Challenge A: A more and more energy efficient railway
Back-up power systems based on fuel cell technology
for signalling equipment electrical supply
Leone M.(*), Stellin M.(*)
Cantamerli G.(**)
(*) Rete Ferroviaria Italiana (RFI), Rome, Italy - (** )Omicron Industriale Srl, Rome, Italy
Abstract
As known, signalling equipments require absolutely no – break electrical power supply because of
their severe impact on safety and availability of railway service.
For this reason the power supply systems for signalling include different levels of redundancy which
are usually achieved by means of traditional batteries and electrical generators coupled with diesel
engines.
Currently another technology is under study and testing, in order to realise an innovative backup
source of energy for railway signalling applications. It consists in Polymer Electrolyte Membrane Fuel
Cells and local Hydrogen generator systems.
This technology has raised great interest, thanks to the several advantages that it can offer than
traditional technologies. First of all the low environmental impact: the energy is obtain by a chemical
reaction whose emissions are not pollutant, consisting only in distilled water and modest heat,
developed during the reaction.
Moreover, the high efficiency of the electrochemical energy conversion process, the modularity of the
structure, that allows to increase the installed power at will, and the long operation time make the fuel
cell system a good candidate to replace both the traditional batteries and the diesel engine - electrical
generator.
For fixed installations, like signalling plants, the local Hydrogen system generator is very useful: it
exploits the water resultant by the energy production process and, with the grid power energy
available in the ordinary operating conditions, restores the hydrogen requested. In this way the logistic
problem of replacing the exhausted hydrogen bottles can be eliminated.
Keywords: Fuel cell, back – up power, no – break power, railway signalling
Introduction
Nowadays the emergency energy of no-break electric power supply systems is generally stored
in accumulator batteries, for a ready but short autonomy, and also produced by an electrical
generator coupled with a diesel motor, to obtain further and more lasting autonomy, as the batteries
are exhausted.
This arrangement is the result of the few consolidated technologies available till now but not
free from grave limits and disadvantages:
1)
2)
3)
4)
Expensive scheduled maintenance that imposes at least four visits per year to test the
battery charge and the functionality of generators;
Low availability due to the presence of several mobile equipments and electronic parts
that are easy victims of usury and then inclined to be affected by failures;
Big areas and specifically sized locals able to support notable weights and to assure
necessary aeration, gas evacuation and acoustic insulation;
Severe temperature ranges and then further need of energy and more costs for the
conditioning of the locals;
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Challenge A: A more and more energy efficient railway
5)
Environmental problems due to the presence of polluting substances like lead in the
batteries and gasses emitted by generators.
In the context of a policy interested to innovative technologies for renewing, improving and revamping
railway infrastructure, RFI is now paying attention to the hydrogen fuel cell as a reserve of energy
instead of accumulator batteries and diesel engine - electrical generator.
Such technology have been already applied with success in the field of telecommunications. The
current intent of RFI is verifying if it is suitable for the railway applications, considering the following
advantages:
a) No need of scheduled maintenance, excepted the activity of substitution of exhausted
hydrogen cylinders; but also this activity is not necessary if the power supply system is also
equipped with a hydrogen generation system;
b) All static devices and no kinematics that expose components to easy usury and failures;
c) Modularity of the structure, that allows to increase the installed power at will;
d) Limited dimensions 1200(l)x800(p)x1270(h) [mm] (cylinders excepted); possibility of both
indoor and outdoor installation;
1270mm
Fuel Cell power
generator
Hydrogen
generator
800mm
1200mm
Figure 1- Rack containing the fuel cell backup power system + the hydrogen generator
e) Wide temperature range allowed for a correct operation (-20°C; +45°C);
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Challenge A: A more and more energy efficient railway
f)
Respect of environment (a study of the Environment Park of Turin has pointed out that within
10 years a fuel cell system, compared to lead batteries, allows to reduce of 90% the emission
of CO2 )
g) Energy saving
Single Fuel cell functionality
A fuel cell is an electrochemical device able to convert chemical energy of the molecules in
electrical energy. It consists in two electrodes, anode and cathode, separated by a polymeric
membrane.
Figure 2 - Structure and electrochemical processes of a fuel cell
By the anode each hydrogen molecule gives an electron to an external circuit; the hydrogen
ions pass through the membrane and by the cathode react with the electron coming from the external
circuit and with the oxygen in the air. Besides electrical energy, this reaction produces only water.
Fuel Cell Power Supply System functionality
CASE 1: Primary energy source available
When the system receives energy by the external energy source and the hydrogen level is low,
the hydrogen generator automatically starts up to maintain the quantity of hydrogen necessary for the
fuel cell operation.
The hydrogen generator consists in an electrolytic device that, by means of an electrolysis process
(the inverse of that happens in a fuel cell), transforms a direct current electrical energy in chemical
energy. The H20 molecule is decomposed in ions H+ and OH-. They react by the electrodes as
follows:
· cathode: 2H+ +2e- → H2
· anode: 2OH- → ½O2 + H2O + 2eThe above reaction consumes energy and needs electrolyte addition. The energy comes from the
primary energy source.
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Challenge A: A more and more energy efficient railway
When the primary electrical power source is available the generator fills the gas cylinders up with the
hydrogen and/or the oxygen produced by the chemical reaction.
On the contrary, when the primary electrical power source is off, the generator stops. All of this
happens automatically.
The fuel cell both with the gas generator is effectively an auto- recharging Uninterruptible Power
System. The water utilized in the electrolysis process comes from a tank belonging to the generator
itself.
The following graph represents the pressure trend inside the hydrogen cylinders during their discharge
(energy production) and recharge (hydrogen production) steps.
Figure 3 – Pressure trend during discharge and recharge of hydrogen cylinders
CASE 2: Primary energy source outage
The back-up power systems insertion takes place in two steps:
-
First transitory step: the final user is fed by a traditional battery but of highly reduced
capacity whose autonomy lasts only few seconds (20”-30”), the only time necessary to
the start up of the fuel cell; meanwhile the Hydrogen generation stops;
-
Second step – the fuel cell operation: the fuel cell starts up and begins to produce
electrical energy from the hydrogen contained in the cylinders and the oxygen in the air
artificially blown through. The ideal operation temperature is given by a cooling circuit
consisting in a liquid-air exchanger.
The water produced in this step is stored in the tank of the gas generator.
The water deficit between the two processes of production of hydrogen and back - up
energy is equal to 20% of the total quantity used. This quantity is restored in occasion of
the scheduled maintenance visits or automatically by the connection to a water system.
To produce 1m3 of hydrogen, the generator consumes 0,85l of water.
To produce 1kWh, a fuel cell consumes 800l of hydrogen.
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Challenge A: A more and more energy efficient railway
Figure 4 Trend of Voltage and of the Electrical Power of a fuel cell
The following flow-chart resumes the previous description: the blue flows represent the case 1, the
green ones the case 2.
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Challenge A: A more and more energy efficient railway
Figure 5 - Functionality of a Fuel Cell System associated to hydrogen generator
The fuel cell supplies direct current energy. The rated power of the whole back-up system can be
chosen within the range of 1,5kW-12kW. The direct current can be converted in the desired shape by
specific power converters.
All the components of the system in a full optional configuration are represented in the following figure.
External Heat Exchanger
Can be separately installed upon customer requests
AC and/or DC Power Electronics
Rectifiers to convert AC into DC. Off and On-line mode
UPS modules to generate proper AC with two options
(230VAC and 400V tri-phase)
The Core
The electrochemical reaction develops here. The fuel cell
stack is hosted here
Auxiliary startup battery
To bridge the start time of the fuel cell stack
Control Interface
For local and remote managing
19” Rack
Can be assembled into 19” rack upon customer requests
requests
Figure 6 – Main components of an indoor installation
Herein a “triple power port inverter”, suitable for RFI application, is described.
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Challenge A: A more and more energy efficient railway
DC/AC power converter
To obtain the desired alternate current and improve the performances of the fuel cell power
system, it has been chosen a triple power gate inverter without single point of failure (TSI), whose
electrical scheme is represented in the following figure.
LEGENDA
Local signaling: Eqpt/operator interface (module status led + module output current bar graph)
DSP: Digital Signal Processor
L-N: AC source port 230Vac
±: DC source port 48Vdc
L’-N’: load port 230Vac
EMI filter: EMI + grid disturbances filter
Boost: boost power stage
Communication bus: module/T2S data transmission line
User interface T2S: System interface (system status led + relay + communications port)
Figure 7 – Inverter Block diagram
The AC grid port is specifically designed to clean up the surges, bursts and all of the well-known
disturbances met on a power network. The AC-to-AC conversion chain isolates the AC output from the
AC input and features a double filtering function. Consequently, the voltage supplied to the critical load
is a pure sine despite all the disturbances (harmonics, surges, glitches. .) usually carried by the grid
and the input current remains sinusoidal even when the load is not linear.
Pure sine wave at the output and ideal power factor at the input are achieved without pumping any
energy from the DC source.
With the TSI concept the filtering of current and voltage is similar to a rectifier combined with an
inverter (on-line mode) but with a significantly better efficiency. Compared to an UPS operating in offline mode, the efficiency is the same range but the rejection of grid disturbances is much higher.
Furthermore, the transfer between energy input sources is disturbance free and can be considered as
a “soft-switching” operation. It is so wise to consider this functioning mode as the normal operation
mode, named as “Enhanced Power Conversion (EPC)” mode.
The AC-to-AC efficiency, which ranges to 96% up is a significant improvement compared to less than
85% overall efficiency given by the rectifier-battery-inverter chain usually in use when similar reliability
performances have to be achieved. So losses are divided by 3.
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Challenge A: A more and more energy efficient railway
The TSI is able to supply 10 times its nominal output current for a time longer than 20ms in case of
downstream short-circuit in the AC distribution. Nominal performances are kept and clean AC power
supply is guaranteed to any other load connected in parallel.
The short-circuit current is also controlled in magnitude to prevent tripping of the upstream breaker.
Full segregation is so ensured and is an additional guaranty that loads are kept free of disturbances
even after failure occurrence.
The internal static switch as well as the inverter of the TSI can be paralleled up to 32 units. The
“synchronization communication bus” is redundant too. The communication is therefore fault-tolerant,
each bus being self-sufficient to handle synchronization, load sharing and data communication.
With TSI one can talk of TOTAL MODULARITY since the static switch has not to be sized according to
the eventual capacity of the AC power system, the evolution of the load being likely unpredictable.
With the TSI the available AC power can be gradually increased to closely follow the load
requirements.
With TSI, the manual by-pass is no longer needed to allow replacement of the static switch. It is just
limited to cabinet maintenance purposes bearing in mind that the TSI module is hot plug and
redundant.
The TSI modules can be arranged in parallel to increase the available output power as well as in 3phase configuration.
Table 1: TSI module technical characteristics
General
EMC (immunity)
EMC (emission)
Safety
Cooling
Isolation
MTBF
Efficiency(Typical)
Dielectric strength
DC/AC
True Redundant
System:
– 3 disconnection
levels on AC out and
DC in ports
- 4 disconnection
levels on AC in
RoHS
EN 61000-4
EN 55022 (Class B)
IEC 60950
Forced
Doubled
240000 hrs
EPC mode 96%
On-line mode91%
4300Vdc
Compliant
Compliant
I/O connection:
-Protected against
inversion of polarity
Terminal block
- Self adaptive to wide
operating conditions
and comprehensive
table of
troubleshooting codes
AC Output Power
Nominal O/P power
2500VA
O/P power (cosφ=1)
2000W
AC Output Specifications
Nominal voltage (AC)
230V
Voltage range (AC)
200-240V
Voltage accuracy
2%
Frequency
50Hz - 60Hz
Frequency accuracy
0.03%
THD (resistive load)
< 1.5%
Load impact recovery
0.4ms
time
Turn on delay
20sec
Nominal current:
-Protected against
reverse current
Crest factor at nominal
power:
-With short circuit
management and
protection
Short circuit clear up
capacity:
-Available while Mains is
available at AC input port
-With magnitude control
and management
10.9A
3.5
10xIn for 20msec
Transfer Performance
Maximum
voltage
0sec
interruption
Total transient voltage
0sec
duration (Max)
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Challenge A: A more and more energy efficient railway
Short time
overload capacity
Permanent
overload capacity
Admissible
load power factor
Environment
150% 5sec
110%
Full power rating
from 0 inductive
to 0 capacitive
Internal temperature management and switch
off
DC Input Specifications
Nominal voltage (DC)
48V
Voltage range (DC)
40-60V
Nominal current
56A
(at 40Vdc)
Maximum I/P current
84A
( for 5sec)
Voltage ripple
2mV
Internal voltage boundaries user selectable
AC Input Specifications
Nominal voltage (AC)
230V
Voltage range (AC)
185-265V
(adj)
Power factor
> 99%
Frequency range
50Hz - 60Hz
(select)
Synchronization range
47-53Hz / 57-63Hz
Altitude above sea level
< 1500m
Ambient temperature
-20 to +50°C
Storage temperature
-40 to +70°C
Relative humidity
95%, non condensing
Signalling & Supervision
Display
Synoptic LED
Alarms output
Dry contacts on shelf
Supervision
Optional device
Weight & Dimensions
WxDxH
102mm x 435mm x 2U
Weight
5kg
Material (casing)
Coated steel
Fuel cell system for RFI application
As illustrated in the following figure (first scheme), a part of the power system for RFI signalling
equipments currently consists in a three phase rectifier – inverter chain. The inverter supplies the load
and in ordinary conditions receives energy by the rectifier. In emergency conditions it receives energy
by traditional batteries.
During the fuel cell system experimentation, the load will be normally fed by the fuel cell system but, if
necessary, commutation on the traditional system will be possible (second scheme in the following).
The definitive solution provides only the fuel cell power system as back-up source (third scheme).
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Challenge A: A more and more energy efficient railway
Figure 8 – Schemes of RFI present and experimental power systems for signalling
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Challenge A: A more and more energy efficient railway
Table 2: Data sheet of the components used in the application
General
Dimensions
1200 x 1200 x 1600mm
Weight
470kg
Cooling
Liquid
Operating temperature
+5°C / +45°C (indoor) - -20°C / + 45°C (outdoor)
Installation
Indoor or outdoor
Noise
0dB (stand by mode)
Relative humidity
0% - 95% non condensing
Altitude
0m asl – 2000m asl
Back up time
4kW/8h
Hydrogen storage cylinder
#4
Stack life expectancy
2000h (continuous production mode) – 87000h (10 years) stand by mode)
Hydrogen production mode
Electrolyser power consumption 3kW
Compressor average power con. 0,7kW
Input voltage
230Vac / 50Hz single phase or 400Vac three-phase
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Specific water consumption
0.85 liters per m of produced H2
H2 outlet pressure
10bar
H2 outlet pressure with booster
150bar
H2 production up to
550l/h
Noise
65dB
Power production mode
Power generation
6kW
Nominal output voltage
48Vdc
Output voltage adj.ble window
46 – 57Vdc
Nominal output current
125A@48Vdc
H2 consumption at max power
5000l/h
Fuel purity
Hydrogen 2.5 – 5.0
Fuel pressure supply
2.5 bar
Noise
65dB
Communication and management
Control panel
LCD status and control console
HMI
Display on board
Communication
Potential free contact, CANOpen (GSM and RS232/RS485), TCP/IP
Safety
Product standard
ISO 22734-1
EMCD
EN61000-6-2; EN61000-6-3
Safety
EN60204-1; EN13611; EN62282; EN60950-1
Storage solution
Auxiliary startup unit
55Ah AGM battery
Cabinet for 4 cylinders
1200 x 550 x 2100mm
For high levels of power (12kW) oxygen cylinders are also necessary. Their number must be equal to
half of that of hydrogen cylinders. In these cases the Direct Oxygen technology (DOX) is adopted
because it allows to obtain more power and requires less auxiliary components (e.g. air blower is no
more necessary).
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