Temperature Management Strategy for Urban Air Mobility Batteries to Improve Energy Efficiency in Low-Temperature Conditions

Abstract

As urban population concentration accelerates, issues such as traffic congestion caused by automobiles and climate change due to carbon dioxide emissions are becoming increasingly severe. Recently, urban air mobility (UAM) has been attracting attention as a solution to these problems. UAM refers to a system that uses electric vertical takeoff and landing (eVTOL) aircraft to transport passengers and cargo at low altitudes between key points within urban areas, with lithium-ion batteries as the primary power source. The lithium-ion batteries used in UAM have characteristics that degrade performance in low temperatures, including decreased power output and diminished energy capacity. Although research has been conducted on preheating lithium-ion batteries to address this issue, sufficient consideration has not been given to the energy used for preheating. Therefore, this study compares the energy recovered by preheating lithium-ion batteries with the energy consumed during preheating and proposes a temperature management method for low temperatures that maximizes the energy gain of lithium-ion batteries.

1. Introduction

As industry develops, the urbanization phenomenon is intensifying around the world. The global urbanization rate doubled from about 25% in 1950 to about 50% in 2020, and is expected to gradually increase to 58% over the next 50 years [1]. Due to the urbanization phenomenon, the problem of traffic congestion has become very serious, causing delays, inconvenience, and economic losses to drivers, as well as air pollution, and in 2014, traffic congestion in the United States consumed 3.1 billion gallons of additional fuel [2]. Urban air mobility (UAM) is attracting attention as a way to improve this problem [3]. UAM is an eco-friendly transportation system that uses low-altitude skyways to resolve traffic difficulties in metropolitan areas and increase speed of transportation, and has been developed around electric vertical takeoff and landing (eVTOL) [4]. Various governments have established and promoted UAM-related policies at national and urban levels [5]. The National Aeronautics and Space Administration (NASA) and the Federal Aviation Administration (FAA) are leading the establishment of UAM-related airworthiness requirements and operation guidelines, and the U.S. Congress has enacted support legislation. The FAA began revising the requirements for the airworthiness of small airplanes in 2016, and released the UAM-related operating guidelines ‘Concept of Operations (ConOps) ver.1’ in 2020, and ‘ConOps ver.2’ in 2023 [6,7,8]. The global UAM market is expected to grow significantly as companies and governments actively move toward the UAM market. According to data from MarketsandMarkets, the global UAM market is expected to grow from USD 3.8 billion in 2023 to USD 28.5 billion by 2030. Major automakers such as Hyundai and GM and aircraft manufacturers such as Joby Aviation and Boeing are also developing UAM through their own development and investment paths [5,9]. The biggest difficulty in developing UAM is securing sufficient mileage. The current lithium-ion battery technology has not yet realized the mileage required for UAM to be competitive [10]. Accordingly, studies on improving the energy density of lithium-ion batteries are continuously being conducted [11]. Another cause adversely affecting the mileage of UAM is the poor low-temperature characteristics of lithium-ion batteries. The lithium-ion batteries used in UAM increase the charging time and decrease the mileage possible due to the increase in internal resistance at low temperature [12]. To improve the low-temperature characteristics of lithium-ion batteries, technologies for preheating batteries are being studied, but studies on energy gain that compare the energy consumed during preheating with the energy recovered through preheating are insufficient, instead focusing on methods for preheating batteries, such as liquid preheating and air preheating. Therefore, this study presents an efficient temperature management method for a lithium-ion battery at a low temperature so that the energy gain of the lithium-ion battery is maximized. This paper is organized as follows. Section 2 summarizes the low-temperature characteristics of lithium-ion batteries. Section 3 reviews the preheating methods for lithium-ion batteries and selects the technique for simulation and experimentation. To verify the performance of the selected preheating method, experiments are conducted in Section 4 and Section 5. Based on the experimental data, an efficient preheating strategy is proven.

2. Low-Temperature Characteristics of Lithium-Ion Batteries

Lithium-ion batteries suffer from significant performance degradation, such as a decrease in capacity and power output at low temperatures, an increase in charging time, and an increase in risk due to dendrite formation [13]. This is because the internal resistance of lithium-ion batteries increases at low temperatures, and Figure 1 illustrates the internal resistance components of lithium-ion batteries.

The internal resistance components of lithium-ion batteries include the movement resistance of lithium ions in the electrolyte, the charge movement resistance between the anode and the cathode, and the diffusion resistance in the solid [14]. The mobility of lithium ions in the electrolyte decreases as the temperature drops due to the inverse relationship between ion mobility and electrolyte viscosity, which increases at lower temperatures [15,16]. The charge transfer resistance between the anode and the cathode is the resistance that occurs when lithium ions are intercalated into and deintercalated from the anode and the cathode, and the diffusion resistance in the solid is the resistance that occurs when the lithium ions are diffused from each active material after being inserted into the anode and cathode active materials [17].

According to Arrhenius’s theory, for a chemical reaction of lithium ions to occur, the reaction molecules must have sufficient kinetic energy [18]. According to the collision theory, a reaction occurs when molecules produce effective collisions, and an effective collision occurs only when molecules with sufficient energy collide in the right direction [19]. Therefore, before impact, the molecules must have sufficient kinetic energy and define this minimum energy as activation energy, $E_A$ [20]. According to Figure 2, there is always a certain number of molecular proportions with kinetic energy above the activation energy $E_A$, and the rate of reaction depends on the temperature [21].

When the temperature rises, the average fluctuation rate of the gas molecules increases, and the distribution of molecular kinetic energy skews toward higher energy levels [22]. In other words, at high temperatures, the distribution of molecules with activation energy increases and effective collisions between reaction molecules may occur more than at low temperatures, resulting in faster reaction rates [23]. From the above theory, Arrhenius derived the Arrhenius equation, which shows the relationship between the reaction rate constant and temperature [18]. Equation (1) represents the Arrhenius equation.

$$k=A{e}^{(-{\displaystyle \frac{E_A}{RT}})}$$

(1)

In the equation, k is the velocity constant, T is the absolute temperature, A is the Arrhenius constant, $E_A$ is the activation energy, and R is the gas constant. The Arrhenius constant A shown in the equation is called the exponential precedence factor or the frequency factor, and has the same unit as the velocity constant. When the velocity constant k is the number of collisions per second (frequency) causing a reaction, the Arrhenius constant A is the number of collisions per second (frequency) occurring in the appropriate direction. Taking the natural logarithm of both sides of Equation (1) transforms it into the form of Equation (2).

$$\mathit{ln}k=\mathit{ln}A-{\displaystyle \frac{E_A}{RT}}$$

(2)

If a graph of $\mathit{ln}k$ vs. 1/T is plotted, a straight line of the form y = ax + b can be obtained, as shown in Figure 3. The slope of the graph in Figure 3b, where lnk is plotted against 1/T, is −$E_A/R$. This demonstrates that the change in the rate constant with temperature depends on the activation energy value. It can be seen that reactions with larger activation energy $E_A$ are more sensitive to temperature changes in terms of the rate constant k. According to the Arrhenius equation, the mobility of lithium ions decreases at low temperatures, which implies an increase in the internal resistance of lithium-ion batteries at low temperatures [24]. Consequently, lithium-ion batteries experience performance degradation at low temperatures due to increased internal resistance, resulting in reduced mileage and increased charging time [13,25].

3. Lithium-Ion Battery Preheating Technology

To improve lithium-ion battery performance at low temperatures, technologies for preheating batteries have been researched and developed [26]. Figure 4 shows a schematic diagram of a lithium-ion battery preheating system. When energy is injected into an external heat source, heat from the lithium-ion battery’s exterior is transferred to the interior through conduction, radiation, and convection [27]. During this process, heat loss and energy loss due to the external heat source occur.

3.1. Classification of Lithium-Ion Battery Preheating Methods

Numerous technologies for preheating batteries have been researched, which can be classified into two categories: external preheating technologies and internal preheating technologies [28]. Figure 5 shows the classification of lithium-ion battery preheating technologies.

3.1.1. External Heating Strategy

External preheating technologies are methods for heating the exterior of the lithium-ion battery, and are divided into BTMS (Battery Thermal Management System)-based preheating technologies and technologies using heat elements [29]. BTMS-based preheating technologies include methods using air, liquid, and PCM (Phase Change Material). Air preheating has the advantages of simple structure, low cost, and high reliability, but it has the disadvantages of lower preheating performance due to low thermal conductivity and fan noise [30]. Liquid preheating methods have the advantages of high thermal conductivity, high heat capacity, and uniform temperature distribution. However, compared to air preheating, the system design is more complex and costly [31]. PCM preheating is cost-effective and has a simple structure. But this method also has the disadvantages of low thermal conductivity and increased weight [32]. Preheating methods using heat elements include resistance heating, Peltier effect, and heat film methods. Resistance heating has a short heat transfer path and high heating efficiency. However, it has poor temperature uniformity and may negatively affect lithium-ion battery life [33]. The Peltier effect method is low cost and easy to install, but it also has poor temperature uniformity and low energy efficiency. Lastly, the heat film method is low cost, easy to install, and has a high temperature rise rate, but requires stability verification. BTMS-based technologies are easy to integrate with existing systems but have relatively lower preheating performance. Technologies using heat elements have relatively superior preheating performance but require stability verification. In conclusion, each technology has its pros and cons, necessitating technology selection suitable for the application situation [26,27,28,29].

3.1.2. Internal Heating Strategy

Unlike technologies that apply heat externally, internal preheating technologies are methods that directly generate heat inside the lithium-ion battery to preheat it. Internal preheating technologies are divided into self-heating techniques and current application techniques [26,27,28,29]. Self-heating technologies include SHLB (Self-Heating Lithium-ion Battery), CCD (Constant-Current Discharge) preheating, and CVD (Constant-Voltage Discharge) preheating techniques. The SHLB technology involves inserting nickel foil inside the lithium-ion battery for self-heating, resulting in an extremely rapid temperature rise rate and high heating efficiency [34]. However, it requires modifying the lithium-ion battery structure and increases costs. CCD preheating generates heat through internal impedance by discharging at a constant current. The CVD method generates heat by discharging at a constant voltage [26,27,28,29]. Both methods do not require an external power source and are cost-effective, but they have the disadvantage of consuming a lot of lithium-ion battery energy [35]. Current application technologies include preheating methods using DC, AC, and Pulse currents. All three methods can provide consistent and even preheating across the lithium-ion battery, but they require an external power source and complex control systems. For internal preheating technologies as well, an appropriate preheating method should be selected considering the advantages and disadvantages of each technology.

3.2. Lithium-Ion Battery Heating Model

While various technologies for preheating lithium-ion batteries have been researched, the liquid preheating method is trending due to its advantages of high thermal conductivity, heat capacity, and uniform temperature distribution. However, the weight increase in the battery pack system due to the weight of the liquid fluid itself and the devices required to heat the fluid could be critical for urban air mobility (UAM), where weight reduction is essential to ensure sufficient flight range. Therefore, we developed a lithium-ion battery preheating model using a heating film method, which offers the advantages of high thermal conductivity and a high temperature rise rate. Additionally, to evaluate the preheating performance of the developed model, we conducted simulations using ANSYS Fluent 2024 R1.

Heating-Film-Based Preheating Model

A lithium-ion battery preheating model using heating film was designed for preheating lithium-ion batteries in urban air mobility (UAM) applications. Figure 6 shows the configuration of the heating-film-based model. Eight lithium-ion batteries were used, with polyimide heating films positioned between aluminum plates. The lithium-ion batteries used were SAMSUNG SDI INR 21700 40T (Yongin, Republic of Korea), and their nominal specifications are presented in

Table 1. Nominal specifications of lithium-ion battery cell.

Item	Specification
Model	INR21700-40T
Capacity	4000 mAh
Nominal voltage	3.6 V
Max continuous discharge	45 A
Cell dimension	Height: 70 mm, Diameter: 21 mm

.

The designed heating-film-based preheating model was simulated using ANSYS Fluent.

Table 2. Boundary conditions of heating-film-based preheating model [36].

Part	Density $(\mathbf{k}\mathbf{g}/{\mathbf{m}}^3)$	Heat Capacity $\mathbf{(}\mathbf{J}\mathbf{/}\mathbf{k}{\mathbf{s}}^{\mathbf{\ast }}\mathbf{k}\mathbf{)}$	Thermal Conductivity $\mathbf{(}\mathbf{w}\mathbf{/}{\mathbf{m}}^{\mathbf{\ast }}\mathbf{k}\mathbf{)}$
Cell	2255	980	18.2
Heating film	1420	1090	0.3
Aluminum plate	2700	900	200

shows the boundary conditions set for the model in the simulation. The simulation result is presented in Figure 7. This result demonstrates that the temperature of the lithium-ion batteries saturates at a constant temperature. Based on this simulation result, it can be anticipated that if an experiment is conducted to preheat lithium-ion batteries using the designed model, the lithium-ion batteries would saturate at a constant temperature during the preheating process.

4. Experimental Method

As can be seen in the schematic diagram of the lithium-ion battery preheating system in Figure 4, input energy is required to preheat the lithium-ion battery, and heat loss also occurs. This means that the amount of energy consumed in preheating the lithium-ion battery may be greater than the energy recovered by the lithium-ion battery through preheating. Therefore, to efficiently preheat the lithium-ion battery at low temperatures and secure the range of UAM, it is essential to consider the capacity gain of the lithium-ion battery. The capacity gain of the lithium-ion battery can be calculated using Equation (3).

$$W_{gain}=W_r-W_c$$

(3)

Here, $W_{gain}$ is the capacity gain achieved by the lithium-ion battery through preheating, $W_r$ is the capacity recovered through preheating, and $W_c$ is the energy capacity consumed in preheating the lithium-ion battery. First, to calculate the capacity recovered by the lithium-ion battery through preheating, the lithium-ion battery was left at set temperatures (−20, −10, 0, 10, 15, 25 °C) for 12 h and then discharged at 1 C to determine the dischargeable capacity. Subsequently, a lithium-ion battery preheating experiment was conducted to calculate the energy consumed in preheating the lithium-ion battery. The lithium-ion battery was left in a chamber set at −20 °C for 12 h, then preheated to room temperature (above 20 °C). Figure 8 shows the overall setup of the lithium-ion battery preheating experiment. The lithium-ion battery was preheated in a chamber maintaining a set temperature of −20 °C, and a DC Power Supply (IDRC-DSP060-025, Beijing, China) was used to heat the polyimide heating film. Thermocouple temperature sensors were attached to three locations on the experimental model, and the lithium-ion battery was preheated based on the average temperature. A Data Logger (GRAPHTEC-GL900, Yokohama, Japan) was used to record the current and voltage consumed in preheating the lithium-ion battery, as well as the time spent on preheating the lithium-ion battery.

5. Results and Discussion

Based on the dischargeable capacity obtained by discharging the lithium-ion battery at 1 C at each set temperature, the energy recovered by the lithium-ion battery through preheating was calculated. Figure 9 shows the graph of lithium-ion battery discharge capacity at different temperatures.

Looking at the experimental results, we can see that, at −20 °C, the lithium-ion battery’s discharge capacity is about 63% compared to room temperature, indicating a significant performance degradation. Also, as the temperature rises, the lithium-ion battery’s discharge capacity increases. At 10 °C, the lithium-ion battery’s discharge capacity is 90.63% compared to room temperature, confirming that preheating the lithium-ion battery can solve the problem of reduced driving range in winter. However, the capacity recovered by the lithium-ion battery for each temperature range is not linear. Furthermore, Figure 10, which shows the lithium-ion battery preheating time graph, demonstrates that the preheating time increases as the temperature rises. Additionally, the graph indicates that when a certain temperature is reached, the lithium-ion battery’s temperature increase saturates, which suggests that the capacity gain decreases rapidly after a certain temperature range.

Figure 10 illustrates that heating from −20 °C to 15 °C takes 12 min; however, beyond 15 °C, the rate of temperature increase declines rapidly. Heat is transferred more efficiently when there is a larger temperature difference between two objects. In other words, as the lithium-ion battery preheating time progresses, the temperature difference between the heat source (polyimide heating film) and the lithium-ion battery decreases. Consequently, the heat transfer to the lithium-ion battery decreases, causing the temperature rise rate to saturate and requiring more time and energy to raise the lithium-ion battery’s temperature.

Table 3. Effects of preheating lithium-ion battery at different achieved temperatures.

Temperature	Capacity	Recovery Capacity	Recovery Energy	Consumed Energy	Energy Gain
−20	5.1 Ah	-	-	-	-
−10	6.16 Ah	1.06 Ah	17.808 Wh	1.66 Wh	16.148 Wh
0	6.8 Ah	640 mAh	28.56 Wh	5.13 Wh	23.43 Wh
10	7.25 Ah	450 mAh	36.12 Wh	11.36 Wh	24.76 Wh
15	7.53 Ah	280 mAh	40.824 Wh	16.66 Wh	24.164 Wh
20	7.78 Ah	250 mAh	45.024 Wh	27.42 Wh	17.604 Wh
25	8 Ah	230 mAh	48.72 Wh	75.11 Wh	−26.39 Wh

shows the capacity gain of the lithium-ion battery for different temperatures.

As shown in Table 3, the amount of energy recovered by the lithium-ion battery increases as its temperature rises. However, the rate of increase in energy consumption is greater. This causes the lithium-ion battery’s energy gain to decrease as the preheating time progresses. In reality, the lithium-ion battery’s energy gain continues to increase until it reaches a certain temperature, after which the rate of increase diminishes, and it becomes negative when it reaches room temperature (about 25 °C). In other words, continuously heating the lithium-ion battery to room temperature may not help in recovering driving range. This is because as the lithium-ion battery temperature rises, the temperature difference between the lithium-ion battery and the heating film decreases. According to Fourier’s law, as the temperature difference decreases, the heat transfer efficiency drops sharply. Equation (4) represents Fourier’s law:

$$Q=-kA({\displaystyle \frac{dT}{dX}})$$

(4)

In Equation (4), Q represents heat flux, k is thermal conductivity, A is area, and dT/dX is the temperature gradient. Additionally, as the lithium-ion battery temperature rises, the temperature difference between the lithium-ion battery and the surrounding environment (−20 °C) increases. According to Newton’s law of cooling, as this temperature difference increases, the heat loss from the lithium-ion battery to the environment increases rapidly. The following Equation (5) shows Newton’s law of cooling.

$${\displaystyle \frac{dQ}{dt}}=hA(T_S-T_A)$$

(5)

Here, dQ/dt is the rate of heat transfer, h is the heat transfer coefficient, A is the surface area, T_S is the surface temperature of the object, and $T_A$ is the ambient temperature. Moreover, the aluminum plate between the lithium-ion battery and the heating film, due to its high thermal conductivity, allows for very efficient heat transfer to the lithium-ion battery in the initial stages of preheating. However, as the lithium-ion battery temperature rises and the temperature difference with the surrounding environment increases, it begins to conduct heat away from the lithium-ion battery to the exterior. In other words, according to Fourier’s law and Newton’s law of cooling, as the lithium-ion battery temperature rises, heat transfer through the aluminum plate and heat loss to the surrounding environment accelerate. As the system approaches thermal equilibrium, heat loss increases dramatically. In conclusion, the increasing heat loss requires more energy to preheat the lithium-ion battery, which results in a decrease in the lithium-ion battery’s energy gain. Based on the experimental results shown in Table 3, we calculated the amount of energy consumed in preheating the lithium-ion battery at temperatures above 20 °C, where the lithium-ion battery’s heat loss is expected to increase rapidly. These calculations are presented in

Table 4. Effectiveness of preheating lithium-ion battery at temperatures above 20 °C.

Temperature	Preheating Period	Consumed Energy	Effectiveness of Preheating
−20 °C~21 °C	1342 s	31.38 Wh	High
−20 °C~21.5 °C	1539 s	36.3375 Wh	High
−20 °C~22 °C	1808 s	42.78 Wh	High
−20 °C~22.5 °C	2062 s	47.34 Wh	Medium
−20 °C~23 °C	2661 s	63.14 Wh	Poor

.

As shown in Table 4, we can see that the lithium-ion battery’s energy gain remains positive up to 22 °C. However, from 22 °C onwards, the lithium-ion battery’s temperature change becomes very small due to heat loss. When the temperature reaches 22.5 °C, the energy recovered by the lithium-ion battery and the energy consumed for preheating become nearly equal. After 22.5 °C, when the temperature reaches 23 °C, the lithium-ion battery’s energy gain becomes negative. In other words, based on the experimental results, heating the lithium-ion battery up to 22.5 °C would be the most efficient strategy to maximize energy capacity gain while maintaining positive energy balance.

6. Conclusions

In this study, we proposed a temperature management strategy for the energy efficiency of UAM batteries at low temperatures to address the reduced mileage problem in cold conditions, aiming to enhance UAM competitiveness. While preheating is being used to solve the winter mileage reduction problem of lithium-ion batteries, for the efficient use of UAM, we must consider the net energy gain: the energy recovered through preheating minus the energy consumed in the preheating process. Based on our experimental results, which determined the energy recovery amounts at different lithium-ion battery temperatures and the energy consumed during preheating, we conclude that preheating lithium-ion batteries up to 22.5 °C would likely yield the maximum energy capacity gain. In other words, if we preheat the lithium-ion battery while considering the point of maximum energy gain, with regard to the heat loss and energy consumption during preheating, we can efficiently manage UAM operations at low temperatures and secure sufficient mileage. This approach allows us to determine the optimal preheating temperature that maximizes the lithium-ion battery’s net energy gain.