**1. Introduction**

Despite the enormous effort put into the reduction of greenhouse gases, CO2 emissions are still increasing. Road transport contributes up to 25 percent to the CO2 emissions and represents one of the fastest-growing economic sectors [1]. A possible strategy to reduce local emissions is to increase the share of electric mobility consistently. Observing the technical developments of zero-emission vehicles in recent years, especially energy storage, has proved to be the bottleneck. Despite intensive research activities, mobile energy storage is still the limiting factor, curbing the success of hybrid and electric vehicles.

Since the direct storage of electrical energy can be realized only by the capacitors and coils, indirect storage methods prevail. This means that in a first step, the electrical energy is converted into another form of energy and subsequently stored for later reconversion into electrical energy. In Figure 1, a short classification into mechanical, electrochemical, chemical, electrical and thermal energy storage systems is given.

When energy storage is discussed in the context of sustainable transportation, the first topic that comes to mind is electrochemical batteries for electric vehicles (EVs). Battery electric vehicles—without a doubt—play an important role in our path towards zero-emission mobility, but many experts agree

that the energy revolution will require a mix of different energy storage solutions and transportation modes, as the "one-size-fits-all-solution" is ye<sup>t</sup> to be invented [2,3].

**Figure 1.** Classification of energy storage systems according to energy type, including examples.

A further classification is made in Figure 2, where different energy storage types are shown as a function of their power rating, energy content, and the consequently-related typical charge and discharge time. The dotted circle in the figure represents the area of particular importance for the use-case sustainable transportation, limiting the number of different storages to batteries, supercaps, flywheels and superconducting magnetic energy storages (SMES). However, the selection can further be reduced as SMES do not ye<sup>t</sup> have the maturity necessary for a real implementation [4,5].

Beyond the obvious use-case of onboard energy storage, stationary buffer storage inside the required electric vehicle fast-charging stations will also be discussed in Section 3.3. Calculations and considerations are based on actual zero-emission buses operating in Graz, Austria. The main influencing parameters and effects related to energy storage aging are analyzed in detail. Based on the discussed aging behavior, advantages/disadvantages, and techno-economic analysis for both use-cases is presented. A final suitability assessment of each energy storage technology concludes the use-case analysis.

**Figure 2.** Power rating, energy capacity and discharge time of different energy storage systems for stationary and mobile transportation applications. Data based on References [6,7].

#### **2. Properties of Di** ff**erent Energy Storage Systems**

To give an overview, Table 1 shows general technical and economic properties of the storage technologies preselected in Section 1.

**Table 1.** General characteristic and economic properties of di fferent energy storage technologies relevant to sustainable transport applications.


Values refer to cell-level for batteries/supercaps and the rotor only for FESS, neglecting periphery and auxiliary systems. Some properties strongly depend on operating conditions, such as ambient temperature. Therefore, it must be mentioned that representative average characteristic values based on References [6,8–17] were used.

#### *2.1. Battery and Superap*

Both Li-Ion batteries and supercaps are mature technologies that have been used in various fields of application since the beginning of the 21st century. However, due to developments in recent years, Li-Ion batteries have become the energy storage device of choice for most transportation applications. Because of their popularity, a lot of scientific [6,7,11] and industrial [8–10,12–15], literature (provided by manufacturers) exists, which can be used to assess certain properties, such as cycle life, aging, etc. This paper will, hence, give only a short overview and primarily focus on the lesser-known properties of Flywheel Energy Storage Systems (FESS)—see Section 2.2.

When it comes to "usable/achievable lifetime", a metric is necessary to 'measure' the state of health of batteries. Typically, capacity and/or internal resistance are used in datasheets of cell manufacturers, giving some indicators to define the end-of-life (EOL) condition, e.g., a decrease of capacity by 20% or increase of internal resistance by a factor of two compared to the begin-of-life (BOL) values. In reality, those limits depend on the actual application, and the datasheet's lifetime values need to be scaled accordingly. Many applications allow a much higher decrease in capacity than defined by the manufacturer. This slightly increases initial costs and weight, but tremendously extends service life, e.g., a typically used value of 33% decrease of capacity results in a BOL to EOL capacity ratio of 1.5 compared to 1.25 for the manufacturer's 20% value. In this case, the battery would weigh (=cost) about 20% more. However, the lifetime would increase by about 65%. In other words, the battery would weight (=cost) less for a given lifetime and reach a higher over-all energy throughput. Only small benefits are gained by pushing it even further. Especially in transportation applications, the initial increase in weight is the limiting factor.

The achievable lifetime and performance of batteries and supercaps depend on many parameters, with temperature as the dominating influencing factor. Even though the values are given in Table 1 sugges<sup>t</sup> a wide operating temperature range, a closer look into actual datasheets reveals the problems within: Temperature must be kept below a certain value in order to reach the highest cycle life. Table 2 illustrates the significant decrease in cycle life when temperatures exceed 25 ◦C.



\* Estimate, based on logarithmic extrapolation.

Low temperatures increase the internal resistance and thereby have a detrimental e ffect on the performance of the system as well. In the case of supercaps, even at the lowest allowed operating temperature, the increase is typically around factor two, e.g., the company AVX states an increase of about 120% at −40 ◦C compared to the reference value at 25 ◦C [8]. This decreased performance is still su fficient for common applications, and only the e fficiency su ffers slightly, but the capacity remains almost the same. Simultaneously, the (increased) losses heat up the supercap and thereby reduce the negative e ffects over time.

However, in the case of a battery, these e ffects are much more severe than in the case of supercaps. Typically, the temperature influence is already noticeable at around 10–20 ◦C. At temperatures below 0 ◦C, charging is often no longer allowed by the manufacturer. Finally, at the low end of the operating temperature range, the discharge performance of the cell is typically less than 10% compared to 20 ◦C values, e.g., References [9,13,15]. Due to this severe decrease in performance, it is often necessary to heat up the cells before the system is put into operation. According to the Graz Public Transport Services (GVB), putting a battery electric bus into operation in the winter may take up to 30 min.

The primary significance of high temperature is the decrease of the cell's lifetime, both calendar and cycle life. As a rule of thumb, one can assume that the calendar life is reduced by a factor of two every 10 ◦C increase in temperature (actual values taken from datasheets vary between 7 and 15 ◦C). The continuous operation at the maximum allowed temperature would reduce the lifetime to just a few months, or a year at most. An additional factor influencing the achievable lifetime is the cell voltage, and in the case of batteries also cycle count, depth of discharge (DOD), as well as charge and discharge rates.

For example, an AVX-SCC series supercap [8] has a base lifetime expectancy of 20 years at 30 ◦C in a fully charged state. The temperature coe fficient is 8 ◦C per factor two in a lifetime, and the voltage dependence is 0.4 V per factor two in a lifetime (a lower voltage increases the lifetime, but reduces available capacity)—see Figure 3.

**Figure 3.** Expected supercap life depending on temperature and maximum voltage. (With kind permission by AVX Corporation) [8].

Similar to the supercap, calendar life of batteries depends on temperature and end-of-charge (EOC) voltage. Unfortunately, in many datasheets, only sparse data sets are given. If anything, one usually finds data regarding temperature dependence. Reasonable EOC voltages for di fferent applications are rarely mentioned in datasheets/literature, with one notable exception being [15]. Depending on the application, Table 3 states di fferent suggested EOC voltages. Still, it is not mentioned how much the lifetime is improved, due to voltage reduction.



Additionally, cycling the battery reduces its lifetime. There is no simple correlation between charge/discharge cycles and occurred damage. As mentioned before, it not only depends on the cycle count, but among other factors, also state-of-charge (SOC), DOD and charge/discharge rates. Still, a few basic and generally valid statements can be made:


One last comment: In the case of rectangular or pouch bag cells, special care has to be taken for correct mounting that homogeneously compresses the cell with a defined pressure. This is equally important for the proper functioning of the cell, as well as to achieve long cell life.

#### *2.2. Flywheel Energy Storage Systems (FESS)*

## 2.2.1. Background Information

Prices of Lithium-Ion batteries are decreasing on the global market and energy densities have reached reasonable values, allowing EVs to travel 200 km and more on one charge [18]. However, there are still significant technical challenges, which need to be solved, or alternatives need to be found. One of the major drawbacks of chemical batteries is limited cycle life, which was described in Section 2.1 and will be discussed in particular in this paper.

It must be stressed that sustainable transportation does not only rely on batteries inside the vehicles. The increasing primary electricity supply through volatile sources, in combination with high grid loads caused by charging power demand, requires decentralized electric energy storage [19]. The requirements for these stationary energy storage systems may di ffer significantly from those of transportation applications. However, in both cases, long cycle life and negligible aging e ffects are usually desired. This is particularly the case when alternatives to chemical batteries come into play.

One may think immediately of gyroscopic reactions as a major disadvantage of FESS. This aspect must be considered during system design, but is an issue that can be resolved [20] as they have been used successfully in various transportation applications (see Figure 4).

**Figure 4.** (**a**) The famed Gyrobus by Maschinenfabrik Oerlikon in 1953 powered solely by a 1500 kg electromechanical steel flywheel. (Image credit by Historisches Archiv ABB Schweiz, N.3.1.54627); (**b**) A modern transit bus accommodating a hybrid drive train with a flywheel energy recovery system by *PUNCH Flybrid*. (Image credit PUNCH Flybrid Ltd., Silverstone, UK).

Flywheel Energy Storage Systems (FESS) has experienced a renaissance in recent years, mainly due to some of their intriguing properties:


Due to the above-listed properties, FESS are increasingly used for grid stability or fast-charging applications, as proposed in References [21,22]. Another example is the currently ongoing Austrian research project "FlyGrid", within which a FESS for a fully automated EV charging station will be developed. One module of this prototype will be used as the reference case and will deliver 5 kWh at 100 kW peak power.

#### 2.2.2. FESS Working Principle

In a FESS, energy is stored in kinetic form; the working principle is based on the law of conservation of angular momentum. In electromechanical FESSs an external torque is applied to a rotor by the use of a motor/generator, hence, only an electrical and no direct mechanical connection for power transmission is required. In order to charge the FESS, the applied torque accelerates the spinning mass (rotor). If the spinning mass decelerates, energy is taken out of the system, and the motor acts as a generator. Electrical energy from the grid or other sources can be converted into kinetic energy charging the FESS. In the case of discharge, the motor/generator decelerates the spinning mass converting kinetic energy back to electrical energy. This principle is demonstrated in a video in the

Supplementary Materials, that belongs to this publication. The amount of stored energy is defined by the rotor's moment of inertia and the rotational speed, according to Equation (1).

$$E\_{\rm KIN} = \frac{I \ast a^2}{2} \,, \tag{1}$$

*EKIN* Kinetic Energy in J

*I* Mass Moment of Inertia of the Spinning Mass/Rotor in kg\*m<sup>2</sup>

ω Angular Velocity in rad/s

Different concepts for Flywheel Energy Storage Systems (FESS) exist, but within this publication, only electromechanical FESS are considered, as they are easily comparable to any other energy storage system with electric connection terminals. Figure 5a shows a schematic diagram of an electromechanical FESS. It consists of a motor/generator with a shaft and an attached spinning mass. The shaft is supported by bearings, which form the connection to the housing. In Figure 5b, the energy content is plotted over rotational speed visualizing the quadratic increase of stored energy. If high specific energies are desired, FESS must operate at extremely high rotational speeds, optimally exploiting rotor material strength. Within this publication, only FESS with high specific energies is addressed. As mentioned in Section 2.2.1 depth of discharge (DoD) does not influence FESS cycle life.

One effect, which must be considered during FESS design is based on Equation (2)—System power is proportional to motor torque and rotational speed. It can be observed that, when the constant power output is required at low rotational speeds, motor-generator torque will reach unnecessarily high values, resulting in heavier and more expensive electric machines. This is why the minimum operating speed is usually kept at around 1/3 of the maximum rpm value.

$$P = \mathbf{M} \ast \boldsymbol{\omega} \,, \tag{2}$$

*P* Power in W

M Motor Torque in N\*m

ω Angular Velocity in rad/s

Around 89% of the total kinetic energy of the FESS is usable when the system is operated between 33 and 100% of the maximum permissible speed. For that reason, FESS usually operate within a certain bandwidth and do not decelerate down to standstill during regular operation.

**Figure 5.** (**a**) Schematics of a flywheel energy storage system, including auxiliary components; (**b**) Energy content as a function of rotational speed.

#### 2.2.3. Self-Discharge of FESS

Losses have an important influence on the suitability of this technology for di fferent use-cases. The following paragraph gives a short introduction to this topic, starting with the three main causes of FESS self-discharge:


Air drag losses during operation are crucial for FESS with high specific energies. Circumferential speeds beyond the speed of sound are common and exceed 1 km/s in some cases, which would cause enormous air drag during operation. This air drag would result in losses and eventually be dissipated into heat, which causes thermal issues leading to system failure. In order to reduce these losses, FESS are usually operated in a vacuum atmosphere, and pressure levels down to 1 μbar are common [23]. At such low-pressure levels, air drag losses play only a minor role. Issues regarding lubrication arising from these vacuum qualities will be addressed in Section 2.2.6. The power consumption of the vacuum pump and other peripheral components must be taken into account when analyzing the overall system losses. The power dissipated in the bearings also plays a crucial role and will be discussed in detail in Section 2.2.5.

Losses do not necessarily represent a problem, when they are below a certain level, but the benchmark for this threshold depends on the actual use-case. With increasing mean power-transfer into and out of the FESS, the acceptable level of system losses increases as well.

This means that for long term storage (low mean power-transfer), the power loss threshold is very low and FESS is not suitable, due to its relatively high self-discharge (hours to days at most). For highly dynamic and predictable load cycles with high mean power-transfer FESS is more suitable. This matter will be demonstrated in Section 3 by means of di fferent use-cases.

In the following sections, crucial FESS components will be dealt with, and details regarding their service life will be discussed.

#### 2.2.4. Rotor Material Selection and Aging

As described in Section 2.2.1 FESS with high energy densities are addressed. Regarding the rotor, rotational speed, and therefore, energy content is limited by permissible stresses (<sup>σ</sup>max) in the rotor. Highest energy densities can be reached when using materials with high σmax ρ ratios, like fiber composite materials [1]. However, other materials like steel are being used in practice as well. Table 4 compares the theoretical specific energies of di fferent rotor materials.


**Table 4.** Possible FESS rotor materials and associated theoretical specific kinetic energy content.

> \* For composite materials a ratio of 60% fibers and 40% (matrix/resin) was assumed.

A spinning mass with a kinetic energy content of 5 kWh would weigh around 9 kg when made of carbon fiber reinforced plastic (CFRP) and 137 kg when 42CrMo4 high-strength steel is used, and the material is fully exploited regarding permissible stress. Commercially available systems reach specific energies regarding the rotor up to 50 Wh/kg when using CFRP, for steel flywheels, according to values are much lower.

However, it must be mentioned that the theoretical specific energy values are reduced by design parameters, such as safety factors, stress concentration (notching), etc. Figure 6 depicts the specific energy content of various real flywheel rotors with Li-batteries and fossil fuels. To show the enormous future potential of FESS technology, the theoretical specific energy potential of a rotor made from a material with properties similar to carbo nano-tubes is shown as well.

**Figure 6.** The specific energy content of selected real world flywheel rotors compared to future rotor potential and other energy storage [24].

Usually, the rotor weight is being compensated using a magne<sup>t</sup> (permanent magnetic thrust bearing), so that the ball bearings are not subjected to the entire rotor weight. The influence of rotor weight compensation is discussed in detail in Section 2.2.5.

While in theory FESS rotors are also subject to aging, due to fatigue stress (or even creep in the case of CFRP rotors) it must be mentioned that these phenomena are usually considered during the design phase by the introduction of a safety factor, and hence, do not result in the capacity fade of the system. In this regard, even rotors made of CFRP can reach high service life when they are designed accordingly, and aging of the matrix is considered [25].
