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Optimizing Thermal Management Systems to Augment the Operational Longevity of Electric Aircraft

Sai Sankalp Shekar

Graduate Student

Department of Aerospace Engineering

University of Illinois at Urbana-Champaign

Matthew Clarke

Assistant Professor

Department of Aerospace Engineering

University of Illinois at Urbana-Champaign

AIAA SciTech Forum

8^th January 2025

Author

Company/Organization

Conference Name, Conference Dates

Conference Location

Presentation Title

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Primary Research Focus

How does a thermal management system's operational capacity affect battery life, and what are the trade-offs between system mass and battery longevity in electric aircraft applications?
What is the primary limiting factor in battery end-of-life: thermal constraints or capacity degradation, and how can this inform optimal thermal management system design?

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Outline

Introduction to RCAIDE
An Electric Aircraft Test Bed
Battery model
Battery Thermal Management System (BTMS) for an e-Aircraft
Case Studies – Lifecycle Analysis
Optimization of Thermal Management System (TMS)
Results
Conclusion

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RCAIDE – (Research Community Aircraft Interdisciplinary Design Environment)

RCAIDE is an open-source, python-based platform for designing, analyzing, and optimizing conventional passenger, drone, supersonic, and all/hybrid-electric aircraft.
RCAIDE is developed upon the principle of continuously building upon previously validated work to reduce latency in the design process.

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Its user-friendly code has a trusted user base of over 10,000 worldwide.

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The Electric Twin Otter

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Viking DHC-6 Series 400 Twin Otter

Retrofitted with an all-electric powertrain
Electric motors with Hartzell HC-B3TN-3D Propellers
MTOW: 5670kg
Integrated battery pack
Integrated battery thermal management system.

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Battery Discharge & Heat Generation Model

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Battery type: LiNiMnCoO₂(NMC)
Cells in series/module: 12
Cells in parallel/module: 220
Number of modules: 12

Heat Generation Rate:

Battery Discharge Model:

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Battery Degradation Model

A battery aging model is based off empirical relations

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Calendar Aging

Cyclic Aging

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Battery Thermal Management Model

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The BTMS consists of

Heat Acquisition System (HAS)
Reservoir
Heat Exchanger System (HEX)
Heating Element/ Heat Pump
Pumps
Regulating Valves
Fans
Necessary Plumbing

The HEX has a dual inlet:

Ram Inlet (Cooling Drag)
Fan (Battery Power)

The battery thermal management system consists of the following components,

The HAS which is a wavy channel liquid cooled system

HEX is a cross flow

Reservoir is exposed to the envirornment.

The Heat Exchanger has a dual inlet which is modelled in our testbed, i.e it can be power by a fan at low speeds, and once a sufficient velocity is achieved the ram inlet opens up to let air through it, and during this period the cooling drag being produced by the HEX is accounted for, which will need to be overcome by the motors to produce additional thrust which is an additional power source from the battery.

The three components that determine the amount of. Heat that can be removed from the battery pack are the HAS, HEX and the reservoir. These components can be sized in such a way that they are capable of removing a certain amount of heat from the battery pack. The way in which these compionents are sized are detailed through an optimization process in two of the authors previous papers.

The heat removal capabilities of the TMS determine the amount of power that they draw from the battery pack and their weight as well,

It is standard practice that a BTMS is designed to be capable of removing the amount of heat that is generated at the top of climb.but however we will in the process of this study understand the long term effects of doing so.

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Operational Overview

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Case Study- Aircraft Configurations

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Configuration 1

Configuration 2

Configuration 3

Config.	HAS Capacity (W)	HEX Capacity (W)	Reservoir Capacity (m³)	TMS Weight (kg)	Aircraft Weight (kg)
1	2918.08	2281.57	0.0021	229.71	5436.18
2	2245.39	6213.33	0.11	374.86	5581.33
3	2040.20	1290.32	0.054	275.46	5481.93

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Performance Over a Single Flight

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Cell level properties for a single flight

Configuration 1 having the lowest heat removal capacity results in the largest temperature profile.
Configuration 2 having the highest heat removal capacity doesn’t result in the smallest temperature profile.
Configuration 3 results in the “best” temperature profile here highlighting the balance between weight and heat removal capacity.

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Lifecycle Analysis

The aircrafts operates six flights a day
The temperature of the battery pack resets to ambient temperature at the start of each day
The battery temperature for subsequent flights is the temperature of the battery after the cooldown period from the previous flight

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Lifecycle Analysis

How do we determine the battery end of life?

Capacity Limited: If the SOC of the battery is less than 20% at the end of any flight
Thermally Limited: If during any point of the flight the temperature exceeds 50 ℃

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Lifecycle Analysis - Results

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At a 106 days Config. 1 has the highest IR Growth and capacity fade
Config. 3 having the longest life naturally has the highest internal resistance growth
Config. 2 & 3 reach a similar capacity fade factor indicating end of life

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Lifecycle Analysis - Results

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Let us now look at the batter characteristics for the three different configurations,

First we will look at the cell temperature, we can look at the range at which the tempertautre of the battery is cycled at. As the range at which it is cycled at increased the degradation of the battery increases as well.

Config 1 reaches the thermal limit quite quickly because the TMS is severely underpowered.

Config 2: even though the TMS is overpowered the penalties associated with this start showing that cause the temperature to grow quite rapidly and it approaches the 50 degree mark.

When we look to the pther graph, here we have the delth of discharge on the vertical axis and we see that Config 2 and 3 hit that limit.

This highlights the siginificance of finding a TMS which sits right in the sweet spot to increase the longetivity

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Preliminary Conclusion

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TMS Heat Removal Ability

TMS Weight

Power Consumption

Aircraft Weight

Additional Power

Battery

Pack

Exit Condition

Thermally Limited

Capacity Limited

Thermal Profile

Current Profile

Battery

Life

System Dependency Diagram

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Preliminary Conclusion

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TMS design and battery life have a nonlinear relationship - small design changes can significantly impact performance

Best TMS design balances peak cooling capacity with overall flight efficiency, not just maximum cooling power

Battery life depends on combined thermal and current profiles across all flight phases

TMS weight and power consumption directly impact battery life - creating a critical design trade-off"

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Design Space Sampling

Design space parameters define system boundaries and operational limits
TMS heat removal capabilities:

Heat Acquisition: 1,000W - 12,500W
Heat Exchanger : 500W - 9,000W
Reservoir Volume: 0.001m³ - 0.216m³

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Design Space Exploration:

Latin Hypercube Sampling (LHS) used to efficiently sample design parameters
Sampling shows uniform distribution across design space
No significant clustering or gaps found, confirming thorough exploration

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Design Space Analysis

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Analysis Results:

75 designs sampled, 72 viable
Peak battery life: 195 days at 5,519.96 kg
Most designs: 160-180 days range

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Sensitivity Analysis – Total Weight

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Sobol sensitivity analysis is performed using the SALib python package
The reservoir has the highest impact on the total weight of the aircraft
There is minimal interdependence between the variable’s impact on the weight

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Sensitivity Analysis – Battery Life

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All the components have a significant impact on the final life of the battery
The second order sensitivity indices are a magnitude of order higher
Since the HAS and the HEX are not physically coupled the 2^nd order index is low

First, looking at how components affect battery life, we found something interesting - all our input variables have similar levels of influence. But what's really crucial is how they work together.

The heat acquisition system has the strongest direct impact on battery life. This makes sense, as it's the component directly managing battery cell temperatures.

Now, here's where it gets interesting... The reservoir's role is quite complex. While making it bigger significantly increases system weight, it doesn't give us proportional improvements in battery life. However - and this is key - the reservoir is vital in how it interacts with other components.

Think of it like this: the reservoir, heat exchanger, and heat acquisition system don't work in isolation. They're constantly interacting, creating a delicate balance in our thermal management system.

This presents us with a fascinating design challenge - we need to optimize these interactions rather than just focusing on individual components."

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Conclusion

How does a thermal management system's operational capacity affect battery life, and what are the trade-offs between system mass and battery longevity in electric aircraft applications?

Analysis demonstrates that optimal balance exists between thermal management system mass and operational capacity. While increased thermal capacity extends battery life, our results show that moderate thermal management capacity could achieve near-optimal battery life while maintaining reasonable system mass.

What is the primary limiting factor in battery end-of-life: thermal constraints or capacity degradation, and how can this inform optimal thermal management system design?

The optimization study using Latin Hypercube Sampling shows that thermal limitations predominantly determine battery end-of-life. However, increasing thermal management capacity beyond certain thresholds leads to diminishing returns due to added mass and power requirements, demonstrating that optimal design requires controlled thermal operation to maximize useful battery life.

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