Charging System for Fuel Cell Applications

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Charging System for Fuel Cell Applications
Vehicles with fuel cells become increasingly important, as OEM have announced tointroduce fuel cell vehicles into the market starting in 201 5. Similarly to a combustion engine, the fuel cell also needs compressed air to provide for a high power density. For a longer period, BorgWarner has now collaborated with different OEM andhas developed a turbocharger with high maturity level, which is scalable to support
various applications.
By Dipl.-Ing. Dietmar Metz, Team Leader Advanced Engineering, BorgWarner,Dipl.-Ing. Jürgen Werner, Program Manager Advanced Engineering, BorgWarner,Dr. Stefan Münz, Director Advanced Engineering, BorgWarner, andDr. Michael Becker, Director Advanced R&D, BorgWarner

Motivation and Requirements for theFuel Cell Air Supply

To fulfill future emission regulations for passenger cars, vehicles which locally generate noemissions will be mandatory for the USA. The
legislation CARB LEV III requires a share of
4.5 % zero emission vehicles (ZEV) for 201 8[1 ]. Moreover, electric vehicles and fuel cell vehicles receive high credits in the USA, so thatthey positively affect the CO2 balance of the
car manufacturers. The capacity of batteries isand will stay limited. Since a breakthrough inbattery technology is not to be expected, fuelcell electric vehicles (FCEV) are seen as the
only solution to allowing emission-free drivingwith a satisfactory driving range for the medium
and long term future.
The fuel cell vehicle generates electricity in thefuel cell. Hydrogen from the vehicle tank reacts
with oxygen from the surrounding air generating electricity. Water vapour is the only reaction product being emitted. Auxiliary systems
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supply air and hydrogen in the correct ratio. Like in a standard international combustion engine compression of the intake air leads to anincrease in efficiency and power density. In order to reduce costs, current applications avoidbatteries with high capacity which could serve
as a buffer during transients. Thus, the fuel cellmust provide its energy dynamically according
to the driver’s demand. This requires a highly

System layout fuel cell with FCAS system1transient operation of the air supply system.
In some respect the requirements for the charging systems of fuel cells and internal combustion engines are similar. In both applications,the air supply system can be considered as akey component. To emphasize its special purpose for a fuel cell application, BorgWarner has
named the charging system Fuel Cell Air Supply (FCAS).

Concept of an Air Supply for FuelCells

In ge neral, most of the known charging tech-nologies for internal combustion engines areapplicable to fuel cells. In comparison to thesupercharger, the turbocharger with radial
compressor and turbine achieves higher system efficiency not only because the turbine recovers the exhaust gas enthalpy of the fuelcell. Other important drivers to substitute thesupercharger in the fuel cell system with a turbocharger are the considerably better NVH behavior and the lower packing space needed,[2].
The process temperature of modern fuel cells
for cars is rather low. Therefore, the exhaustgas enthalpy provided to the turbine is not sufficient to drive the compressor. Hence, a powerful electrical motor is necessary to drive thecompressor; in fact, it is an essential component of the FCAS system. Even a variant of theFCAS system without a turbine is available aspart of the FCAS family.

P c stands for the compressor power, T1 for thecompressor intake temperature, πc for pressure ration of the compressor, Кair is the isentropic
exponent, ṁ air the air mass flow compressor
and n c the compressor efficiency.
The steady-state required electric power canbe derived from the compressor drive power
and the power losses from the electrical motor
and the power electronics. If a turbine isavailable, the generated power can be subtracted. The targeted transient behavior determines the dimensioning of the electrical motor
since the maximum power demand duringtransients can be as much as 1 50 % above thestationary demand.

Layout of the FCAS

The FCAS consists of:• a bearing system with two journal bearingsand one bi-directional thrust bearing• electric drive with stator and rotor• water cooling• radial compressor• optional turbine with variable turbine geometry.

Compressor and Turbine

The radial compressors used for the FCAS as
well as for the internal combustion engine aresimilar. Basis for the development of the FCAS

The required compressor drive power is calculated with Eq. 1 :Fuel cell air supply (FCAS)
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2compressor was the huge range of available
variants which allows to optimally match thecharacteristics of the fuel cell regarding specificmass flow and pressure ratio. In general, thecompressor map is shifted towards smaller
mass flows and pressure ratios. The mass flow
range is smaller, so the compressor map canbe narrower which leads to higher efficiencies.
The turbine geometry can be taken over fromthe conventional turbocharger with just thematching being different. For cost reduction

purposes and because of lower temperatures,the turbine material can be downgraded to for
example aluminium. The lower enthalpy can be
optimally used by means of the variable turbine
geometry with adjustable guided vanes (VTG).
For the FCAS system, the newest VTG generation 6 was chosen, which features high efficiencies with closed vanes. On the turbine
side, a comprehensive modular choice of variants is available as well as to match the FCAS
to the desired fuel cell application.

Electric DriveComparison of FCAS and standard
turbocharger

For the FCAS the most important factors are
best efficiency factors and high power density.
Hence, a permanently excited synchronouselectrical motor was selected for driving theFCAS system. Two variations which differ onlyin their design length are available for the usewith and without turbine. In addition, the power
output of both variants is scalable. In each case the operating voltage ranges from 280 to400 V.

Cartridge VTG generation 6 (guide vanes in the center)
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The following power output can be achieved insteady state operation:• FCAS system with turbine up to 1 0 kW
• FCAS system without turbine up to 20 kW.
The efficiency of a turbo machine is dependingon the Mach and Reynolds number. For acompact and efficient machine, high speeds
are an advantage. However, with rising speed
the centrifugal forces on the permanent magnets in the rotor also increase limiting the maximum circumferential speed. A further limit isset by the first critical bending speed of the rotor which must be above the operating speed.
With increasing rotor speed the cooling systemof the electrical motor becomes more challenging. A target speed of 1 20,000 rpm at rated
power has turned out to be the best compromise between costs, efficiency, overall size andtransient behavior. It is significantly higher thanother well known applications [3].

Oil-free Bearing

The fuel cell must not be polluted with hydrocarbons or other chemicals. A perfect oil sealing is difficult to achieve with the standard
oil-lubricated journal bearings usually being
used for turbochargers. Hence, a new kind ofbearing system had to be developed. Due tothe high complexity and the low load-carrying
capacity, magnet bearings were not an option.
Instead, foil bearings, also called air bearings,were applied [4].
Foil bearings have the following advantages:• oil supply not required• low noise emission• high speeds possible• low power losses at very high speeds• maintenance free
• robust against acceleration and vibration.
There are also disadvantages:• increased start up torque
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Measurement of shaft motion comparison ofFCAS vs. standard turbocharger• overall size and weight• minimum engine speed necessary• so far lack of experience in terms of series
application.
Based on the state of the art technology, a hydro dynamic air bearing was developed specially for the FCAS. A robust design forautomotive applications was essential. A highlife span with up to 1 00,000 stop-starts had tobe achieved. Furthermore, good damping qualities were required with high efficiency. The
cooling of the foil bearings had to be realized
without external air supply. Oil free bearings
offer a new degree of freedom for the design ofthe turbo machine. On a standard turbocharger, an oil spill across the piston rings to thecompressor must be avoided by carefully balancing the static pressure inside the bearing
housing and the compressor. The bearing has
to be oriented horizontally. With the oil free
FCAS bearing system both is not an issue
anymore allowing more flexibility in the compressor design and in the orientation of the device. A defined purging with air from thecompressor is asked for and it also reduces thebearing temperature. Another advantage of the4Measurement of transient ramp-up with FCAS

air bearings appears to be the much higher rotor stability compared to a conventional turbocharger. The shaft motion is reduced by up tothe factor of 1 0, so smaller gaps are realized in
the compressor and the turbine which result inhigher efficiency.

Power Electronics

Only direct-current is available in the fuelcell vehicle. Therefore an inverter (powerelectronics) is employed to provide current
and to control the synchronous electrical
motor of the electric turbocharger. The targeted high speed imposes high demandson the power electronics. The followingtasks must be fulfilled:• enabling maximum power with high efficiency• scalability for different system power requirements (modular system)• high quality current signal for the electrical motor (high frequency) to minimize losses inside rotor and stator of the FCAS
BorgWarner Knowledge Library 201 4system• robustness and suitability for automotiveapplication• compactness and low cost.
The inverter technology known from industrial applications and automotive drives can
not be used for the FCAS. However, it was
the base for further development. The essential part in the inverter developmentwas the use of semiconductors, which areapproved for automotive applications. Furthermore, other aspects such as lifetime,
conformity with the automotive standardsof the electromagnetic compatibility (EMC)
and high voltage safety had to be considered. The inverter is an important cost factorof the system and cost optimization is themain focus for all further development.

Demands for Transient Response in
Automotive Application

The fuel cell must be able to deliver the dynamic power required to fulfill the demand of the5

driver. The available system power is directly
correlated to the supplied air mass flow and theboost pressure. Therefore, the air supply system needs to be highly transient too. For a turbocharger, the pressure is directly linked to thecircumferential speed of the compressor. Thus
high speed gradients are necessary for a goodtransient behavior. The ramp-up time can beestimated in advance by taking into accountthe limits for the electric power and the demanded compressor power, the bearing friction
power to be overcome and the efficiency in theelectric chain. The ramp-up time was an important feature in the development of theFCAS. In experimental investigations a rampup time t90 % of 1 s was realized.

Conclusion

Based on the experience with conventional
turbochargers, BorgWarner Turbo Systems hasdeveloped an air supply system for fuel cellvehicles. The system will be a major contribution for enabling future zero emission propulsion
concepts. The so-called fuel cell air supply features innovative technologies like a high-speed
electrical motor and air foil bearings to fulfill thespecific requirements of the fuel cell.

References[1 ] California LEV Regulations with amendments effective 8/7/1 2
http://www.arb.ca.gov/msprog/levprog/cleandoc/cleancomplete_lev-ghg_regs_3-1 2.pdf[2] Venturi, M.; Sang, J.; Knoop, A.; Hornburg,G.: Air Supply System for Automotive Fuel CellApplication; SAE Paper 201 2-01 -1 225, 201 2[3] Mengelle, T.; Aury, J.: Motorized Centrifugal
Air Compressors for Fuel Cell Applications.
Toulouse, 2008
[4] Agrawal, G.L.: Foil/Gas Bearing Technology– An Overview. ASME Publication 97-GT-347,
1 997

BorgWarner Knowledge Library 201 4Contact

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