Technical and economic comparison of different electric bus concepts based on actual demonstrations in European cities

Abstract

Battery‐powered electric buses are increasingly being used around the globe. This work investigates four technological concepts for the rollout of electric buses from a technical and economic perspective: very fast and moderate opportunity charging, overnight charging and trolley hybrid buses (power supply via catenary/battery). The investigations fund on real demonstrations in four European cities. They were carried out in direct cooperation with the respective public transport operators to obtain realistic results. The focus of this work is on an economic comparison based on the total cost of ownership (TCO), including all investment and operating costs in the bus service. Thereby, the battery represents a major asset and is examined more closely, especially with regard to the expected service life. The TCO of corresponding electric and diesel buses, scaled up to a complete line, are directly compared. This includes penalty costs for the emissions of noise and pollutants. Sensitivity analyses on the most relevant variables are conducted to take into account risks and uncertainties. It shows that electric buses can nowadays already be economically competitive under favourable assumptions, regardless of the concept. Trolley hybrid buses turned out to be the most cost‐effective variant in their respective country.

Full text

IET Electrical Systems in Transportation
Research Article
Technical and economic comparison of
different electric bus concepts based on
actual demonstrations in European cities
ISSN 2042-9738
Received on 8th March 2019
Revised 27th June 2019
Accepted on 24th September 2019
E-First on 21st February 2020
doi: 10.1049/iet-est.2019.0014
www.ietdl.org
Fabian Meishner1,2 , Dirk Uwe Sauer1,2,3,4
1Chair for Electrochemical Energy Conversion and Storage Systems, Institute for Power Electronics and Electrical Drives (ISEA), RWTH
Aachen University, Jägerstrasse 17-19, 52066 Aachen, Germany
2Jülich Aachen Research Alliance, JARA-Energy, Templergraben 55, 52056 Aachen, Germany
3Helmholtz Institute Münster (HI MS), IEK-12, Forschungszentrum Jülich, 52425 Jülich, Germany
4Institute for Power Generation and Storage Systems (PGS), E.ON ERC, RWTH Aachen University, Mathieustrasse 10, 52074 Aachen,
Germany
E-mail: fabian.meishner@isea.rwth-aachen.de
Abstract: Battery-powered electric buses are increasingly being used around the globe. This work investigates four
technological concepts for the rollout of electric buses from a technical and economic perspective: very fast and moderate
opportunity charging, overnight charging and trolley hybrid buses (power supply via catenary/battery). The investigations fund
on real demonstrations in four European cities. They were carried out in direct cooperation with the respective public transport
operators to obtain realistic results. The focus of this work is on an economic comparison based on the total cost of ownership
(TCO), including all investment and operating costs in the bus service. Thereby, the battery represents a major asset and is
examined more closely, especially with regard to the expected service life. The TCO of corresponding electric and diesel buses,
scaled up to a complete line, are directly compared. This includes penalty costs for the emissions of noise and pollutants.
Sensitivity analyses on the most relevant variables are conducted to take into account risks and uncertainties. It shows that
electric buses can nowadays already be economically competitive under favourable assumptions, regardless of the concept.
Trolley hybrid buses turned out to be the most cost-effective variant in their respective country.
1 Introduction
The rollout of (battery) electric buses plays a major part in the
reduction of hazardous emissions [most importantly CO2, NOx,
particulate matter (PM), noise], which are caused by the operation
of diesel buses in urban areas. The use of an electric bus makes it
possible to obtain potential emission savings that could be achieved
by replacing at least 30 internal combustion passenger cars with
electric cars [1]. The emission of fine dust by mechanical braking
is also reduced in general comparison to vehicles with combustion
engines, since a large part of the kinetic energy can be recovered
electrically (regenerative braking). To promote electrified public
transport, alongside practical operation in daily road traffic, the
economic perspective is of major significance. Within this work,
four different technological concepts of electric buses, already
driving in European cities, were directly compared and evaluated
regarding their economic efficiency compared with diesel buses.
In recent years, tests with electric buses have been launched in a
large number of European cities. An important major research
project was the EU funded ZeEUS [2]. As an important result, the
‘E-Bus Report’ [3] provides a comprehensive overview of the
projects implemented and of manufacturers/products available in
Europe (as of the end of 2017). Topics relating to the cost-
effectiveness and reliability of the new systems were also dealt
with, but they were not published for various reasons. The
technical maturity of the demonstrations was often not yet
finalised, resulting in unsatisfying results.
Furthermore, the investigations took place in a strongly
evolving market environment, in which the publication of sensitive
data had to be approached with caution. In the context of another
EU funded project named Eliptic [4], an outlook was given on
possible business cases for electric buses. This was based on actual
demonstrations and was intended to show whether and how the
operation of electric buses can be economically viable in the future.
The most important findings are presented in this paper, which is
an extension of the work presented at the IEEE VPPC 2017
conference [5].
The impact of two different charging concepts (overnight
(ONC) and opportunity charging (OC)) on life cycle costs (LCC)
has been investigated in [6]. The study was based on simulations of
routes in Finland and California, taking cost data from the
literature. The results showed that the LCC of electric buses were
on average still higher than those of diesel buses. In some
scenarios; however, they came very close to profitability. This was
especially true for ‘end station’ concepts, where the (fast) charging
stations are located at the respective terminal stops of the routes. A
comparison of the economic efficiency of diesel and battery-
electric buses including prospects was summarised in an extensive
analyst report by Bloomberg [7]. It was based on general
assumptions without reference to a specific application and already
stated a greater economic efficiency of the electric bus in medium
and big cities in the USA. The capital costs of the vehicle were the
highest asset. It turned out that the electric bus had lower-energy
and operating costs. These included aspects such as ‘labour,
insurance, repair and maintenance’. The profitability increased
with the annual mileage. The results of a stakeholder survey on
different e-bus charging technologies in northern Europe (including
inductive charging) have been presented in [8]. Advantages and
disadvantages of the operation were quoted. A theoretical cost
comparison of different charging infrastructure (CI) concepts can
be found in [9]. A distinction was made between stationary and in-
motion recharging, either conductive (trolleybus) or inductive, as
well as battery swapping stations. Here, too, cost information from
the literature was used, e.g. for the charging stations.
Interestingly, the battery swapping stations had the lowest total
costs in the investigated scenario (Orange Line, Los Angeles
Metro). The authors also describe the reasons why these are not
used anyway. These include the lack of interoperability between
manufacturers and problems with different battery degradation.
The line was characterised by a high vehicle frequency (16
vehicles/h). A comparison to combustion engine propulsion buses
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was not made. The identification of the most relevant key
performance indicators and the general evaluation of three
European electric bus projects (EBSF, 3iBS and ZeEUS) were dealt
with in [10]. This comprised financial, environmental, social and
safety issues, among others. The economic advantages of the
application of trolleybuses were already illustrated in the middle of
the last century [11]. This was probably also the main reason for
their extensive worldwide deployment, which, however,
experienced a decline in the past years due to more attractive
alternatives. The use of powerful traction batteries on these
vehicles is a logical step. A study in Zurich (Switzerland) showed
that up to 20% of energy could be saved by equipping the
trolleybuses with batteries [12]. The replacement of diesel buses by
trolley hybrids was discussed in [13]. The trolleybus was presented
as the most economical variant, especially on longer distances
(number of departures).
In comparison to the studies mentioned above, which are often
based on purely theoretical approaches and data, we presented the
results of four real demonstrations in this paper. These have
fundamentally different approaches, which are classified in Chapter
2. To make a meaningful economic assessment, we have carried
out extensive simulations. These also take into account the load
profiles and corresponding service lives of the most important
technical components such as the battery. Especially, the battery
was part of a more detailed investigation. Where possible, we
performed an ageing prediction and considered different battery
lifetime scenarios. To make a meaningful national economic
comparison, which is lacking in many studies that are limited to the
business part only, we have directly integrated the environmental
costs for electric and diesel buses. The basic methodology is
presented in Chapter 3. The input parameters, especially in terms
of cost, and the results have been obtained from and validated by
the respective public transport operators (PTOs) in a set of
workshops and meetings over 3 years. The collected data and
results of our economic investigations for each scenario can be
found in Chapter 4. We excluded a direct cost comparison of the
different concepts. We aimed to provide a realistic insight into the
actual application of electric buses in our partner cities, which is
the reason why we took as much realistic data as possible. Since
the examinations took place in different countries with different
wage levels and energy prices, this impeded a direct comparison.
Nevertheless, it was important for us to give an overview of the
current economic efficiency of electric buses by providing realistic
figures. Chapter 5 discusses this issue, as well as the results and
outlook of the various concepts.
2 Technological concepts
As mentioned before, the studies in this work are based on actual
test operations conducted in four different European cities within
the frame of the European project Eliptic [4]. Fig. 1 shows a basic
classification. A distinction is made between the applied charging
and battery concept.
Regarding charging, we investigated opportunity charging,
overnight charging and charging in motion. In the first case, a
distinction is also made about the grid connection. In this case, the
use of existing (AC or DC) infrastructure of the local tram/metro
network was possible. The corresponding batteries are roughly
divided into high-power, medium-power and high-energy (low-
power) packs.
A more detailed description of the projects, which is not
possible here for capacity reasons, can be found in [14, 15].
2.1 Scenario ‘very fast opportunity charging’
The project took place in Barcelona (Spain). It examined the
operation of two articulated electric buses (18 m) on the H16 line.
The recharging was performed using fast-charging stations at the
respective terminals. The rated electrical charging power was 400
kW. One charger was connected to the municipal medium-voltage
grid (AC). This necessitated the installation of a new transformer.
At the other terminal, the charging power was drawn from the
existing underground infrastructure (auxiliary network, AC).
This intended potential savings to be made on electrical
components during installation. In this case, however, it led to
costly construction measures and cable laying. Also, overnight
charging was enabled by two smaller units with an electrical output
of 50 kW. The concept of regular intermediate charging at the
terminal stops made it possible to keep the traction batteries
relatively small. This was supported by the premise that the buses
are charged at the terminals without exception.
The batteries consisted of cells with a lithium-titanate-oxide
(LTO) anode instead of the most commonly used graphite (C).
Owing to the high-power density and the associated high charge/
discharge rates of this cell chemistry, the 125 kWh pack could be
dimensioned comparatively small in energy terms [16, 17].
2.2 Scenario ‘moderate opportunity charging’
The connection of fast-charging stations to the existing DC tram
network was being investigated in Oberhausen (Germany). For this
purpose, two standard electric buses were purchased, each of which
started operation on one line. In addition to being charged
overnight in the bus depot, an electrical output of 220 kW can be
regularly drawn at the respective terminal stops. The electrical
connection is made either directly to the overhead line or to the
busbar of a substation. The advantage of this implementation is the
use of the existing infrastructure. This potentially saves space and
costs. When considering this scenario, it must be borne in mind
that it was a technical first demonstration. For this reason, a final
evaluation was not possible yet as the technology used is still partly
subject to development. The 200 kWh battery pack, manufactured
by a Polish supplier, consisted of lithium iron phosphate (LFP)
cells from A123 Systems [18]. These are characterised by a higher-
energy density compared with LTO cells, but still offer an
acceptable power density to allow moderate opportunity charging.
2.3 Scenario ‘overnight charging’
The testing of a pure overnight charging approach took place with
a standard electric bus in Bremen (Germany). The concept was
characterised by lower infrastructure and higher battery costs.
The 230 kWh battery was recharged in the depot using a type 2
plug with up to 50 kW. The energy was drawn from the medium-
voltage grid of the local electricity supplier. To ensure a daily range
of 230 km without interim recharging, the passenger compartment
was heated with a small diesel generator in winter. This enabled
energy savings of up to 50% compared with an operation with
electrical heating. This allowed the comparatively compact
dimensioning of the energy storage, which consists of LFP cells.
The manufacturer pursues a concept, in which only the strongly
aged cells are successively replaced and not the entire battery pack.
Fig. 1 Classification of technological concepts
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This should bring a cost advantage, which still has to be verified in
real operation (see Section 4.3).
2.4 Scenario ‘trolley hybrid’
The application of trolley hybrids, i.e. trolleybuses equipped with
traction batteries was investigated in this scenario. Trolleybuses
operate since the middle of the last century. The combination with
modern batteries increases the operational flexibility, as it allows
driving on routes without an electrical supply grid. The
demonstration took place in the Hungarian city of Szeged. The
examined bus line, having a circulation length of 13.3 km of which
5.8 km run under catenary, is in fact a diesel bus line. During the
test period, it was served with electric buses. For this, two 18 m
trolleybuses have been equipped with an additional traction battery.
The usable energy of the 81 kWh Li-ion battery pack, having a
Nickel-Manganese-Cobalt (NMC) based cathode and graphite
anode, is limited to 36.2 kWh. This was due to ageing constraints.
Recharging took place during operation under catenary using an
80 kW onboard charging device.
Table 1 summarises the most important parameters. The yearly
vehicle mileage, which refers to 100% availability (a value which
is not reached in real operation), and passenger capacity are
important factors. To gain comparability, the costs are referred to
them in the following. For more detailed information (exact route
profile, detailed description of the buses) of the demonstrations, see
[15].
3 Proposed simulative approach
To assess the technological concepts and their economic benefits,
we have carried out detailed simulations involving all pertinent
technical components. We have a special focus on the battery and
our approach allows us to predict the ageing of the battery during
operation. The simulation was implemented in MATLAB/Simulink.
In a first step, the load profiles of all technical components
(especially battery, power electronics, connectors etc.) were
calculated. This was done for each scenario. The current vehicle
plans and technical configurations served as the basis.
Subsequently, the total cost of ownership (TCO) were calculated.
For capacity reasons, a detailed description cannot be given here,
but the most important elements are presented (namely, the TCO
calculation and composition). For more information on the
simulation process and its input parameters, the authors refer to
[14] (Chapter 2.2).
3.1 TCO calculation and composition
We applied the net present value (NPV) method to calculate the
TCO. The calculation of present values enabled us to compare
arising cash flows, which differ in amount, timing and duration.
The NPV of an investment is defined as the total of all cash flows
(CFt) over a set period (T) discounted by the discount rate (r) to the
starting date (t = 0) [19]
NPV = ∑
t= 0
T
CFt1 + r−t
(1)
The TCO includes all costs/revenues for the pertinent technical
equipment, arising during the investigation period. It was 12 years
for the battery-electric buses and 18 years for the trolley hybrid
bus. Table 2 provides a classification of the considered costs. From
the PTOs point of view, the transition to electric buses is not yet
taking place primarily for financial reasons. Therefore,
environmental aspects [external costs (ECs)] play an important role
in overall economic consideration. In addition to avoiding CO2
emissions that promote the greenhouse effect, an important goal of
electric mobility is the local reduction of air pollutants, especially
in densely populated areas. These cause damage to health and the
environment, which is ultimately a financial issue from a national
economic point of view. The quantification of the consequential
damage is reflected in correspondingly high environmental costs,
which were analysed and determined in reports by the Federal
Environment Agency (“Umweltbundesamt”, UBA, [1]) and the
European Commission [20] (see Table 3).
PTOs do not have to pay for these costs, making diesel buses
still an economical alternative. Nevertheless, a business case can
already be generated today via politics with appropriate subsidies
for clean buses.
4 Simulation results/economic assessment
The economic assessment of the four demonstrations/technological
concepts includes four steps:
(i) TCO calculation of the initial scenario (operation as described
in Chapter 2). This includes a consideration of the external/
environmental factors. To comp are 12 and 18 m buses in a
meaningful way, the value is referred to in €/passenger km.
Table 1 Main parameters of the four scenarios
‘Very fast
opportunity
charging‘
‘Moderate
opportunity
charging’
‘Overnight
charging’
‘Trolley
hybrid’
city of
demonstration
Barcelona
(Spain)
Oberhausen
(Germany)
Bremen
(Germany)
Szeged
(Hungary)
number of
lines/buses
one line/two
buses
(articulated)
two
lines/one
standard
bus each
one line/one
standard
bus
one line/two
buses
(articulated)
passenger
capacity
110 70 80 120
distance per
vehicle
200 km/
day∼73,000
km/year
240 km/
day∼87,000
km/year
155 km/
day∼57,000
km/year
240 km/
day∼90,000
km/year
battery 125 kWh/Li-
ion (NMC/
LTO)
200 kWh/Li-
ion (LFP/C)
230 kWh/Li-
ion (LFP/C)
81 (36.2
usable)
kWh/Li-ion
(NMC/C)
charging
power
2 × 400/50
kW
2 × 220/20
kW
1 × 50 kW 80 kW
power supply
grid
metro aux./
public (both
AC)
tram (DC)/
public (AC)
public (AC) trolleybus
grid (DC)
route
characteristics
low speed/
high traffic
medium
speed/low
traffic
medium
speed/low
traffic
medium
speed/low
traffic
Table 2 TCO composition
ECs • penalties for the emission of noise and
pollutants in inner cities
• production not regarded
energy (E) • diesel (/l; incl. taxes and excise taxes)
• electricity (/kWh; incl. subsidies and taxes)
• grid utilisation fees (/kW and /kWh)
CI • charging devices [asset and maintenance
(maintenance charging infrastructure (MCI))]
• coupling devices (asset; stationary part)
• grid connection costs (one-time expenses)
• constructional measures (one-time
expenses)
Battery System (BS) • cells, packaging, system components (asset)
(BS1)
• cell/pack replacement (BS2)
maintenance
(vehicle)
• maintenance costs (/km)
• insurance costs (annual costs)
vehicle • vehicle (asset; incl. e-machine/diesel engine)
• coupling device (asset; onboard part)
• no BS (see above)
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(ii) Scaling the number of electric buses in
operation→consideration of a complete line electrification.
(iii) Comparison to diesel buses (EU6).
(iv) Sensitivity analysis on the most important parameters (impact
assessment).
Table 4 states the general simulation parameters. About 12 years
were assumed to be the standard service life for a bus (of course
this depends on the country and PTO) and 18 years for a trolleybus
(due to the lower wear of the electrical components).
Predicting the development of electricity and diesel prices over
a longer period is impossible. Therefore, we chose for a
conservative approach assuming a price increase of 50% for both
in the period under consideration. The cost per passenger kilometre
is a suitable measure for comparing different buses. For this, we
have assumed an average passenger load of 40% in all scenarios.
The final NPV is calculated according to (2). It is composed of
the investment costs (i) for vehicles (Vi), the initial battery systems
(BS1i), and the CI incurred at the beginning of the period under
consideration. The annual running costs (a) for the maintenance of
the vehicles (MVa) and infrastructure (MCIa), as well as energy
(Ea) and ECs (ECa) are added to this. Furthermore, in scenarios
with battery replacement, the costs for an additional pack (BS2i)
are included. The residual value (rv) of the first pack (BS1rv),
which amounts to 5% of the initial costs, is subtracted from these.
The residual values of the vehicles (Vrv), battery systems (BS1rv,
BS2rv) and infrastructure (CIrv) are deducted at the end of the
period. In the case of diesel buses, the costs for CI and BSs are
omitted.
NPV = Vi+ BSli + CIi
+∑
t= 0
T
MVa+ MCIa+Ea + ECa× 1.04−t
+ BS2i− BS1rv × 1.04−t=ts
Vrv + CIrv + BS2rv × 1.04−t=tend
(2)
4.1 Scenario ‘very fast opportunity charging’
Table 5 lists the manufacturers involved and the main cost
parameters of the demonstration in Barcelona. A more detailed list
can be found in [14]. The vehicle and battery assets are high. This
results in more than twice the investment costs for one electric bus
compared with a diesel bus. Owing to lacking long-term
experience and the initial need of adaptation of the workshop and
staff, diesel and e-bus maintenance are assumed to be equal first,
but with a distinct decreasing trend for the electric system. This
accounts for all scenarios we investigated in this paper. The
installations of the fast-charging stations at terminals 1 and 2 (T1
and T2) make up a comparatively big part of the initial investment.
This is explained by major efforts for constructional work such as
opening the walkway to install a transformer underneath it at one
terminal. This was necessary to withdraw the energy from the
municipal power grid. The other terminal is connected to the
subway's auxiliary network. Owing to the long distance of a few
kilometres, there was a great effort involved in laying the cables,
which resulted in high costs. Owing to the required high
performance, the prices for charging stations are correspondingly
high.
Fig. 2 shows the electric power [traction (yellow), aux.
consumers (red), charging (green)] and battery state of charge
(SOC) (blue) curves for simulation under worst-case conditions
[full passenger load (110) and auxiliary power (25 kW)]. Negative
power values indicate a discharge of the battery and positive values
indicate a charge. It shows optimisation potential regarding the
battery sizing/design, as there is still the unused capacity of the
125 kWh battery. On the other hand, the oversizing provides a
safety buffer and reduces the cycle depth, which extends the
service life of the battery.
Fig. 3 shows the expected cyclical lifetime of the LTO battery
(‘Wöhler curve’) [21]. During average operation (2 kWh/km
consumption), the cycle depth/depth of discharge (DOD) is ∼25%.
This translates to around 16,500 equivalent full cycles (EFCs) that
are executed on the battery within the 12 years of operation (dotted
green line). For this reason, we assume that the end of life of the
battery is determined by calendar ageing. This depends strongly on
the average SOC and the temperature. These parameters are
Table 3 ECs for pollution in urban areas per weight and km
(noise)
Pollution type CO2NOxPM10 Noise
costs [Germany] [1] €145/t €10.300/t €36.300/t €0.0968/km
costs [EU] [20] €30/t €4.400/t €87.000/t €0.0768/km
Table 4 General simulation parameters
General parameters
discount rate 4%
period under review 12/18 years (trolley hybrid)
electricity price trend + 50% within 12 years
diesel fuel price trend + 50% within 12 years
average passenger load 40%
Table 5 Involved suppliers and cost parameters
Technology Company
battery-electric buses
(18 m)
Solaris Bus & Coach S.A. [3]
pantograph SCHUNK Gmbh
charging devices Ekoenergetyka
cost type E-Bus per unit diesel bus per unit
vehicle (w/o bat.) €532,000 (12 years
lifetime)
€350,000 (12 years
lifetime)
maintenance T0€4500/quarter €4500/quarter
battery first: €187,500 and
second: €144,000
—
energy/diesel T0−Tend €0.06/0.1–
0.09/0.15/kWh
€0.93–1.40/l
CI OC T1 – connected to the public grid
transformer +
construction
€60,000
installation costs €95,000
construction costs €75,000
coupling system €18,500
charging station €170,000
CI OC T2 – connected to the metro grid (aux.)
installation costs €400,000
construction costs €75,000
coupling system €18,500
charging station €70,000
CI depot – connected to the metro grid
installation costs €15,000 initially
charging station €35,000/unit
Fig. 2 Simulated worst-case operation on weekdays (4.5 kWh/km specific
energy consumption; 2 kWh/km in average case)
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difficult to estimate from the outside. Therefore, we assume two
scenarios in the following: without and with (after 6 years) battery
replacement.
Fig. 4 shows the final TCO, stated in €/passenger km. It has
four bars, of which the three bars on the left-hand side represent the
electric bus under different assumptions. The leftmost states a full
line electrification (eight electric buses in operation instead of two)
without a battery replacement within the 12 years’ timeframe and
an operational availability equal to that of the diesel bus. The
second bar corresponds to the first one, but includes the battery
replacement after 6 years. Taking past price trends into account, the
second battery is thereby assumed to have around 25% reduced
invest costs compared with the first one, which is given a rest value
of 5%. The third bar shows a scenario that seems most realistic at
the moment. Owing to a lower technical reliability of the electric
bus system at the beginning, diesel bus reserves have to be
maintained, which are expressed in an increased vehicle price
(factor 1.5). By including external aspects (orange bar), the
economic efficiency of a line electrification can be displayed. The
costs of the electric bus scenarios with comparable availability are
equal or even lower than for the diesel bus. However, this requires
a comparable (high) reliability of the electric bus system, which
should be achievable in the coming years. The cost reduction,
which compensates the high investments, is mainly caused by the
distinctly lower-energy costs (around €0.10/kWh and ∼2 kWh/km
consumption compared with around €1.20/l and ∼60 l/100 km for
diesel). This case shows that by the expansion of the service,
electrified public transport can be realised at competitive costs.
To counteract uncertainties of some assumptions and
estimations, important parameters are reviewed more deeply. Two
of the most uncertain factors are the price trends of fuel and
electricity. While both are assumed to grow by about 50% during
the period under review, the variation of the price trends between
30 and 150% is performed. This intends to cover different price
development scenarios and evaluate their influence on the
economic efficiency of the systems.
Fig. 5 shows the results. We can see that, under favourable
conditions, an economic efficiency can be achieved for all
electricity price trends (negative TCO difference). Assuming an
increase in electricity prices of 30% (blue line), the electric bus,
assuming no battery replacement, becomes economical from a
diesel price increase of about 80%. For 150% (green line), the
corresponding diesel value is 120%. Here as well, the strong
dependence of the diesel price is evident. It is apparently the most
relevant factor from a financial point of view. The repurchase of
batteries is impactful, but not overall decisive. It further reduces
the efficiency of the electric bus (purple line) by around 10
cents/km. Although battery replacement is planned to increase
reliability and to ensure a trouble-free operation of the buses, the
necessity of more than one battery per bus over 12 years is, as
stated before, uncertain. Assuming the same vehicle price, a
scenario that should appear realistic in the future due to growing
production figures, the electric bus would be economically viable
across all energy price influences (light blue line).
4.2 Scenario ‘moderate opportunity charging’
Table 6 lists the most important cost parameters and manufacturers
involved. Here, too, the investment costs for a standard electric bus
with battery are about twice as high as for a diesel bus. The
technical feasibility of connecting two fast-charging stations to the
local tram grid (DC) was to be demonstrated in this scenario.
Although this prevented a new connection to the medium-voltage
grid, the investment costs were nevertheless quite high compared
with a ‘standard’ grid connection. This was caused by the
necessary work for a direct connection to the overhead line.
Furthermore, comparatively expensive DC chargers had to be
installed, which had to be galvanically isolated and have to operate
with a strongly fluctuating input voltage.
Fig. 6 shows the simulation results of the worst-case operation,
which is characterised by an auxiliary power of up to 25 kW during
operation. The DOD of the battery is up to 20%. This resulted in a
buffer of more than three round trips. Thus, it was quite obvious
that the capacity was oversized even under permanent worst-case
conditions and caused (too) high initial costs for this project. As
already mentioned, however, this is a technical first demonstration,
which should not be assessed solely on the basis of economic
factors. In this case, the oversizing allowed a longer battery life to
be guaranteed (lower cycle depth and current rates), which was
seen as a more important point. Fig. 7 underpins this statement
[18]. We assumed a cyclic lifetime of 20,000 EFCs at a DOD of
10%, which is the cycle depth for an average operation. Having 19
Fig. 3 Expected cycle lifetime of NMC/LTO cells in average operation
Fig. 4 TCO comparison: very fast opportunity charging
Fig. 5 Sensitivity analysis in €/vehicle-km (same reliability): very fast
opportunity charging
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cycles/day, this results in around 8300 EFCs over 12 years (green
dotted line). This is again, far below the critical limit. As in the
scenario before, it is presumable that the battery end of life will be
determined by calendric ageing.
Fig. 8 shows the final TCO, stated in €/passenger km. The bars
are analogous to the previous scenario. Again, approaching
profitability of electric buses can be depicted, when we include
economies of scale to the calculations (operation of three buses per
line instead of only one), assess external aspects and assume same
reliability (bars 1 and 2 versus 4). Despite the high initial costs, the
almost competitiveness of the electrical system is achieved, as in
the previous scenario, by the lower-energy costs (€0.15 ct./kWh
and ∼1.3 kWh/km consumption compared to €1/l and ∼38 l/100
km for diesel in year zero).
In the last step, the price trends for fuel and electricity are
subjected to a sensitivity analysis. Both are estimated to grow by
around 50% over the next 12 years, which are seen as high, but
decent numbers. Nevertheless, for the sensitivity analyses, both
trends are again varied between 30 and 150% to identify the
dependency of the TCO on the prognoses. The results are displayed
in Fig. 9, stated in €/vehicle km. A positive value [delta TCO
(electric–diesel)] means a more economical operation of the diesel
bus.
While the cost difference between the trends (60% step width)
for electricity is around €0.05/km for each solution, the
corresponding cost difference for diesel amounts up to nearly
€0.09/km. This is caused by the higher-energy consumption of the
combustion engine. However, only under the most favourable
forecasts (30% electricity and 150% diesel trend), the electric bus
can achieve economic efficiency (deep blue bar). Another
uncertain factor, which is still part of broad scientific research, is
the durability of battery cells used in electric vehicles.
The initial simulations assumed that a replacement of the LFP
batteries would probably not be necessary during the period under
review, especially concerning cyclic ageing (Fig. 7). The
repurchase of the batteries after 6 years is analysed (violet graph,
repurchase costs see Table 6). With this assumption, the electric
bus will not yet become economical, even with the most favourable
energy cost developments. A further sensitivity was carried out on
the vehicle price (light blue line in Fig. 9). Assuming equal cost for
electric (chassis including powertrain without battery) and diesel
buses, the profitability is already evident from a diesel price
increase of about 105% over 12 years (at an electricity price
increase of 50% and no battery replacement).
Table 6 Involved suppliers and cost parameters.
Technology Company
battery-electric buses
(12 m)
Solaris Bus & Coach [3]
battery cells A123 Systems Inc. [18]
pantograph SCHUNK Gmbh
charger Ekoenergetyka
CI Siemens AG
cost type E-bus per unit diesel bus per unit
vehicle (w/o battery) €300,000 (12 years
lifetime)
€240,000 (12 years
lifetime)
maintenance T0€4500/quarter €4500/quarter
battery first: €200,000 and
second: €125,000
—
energy/diesel T0−Tend €0.15–0.225/kWh €1–1.50/l
CI
installation costs OC €367,000
installation costs depot €1000
coupling system OC €18,500/per unit
charging station OC €90,000/per unit
charging station depot €16,000/per unit
Fig. 6 Simulated worst-case operation on weekdays (2.6 kWh/km specific
energy consumption, 1.3 kWh/km in average case)
Fig. 7 Expected cycle lifetime of LFP/C cells in average operation
Fig. 8 TCO comparison: moderate opportunity charging
Fig. 9 Sensitivity analysis in €/vehicle-km (same reliability): moderate
opportunity charging
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149

4.3 Scenario ‘overnight charging’
Table 7 summarises the most relevant investment costs. The
infrastructure assets are rather low for the concept of pure
overnight charging, especially for only a small number of electric
buses. A bigger fleet would necessitate a more extensive depot
reconstruction. This is not considered here. The vehicle and
specific battery costs are the smallest of the four concepts/scenarios
presented. Since recharging takes place at comparatively low
current rates, comparably cheap Chinese LFP cells are used,
compared with the rather expensive cells in the previous scenarios.
However, they have disadvantages in terms of performance, energy
and service life. These are reflected in our investigations in
increased maintenance costs for the battery of around €1000/year,
which is to be regarded as a fairly small value. In this case, the
prediction of ageing is difficult because we had no more precise
information about the cells. We considered a scenario with/without
battery replacement, too.
Fig. 10 shows the simulated worst-case operation of the test
operation. It can be seen that under these conditions the battery is
completely discharged. The required power of the auxiliary units is
comparatively low, as fossil fuels are used to heat the passenger
compartment. Without this additional heating device, the
achievable range would be significantly reduced.
Fig. 11 shows the final TCO related to passenger's kilometres.
Even under favourable assumptions (same reliability of electric and
diesel bus and a small share of battery replacement costs), the
electric bus cannot compete from a business perspective.
Only when we consider the external aspects, a close to cost
parity can be depicted for the best case electric scenario (leftmost
bar). This is mainly caused by the comparably high electricity price
of €0.22/kWh the operator currently pays for taking energy from
the public power grid. The sensitivity analysis in Fig. 12 underlines
the statement. In this scenario, the electric bus only becomes
economical under the most favourable assumptions (dark blue plot:
30% price trend for electricity and more than 120% for diesel, light
blue: same vehicle acquisition costs and 50% electricity/120%
diesel price trend).
4.4 Scenario ‘trolley hybrid’
Table 8 shows the major cost parameters of the ‘trolley hybrid’
scenario, examined in the Hungarian city of Szeged. Here, too, the
electrical system (vehicle incl. battery) is more than twice as
expensive as the diesel bus. As said before, it is assumed that the
maintenance costs for the diesel and electric buses are the same,
taking into account the additional BS, which should cause higher
costs for the first time. In general and for future studies,
maintenance costs for a trolley hybrid bus are expected to be
significantly lower than for diesel. In Hungary, the electricity
prices are around 30% less than in Germany and the diesel costs
are around 10% lower. This has a significant impact on the overall
profitability. The installation costs of a catenary system (including
substations) are included in the calculations, varying from
€300,000 to 900,000/km. The first value applies to non-complex
straight lines. Depending on the type and complexity of the track
(number of bends, crossings etc.), it can rise to a level of
€900,000/km or more [13, 23].
Fig. 13 shows the simulated worst-case operation on the
demonstration route. The battery SOC falls below 40% in the first
cycles, which would be a critical value since the BS is restricted to
an operation between 40 and 85% SOC (see Table 1). In real
operation, the battery management system should be able to allow
the operation in such exceptional situations, as they outweigh a
potentially minimal increased ageing. Fig. 14 shows the expected
cycle life of the NMC/C battery [24]. During average daily
Table 7 Involved suppliers and cost parameters
Technology Company
battery-electric bus and
equipment (12 m)
Bozankaya/Sileo GmbH [3, 22]
cost type E-Bus per unit diesel bus per unit
vehicle (w/o bat.) €280,000 (12 years
lifetime)
€240,000 (12 years
lifetime)
maintenance T0€4500/quarter €4500/quarter
battery T0€138,000 —
energy/diesel T0€0.22/kWh €1/l
CI (depot)
installations €10,000
charging station €40,000/unit
Fig. 10 Simulated worst-case operation on weekdays (1.5 kWh/km specific
energy consumption, 1 kWh/km in average case)
Fig. 11 TCO comparison: overnight charging
Fig. 12 Sensitivity analysis in €/vehicle-km (same reliability): overnight
charging
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operation, the battery is charged with 19 cycles at a cycle depth of
around 15%. This translates to 2.8 EFC/day around 18,400 EFCs
within the 18 years of operation. This value is only a bit above the
expected limit, which again indicates that the battery end of life
would be determined by calendric ageing. We assumed one
obligatory battery replacement as the most realistic scenario and
included it in our calculations.
Fig. 15 presents the final TCO, stated in €/passenger km. The
bars show a cost comparison of the trolley hybrid demonstration
without and with (€300,000 respective 900,000/km) infrastructure
construction costs (first, second and third bars) as well as the
respective diesel bus operation (fourth bar). In contrast to the
previous investigations, a period of 18 years is considered here, as
this is the usual service life of a trolleybus. It is assumed that the
diesel buses will be replaced after 12 and the battery will be
replaced after 9 years.
It includes, as in the former investigations, scaling effects of
infrastructure (operation of five buses on this line instead of two)
and external/environmental aspects of operation (production not
regarded). These are very low here, as they are mainly caused by
CO2 penalty costs (the EU value, which is used here, is
significantly smaller compared with the German, applied in the
first three scenarios, see Table 3). CO2 emissions from electricity
generation are also lower due to the high proportion of nuclear
power in the electricity mix.
Trolley hybrids are the most economical option for existing
infrastructure (leftmost bar). However, also the new installation
represents an economical alternative to the diesel bus. This is
especially due to the significantly lower-energy costs of electric
buses (light blue bar).
The sensitivity analysis (Fig. 16) shows the TCO difference
(electric–diesel), stated in €/vehicle km. It relates to a scenario with
€300,000/km costs for the construction of a new trolleybus grid
including two battery replacements (6 and 12 years). While the cost
difference between the trends for electricity (60% step width) is
around €0.05/km for each solution, it is correspondingly up to
almost €0.10/km for diesel. This is caused by the higher-energy
consumption of the combustion engine (60 l/100 km versus 17
kWh/100 km).
It can be seen that, depending on the different cost
developments, the trolley hybrid bus can still be economical, even
if infrastructure had to be constructed.
The vehicle price has a very big influence on the TCO of
electric bus projects. Assuming that the purchase price of the
electric vehicle (chassis with powertrain, but without battery)
approaches that of the diesel, a distinct economic efficiency
becomes clear, regardless of all energy price developments (light
blue curve).
Table 8 Involved suppliers and cost parameters
Technology Company
trolleybuses (18 m) Ikarus Skoda TR187.2
battery cells Kokam [24]
charger onboard
CI trolleybus grid (DC)
cost type E-Bus per unit diesel bus per unit
vehicle (w/o bat.) €680,000 (18 years
lifetime)
€350,000 (12 years
lifetime)
maintenance T0€2500/quarter €2500/quarter
battery T0€60,750 —
energy/diesel T0−Tend €0.10–0.18/kWh €0.9–1.49/l
CI (depot)
installation trolley grid €0–300,000–900,000/km
maintenance grid €1000/month (proportionately)
Fig. 13 Simulated worst-case operation on weekdays (3.7 kWh/km specific
energy consumption, 1.7 kWh/km in average case)
Fig. 14 Expected cycle lifetime of NMC/C cells in average operation
Fig. 15 TCO comparison: trolley hybrid
Fig. 16 Sensitivity analysis in €/vehicle-km (same reliability): trolley
hybrid
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151

5 Discussion
As mentioned in Section 1, a direct comparison of the four
scenarios makes no sense due to different energy and wage prices.
This accounts especially for a comparison of the German/Spanish
projects with the Hungarian trolleybus.
From a mere business perspective, only the Trolley hybrid
concept is beneficial against the operation of diesel buses in its
country (see Fig. 15) This applies if the infrastructure already
exists or can be set up cost-effectively (€300,000/km). In general,
this scenario is by far the most cost-effective. This is due to several
factors:
(i) The long depreciation period of trolleybuses (18 years) and the
electricity grid (30 years) play a major role. The installation of 3
km of catenary infrastructure is distributed over an average of five
buses. This number is still very small, assuming that the
installation is in the city centre and can potentially be used by
many lines. The share for the infrastructure (including its
maintenance) in the TCO loses thus clearly in weight.
(ii) Compared to Germany/Spain, the electricity costs are quite low
at comparable diesel prices. There are almost no ECs, as CO2
emissions mainly cause these. These are low in Hungary due to the
electricity mix (big share of nuclear power) and are distributed
over the largest number of passenger kilometres. The
environmental damage caused by the use of nuclear power is not
taken into account in this calculation.
(iii) Furthermore, the maintenance costs of the vehicles are also
lower, as the trolleybus is an established system with decades of
experience. Also, wage levels in Hungary are well below those in
Spain and Germany.
Despite these obvious advantages, the concept does not receive a
predominantly positive response. The main reason is that the
installation of new overhead lines is politically and from an urban
planning perspective difficult to enforce. This is especially true for
cities that do not have catenaries nowadays such as Aachen
(Germany), the current residence of the authors (a city with
comparable size with Szeged).
Taking the Hungarian Trolley hybrid out of consideration, and
looking only at the remaining three electric bus concepts, it
emerges that the gap to profitability is already quite small
(requirement: same availability as the diesel bus). It can surely be
surmounted shortly by reducing the investment costs for electric
buses, which are currently about twice as high as for a comparable
diesel bus. When external factors were included, namely CO2
emissions, noise and air pollution in urban areas, as well as the
assumption of a long (but realistic) battery lifetime, the electric bus
system could already be outlined as overall beneficial in all cases.
This is mainly due to significantly lower-energy costs. The vehicle
and battery accounted for the major part of the TCO in all
scenarios. Owing to a large number of vehicle/passenger km, the
infrastructure costs were greatly reduced. This is an important
statement, as the high initial costs often act as a deterrent. The
results are consistent with comparable studies [6, 7], which also
state the today's economic efficiency of the electric bus in
favourable, but not unrealistic, scenarios.
The ‘very fast opportunity charging’ scenario was characterised
by disproportionately high vehicle and infrastructure costs. It
successfully deployed the fast-charging concept, which is
important for line operation with short pause intervals and high
daily mileage. The ‘overnight charging’ was characterised by the
smallest battery price and its replacement concept, which still has
to prove in real operation. A disadvantage was the small daily
range and the necessity of a fossil auxiliary heating to guarantee
passenger comfort in winter. ‘moderate opportunity charging’,
however, seems to be practical for urban lines and less densely
populated areas with fewer passenger expectancy.
In conclusion, it must be said that all investigated scenarios
were pioneer demonstrations, and thereby still suffered from minor
problems and risks, mainly caused by not finally mature
technology of buses and CI. This fact was taken into account by
including a vehicle reserve. It currently makes the electric bus
uneconomical. However, reliability differs greatly between
manufacturers and cities. Fleets with very high reliability (95%)
are already in operation, e.g. in Eindhoven (Netherlands) [25]. At
this point, it is the task of the big manufacturers to provide fully
developed and mature system solutions for the electric bus.
6 Conclusion
Four different concepts for the introduction of battery-electric
buses were successfully demonstrated. The opportunity charging of
battery-electric buses, taking the energy (also) from the local public
transport grid has been successfully demonstrated in Barcelona
(Spain) and Oberhausen (Germany). The concept of overnight
charging, which is considered to be the most comfortable solution,
since infrastructure investments are rather low, was tested in
Bremen (Germany). It is generally preferred by many cities, but
suffers from the disadvantage that a very large battery or an
additional fossil heating has to be installed to achieve the required
range, especially in winter. In Szeged (Hungary), the operation of
Trolley hybrid buses on a former diesel bus line has been
successfully demonstrated.
The investigations were performed for small fleets and only
one, respective two lines. The conversion of a whole bus fleet from
diesel to electric will be a more complex task, as there have to be
profound changes in the depot and its power supply. Besides,
scaling effects will have more impact and further reduce the cost of
electric buses/vehicle and passenger km.
The combination of different approaches can be advantageous
for other projects. To name an example, the presented overnight
charging scenario could benefit from the adaptation of aspects from
the opportunity charging concepts. Thus, the purchase of energy
from the local tram/metro network (if available, as in this case)
represents a potentially cost-effective alternative.
To determine the costs of larger bus fleets is of further interest.
The cases examined concerned only a few electrically operated
buses. Owing to the small number, they can obtain their energy
from the public or tram/metro power grids without major
upgrading measures. However, supplying an entire fleet with more
than 100 buses could require connection to a high-voltage grid to
enable simultaneous charging, especially overnight. Such a concept
could nevertheless prove profitable as the infrastructure costs are
spread over a large number of vehicles.
7 Acknowledgment
The presented work has been kindly financed by the European
Commission within the frame of the Eliptic project, under Grant
agreement no. 636012.
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