to work through these steps and perform in-depth analyses at different scales.

Figure 1. A cell scale and pack scale model in the COMSOL Multiphysics® software. (Image: COMSOL)

Power–Energy Tradeoff

In the context of eVTOL aircraft, it is essential to highlight the dual demands of high power for takeoff and landing, as well as high energy for sustained flight. Ideally, there needs to be a balance between power-optimized and energy-optimized cells in an eVTOL system. Power-optimized cells provide high power density and can deliver high current loads, which are imperative for rapid acceleration, takeoff, and maneuvering. They are critical during the takeoff and landing phases, when high bursts of power are required. In contrast, energy-optimized cells provide high energy density and can store more energy, translating to longer flight times and greater range. These cells are important for maintaining prolonged flight duration, ensuring the aircraft can travel significant distances without requiring frequent recharges.

Figure 2. The cell voltage versus the state of charge (SOC) for various C-rates of an energy-optimized cell (solid lines) and the corresponding open-circuit voltage (OCV) versus SOC (dashed line). (Image: COMSOL)

The COMSOL Multiphysics software can play a crucial role in evaluating and optimizing battery design to achieve the crucial balance between these dual demands. Performing studies such as rate capability analyses provides valuable insight into how batteries perform under varying charge and discharge rates. By simulating a range of different C-rates, engineers can evaluate factors like capacity retention and voltage stability (Figure 2), enabling them to predict how different battery configurations and materials impact performance under real-world eVTOL conditions. By adjusting parameters such as electrode thickness, electrode porosity, and electrolyte composition, engineers can optimize battery designs to strike a balance between power density and energy density and enhance the overall reliability, longevity, and safety of eVTOL battery systems.

Figure 3. A Ragone plot depicting volumetric energy against average volumetric power, comparing a cell optimized for energy with one optimized for power. (Image: COMSOL)

In the software, the evaluation can be expanded to gain a deeper understanding of specific results and comprehensive insights into key aspects of the underlying phenomena occurring in the cells. For instance, if a significant capacity decrease occurs at a certain C-rate, designers can investigate the underlying cause by analyzing internal variables of the cell, such as the electrolyte salt concentration and the potential drop across it. Such detailed analyses can help designers understand why the capacity utilization decreases considerably with increased load. Or, if the goal is to compare two different battery designs, generating a Ragone plot (Figure 3) helps visualize how these batteries perform in terms of storing energy (energy density) versus delivering power (power density) under different operating conditions, enabling a timely comparison.

Battery Management Systems

All applications using batteries need a battery management system (BMS). The BMS continuously monitors critical performance parameters such as voltage, current, temperature, and state of charge (SOC) to keep a battery within safe operational limits, protecting it from conditions like overcharging, over discharging, and overheating. It also balances the cells within a battery pack to ensure uniform performance and extend the overall battery life. This information is communicated to users and operators in real time, keeping them well informed about the status and health of the battery systems.

As batteries operate, various degradation processes occur, such as mechanical degradation, the loss of active material, and electrolyte depletion. These processes lead to several unwanted effects that cause capacity loss in batteries. They diminish the battery’s performance, reducing its ability to deliver power (power fade) and store energy (capacity fade) over time. Early detection of battery degradation helps mitigate risks like in-flight failures or power loss and assists operators in planning battery usage and maintenance. Tracking degradation metrics such as impedance, capacity retention, and voltage profiles during cycles via the BMS provides insight into the extent of degradation over time and ensures the safe, reliable, and efficient use of batteries in various systems.

In COMSOL Multiphysics, conducting a dynamic degradation and life analysis enables the integration of diverse degradation mechanisms into a simulation. This approach facilitates the study of the evolution of parasitic processes over time, their impact on regular battery operation, and the resulting capacity loss. It also enables investigation of subsequent changes in cell chemistry and the cell’s structural integrity. For example, an aging model can be developed to simulate the formation of a parasitic solid-electro-lyte-interphase (SEI) film on the negative electrode of a lithium-ion cell over 2,000 cycles, resulting in irreversible loss of cyclable lithium (Figure 4). This model also captures the resistance of the growing SEI film as well as the effect of the reduced volume fraction on charge transport within the electrolyte.

Figure 4. The relative capacity versus cycle number of an example model. (Image: COMSOL)

Monitoring a battery’s temperature is also critical for optimizing its performance, evaluating and preventing degradation, and ensuring safety. In a battery pack, if one cell overheats, the other cells in that pack and in connecting packs will follow suit, leading to battery failure and potential safety issues, such as fire. Looking at heat generation among cell packs can also guide decision-making regarding the setup of how the batteries will be housed in the eVTOL system. If the batteries are interconnected, there is an increased risk of all batteries malfunctioning while the system is in the air.

Using the COMSOL software, a thermal analysis can be performed to identify hot spots and determine the maximum temperature threshold at which cells may overheat — providing designers with a better understanding of what design adjustments are needed to prevent thermal runaway. Additionally, engineers can simulate an individual battery cell experiencing thermal runaway due to abuse like short circuits or excessive heating in order to forecast when and how the runaway will occur, as well as how quickly it will spread throughout the entire battery pack. The software can also be used to virtually test different cooling mechanisms and thermal management strategies.

Versatility of COMSOL Multiphysics for Battery Design

Like with EVs, lithium-ion batteries are widely considered as the frontrunner battery choice for eVTOLs, but alternative technologies, such as solid-state batteries, sodium-ion batteries, and fuel cells, are also being considered (Ref. 1). The Battery Design Module, an add-on product to COMSOL Multiphysics, includes built-in features for modeling lithium-ion batteries and general battery and electrochemistry features for constructing a battery model for any chemistry, including sodium-ion and solid-state batteries. Additionally, the Fuel Cell and Electrolyzer Module offers specialized functionality for modeling fuel cells.

Figure 5. A model of thermal runaway propagation in a battery pack. (Image: COMSOL)

Depending on the purpose of the modeling, battery modeling in COMSOL Multiphysics can be carried out at different scales, ranging from highly sophisticated microscopic models aimed at a detailed understanding of battery behavior to simplified, lumped models used for simulating battery packs integrated within larger systems, such as eVTOLs. The COMSOL software also provides unique functionality for performing multiphysics analyses, facilitating a thorough examination of how electrical, thermal, and mechanical factors interact across these scales. (Ref. 2)

eVTOL Battery Simulation Takes Off

Batteries for use in eVTOL systems need to achieve sufficient power–energy tradeoff, and multiphysics simulation offers a comprehensive and timely way to work through all of the design requirements. Reliable design software is especially important right now given the expected increase in eVTOL aircraft design in the aerospace and defense industry and the growing discussion around eVTOL aircraft for public transport. Before these aircraft take flight, suitable battery designs are needed.

References

“The Key Things to Know about eVTOL Batteries” Asian Sky Group; 2022.
H. Ekström and E. Fontes , “Battery Modeling,” COMSOL.

This article was written by Niloofar Kamyab, Ph.D., Applications Manager, Electrochemistry, COMSOL Inc. (Burlington, MA). For more information, visit here .