System Design and Supervisory Controller Development for a Fuel-Cell Vehicle

System Design and Supervisory Controller Development for a Fuel-Cell Vehicle

Copyright @ IF AC Mechatronic Systems, Darmstadt, Germany, 2000 SYSTEM DESIGN AND SUPERVISORY CONTROLLER DEVELOPMENT FOR A FUEL-CELL VEHICLE Paul Rod...

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Copyright @ IF AC Mechatronic Systems, Darmstadt, Germany, 2000

SYSTEM DESIGN AND SUPERVISORY CONTROLLER DEVELOPMENT FOR A FUEL-CELL VEHICLE Paul Rodatz, Lino Guzzella, Leonardo Pellizzari

Swiss Federal Institute o/Technology (ETH) Engine Systems Lab (LMS), Zurich

Abstract: A quasi static approach has been utilized to simulate the performance of a fuelcell vehicle with supercapacitor storage. The vehicle is being developed at the Paul Scherrer Institut, together with the Swiss Federal Institute of Technology and industrial partners. This paper describes the underlying model, with emphasis on the fuel-cell auxiliary components. The configuration of the fuel-cell system is discussed and suggestions for the best operating conditions are given. Further a proposal of a driving strategy is presented. Copyright © 2000 IFAC Keywords: Fuel-cell, Systems Design, Supervisory Control, Supercapacitor

1. INTRODUCTION

a variety of auxiliary components, which need to be monitored and controlled. Aside the control of the reactants flow, a reliable humidification and cooling of the fuel-cells must be accomplished. Water and heat management are crucial for the fuel-cell operation and efficiency. Deionized water is used to cool the fuel-cells and humidify the oxidant (air).

With rising concern for the environment the call for clean and fuel-efficient vehicles is growing louder everyday. In May 1999 the Paul Scherrer Institut, together with the Swiss Federal Institute of Technology (ETH) and industrial partners started a project to develop a fuel-cell powered electric vehicle with supercapacitor storage. The major components of the power train are: PEM fuel-cell • Supercapacitor Electric Motor DC/DC Converter Controller Unit.

2. QUASI-STATIC MODEL The system performance is simulated using a quasistatic (QS) approach incorporated in a software package (the QSS-Toolbox) developed at ETH ZUrich (Guzzella and Amstutz, 1999). The main advantages of the quasi-static approach are the flexibility, the simplicity and the speed it offers. Using predefined blocks various system structures can be build in a short time. Moreover, with the knowledge of the basic assumptions and techniques that are used, the programming of new blocks is very straightforward.

Fuel-cells are electrochemical devices which produce electrical energy directly by oxidizing hydrogen without intermediate thermal or mechanical processes. Proton exchange membrane (PEM) fuel-cells are very efficient, compact and of low weight and operate at almost ambient temperature. Because PEM fuel-cells use hydrogen as fuel, the only by-products are water and heat.

The central idea of the QSS-Toolbox is the inversion of the physical causality chain. In contrast to the causality of a physical representation, where a given force results in a speed, the QS representation finds the underlying force for a given speed. Therefore the main input into the model is a cycle, in which the vehicle speed v(k), k = 0,1,... and height (elevation) h(k) profile is given over the time. Normally legislation cycles (such as the FTP in the U.S. or the MVEG-95 in Europe) are used, but also information derived from topographic and route maps can be used. The information is stored in n-dimensional vectors with constant or varying time intervals. Varying time

Supercapacitors are storage devices which have almost the same behavior as normal capacitors. Their storage capacity is however significantly higher, making them suitable as short term storage elements. The fuel-cell output voltage is converted by a DC/DC converter to supply the drive chain. During recuperation the DC/DC converter is used to charge the supercapacitor. The fuel-cell system consists of the fuel-cell itself and

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intervals are very useful when optimizing longer routes, without losses in accuracy, i.e. in phases of acceleration the intervals can be shortened.

T wheel(k)= r · [m f + mr] . a(k) + r· [P;ir. A f

1 -

-

k = 0, ... n-l

-

v(k) = Z(v(k + 1) + v(k))

k = 0, .. . n-l

L (v(k+ 1)+v(k))

v2(k) + mf . g. Cr(V(k))] (5)

(1)

n-I

x(k) = ~

.

The transmission is modeled using a Willans approach (Heywood, 1988).

't

1 -

w

where r is the tire radius. The slope y is in most cycles neglectable and therefore has been omitted in equation (5).

Using the speed information acceleration a(k) and distance x(k) vectors have to be calculated: a(k) = -(v(k + 1) - v(k))

.C

k = 0, ... n-l

TOU!

= e g(i, ((Jin) . Tin -

((JOU!

= i · ((Jin

To(i, ((Jin)

(6)

k=O

where eg is the internal efficiency, To the idling losses and i the gear ratio. Equation (6) is valid for positive wheel torque, although for negative torque (during recuperation while braking) an analogous expression can be formulated .

where't is the step size or interval length. In the following the main blocks that are used in the modeling of a fuel-cell vehicle are explained.

2.1 Vehicle

2.2 Electric Motor

Using the speed and acceleration information (1) the forces that apply to the moving vehicle can be calculated. They are made up by the aerodynamic resistance Fa(t), wheel friction Fr
Two different approaches can be used to model the electric motor/generator. The first is based on measured motor-efficiency-maps. Unfortunately, at low loads the efficiency tends to very small values and is hard to measure or estimate correctly. Hence, this approach is not recommended if high accuracy is demanded.

1 2 = -P . . Af - C w • V 2 al£

Fr(y, vf ) = cos(y)· mf · g . cr(v)

(2)

The second approach uses a Willans formulation

F g(Y) = sin(y)· mf · g

(7)

The aerodynamic resistance Fa(t) is a complex phenomenon, which can only be described thoroughly with the results of wind tunnel experiments. In equation (2) the vehicle is idealized as a geometric body with a cross-area Af and a constant drag coefficient cw·

where kl is a model constant. Notice, that the Willans approach is well suited for problems, that need to be scaled, as the scaling factor becomes a system parameter.

The wheel friction coefficient c r is a function of the rev of the tire, the tire pressure and temperature as well as the road condition (wet road increases the coefficient by 20%). A common approach to calculate the coefficient is given by the equation below:

2.3 Fuel-cell

The time constants of the dynamic effects in the fuelcell (charging and discharging of fuel-cell capacitance; dissociation and electron transfer processes; diffusion of reactants) are compared to the time constants of the other system parts very short (Rizzoni, et aI., 1999). Therefore they can be neglected and the fuel-cell can be described as a static device by its current to voltage characteristics.

(3)

where crQ and CrI are coefficients which depend only on the tire pressure. Further the acceleration force is given by

Using a greatly simplified model of a fuel-cell, the voltage drop across the cell can be modeled as an ohmic resistance R fc ' which depends on many factors, such as current, reactants partial pressure and humidification, fuel-cell temperature etc. The fuel-cell internal voltage can be approximated by

(4) where mf is the vehicle mass and mr the equivalent mass of all rotation inertias. These forces define a torque at the wheels that is necessary to realize the given drive cycle.

(8)

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where Tlis is the isentropic efficiency, Tlm the mechanical efficiency and TlEM the efficiency of the electric motor, which is powering the compressor. Tlm and TlEM are incorrectly assumed to be constant, Tlis is described by a characteristic flow rate - compression ratio map. IIcp is the compression ratio and T bC the temperature before the compressor (GuzzeUa and Amstutz, 1998). 14"

The hydrogen recirculation pump is modeled similar to equation (10) with the respective values for hydrogen. Since the pump will be coupled to a low voltage motor the efficiency Tloc of the DC/DC converter has to be considered.

Fig. 1: Fuel-cell voltage with variable system pressure (computation); stoichiometric coefficient: "H2= "Air = 2; relative humidity:
where T bP is the temperature in front of the pump, which can be assumed to be equal to the fuel-cell operating temperature T FC'

Various models have been proposed to predict the fuel-cell performance more accurately. They all have in common that the fuel-cell voltage is described by a function of the thermodynamic equilibrium potential E, the activation overvoltage Vac! and the ohmic resistance losses Vohmic '

v

= E + V act + Vohmic

For the layout of the cooling system it is assumed, that all waste heat which is produced by the fuel-cell has to be transported by the cooling water or by natural dissipation at the fuel-cell stack. The power needed to drive the cooling pump is given by:

(9) P

1.C. Amphlett et.at. (1995) developed a combined mechanistic empirical model, that takes into account the effects of the partial pressure of hydrogen and oxygen, the humidity and the fuel-cell operating temperature. Utilizing this model yields the results shown in figure 1.

P, cool

I (I

ffiw ) --~p 1'] m 1'] EM 1']i Pw cool

= --

(12)

where Tli is the pump efficiency, Pw the water density, mw the water mass flow rate and ~Pcool the pressure drop in the cooling system.

At low current densities the increase of system pressure from 1.2 bar to 2.2 bar leads to an improvement in fuel-cell output of approximately O.05V or 5%, whereas at high current densities an improvement of 0.1 V or 20% is achieved.

Beard and Smith (1971) describe a method to calculate the heat dissipation from radiators. Using this method the maximum heat dissipation dependent on the vehicle speed is determined. When the waste heat output of the fuel-cell exceeds this value, the power demand of the ventilator has to be taken into account.

2.4 Fuel-cell Auxiliary Components

2.5 Supercap

In this chapter the configuration of the fuel-cell system will be explained. The air-side is an open circuit, whereas the hydrogen-side is a closed circuit, i.e. excess hydrogen is recirculated. The main energy consuming auxiliary components of the fuel-cell system are the air-compressor, the hydrogen recirculation pump and the cooling water pump.

The supercap is modeled by an equivalent circuit of one RC module.

The air-compressor with no recuperation of compressed air energy considered is described by the following equation:

Fig, 2: Equivalent circuit of supercap

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low fuel-cell output power indicates that the auxiliary components need more power than the fuel-cell can supply. Therefore the fuel-cell should be operated in this region at almost ambient pressure or switched off altogether and the power needed for driving being supplied by the storage device. (The efficiency of a real fuel-cell drops more sharply at ambient pressure than indicate in figure 1, therefore the fuel-cell should not be operated at ambient pressure.)

The instantaneous power of the supercap is then calculated by

(13)

where QscCt) is the capacitor charge given by

(14)

When the supercharging of the incoming air is accomplished by a turbocharger, the power recuperated by the turbine can be subtracted from the power needed at the compressor. Due to the low temperature level of the out flowing air, the amount of energy recuperated in a turbine will not be sufficient to drive the compressor. Therefore, the turbocharger needs to be assisted by an additional power source. A configuration with an electric motor has the benefit of a faster response time to dynamic changes in the desired flow rates. Another option could be to install a thermal dri ven turbocharger using catalytic burning of excess hydrogen. In this case no electric power needs to be branched off the fuel-cell power output. Hence more power is available tb drive the E-motor of the vehicle, resulting in a higher top speed.

Note that the internal voltage llj(t) is the voltage over the capacity and not directly accessible to measurement. Nevertheless u#) may be used to describe the amount of energy stored in the supercap

(15)

The driving strategy discussed in the following chapter uses Uj( t) as an indicator for the energy level of the supercap. This energy level needs to be monitored to avoid overcharging of the supercaps as well as vcry low discharging of the supercap, since at low energy levels its efficiency drops sharply.

Because only the pressure loss over the fuel-cell (Ap'" 0.1 bar) has to be compensated by the hydrogen recirculation pump, this required power can almost be neglected.

3. DISCUSSION OF FUEL-CELL SYSTEM To maximize the efficiency of PEM fuel-cells the reactants should be supplied at higher than ambient pressure. Because the hydrogen is drawn from high pressure tanks, almost arbitrary operating pressures can be realized by an actuated pressure reducing valve. In contrast the oxygen is taken from the ambient air, and thus needs to be compressed. The power demand of the compressor has to be weighted against the increase in fuel-cell power, At low flowrates the increase in fuel-eel! efficiency is offset by compressOr losses, resulting in a very poor fuel-cell system efficiency. Figures 3 and 4 show the optimal operating pressure for a fueJ-ce.lJ stack. The zero efficiency at

The fuel-cell is best operated at a temperature in the region of 70°C, since its efficiency increa."es with rising temperature. Unfortunately, with higher temperatures the membrane wiHbe damaged and the danger of a leak between hydrogen and oxygen emerges. Therefore great precaution has to be taken, that no hot spot occurs in the fuel-cell. Hence no large gradient between fuel-cell and cooling water is allowed. This requires a small temperature difference of the water between entrance and exit of the fuel-cell, These requirements result in a high flow rate. The circum-

Fig. 3: Fuel-cell system efficiency without recuperation of compressed air energy. Thick line shows optimal operating pressure.

Fig. 4: Fuel-cell system efficiency with 50% recuperation of compressed air energy. Thick line shows optimal operating pressure.

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DCIDC Converter. 0.4 %

DCIDC Converter. 0.4 % AirCompressor. 14.9 %

Air Compressor: 20.9 %

Hydrogen Pump: 0.5 % Cooing Pump: 1.6 %

Hydrogen Pump: 0.5 % Cooling Pump: 1.6 % Fuel-Cel System: 23.4 %

S a. S

o

5 .5 %

Transmission: 5.8 % Auxiiary Units: 1.9 % Power Electronics : 4.0 % E-Motor: 7.5 %

Power Electronics: 4 .0 %

Fig. 5: Sankey diagram of fuel-cell vehicle at maximum fuel-cell power and top speed (energy loss of fuel-cell is not considered)

Fig. 6: Sankey diagram of fuel-cell vehicle with recuperation of air energy at maximum fuel-cell power and top speed (energy loss of fuel-cell is not considered)

stances are further aggravated by the low temperature level of the cooling water, since large heat exchangers and powerful ventilators are needed to dissipate the heat to the surrounding.

absorb recuperated energy. To cope with this problem, the demands need to be weighted with different priority, to satisfy the power demand at all times being the most important.

Figures 5 and 6 show the computational results of the fuel-cell vehicle. In figure 5 no recuperation of compressed air energy is assumed, whereas in figure 6 50% of the maximal possible energy is recuperated. When recuperation is considered more power is available at the electric motor, resulting in an increase in losses at the electric motor and in the transmission. Nevertheless approximately 5% more power is available at the wheel, enabling better acceleration and greater top speed.

A number of security aspects have to be considered at the highest priority level. These include: supercaps may not be overcharged supercaps may not be discharged below 25 % of maximum energy storage capacity (because of deteriotating efficiency) charging and discharging power of supercaps may not exceed certain limits to avoid overload of power electronics the current drawn from the fuel-cells may not be higher than the fuel-cells are able to supply fuel-cells may never be supplied with negative current (resulting in electrolysis) to allow enough time for diffusion of reactants the fuel-cells should not exhibit major leaps.

4. DRIVING STRATEGY Because the supercapacitor is only suitable for short term energy storage, a predefined driving strategy is required. The driving strategy optimizes the energy exchanges between fuel-cells, supercapacitors and electric motor by monitoring component status and controlling the operating parameters. The purpose is to reduce hydrogen consumption according to energy management strategies specified and tested by simulations.

The starting point of the driving strategy discussed in this paper is the idea to keep the total energy stored in the vehicle constant, i.e. the energy would be either stored in the supercaps or in the moving vehicle (kinetic energy). The fuel-cells would be used to compensate energy losses in the power train. However simple calculations show that the storage capability of ordinary supercaps is not sufficient to absorb all kinetic energy. To connect too many supercaps in parallel or serial is not reasonable, because the increased vehicle mass will result in a higher power demand.

There are many requirements for an ideal driving strategy, with the main three being: 1. satisfy power demand 2. recuperate as much breaking energy as possible 3. vehicle operation with minimal fuel consumption

An optimal driving strategy can only be realized, when the driving profile is known in advance. At present, even with sophisticated navigation systems the short-term « 30sec) dri ving profile cannot be predicted. Therefore a real driving strategy will always result in a compromise between good acceleration or

These demands can be under certain circumstances incompatible, e.g. to allow good acceleration the fuel~ells need to be assisted by the supercaps, which ;hould therefore be fully charged and thus cannot

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5. CONCLUSION

I

___

~ I

I

I

__ __ :__ I

I

1

~~ ~!.~njl _: ____ ~

Fuel Ca.on

In this paper the performance of an electric vehicle powered by supercharged fuel-cells and supercaps has been simulated using a quasi-static approach. The main advantages of this approach are the high flexibility and speed it offers. The configuration of the fuel-cell auxiliary components and the power distribution have been discussed in detail. It has been shown that the benefits of a supercharged fuel-cell can be offset by losses in the air compressor. Hence a carefully designed driving strategy is required, which optimizes the interaction between fuel-cell and auxiliary components. A potential solution has been given by the driving strategy presented, which will serve as a basis for further deliberations.

___ " _

I

I

I

I

I

I

I

I

I

---r---~--------l--- - ~

vehicle speed [kmlhJ

Fig. 7: Basic outline of driving strategy, the bulleted line represents a mini-cycle of acceleration up to 80 km/h and appendant breaking.

The next step will be the realization of a driving strategy with the driving profile a priori known (non causal). Although it has no benefit for the implementation of a real vehicle, it can serve as a lower fuel consumption limit. The quality of a causal driving strategy can then be judged by its deviation from the non-causal strategy.

yielding recuperation of breaking energy. Based on the assumption that with increasing vehicle speed the need for strong acceleration decreases and the amount of energy which can be recuperated increases the following foundation for a driving strategy can be put up. At low vehicle speed (e.g. below 60 km/h) the supercaps should be charged to a high degree, whereas with increasing speed they are discharged gradually to allow the recuperation of as much breaking energy as possible. The exact gradients have to be determined by simulations and experiments and are highly dependable on the system configurations.

REFERENCES Amphlett, J.c., R.M. Baumert, R.E Mann, B.A. Peppleyand P.R. Roberge (1995). Performance Modeling of the Ballard Mark IV Solid Polymer Electrolyte Fuel Cell. 1. Electrochem. Soc., Vol. 142, No.I, pp. 1-15. Beard, R.A. and GJ. Smith (1971). A Method of Calculating the Heat Dissipation from Radiators to Cool Vehicle Engines. SAE Technical Paper

Figure 7 shows the basic outline of the driving strategy discussed above. The vehicle can only be operated in the area left of the maximum speed and below the maximum storage capacity. Under stationary condition the gray highlighted area indicates the zone where the vehicle is powered only by the fuel-cell. Above the gray highlighted area the fuel-cell is switched off and the supercaps will supply the energy. Below the gray highlighted area the fuel-cells are used to power the vehicle and charge the supercaps.

710208.

Guzzella, L. and A. Amstutz (1998). Control of Diesel Engines. IEEE Control Systems Magazine, Vol. 8, No. 9, October, pp55-71. Guzzella, L. and A. Amstutz (1999). CAE-Tools for Quasistatic Modeling and Optimization of Hybrid Powertrains. IEEElASME Transactions on Mechatronics, Vol.48, No.6, pp1762-1769. Heywood, J.B. (1988). Internal Combustion Engine Fundamentals. McGraw-Hill Book Combany Rizzoni, G, L. Guzzella and B. Baumann (1999). Unified Modeling of Hybrid Electric Vehicle Drivetrains. IEEElASME Transactions on Mechatronics, Vol. 4, No. 3, pp 246-257.

Under dynamic driving conditions it will be necessary to depart from this schema, e.g. with high acceleration the fuel-cell power output will not satisfy the power demand. Therefore the fuel-cells need to be assisted by the supercaps regardless of the momentary speed and stored energy. This behavior is demonstrated by the bulleted curve in figure 7, which represents an acceleration up to 80 km/h in 16 sec starting with fully charged supercaps, and the appendant breaking to standstill. When accelerating, the fuel-cell remains off, until the supercap state enters into the 'fuel-cell only' area. The continuing decrease of the supercap storage capacity indicates, that the momentary fuelcell power output is not sufficient to satisfy the power demand and therefore is assisted by the supercaps. While breaking, the fuel-cell is switched off, because the supercaps will be charged using the recuperated energy.

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