1

Abstract— In this study is aiming at the design and analysis of a

fast-charging system (FCS) to decrease charging time and reduce

the high demand effect on the power grid using Flywheel and

with/without battery energy storage structure. In addition, the

electric bus charging and battery sizing analysis to better charging

system design. The flywheel energy storage has advantages to fast-

charging applications. Besides, the target fast-charging system will

support transportation electrification infrastructures, reduced

operational costs, and CO2 emissions. The analysis of the FCS

system will provide resilient features to ensure minimum

operation interruptions. The design of the system can use flywheel

technology to reduce the adverse effects of the high demand in the

power grid. Also, the fast charging system can affect the EV

battery size, especially the public bus. Thus, the relation between

the Electric bus battery and the charging process has been

analyzed. The design of the fast charging system aims to optimize

power-flow and minimize the sizing and protect battery life on

both (the vehicles and the storage) sides. The fast charging system

with flywheel energy storage has been analyzed with two different

perspectives. The proposed FCS has high power charging

technology with energy storage platforms and infrastructures.

Index Terms— Electric vehicles, Energy Storage, Fast charging systems,

Flywheel, Battery sizing

I. INTRODUCTION

ITH the impact of globalization, humanity has become

more conscious and people became more prone to

environment friendly energy sources instead of fossil fuels. In

addition, for transportation need which is indispensable for

people, the electrical vehicles provide an eco-friendlier option

rather than fossil fuel vehicles. When these advantages are

considered, the electrical vehicles (EV) are expected to spread

more in the near future.

The prevalence of the electrical vehicles increased the

importance of the studies made in this field. Especially, for a

more efficient and long drive performance of EV’s, the

innovations in the inverter/converter and battery technologies

boosted [1], [2]. Besides, significant studies are performed on

the EV’s charging technologies. Those studies show that, the

fast charging stations designed for EV’s, causes an over load

demand on the grid which’s effects should be reduced. Also,

there are some other studies are made on the interaction

between the EV’s and the grid in which the EV’s are not only

the load but also a source for the grid as a power supplier [3][7].

In one of these studies, the charging of the EV’s in different

power levels are examined depending on the transportation

time, distance and parking period [3]. Also, an optimum power

flow scenario developed on the basis of a MILP substructure

and a micro grid structure that EV supplies building/grid loads

[6]. The EV charging topology is divided three levels. These are

classified as level 1, level 2 and level 3. The level 3 charging

mode includes direct current (DC) which provides fast charging

by using high power. That is the reason of the effects of DC fast

charging to the grid. There are some different suggestions to

solve this situation. In one of them, a hybrid charge station

structure supplied by photovoltaic (PV) solar panels is proposed

for DC fast charging [8].

The charging stations designed to charge the electrical

vehicles are classified under different standards. These

standards are set by the international institutions like

International Electro-technical Commission (IEC), Society of

Automotive Engineers (SAE) and CHAdeMO [19] depending

on the defined technical data and qualifications. The

interconnection of the electrical vehicles with the grid is also

defined under the given standards. Especially the level 1 and

level 2 type of charging stations, which use AC and have lower

power values, are preferred for houses. In addition, charging

time of the electrical vehicles in these stations can last longer

(approximately 6-8 hours). That is the reason of the tries for

increasing the number of the DC fast charging stations which

can rise to higher power values. Decreasing the charging

duration is very important for the electrical vehicles to become

widespread. Because long charging durations affect the driving

comfort of the vehicles very much and perceived as negative

for the end users. As a solution for this situation, by using DC,

the EV’s batteries can be charged in a short time with high

current values. Today Tesla, the electrical vehicle producer that

set the fast charging stations and spreading them rapidly, fast

charging stations can raise 120kW charging power and can

fulfill the battery of a 90-kWh vehicle 50% in 20 minutes and

80% in 40 minutes [9]. Also, in some other charging station

examples, the charging power values can raise 350kW power

values instantaneously. These new generation charging stations

are named ultra-fast charging stations. The DC fast charging

station is named also extreme fast charging [10]. The type of

fast charging needs high power. So, some of the application is

used the Medium voltage level grid connection [10]. Also,

ultra-fast charging stations have some adverse effects on the

grid. Thus, these charging stations need the local power supply

like energy storage to reduce power demand from the grid. The

flywheel has developed rapidly as an alternative energy storage

unit. Flywheel stores the energy in the form of the kinetic

energy of a spinning mass. Conversion from the kinetic to

electric energy is accomplished by electromechanical systems

Hossam A. Gabbar1,2*, Onur Elma1, Md Ibrahim Adham1 1Faculty of Energy Systems and Nuclear Science, Ontario Tech University, Oshawa, ON, Canada

2Faculty of Engineering and Applied Science, Ontario Tech University, Oshawa, ON, Canada *Corresponding Author: Hossam A.Gabbar, Email: hossam.gaber@ontariotechu.ca

ANALYSIS AND DESIGN of FAST CHARGING SYSTEM

with FLYWHEEL ENERGY STORAGE PLATFORM

W

2

[11-12]. In the flywheel storage system, system control is the

most important part to operating states. One of the control

topologies is magnetic integration technique [12]. DC-DC

converter is a critical part of the fast charging system which has

also flywheel and/or battery storage system. There is different

type of DC-DC converter topologies are used for the fast

charging applications [13].

Also, the fast charging systems need high power density.

Thus, some of the hybrid resources and/or energy storage

system are integrated with FCS to reduce high demand on the

grid [14-16]. Multi-flywheel system is used to study the effect

of the hysteresis control on the FCS dynamics stability [15]. In

another study, an advanced computational intelligence

technique based on an enhanced artificial immune system

(EAIS) is used to improve the performance of the FFCS to

support transportation electrification [17]. The wireless

charging another popular charging method. Flywheel based

wireless charging system has been proposed and analyzed [18].

Thus the flywheel has a good potential to use in charging

system especially for stability, storage and reducing charging

time.

The Figure 1 is given in the proposed hybrid fast charging

configuration. The proposed system uses the battery and

flywheel to reduce effects of the peak demand on the grid side.

Fig. 1. Proposed hybrid fast charging configuration

II. FAST CHARGING STANDARDS

The charging time is one of the most important disadvantages

for electric vehicles. However, advances in the field of power

electronics and battery packaging and management have

enabled to be shortened of the charging time. The fast chargers

which have high power rates has been developed. These off-

board high power chargers are called ultra-fast chargers. New

standards are needed for equipment that can provide this

charging power. For this purpose, CHAdeMO which is the one

of the fast charger standard associations have developed a next-

gen ultra-high power charging standard [19]. Another charge

connection standard developed for e-buses is OppCHARGE

and it is becoming more widespread in Europe [20]. In addition,

Society of Automotive Engineers (SAE) has been working on

ultra- fast charging standard which will be up to 1-1.2 MW [21].

The charging standards which are related to Electric bus (EB)

are given in Table 1 [19]–[21].

TABLE I CHARGING STANDARD RELATED ELECTRIC BUSES

Levels Maximum

power rating

(kW)

Maximum

current rating

(A)

Maximum

voltage

rating (V)

SAE STANDARD

AC Charging 133 160 480

DC Charging

Level 1

Level 2

350

1200

600

1200

500

1000

CHADEMO

DC Charging

Level 1

Level 2

62.5

400

125

400

500

1000

OPPCHARGE (IEC) STANDARD

DC Charging

Level 1

Level 2

150

300 & 450

200

600

450-750

The electric vehicles need an energy to charge their batteries.

This process is called charging and the all charging components

and equipment are manufactured depend on national and

international standards. These standards cover the different type

of layers of the charging process. The standards can be divided

4 layers which are connector, communication, safety, charging

topology as given Figure 2.

Fig. 2. The divided standards of EV-charging

The fast charging systems need these standards to spread.

The big vehicles like bus, truck need more energy in order to

fulfill their missions. Thus, the charging time is a critical

challenge for that type of vehicles. The fast charging system can

be reduced charging time depend on power rate and charging

structure. Especially the buses for using public transportation.

Their operation times are long and the charging times are

limited. Therefore, there are two option to get operational

energy. First is a huge battery size which is expensive and have

some limitation to mount to the bus. The second, the ultra-fast

charging systems which have high power rate to charge the

battery very limited time. When the charging time is limited,

the connection should be very quick. So, the overhead charging

connectors are suitable for these types of charging.

III. FAST CHARGING FOR E-BUS

E-buses needs high power density for enough driving range.

3

The charging power range is really high. So, they need a special

charging equipment and systems. Pantograph, a kind of

connector, is used to reach higher powers especially in charging

systems for public transportation.

These connector connections are designed in three different

types under overhead chargers which are downward

pantograph, upward pantograph and side insertion. There are

different options for charging electric buses used in public

transportation. These options are given in Figure 3.

Fig. 3. Charging options for E-buses

A. On-route Charging

The electric bus is driven underneath the charging station,

which consists of a bipolar catenary. The bus driver starts the

charging procedure by raising the pantograph to the catenary,

and stops it by lowering the pantograph again.

Overhead charging (500-600 kW)

o Terminal charging (5-30 mins) (may categorize

under off-route charging)

o Flash charging (20 secs-1 min)

Wireless charging (limited charging power)

Mobile charging (-)

Battery changing (no charging limit)

B. Depot Charging

Plug-in charging (50-150 kW)

Overhead charging (300 kW)

The charging options for Electric buses affect from public

transportation strategy, routes, locations, grid connection and

etc. Thus, these affects should be analyzed with case studies.

Grid connection is a critical part of the fast charging structure

for electric buses.

IV. ANALYSIS OF THE ULTRA-FAST CHARGING TOPOLOGIES

High required charging rates lead to high power demands,

which may not be supported by the grid. Especially for

commercial vehicles the power limits are higher than personal

vehicles. The parking area can be converted to a hybrid fast

charging station or there will be mobile charging units. The fast

charging systems need DC charging to charge the battery with

high power limits. The dc/dc converter in between the energy

storage system and the rectifier is used primarily to control the

charging and discharging rates of the battery storage. It can be

also used to reduce or eliminate the grid harmonics, resulting in

a higher power factor for a more efficient charging

configuration. The DC/DC converter has to be bidirectional,

enabling the battery storage charging from the grid and

discharging through the EVs. The single- and dual stage

DC/DC converter topologies are analyzed to be used in an EV

charging station infrastructure. The different converter options

considered for the comparison can be classified into two main

categories which are given in Figure 4.

Fig. 4 The classified bidirectional converters

The UFC systems need DC charging to charge the battery with

high power limits. The dc/dc converter in between the energy

storage system and the rectifier is used primarily to control the

charging and discharging rates of the flywheel. It can be also

used to reduce or eliminate the grid harmonics, losses and other

adverse effects. The DC/DC converter has to be bidirectional,

enabling the flywheel energy storage charging from the grid and

discharging through the EVs. In the flywheel based ultra-fast

charging design, dual stage Cascaded Buck Boost (CBB)

topology can be chosen. The CCB type bidirectional converter

is shown in Figure 5. This topology has more benefits about

efficiency and suitable for the EV applications [27].

Fig. 5 The CCB type bidirectional converter

The proposed ultra-fast charging system has some of

substructure and power devices. The schematic of the proposed

system is given in the Figure 6.

4

Fig. 6 Proposed schematic for ultra-fast charging design

The proposed schematic has been used to designing steps of the

proposed system. The sub-structure in the system can be

analyzed.

A. Fast Charging Technologies with Wireless

Wireless charging can be enabled by the inductive charging

method which has already existed but still not standardized in

the industry yet. This method is facilitated by transferring the

energy from the power supply underneath the EV to the EV

battery through the magnetic induction capability based on the

principle of electromagnetic induction at high frequency. The

main components of inductive charging are two coils; the

primary coil, which is placed on the road interface (charging

pad) in the building construction linked to the wall socket

(power network), and the secondary coil which is placed on the

EV battery pack plate.

Currently, conductive charging is considered the most

common charging technique which is categories into two types

namely; AC and DC changings. This conductive method

provides much more flexibility to the user of choosing where to

charge whether at home, workplace or at a public charging

station [28].

AC conductive charging, also known as Level-1 or Level-2,

can be implemented almost anywhere provided that a standard

electrical outlet with AC power is available. A Battery

Management System (BMS) is usually used to facilitate

communication as provided by the internal wiring network.

This provides high performance and low-cost advantages as

BMS is suitable for Plug-in Hybrid Vehicle (PHV) application

in which the specific energy is lower. However, this conductive

charging has two disadvantages namely; power output

limitation due to the size and weight restriction on the on-board

charger, as well as relatively long charging time. Additionally,

the AC power of the utility grid outlet has to be converted to

DC power using an on-board inverter coupled with a DC-DC

converter in order to charge the EV battery via its DC positive

and negative terminals [11-12].

DC charging solution, also known as Level-3; on the other

hand, is suitable for high power charging applications. The

power output of the DC chargers is limited only by the ability

of the EV battery to accept the charge. Rapid charging is

enabled utilizing a DC fast charger which usually takes less

than 1 hour for a single charge. The biggest advantages of such

DC charger is that it can be designed with either high or low

charging rates and it is not limited to its weight and size. DC

charging solutions use off-board chargers which are located

outside the EV and this setup provides flexibility in terms of

power that can be delivered. The higher the power of the DC

charger, the faster the charging gets. Nevertheless, DC charging

requires high investment for installation compared to AC

charging, and could be availably accessed at public charging

stations only. In order to ensure safe and reliable DC charging

services, there are some challenges that needs to be overcome.

First is the adverse impact on the electricity grid including

harmonic contamination and high current demand

superimposing on peak hours. Second is the availability of this

method which is restricted due to the limitations of the electrical

supply network. Finally, since the off-board charger and BMS

are physically separated, complex communication network is

required to ensure correct charging conditions depending on the

battery type, voltage, temperature, and state of charge supplied

by BMS, and their off-board charger should adapt a proper

charging method [28].

EV battery chargers can be classified into on-board and off-

board with either unidirectional or bi-directional power flow

capabilities. Unlike on-board chargers that are widely installed

in many places, off-board chargers are less constrained by size

and weight. However, the charging time and battery life are

closely linked to the characteristics of the off-board charger

used. Different schemes and topologies have been proposed for

offboard chargers [29].

DC fast charger typically operates at Level-3 charging powers

and is designed to charge EVs quickly within an electric output

ranging between 50 kW and 450 kW. With higher power

operation, the AC/DC converter, the DC/DC converter and the

power control circuits become larger and more expensive. This

is why DC fast charger is implemented as off-board chargers

rather than on-board chargers so that it does not take up space

within the vehicle and also the fast charger can be shared by

many users.

To analyze the power flow for DC charging from the DC

charger to the EV battery, in the first step the alternating current

or AC power provided by the AC grid is converted into direct

current or DC power using a rectifier inside the DC charging

station. Then, the power control unit appropriately adjusts the

voltage and current of the DC converter to control the variable

DC power delivered to charge the EV battery. There are safety

interlocks and protection circuits used to de-energies the EV

connector and to stop the charging process whenever there is a

5

fault condition or any improper connection between the EV and

the off-board charger. The battery management system (BMS)

of the EV battery plays a key role of communicating between

the charging station and EV to control the voltage and current

delivered to the EV battery and to operate the protection circuits

in case of any unsafe situation. For instance, control area

network shortly referred to as CAN or power line

communication referred to as PLC are used for communication

between the EV and the charger [30].

The Combined Charging System (CCS) called the combo one

connector which is mainly used in the USA. The second is the

CCS combo-2 connector which is mainly used in Europe. The

third is the CHAdeMo connector used globally by cars built by

Japanese car manufacturers predominantly. Fourth is Tesla DC

charging connectors which are used for AC charging as well for

Tesla vehicles. Finally is the Chinese DC connector based on

China's GB/T standards.

Although DC fast charging is quite attractive as it delivers

high power going all the way up to 400kW to the EV battery

which results in very short recharging times, DC charging has

a number of limitations as the power cannot be increased

infinitely. This is due to three technical limitations of fast

charging. First, the high current charging leads to high overall

losses in charger and battery (I2R). Suppose the internal

resistance of the EV battery is R, and the power losses in the

battery can be expressed simply by using I2R, where I is the

charging current, then the losses would increase by a factor of

4 times whenever the current is doubled.

The second limitation of DC fast charging is coming from the

EV battery. When fast charging the EV battery, the State-of-

Charge SOC of the battery can only go up to 70-80% SOC. This

is because fast charging creates a lag between the voltage and

SOC and this phenomenon increases as the battery is being

charged faster. Hence, fast charging is typically done in the

constant current (CC) region of the battery charging. And after

that, the charging power is reduced gradually in the constant

voltage (CV) region of the battery charging. Moreover, the

battery charging rate (C-rate) increases with fast charging and

this then leads to reduction in the battery lifetime. The third

limitation is coming from the charging cable.

For any EV charger, it is important that the cable is flexible

lightweight so that user can carry the cable and connect to the

car. With higher charging powers, thicker and thicker cables are

needed to allow more charging current. Else, it will heat up due

to the losses. DC fast charging systems today can already

transmit charging currents up to 250A without cooling.

However, in the future with currents above 250A, the charging

cables would become too heavy and less flexible for usage. This

applies to CHAdeMO 3.0 that is 350-400kW charging enabled

up to 600A and 1.5 kV [31]. The solution would then be to use

thinner cables for the given current with cooling systems built-

in and thermal management to ensure that the cable do not heat

up which is more complex and costly to implement compared

to using a cable without cooling.

Canada is home to four manufacturers of electric vehicles,

especially battery electric buses namely; Green Power Motor

Company [32], The Lion Electric Company [33], New Flyer

Industries [34] and Nova Bus [35]. Nova Bus battery has

powerful modular options capable of storing up to 594 kWh of

on-board energy.

For such public transit buses, there are some world-leading

electrification technology developers including BTC Power

[38], BAE Systems [39], Proterra [40], SIEMENS [41] and

ABB [42].

The most popular of this charge system is Versi Charge Ultra

50/175 developed by Siemens-USA, rated at 50 and 175kW,

taking only half hour to fully charge the bus battery.

Siemens Si-charge is currently being developed for overhead

and depot chargers rated at 50-600kW, with flexible dispenser

configurations and semi-parallel charging [37].

The charging station main power source can be either a high-

voltage or a low voltage supply, depending on if the connection

point is 10 kV or 400 V, respectively. In a high voltage

connection, a 10/0.4 kV substation and a connecting 10 kV-

cable is usually installed in addition to a 400 V service.

In order to decrease the power demand from the buses on the

utility grid especially at peak loads, an energy storage system

as a buffer can be connected to the charging station. Charging

the energy storage system when no bus is at the station is

usually implemented in practice, and later, when a bus arrives

for charging, it helps provide adequate charging power. This

method, the energy out take from the power grid is spread out

over a longer time, which decreases the required power. The

storage can be optimally sized to provide the full power demand

or only a portion. It can be designed to recharge fully in between

each bus or make it so large that it does not have to recharge

fully during the busiest hours.

The flow of power from the power grid and the energy storage

system and the electric vehicle battery can be managed via a

DC bus and two power converters as shown in Figure 32 below:

Using flywheel as the energy storage system as a buffer, the

charging topology with the power electronic converter is quite

complex due to the control scheme required in order to regulate

the dynamics of the flywheel. Flywheel-based fast charging

system structure including system level control structure is

shown in Figure 7 below.

Fig. 7 Flywheel-based fast charging system structure

A hysteresis control strategy employs a droop-based DBS

control method to avoid digital communication between the

grid and flywheel energy storage system converters. The

proposed algorithm provides a good response to system-level

signals while not interrupting the predefined charging profiles

of the electric vehicle battery.

Using a super capacitor energy storage system in the charging

system can be implemented with the flywheel energy system as

proposed by G. Joos and M. de Freige [36].

Today’s EVs mostly utilize electrochemical storage devices

to store energy and power its powertrain. These devices must

fulfill certain requirements, so that the EV can perform in an

6

efficient and satisfactory manner. The key requirements are as

follows:

1 High specific energy to ensure a satisfactory range.

2 High specific power to meet acceleration expectations.

3 Maintenance free and long lifetimes.

4 Safe operation under a wide range of conditions including

cold and hot weathers.

5 End of life disposal has minimum environmental impact.

6 High efficiency in charge and discharge cycles.

Batteries that can only discharge once are called primary cells,

for example AAA Alkaline batteries. Batteries that can be

recharged are called secondary batteries. Examples of which are

lead-acid, nickel metal hydride and lithium ion batteries.

Batteries consist of a positive and a negative pole or electrode.

In a charged battery, energy is stored in a chemical form in the

electrodes which is released as electrical energy when

discharged. Vice versa, secondary batteries can be charged

using electricity which is when electrical energy is converted to

chemical energy, stored in the battery.

V. ANALYSIS OF THE ADVANTAGES OF THE FUFC

In this section, the flywheel-based energy storage application

for fast charging has been analyzed. Flywheels store energy as

kinetic energy, which works by accelerating a rotating mass to

high speed and using the momentum to generate electricity

when needed. Flywheels have higher efficiency and have a

higher life cycle than batteries with little decrease in efficiency

[22]. The total charge/discharge of the flywheel is better than

the battery. In addition, flywheels operate in ambient

temperature and with tremendously less cooling system

requirements. Thus, they have less footprint and generate less

heat than batteries [23]. On the other hand, batteries may be

broken via undetectable internal failures, even with regular

maintenance. Thus, flywheel energy storage system has some

advantages for ultra-fast charging applications.

A. Proposed Flywheel Based Ultra-Fast Charger

It can slow charge from the grid, reducing demand, and then

deliver high bursts of power for short durations. The main aim

of fast charging with FESS is the peak demand control and

reduce effects of the peak demand on the grid. In addition, the

get cost benefits for fast charger owners. The scenario of flash

charging with flywheel is given Figure 8.

Fig. 8. Proposed flash charging with Flywheel

B. Flywheel Energy Storage (FES)

Flywheels store energy as kinetic energy, which work by

accelerating a rotating mass to high speed and using the

momentum to generate electricity when the energy needed.

Flywheels have higher efficiency and have a higher life cycle

with little decrease in efficiency. The total charge/discharge of

the flywheel is better than the battery. In addition, flywheels

operate in ambient temperature and in tremendous less cooling

system requirements. Thus, they have fewer footprints and

generates less heat than batteries [23]. On the other hand,

batteries may be broken into undetectable internal failures, even

with regular maintenance. The efficiency of flywheel which

takes into account the required energy in order to keep the

flywheel spinning, and/or standby loss of the flywheel. The

flywheel standby loss is between 0.2% to 2% according to

manufacturers [24]. The structure of the FES is given in Figure

9.

Fig. 9. The structure of the flywheel

C. Case Study: Performance Analysis of FESS for on-route

charging

The on-route ultra-fast chargers are one of the charge

solutions for public transportation. The proposed ultra-fast

charger has FES, bidirectional DC-DC converter, rectifier and

contactors. The both charge and discharge process of the FES

are done via DC-DC converter. Thus, system not needs another

inverter to connect to the grid that helps to reduce the system

capital and maintenance costs. The flywheel is used as an

energy storage in the proposed system. Because of advantages

of high-power density, charge/discharge cycles and fast

response time. The system performance is analyzed via bus stop

in the one of the busiest bus routes in the Toronto, Canada.

Toronto is a biggest city in the Canada and one of the crowded

cities in the North America. The route 32 is the busiest bus route

under the Toronto Transit Commission (TTC) [25]. The one of

the bus stops is chosen which is in the Eglinton Avenue and

Dynevor road. This bus stop is located the nearest the one of the

busiest intersections between Eglinton Avenue and Dufferin

Street.

The buses stop on this stop every 4-5 minutes in the rush

hours. The each pick up/ drop off might be around 1-1.5 minute.

7

When the bus come to the bus stop the charger pantograph is

down and connect to the bus and the charging process starts.

The connection and disconnection times may take 20 seconds.

So the charging time is around 1 minute. The charger power

rate is 600 kW that power rate is already used in some of on-

route charger [26].

When the charging to the e-bus, the switch 1 is active and the

flywheel supply to the e-buses. After the e-bus charging, there

is a 3 minutes time to charge flywheel from the grid. The switch

1 is inactive and Switch 2 is active and the flywheel is charging

from the grid. The process of the charging/discharging are

given in Table 1.

TABLE II. The charging periods of the system

From FES to E-bus From Grid to FES

S1 S2 S1 S2

Power

rate

Charging

time

Energy Power

rate

Charging

time

Energy

600 kW 1 minute 10 kWh 200 kW 3 minute 10 kWh

As seen in Table II, the proposed FES system can reduce 66%

peak demand on the Grid side. The fast response of the flywheel

and advance control of the charge/discharge process can enable

this to be feasible. The charged energy is 10 kWh which can

contribute 6-8 km range to the E-bus. The total route distance

is 15 km, so the on-route charge can give the around 50% of

energy to complete the route.

VI. ANALYSIS OF PUBLIC BUS BATTERY SIZE DEPEND ON

ROUTES AND OTHER PARAMETERS

The mostly fast chargers have 50-150 kW power rates in the

market. These power rates may give a suitable charging time

for personal EVs but not for commercial electric vehicles. Thus,

the DC fast charger should be improved to charge bigger battery

capacities in acceptable charging times. Especially, the regular

DC fast chargers cannot give acceptable charging time for the

buses and/or heavy trucks. The new battery and power

electronic technology make possible to use much more power

rate for fast charging. These high-power rate chargers are called

ultra-fast charging and extremely fast charging. The proposed

analysis, the ultra-fast charging system has been modeled to

reduce the charging time of public buses. The updated charger

standards have been taken into consideration such as

CHAdeMO and SAE. Accordingly, the 450 kW ultra-fast

charging system is analyzed. In addition, the public bus routes

have been used to optimize bus battery capacity according to

the bus schedule, stop time and the flash charging power limit.

The optimum battery size is calculated for the E-buses.

Size of EB battery has been evaluated based on number trip,

waiting time, power rating of the charging station, route length,

peak period operation, off-peak period operation and number of

terminal charging station. 30% margin has been added while

calculating the final battery size considering that the bus battery

will charge & discharge between 20% to 90% of SOC. The

required battery size of EB has been calculated for scenario 1

and scenario 2 and compare them with no terminal charging

station scenario. To make the study versatile, instead of using a

specific route length, 100 different route lengths within specific

boundary have been considered with Monte Carlo method for

both scenarios.

The amount of energy that will be supplied from the Fast

Charging Station (FCS) to EB while waiting in the terminal

during peak period operation is,

EP=EP1=EP2 = (2÷60) ×450 = 15 kWh.

Here, EP is the amount of energy from FCS during peak

period operation (kWh), EP1 is the amount of energy from FCS

at terminal 1 during peak period operation (kWh), EP2 is the

amount of energy from FCS at terminal 2 during peak period

operation (kWh)

The amount of energy that will be supplied from FCS to EB

while waiting in the terminal during off-peak period operation

is, EOP = EOP1 = EOP2 = (6÷60) ×450 = 45 kWh.

Here, EOP is the amount of energy from FSC during off-peak

period operation (kWh), EOP1 is the amount of energy from FCS

at terminal 1 during off-peak period operation (kWh), EOP2 is

the amount of energy from FCS at terminal 2 during off-peak

period operation (kWh)

Considering both peak period and off-peak period

operation, the battery size of EB for scenario 1 can be calculated

by as follows:

BSP = [[{(RL×2×M) – EP}×NTP]+EP]×1.3 (1)

BSOP = [[{(RL×2×M) – EOP} ×NTOP] +EOP] ×1.3 (2)

Here, BSP is the battery size of EB for peak period

operation (kWh), BSOP is the battery size of EB for off-peak

period operation (kWh), RL is the route length/distance

between terminals (km), M is the energy consumption rate of

EB per km (kWh/km), NTP is the number of trips during peak

period, NTOP number of trips during the off-peak period.

Similarly, the battery size of EB can be calculated for both

peak period and off-peak period operation for scenario 2 is as

follows: BSP = [[{(RL×2×M) -EP1-EP2} ×NTP] +EP1+EP2]×1.3

(3) BSOP = [[{(RL×2×M) – EOP1- EOP2} ×NTOP] +EOP1

+EOP2] ×1.3 (4) Size of EB battery if there is no charging station in the

terminal can be obtained by the below equation: FBSWCS = RL×2×M×1.3 (5)

The final battery size of EB considering both peak period and

off-peak period operation for scenario 1 can be calculated as

follows: 𝐹𝐵𝑆 =

{ BS𝑝, RE1 ≤ EOP

[(BSP ÷ 1.3) + {(RE1 − EOP) × NTop}] × 1.3, RE1 > EOP

(6)

here, RE1 is the amount of energy required by EB for the 1st

trip for scenario 1 (kWh).

Final battery size for scenario 2 considering both peak period

and off-peak period operation can be found as follows:

8

𝐹𝐵𝑆 =

{

BS𝑝, RE1−2 ≤ Eop1

[(BSp ÷ 1.3) + {(RE1−2 − Eop1) × NTop}] × 1.3, RE1−2 > Eop1

(7)

Here, RE1-2 is the amount of energy required by EB for the 1st

trip for scenario 2 (kWh).

The algorithm for calculating the EB battery size is has been

shown in Figure 10.

Fig. 10 The flowchart of the batter size calculation

Two scenarios have been considered in this study to find out

the size of EB battery. In the first scenario, single terminal with

a charging station of 450 kW has been considered. 100 different

routes have been taken which length are between 10 km to 20

km. Mileage or energy consumption rate is one of the deciding

factors for the battery size of EB. Mileage or energy

consumption rate is the amount of energy needed by a EB to

travel one km and expressed as kWh/km. The real public bus

route data is used for both scenarios which is given in TABLE II.

TABLE III . PUBLIC BUS ROUTE DATA

Bus route

number

Route

distance

Bus time-

schedule

Bus energy

needs each route

401 24 km 10-12 mins 28.8 kWh

417 6.5 km 30 mins 7.8 kWh

915 21 km 30 mins 25.2 kWh

910 18 km 30 mins 21.6 kWh

950 40 km 30 mins 48 kWh

304 22 km 30 mins 26.4 kWh

A. Case Study I

In this case, single terminal with a charging station of 450 kW

has been considered. 100 different routes have been taken

which length are between 10 km to 20 km. As single terminal

has been considered so the length of each trip will be between

20 (10*2) km to 40 (20*2) km. Since bus will get charge from

terminal charging station, waiting time in the terminal is very

important & this waiting time depends on which part of the day

has been considered. Waiting time will be less if there is a rush

of commuters as the EB needs to commute high number of

passenger & will be high if there is less commuter rush. Both

scenarios have been considered & divided into rush period/peak

period operation & off-peak period operation. In general, the

first peak period starts in the morning from 6am & continues

till around 10am & the second rush period is from 4 pm to 8

pm. During peak period, EB waiting time will be minimum &

in this case it has been considered as 2 min & during off-peak

period the waiting time is 6 min. For connecting &

disconnecting the charger with EB, there will be required some

extra time & this will not be more than 30 sec. Waiting time has

been considered excluding this preparation time. Mileage or

energy consumption rate is one of the deciding factors for the

battery size of EB. Mileage or energy consumption rate is the

amount of energy needed by an EB to travel one km and

expressed as kWh/km. In this case a 12-meter bus with energy

consumption rate of 1.2 kWh/km has been considered. Also,

while calculating the final size of the battery, 30% margin has

been added considering that the bus battery will charge &

discharge between 20% to 90% of SOC. Parameters that have

been considered has been summarized in the Table IV. TABLE IV. Parameters of Case-1 and Case-2

Parameters Value

Case 1 Case 2

Number of Terminal/s 1 2

Route Distance/ Distance between Terminals 10 to 20

km

10 to 20

km

Number of Charging Station 1 2

Power Rating of the Charging Station 450 kW 450 kW

E-Bus waiting time during peak hour

operation in terminal 2 min 2 min

E-Bus waiting time during off-peak hour

operation in terminal 6 min 6 min

Number of trips during peak period

operation 5 5

Number of trips during off-peak period

operation 5 5

Energy Efficiency of EB 1.2

kWh/km

1.2

kWh/km

B. Case Study II

In case study 2, two terminals have been considered & each

terminal has one 450 kW charging station. Distance between

the two terminals is 10 km to 20 km. Parameters of case 2 have

been given in Table IV. Flowchart of calculating the bus battery

size has been shown in Figure 11 and 12, respectively.

Fig. 11 Analysis of battery size of bus with single terminal

9

Fig. 12 Analysis of bus battery size of bus with two terminals

VII. RESULTS AND DISCUSSION

The first analysis for flywheel-based fast charger unit is built

in the bus stop which can be possible to flash charging without

any over demand on the grid side. Also, the proposed system

can reduce the charging demand around 66%. This show that

the flywheel can help to control high demand caused by ultra-

fast chargers.

In addition, another analysis for the fast-charging and the EV

bus battery sizing has been done. That can show the relation

between route, battery size and charging system structure.

As seen in Figure 11 shows the battery size for peak period

operation, off-peak period operation, and total battery size

considering both peak & off-peak period operation for case 1.

If the EB gets more energy compared to the required energy for

a specific route length while waiting in the terminal, total

battery size does not depend on number of trips for that certain

route. For case 1, if the route length is less than 18.75 km,

battery size will not depend on the number of trips during off-

peak period. To clarify more, for route length 17.22 km the total

bus battery is 190.632 kWh. Now if 10 trips during off-peak

period is considered instead of 5 trips then the total bus battery

size will be same as 190.632 kWh. In this paper peak period

operation has been considered firstly then off-peak period has

been considered. EB battery size will be increased to a large

extend if there is no on route ultra-fast charging station in the

terminal. Also, for route length 17.22 km bus battery size will

be 413.28 kWh if there is no on route ultra-fast charging station

in the terminal but if a 450 kW ultra-fast charging station is

considered in the terminal & EB is operated maintaining the

waiting time in peak and off-peak period operation then the bus

battery will be 190.632 kWh. So, for this case EB battery size

can be reduced by 222.648 kWh by putting an ultra-fast

charging station in a terminal. As bus battery comprises around

30% to 40% of total cost, so the cost EB can be reduced to a

great extent by using this on route ultra-fast charging station.

As seen in Figure 12, if there are two terminals, then the

battery size can be reduced further. In case -1 when the route

length is 15.31 km the final battery size is 160.836 kWh for 10

trips (5 trips during peak period & 5 trips during off-peak

period) but in case-2 for the same route length and for the same

number trips the battery size is 82.836 kWh which is almost half

compared to case-1. Also, it can be noted that if the distance

between the two terminals is less than 11.10 km the battery size

does not depend on the number of trips which means if the

number of trips is increased during both peak period & off-peak

period, the battery size will not increase. EB battery size is

noticeably high if there is no charging station in the two

terminals compared to 450 kW charging station in each

terminal. For example, when the distance between 2 terminals

is 15.31 km, the bus battery size is 82.836 kWh if there is

charging station at each terminal but if there is no charging

station, the battery size is more than four times higher which is

367.44 kWh. So, just putting 2 charging stations, battery size

can be reduced greatly & hence the total cost of the EB fleet can

be reduced to a large extent.

VIII. CONCLUSION

The ultra-fast charger is a critical key factor for transportation

electrification. Public transportation has a huge potential to

reduce carbon emissions. The transition to electric buses has

some challenges. One of them is the effects of fast charging on

the grid side. The proposed flywheel-based ultra-fast charging

system gives a solution to reduce peak demand on the grid side.

The system can reduce peak demand by 66% based on the case

study.

In addition, another analysis has been done which is about

fleet charging and electric bus battery size. The benefit of

having an ultra-fast charging station in terminals on EB battery

has been evaluated and quantified. Also, a comparison has been

made between having a single charging station in one terminal

and having two charging stations in two terminals. It has been

found that the EB battery will be 4 times higher if there is no

charging station in terminals compared to two charging stations

in two terminals and the EB battery will be around 2 times

higher if there is a single charging station compared to two

charging stations in two terminals. This study also showed the

optimized route length for the single terminal charging station

and two charging stations in two terminals. This will help the

EB fleet operator to determine the optimized route length for

their operation which will increase their revenue. Also, the EB

battery is directly proportional to the number of trips.

The ultra-fast charger needs to be improved and this is really

important to increase EVs on the road.

ACKNOWLEDGMENT

The study is supported by CUTRIC and Mitacs -IT15756.

Thanks to Faisal Mumtaz from Smart Energy Systems Lab at

Ontario Tech University for his support to this work.

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