Leveraging disruptive battery cooling technology to make electric aircraft a reality

High-power density batteries require more efficient cooling

Batteries are key to the future of electric aircraft. As the transition toward fully electric or hybrid powertrain ramps up, batteries with high capacity and C-rates are crucial for developing new products.

Nevertheless, batteries generate a lot of heat when they are in operation. It is critical to have intense and homogeneous cooling of battery cells for high performance and durability of the cells overcharging and flight missions. C-rates and corresponding heat generation in the cells are enormous, especially during takeoff. Safety is also crucial, particularly for aircraft applications. Engineers need to pay attention to battery thermal runaway prediction and mitigation and cell venting, which manifests itself by hot gas release and subsequent pressure rise in the battery module.

Ricardo is a global strategic engineering and environmental consultancy company involved in these issues, specializing in transport, energy and scarce resource sectors. Jan Majer is an analysis function lead at Ricardo and is responsible for global analysis and simulation teams. Project InCEPTion turned to Ricardo to develop a new, innovative solution for battery cell cooling that could be applied to the new electric propulsion module.

Project InCEPTion (Integrated flight Control, Energy storage and Propulsor Technology for Electric Aviation) aims to develop a propulsion system module – electric ducted fan (EDF) – that incorporates a battery pack, motor and power electronics into a single module. This scalable, power-dense, quiet and efficient module combines batteries and fuel cells to accelerate the electrification of various classes of aircraft from electric vertical takeoff and landing (eVTOL) aircraft, general aviation electric conventional takeoff and landing (eCTOL) up to subregional aircraft.

Figure 1. Using Simcenter Amesim to show internal cell heat flow analysis.

Immersion cooling is the future

The project team decided early on that immersion cooling was the preferred method for this project. As Majer explains, with conventional systems, battery cells sit on a cold plate and are only cooled from one side, whereas the other end of the battery remains hot. “With this new solution, the battery is submerged in dielectric fluid to give even cooling over the entire surface,” says Majer. “This makes battery systems more efficient with fast or ultra-fast charging. Everyone wants the fastest possible charging these days but the quicker you charge, the more heat you generate, so efficient cooling is essential.”

Majer notes that currently, cold-plate and air cooling are commonly used in various applications and immersion cooling can be critical for next-generation aircraft that require more power. Traditional cooling systems would not be sufficient for instances where you need to cool a battery very quickly, such as during takeoff.

Thermal and multiphase analysis

The thermal and fluids team at Ricardo used Simcenter™ software, which is part of Siemens Xcelerator business platform of software, hardware and services, to develop the new battery cooling system.

The team also used Simcenter Amesim™ software to assess flow, thermal and hydraulic performance during mission profiles. The thermal representation of the battery cell was based on fine discretization in all directions with radial and axial thermal conductivities.

The team used Simcenter STAR-CCM+™ software to conduct 3D computational fluid dynamics (CFD) analysis to estimate heat transfer from cell surface to fluid and pressure drop. Then they carried out optimization loops to equalize coolant flow velocities and improve cooling homogeneity. This led to a reduction in pressure drop due to the removal of excessively high-velocity spots, causing lower friction.

Next, using Simcenter Amesim enabled Ricardo engineers to guide pressure relief valve (PRV) specifications in the early design stages. Engineers used Simcenter Amesim and its thermal and hydraulic libraries, including multiphase capabilities, to model gas venting into a cooling fluid (mixture of liquid, gas and vapor).

During venting, the team tracked internal pressure in the modules to assess the risk of structural damage or explosion. This allowed engineers to assess the performance of the PRV to confirm that its size and opening characteristics were sufficient to prevent failure.

“Using Simcenter Amesim and Simcenter STAR-CCM+ provides us access to submodels representing real-world physical phenomena,” says Majer. “Using Simcenter enables to quickly build and set up the model through intuitive and straightforward user interfaces, extensive user manuals and other documentation. Instead of assessing and verifying existing solutions, simulation proactively feeds our design process.”

Figure 2. Using Simcenter STAR-CCM+ to visualize heat transfer throughout the battery pack.

Thermal runaway and cell venting simulation

With these analyses complete, using Simcenter STAR-CCM+ software was key to maximizing the resilience against thermal runaway propagation and damage due to venting gas flow and pressure elevation.

Figure 3. Cell vent gas profile.

Figure 4. The modules’ pressure distribution versus PRV opening.

Engineers tested CFD submodels with various levels of complexity to assess the risk of cooling evaporation and boiling on the hot cell surface. Engineers found that a two-phase volume of fluids (VOF) based model showed superiority over single-phase models by predicting the sudden movement of hot coolant and the unsteady coolant movement associated with rapid boiling.

The team modeled venting gas flow to confirm the module filled with coolant could provide sufficient flow toward the PRV and understand the amount of coolant released through the valve, which has a significant impact on the resilience against thermal runaway propagation. The model was also used to assess the module’s performance with different spatial orientations to help determine its ideal positioning.

Better cooling enables better batteries

Using Simcenter Amesim to analyze the 1D thermo-hydraulic battery pack cooling provided the team with vital insights into cell temperature distribution. Engineers compared resulting values against cell supplier targets and used their detailed understanding of the interaction flow and temperature fields to further optimize the design.

Using Simcenter STAR-CCM+ allowed engineers to complete the 3D CFD analysis of coolant inside the module under normal operating conditions over flight missions. The team used Simcenter STAR-CCM+ to optimize the design with a homogeneous flow pattern that reduced pressure drop to save hydraulic power and increase the efficiency of the powertrain.

For the thermal runaway, it helped identify the worst-case scenario, with the module at a 90-degree inclination angle with two cells in the end row venting. This could lead to gas bubbles surrounding the cells that are then at high risk of thermal runaway propagation due to the absence of coolant.

By turning the coolant pump back on, active cooling will move the vent gas away from the cells at risk, mitigating thermal runaway propagation.

These simulations enabled the team to make critical design decisions, such as cell arrangement, venting system or positioning and dimensioning of safety PRV early in the design process. This accelerated development time and gave the project team increased confidence in the structural integrity of the module and its cooling performance.

“I expect that immersion cooling technology will support the electrified aircraft propulsion soon because they’re so much more efficient and allow for more powerful batteries,” says Majer.

Figure 5. Venting gas flow through the module.

Figure 6. Coolant vapor as a result of a boiling VOF-based model.

Figure 7. Venting gas flow through the module with 90-degree module inclination.

Using Simcenter to drive engineering innovation

The team used Simcenter to enable a concurrent way of designing, analyzing and testing an aircraft to achieve full traceability.

“Using Simcenter enabled us to reduce our modeling phase time by 30 percent,” says Majer. “Leveraging Simcenter integrated tools proved to be very intuitive and user-friendly, resulting in short preparation and run times.

“Using Simcenter during the preconcept and concept phases enabled us to quickly assess the modeling, whilst in the definitive phase of the project, detailed 3D modeling captured all the usable physical phenomena in detail. For this project, we used various available physical submodels that supported accurate modeling of complex phenomena.”

Since the thermal management of the batteries is a critical safety issue, their design, development, verification and certification fit into the Siemens Xcelerator product design and engineering, model-based systems engineering (MBSE) and verification management digital threads. Ricardo used Simcenter performance engineering skill tools to provide required engineering data to support aircraft programs and processes. Leveraging Simcenter solutions enabled Ricardo to size and optimize the detailed design and provide the elements for proof of compliance based on virtual test data. This effectively contributed to aircraft program execution excellence by enabling the team to stay on track and budget.