System-level thermal and electrical modeling of battery systems for electric aircraft design

TL;DR

The paper addresses the challenge of sizing and validating battery thermal management for an 8-seat electric aircraft powered by high-energy NMC Li-ion cells. It develops a system-level framework that couples an ECM-based single-cell battery model with a lumped thermal balance and a Vapor Cycle Machine (VCM) BTMS, together with a thermal runaway module, to minimize internal battery energy $\Delta E_b$ over a predefined flight via design variables $p=(T_{fl}, \dot{V}_{fl}, P_{BTMS,rated})$ and state $x(t)=E_b$. Results show a baseline 304 kWh battery with two parallel banks can meet range targets, but BTMS sizing matters: water cooling completes the mission with a 16.5% weight increase, reducing the unconstrained range from about 480 km to 410 km, whereas air cooling may fail to complete the cycle; heating-induced thermal runaway scenarios indicate robustness of the design. Overall, the framework provides actionable insight for BTMS sizing and safety assessment in electric aircraft, and points to future work on battery chemistries, HVAC integration, and experimental validation of TR mechanisms.

Abstract

This work introduces a framework for simulating the electrical power consumption of an 8-seater electric aircraft equipped with high-energy-density NMC Lithium-ion cells. We propose an equivalent circuit model (ECM) to capture the thermal and electrical battery behavior. Furthermore, we assess the need for a battery thermal management system (BTMS) by determining heat generation at the cell level and optimize BTMS design to minimize energy consumption over a predefined flight regime. The proposed baseline battery design includes a 304-kWh battery system with BTMS, ensuring failure redundancy through two parallel switched battery banks. Simulation results explore the theoretical flight range without BTMS and reveal advantages in increasing battery capacity under specific conditions. Optimization efforts focus on BTMS design, highlighting the superior performance of water cooling over air cooling. However, the addition of a 9.9 kW water-cooled BTMS results in a 16.5% weight increase (387 kg) compared to no BTMS, reducing the simulated range of the aircraft from 480 km to 410 km. Lastly, we address a heating-induced thermal runaway scenario, demonstrating the robustness of the proposed battery design in preventing thermal runaway.

Figures (9)

• Figure 1: Power block diagram showing the electric aircraft powertrain topology. It consists of a BTMS, a battery system (BAT), two inverters (INV), two electric motors (EM) and two propellers (PROP). The power flows (indicated by arrows), $P_i$, are battery output power, BTMS, inverter, and machine input power flows indicated with the subscripts for $i =$ {b, BTMS, DC, AC}. Others are auxiliary loads, $P_\mathrm{aux}$, and in-/output power to the propellors, $P_\mathrm{M}$ and $P_\mathrm{P}$, respectively. The placement of some components in this schematic does not correspond to their actual location in the aircraft and is for visual convenience only.
• Figure 2: Applied coordinate system for aircraft in flight for the use of a longitudinal point mass model, with components $F_\mathrm{L}$ and $F_\mathrm{D}$, respectively, representing total lift and drag force. The flight path angle $\gamma$ represents the direction of the flight, while the angle of attack $\alpha$ represents the angle of the wings w.r.t. the incoming air.
• Figure 3: Second-order equivalent circuit model diagram, consisting of an open-circuit voltage source $U_\mathrm{oc}$, internal resistance $R_\mathrm{0}$ and two RC pairs.
• Figure 4: Fit of HPPC cycle voltage for the 11.84 Ah NMC cell at 23 $^\circ \mathrm{C}$. The RMSE of the fit is 10.08 mV (0.58%).
• Figure 5: BTMS system layout. Battery coolant loop left of the chiller, VCM on the right side. The actively controlled components are the pump and the compressor. The chiller represents a heat exchanger between the two loops.