A Model-based Scenario Analysis for Assessing the Benefits of Fuel Cell Vehicle Hybridization

9th 9th IFAC IFAC International International Symposium Symposium on on Advances Advances in in Automotive Automotive Control Control Available online at www.sciencedirect.com 9th IFAC International Symposium on Advances in Automotive Orléans, France, France, June June 23-27, 23-27, 2019 2019 Orléans, 9th IFAC International Symposium on Advances in Automotive Control Control Orléans, France, June 23-27, 2019 Orléans, France, June 23-27, 2019

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IFAC PapersOnLine 52-5 (2019) 309–315

A A Model-based Model-based Scenario Scenario Analysis Analysis for for Assessing Assessing the the Benefits Benefits of of Fuel Fuel Cell Cell Vehicle Vehicle Hybridization A Scenario Analysis AnalysisHybridization for A Model-based Model-based Scenario for Assessing Assessing the the Benefits Benefits of of Fuel Fuel Cell Cell Vehicle Vehicle Hybridization Hybridization M. Aliberti, I. Arsie, A. Cricchio, C. Pianese, P. Polverino, M. Sorrentino11

M. Aliberti, I. Arsie, A. Cricchio, C. Pianese, P. Polverino, M. Sorrentino  M. Aliberti, I. Arsie, A. Cricchio, C. Pianese, P. Polverino, M. Sorrentino1 M. Aliberti, I. Arsie, A. Cricchio, C. Pianese, P. Polverino, M. Sorrentino1  Department via  Department of of Industrial Industrial Engineering, Engineering, University University of of Salerno, Salerno, via Giovanni Giovanni Paolo Paolo II II 132, 132, 84084, 84084, Fisciano Fisciano (SA), (SA), ITALY. ITALY. Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II 132, 84084, Fisciano (SA), ITALY. Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II 132, 84084, Fisciano (SA), ITALY. Abstract. Abstract. Abstract. The model-based scenario scenario analyses analyses of of micro micro and and mild mild tri-hybrid tri-hybrid vehicle vehicle configurations, configurations, The paper paper presents presents model-based Abstract. related to existing conventional vehicles equipped with small power proton exchange membrane (PEM) related to existing smalland power exchange membrane (PEM) The paper presentsconventional model-basedvehicles scenarioequipped analyses with of micro mildproton tri-hybrid vehicle configurations, fuel cell (FC) systems and, eventually, additional battery pack. The analyses are intended to support shortThe paper presents model-based scenario analyses of micro and mild tri-hybrid vehicle configurations, fuel cellto(FC) systems and, eventually, additional battery pack. The analyses intendedmembrane to support(PEM) shortrelated existing conventional vehicles equipped with small power protonare exchange to medium-term development of fuel cell powered Light Duty Vehicles (LDVs) technology and are useful related to existing conventional vehicles equipped with small power proton exchange membrane (PEM) to medium-term development of fuel cell poweredbattery Light Duty (LDVs) are useful fuel cell (FC) systems and, eventually, additional pack. Vehicles The analyses are technology intended to and support shortfor carmakers carmakers to quantitatively quantitatively assess the potential potential advantages associated to the the afore-mentioned afore-mentioned fuel cell (FC) systems and, eventually, additional battery advantages pack. The analyses are intended to support shortfor assess the associated to to medium-termto development of fuel cell powered Light Duty Vehicles (LDVs) technology and are useful opportunities in development terms of of short-term short-term clean and cost-efficient cost-efficient production of hydrogen. hydrogen. Theand analyses are to medium-term of fuel cell powered Light Duty Vehicles (LDVs) technology are useful opportunities clean production of analyses are for carmakersintoterms quantitatively assess theandpotential advantages associated to the The afore-mentioned performed making use of of aa comprehensive comprehensive vehicle model, based on on a hybrid hybrid to (black-box and lumped lumped for carmakers to quantitatively assess the vehicle potential advantages associated the afore-mentioned performed making use model, based a (black-box and opportunities in terms of short-term clean and cost-efficient production of hydrogen. The analyses are parameters) approach, approach, medium passenger passenger carcost-efficient equipped with with turbocharged SI engine. engine. The model has has opportunities in terms of ofashort-term clean and production of hydrogen. The analyses are parameters) car equipped aa turbocharged The model performed making useofofa medium a comprehensive vehicle model, based on a hybridSI(black-box and lumped been enhanced to account for the additional components of two different powertrain configurations, namely performed making use of a comprehensive vehicle model, based on a hybrid (black-box and lumped been enhanced to account the additional components of two different powertrain namely parameters) approach, of afor medium passenger car equipped with a turbocharged SI configurations, engine. The model has conventionalapproach, powertrain with ICE passenger and fuel fuel cell cell Auxiliarywith Power Unit (APU) (APU)SI and and hybrid powertrain parameters) of awith medium car equipped a turbocharged engine. The powertrain model has conventional powertrain ICE and Auxiliary Power Unit hybrid been enhanced to account for the additional components of two different powertrain configurations, namely ICE/FC. Simulations have been carried out out vs. standard standard driving cycles for two twoconfigurations, energy management management been enhanced to account forbeen the additional components of two different powertrain namely ICE/FC. Simulations have vs. for conventional powertrain with ICEcarried and fuel cell Auxiliarydriving Power cycles Unit (APU) andenergy hybrid powertrain strategies. The results allow evaluating the impact of powertrain configuration on CO emissions, in case case conventional powertrain with ICE and fuel cell Auxiliary Power Unit (APU) and hybrid powertrain 2 in strategies. The results allow evaluating theout impact of powertrain configuration 2 emissions, ICE/FC. Simulations have been carried vs. standard driving cycles for on twoCOenergy management of both Tank-to-Wheel (TTW) and Well-to-Wheel (WTW) analyses. ICE/FC. Simulations have been carried out vs. standard driving cycles for two energy management of both Tank-to-Wheel (TTW) and Well-to-Wheel (WTW) analyses. strategies. The results allow evaluating the impact of powertrain configuration on CO emissions, in case strategies. The results allow evaluating the impact of powertrain configuration on CO22 emissions, in case of both Tank-to-Wheel (TTW) and Well-to-Wheel (WTW) analyses. © both 2019, Tank-to-Wheel IFAC (International Federation of Automatic(WTW) Control)analyses. Hosting by Elsevier Ltd. All rights reserved. of (TTW) and Well-to-Wheel

Keywords: Fuel cell, cell, Hydrogen, Hydrogen, Control, Control, Modeling, Modeling, Hybrid Hybrid Vehicles. Vehicles. Keywords: Fuel Keywords: Fuel cell, Hydrogen, Control, Modeling,  Hybrid Vehicles. Keywords: Fuel cell, Hydrogen, Control, Modeling, Hybrid Vehicles. overcome overcome the the above above constraints, constraints, governmental governmental and and research research 1. INTRODUCTION  1. INTRODUCTION institutions are now strengthening the cooperation with main  institutionsthe areabove now strengthening the cooperation main overcome constraints, governmental andwith research as demonstrated by awareness of EU overcome above constraints, governmental and research 1. INTRODUCTION emissions is is OEMs, Light duty vehicles (LDVs) (LDVs) impact impact on on global global CO CO22 emissions OEMs, as the demonstrated by the the increasing increasing awareness EU Light duty vehicles institutions are now strengthening the cooperation withofmain 1. INTRODUCTION funding bodies the high high potential potential offered by bywith FCHEVinstitutions are towards now strengthening the cooperation main as high as 16% (Environmental Protection Agency, 2014). The funding bodies towards the offered FCHEVas highduty as 16% (Environmental Protection Agency, 2014). The OEMs, as demonstrated by the increasing awareness of EU Light vehicles (LDVs) impact on global CO2 emissions is based hybridization, even short-term perspective as demonstrated byin increasing awareness of(The EU associated growing number impact of environmental environmental issues has made made Light duty growing vehicles (LDVs) on global CO is OEMs, based even intheaa potential short-term perspective (The 2 emissions associated number of issues has fundinghybridization, bodies towards the high offered by FCHEVas high as 16% (Environmental Protection Agency, 2014). The Fuel Cells and Hydrogen Joint Undertaking, 2015). Moreover, funding bodies towards the high potential offered by FCHEVthehighsustainable sustainable transportation paradigm become an Fuel as as 16% (Environmental Protection Agency, 2014). The and Hydrogen Moreover, the transportation paradigm become an basedCells hybridization, evenJoint in Undertaking, a short-term 2015). perspective (The associated growing number of environmental issues has made recent proposals of major automakers (Toyota, Honda and based hybridization, even in a short-term perspective (The increasingly popular solution, due to the favorable features of associated growing number of environmental issues has made recent proposals of major automakers (Toyota, Honda and increasingly populartransportation solution, due toparadigm the favorable features an of Fuel Cells and Hydrogen Joint Undertaking, 2015). Moreover, the sustainable become BMW) indicate that hydrogen technology is overall ready to Fuel Cells and Hydrogen Joint Undertaking, 2015). Moreover, high efficiency efficiency and low low (or zero) zero) emissions emissions (Onatbecome et al., al., 2015; 2015; the sustainable transportation paradigm an BMW) indicate that hydrogen technology is overall ready to high and (or (Onat et recent proposals of major automakers (Toyota, Honda and increasingly popular solution, due to the favorable features of start competing with alternative sustainable mobility (Greene recent proposals of major automakers (Toyota, Honda and Katrasnik, 2013). Moreover, the recent events involving increasingly popularMoreover, solution, due torecent the favorable features of BMW) start competing with alternativetechnology sustainableismobility (Greene Katrasnik, 2013). the events involving indicate that hydrogen overall ready to high efficiency and low (or zero) emissions (Onat et al., 2015; et 2013). indicate that hydrogen technology is overall ready to traditional cars and emission levels, together with the well-known well-known high efficiency low (or zero)together emissions (Onat et al., 2015; BMW) et al., al.,competing 2013). traditional cars emission levels, with the start with alternative sustainable mobility (Greene Katrasnik, 2013). Moreover, the recent events involving start competing with alternative sustainable mobility (Greene progressive reduction of fuels caused involving this Katrasnik, 2013). Moreover, the reserves, recent events progressive reduction of fossil fossil reserves, this latter latter It et al., 2013). is finally traditional cars emission levels,fuels together withcaused the well-known It is 2013). finally worth worth remarking remarking the the increasing increasing integration integration et al., to suddenly become more severe. Consequently, main OEMs traditional cars emission levels, together with the well-known to suddenly become more severe. Consequently, main OEMs between energy (RE)-based on-site generation and progressive reduction of fossil fuels reserves, caused this latter between renewable renewable energy (RE)-based on-site generation and are now switching from medium-term development outlook progressive reduction of aafossil fuels reserves, caused this latter It is finally worth remarking the increasing integration are now switching from medium-term development outlook electric mobility, especially to maximize the benefits It is finally worth remarking the increasing integration to suddenly become more severe. Consequently, main OEMs electric mobility, especially to maximize the benefits of of towards a shorter-term one.severe. Consequently, main OEMs between renewable energy (RE)-based on-site generation and to suddenly become more towards shorter-term as aa successful energy storage means and carrier, as between renewable energy (RE)-based on-site generation and are now aswitching fromone. a medium-term development outlook hydrogen hydrogen as successful energy storage means and carrier, as are now switching from a medium-term development outlook electric mobility, especially to maximize the benefits of addressed in Fuel Cells and electric mobility, especially toHydrogen maximizeJoint theUndertaking, benefits of towardscomparing a shorter-term one. When all XEV in a(The (The Fuel Cells and Hydrogen Joint Undertaking, When comparing all existing existing XEV powertrains, powertrains, hydrogenhydrogen- addressed hydrogen as successful energy storage means and carrier, as towards a shorter-term one. 2018). Indeed, satisfying both hydrogen demand for mobility hydrogen as a successful energy storage means and as fueled vehicles (either Fuel Electric Vehicles FCEVs Indeed, satisfying both hydrogen demand forcarrier, mobility fueled (either Fuel Cell CellXEV Electric Vehicles --hydrogenFCEVs -- 2018). addressed in (The Fuel Cells and Hydrogen Joint Undertaking, When vehicles comparing all existing powertrains, and residential electricity demand by clean microgrids could addressed in (The Fuel Cells and Hydrogen Joint Undertaking, or Fuel Cell Hybrid Electric Vehicles FCHEVs) exhibit more When comparing all existing XEV -powertrains, hydrogenand residential electricity both demand by clean microgrids could or Fuelvehicles Cell Hybrid Electric FCHEVs) more- 2018). Indeed, satisfying hydrogen demand for mobility fueled (either Fuel Vehicles Cell Electric Vehiclesexhibit - FCEVs aa significant leveraging towards achieving techno2018). Indeed, satisfying botheffect hydrogen demand for mobility advantages as compared to both electric and thermal hybrid fueled vehicles (either Fuel Cell Electric Vehicles - FCEVs - have haveresidential significant leveraging effect towards achieving technoadvantages as compared to both electric and thermal hybrid and electricity demand by clean microgrids could or Fuel Cell Hybrid Electric Vehicles - FCHEVs) exhibit more economic feasibility of renewable-based generation of and residential electricity by clean microgrids could vehicles (i.e., Hybrid Electric Vehicles -- HEVs), due to, or Fuel Cell Hybrid Electric Vehicles - FCHEVs) exhibit more economic feasibility of demand renewable-based generation of vehicles (i.e., Hybrid Electric Vehicles HEVs), due to, have a significant leveraging effect towards achieving technoadvantages as compared to both electric and thermal hybrid hydrogen (Sorrentino et al., 2018). have a significant leveraging effect towards achieving technorespectively, longer range and higher fuel-economy (km/kg) advantages as compared to both electric and thermal hybrid hydrogen (Sorrentino et al., 2018). respectively, range and higher fuel-economy feasibility of renewable-based generation of vehicles (i.e.,longer Hybrid Electric Vehicles - HEVs), (km/kg) due to, economic feasibility of renewable-based generation of and reduced impact (Sorrentino et al., 2016). vehicles (i.e.,environmental Hybrid Electric Vehicles - HEVs), due to, economic and reduced environmental impact (Sorrentino et al., 2016). hydrogen (Sorrentino et al., over 2018). The literature has respectively, longer range and higher fuel-economy (km/kg) hydrogen The literature has indicated, indicated, over the the last last decade, decade, an an increasing increasing (Sorrentino et al., 2018). Nevertheless, FCHEV technology has fuel-economy not significant respectively, longer range and higher (km/kg) Nevertheless, FCHEV technology not yet yet had had towards advanced control and design of fuel and reduced environmental impacthas (Sorrentino et significant al., 2016). interest interest towards advanced control and design of fuel cell cell market developments, mainly due to the currently very The literature has indicated, over the last decade, an increasing and reduced environmental impact (Sorrentino et al., 2016). market developments, mainly due very vehicles (Hu et al., 2015; Song et al., 2018; Sorrentino et al., The literature has indicated, over the last decade, an increasing Nevertheless, FCHEV technology has to notthe yet currently had significant vehicles (Hu et al., 2015; Song et al., 2018; Sorrentino et al., expensive hydrogen as of interest towards advanced control and design of fuel cell Nevertheless, FCHEVproduction, technology as haswell not yet had absence significant expensive hydrogen production, as well as the the absence of 2019). Nevertheless, current drawbacks associated to fuel cell interest towards advanced control and design of fuel market developments, mainly due to the currently very 2019). Nevertheless, current drawbacks associated to fuel cell distribution and infrastructures thatcurrently guarantee the vehicles (Hu et al., 2015; Song et al., 2018; Sorrentino et al., market developments, mainly due to the very distribution and refueling refueling infrastructures guarantee the unreadiness of reliable capillary hydrogen vehicles (Hu al., 2015; Song et al., and 2018; Sorrentino et al., expensive hydrogen production, as well that as the absence the of cost cost and and the et unreadiness of drawbacks reliable and capillary required dissemination and fuel supply large scale. To Nevertheless, current associated tohydrogen fuel cell expensive hydrogen production, as wellin asaa the absence of 2019). required dissemination and fuel supply in large scale. To 2019). Nevertheless, current drawbacks associated to fuel cell distribution and refueling infrastructures that guarantee the distribution and refueling infrastructures that guarantee the cost and the unreadiness of reliable and capillary hydrogen required dissemination and fuel supply in a large scale. To cost and the unreadiness of reliable and capillary hydrogen required dissemination and fuel supply in a large scale. To 1 1 Corresponding author, [email protected] Corresponding author, [email protected] 1 author, [email protected] 1 Corresponding 2405-8963 © 2019, IFAC IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Copyright © 309 Corresponding author, [email protected] Copyright © 2019 2019 IFAC 309 Peer review under responsibility of International Federation of Automatic Control. 10.1016/j.ifacol.2019.09.050 Copyright © 2019 IFAC 309 Copyright © 2019 IFAC 309

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threshold, by imposing a negative torque request to the EMG;

supply worldwide (De-León Almaraz et al., 2014), short-term diffusion of fuel-cell propulsion may benefit from the integration with existing conventional and/or thermal hybrid powertrains. Therefore, with the aim of supporting short- to medium-term development of fuel cell powered Light Duty Vehicles (LDVs) technology, the present research focuses on performing model-based scenario analyses of micro and mild tri-hybrid vehicle configurations, obtained by integrating existing conventional vehicles with small power proton exchange membrane (PEM) fuel cell systems and, eventually, additional battery pack. In this way, it will be possible for car makers to quantitatively assess the potential advantages associated to the aforementioned opportunities in terms of short-term clean and cost-efficient production of hydrogen.

b) Torque gap filling: when shifting gear, the ICE is not capable of transmitting tractive power to the wheels, thus the electric motor can compensate for this torque lack; c) Regenerative braking: when braking, the electric motor can switch its operating mode to generator and impose a resistant torque to the wheels, thus allowing converting part of the kinetic energy lost by the vehicle into electric power; d) Electric-only mode: the driver may choose to activate, when the power demand is limited and SoC is above a certain threshold, the pure-electric drive mode;

The article is organized as follows. The modeling environment is firstly described, also introducing the steps linking the original conventional vehicle simulated configuration to the micro and mild tri-hybrid ones. Then, the considered scenarios are detailed and motivated in terms of potential real-world short-term applications. Finally, before the concluding remarks, the main outcomes resulting from the performed model-based evaluation of assumed scenarios are presented and critically discussed.

e) Boost: the electric motor can provide additional torque supporting the ICE, for example to make the overtaking maneuvers safer. Finally, the model of FCEV has been obtained adding to the previous configuration a Fuel Cell System (FCS), which is described in detail in the next section. A scheme of the powertrain of the FCV is shown in Fig. 1.

2. MODELS The reference vehicle that has been modeled for the Tank-toWheel (TTW) analysis is a C-segment car, equipped with a 1.4 l Turbocharged SI engine, whose main characteristics are shown in Table 1. In its standard configuration, the vehicle is basically equipped with the conventional Internal Combustion Engine (ICE), a 12V battery and an alternator to recharge the latter. Then, a hybrid powertrain has been considered, adding an Electric Motor Generator (EMG) and a second battery, both working at a voltage of 48V. In the layout that has been considered, the EMG is connected directly to the secondary shaft of the transmission: this results in a shorter kinematic chain between the motor and the wheels and a more efficient torque transmission.

Battery

DC/DC

Table 1. Main Characteristics of the reference C-segment vehicle. Parameter Vehicle mass Drag coefficient (Cx) Frontal area Wheel radius ICE max. power ICE max. torque EMG max. power EMG max. torque EMG voltage

Fig. 1. Scheme of the hybrid FC powertrain.

Value 1290 kg 0.343 2.006 m2 0.31 m 125 kW @ 5500 rpm 250 N*m @ 2500 rpm 22 kW 60 N*m 48 V

2.1 Vehicle model In order to simulate the behavior of the vehicle in all the considered configurations, described in section 2.3, a preexisting and experimentally validated model implemented in MATLAB/Simulink® has been used (Arsie et al. 2015): its modular structure, shown in Fig. 2, allows switching from a configuration to another just by enabling/disabling certain blocks.

Such a configuration allows using a series of features, such as: a) Battery recharging: the goal is to recharge the battery when its State-of-Charge (SoC) falls below a certain 310

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311

tf

 v  t  dt

FE 

ti

Vf

 km   l 

(5)

 gCO2  gCO2  2330 V f    l 

(6)

Table 2. Constants used in eqs. (1) through (12). Constant PEM fuel cell system kFC Hydrogen storage ksto Internal combustion engine kICE Gear box kgear Gasoline stored Vf Gasoline density ρf

Fig. 2. Scheme of the complete fuel cell hybrid vehicle model. The main input of the model is the speed required by the driving pattern of the considered homologation cycle, more precisely the error between reference and actual vehicle speed. This error signal feeds the driver, represented through a PID controller and whose output is the action on the pedal: this translates in a torque demand, which splits between ICE and EMG in a measure that depends upon the adopted energy management strategy, or in a braking demand, which splits between conventional and regenerative kind.

(1)

mH 2 , sto

(2)

msto 

ksto

M

tf

(3)

On the other hand, to estimate hydrogen fuel economy, an efficiency map has been inferred by fitting the experimental data. To consider the whole balance of plant (BoP) of the FCS (see Figure 3), both gross stack power (Pstack) and auxiliary power adsorption (Paux) have been measured to calculate net efficiency, as follows:

d NEDC

g

0.478 60 0.75

Fig. 3. Components in the Balance of Plant of a Fuel Cell System for automotive applications.

where dNEDC is the length of the New European Driving Cycle (NEDC). The results of the simulation are fuel consumption, otherwise gasoline-only related CO2 emissions and fuel economy (FE), and H2 consumption, computed as follows:

mH 2   mH 2 dt

[kg/kW] [l] [kg/l]

About the FCS, the use of proton exchange membrane (PEM) fuel cells has been considered. A schematic representation of a generic automotive PEM FC-based system is given in Fig. 3. In order to model FCs energetic behavior, experimental data acquired at the PEM FC test bench available at the Energy and Propulsion Laboratory - University of Salerno (eProLab UNISA) have been used. First, the polarization curve has been traced to determine the possible operating points of the single cell (Fig. 4) in terms of current I, voltage V and gross power PFC.

Considering for the constants the values listed in Table 2, the term ksto is the gravimetric density of the storage system, assuming that compressed H2 at 700 bar and ambient temperature is used, while mH2,sto is the mass of the gas to store in order to achieve a prefixed autonomy of M = 500 km, computed as:

mH2 ,sto  mH2

Value 20 0.044 2

2.2 Fuel Cell System model

It is clear that the global torque demand changes in the different configurations, mainly because of the variation in the mass of the vehicle. Referring to the parallel FCHV architecture, the new vehicle mass can be obtained by adding the mass of hybridizing devices to the conventional vehicle body mass: for a FCS of power PFC and the associated hydrogen tank, they can be computed as follows:

mFC  k FC PFC

Unit [kg/kW] [kgH2/kgsto] [kg/kW]

(4)

FCS 

ti

Pstack  Paux mH2 HHVH2

(7)

Then the obtained points have been curve-fitted by the rational polynomial function shown in Fig. 5, as proposed in Sorrentino et al., 2013: 311

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As mentioned above, the vehicle model has been suitably modified to simulate a hybrid powertrain and introduce the FCS, in order to analyze two different configurations:

0.95

Voltage [V]

0.9

a) Conventional powertrain with ICE and fuel cell Auxiliary Power Unit (APU): it is basically the reference vehicle in its default configuration, where the alternator is replaced by a small FCS which acts as an APU, recharging the standard 12V battery. This configuration is expected to reduce the energetic demand of the vehicle since it lowers the load on the ICE and raises the overall efficiency in the production of electric power from about 23% (considering for the alternator and the ICE an efficiency of about 77% (BOSCH) and 30%, respectively, to that of the FCS, in this case 45,7%.

0.85

0.8

0.75

0.7 0

20

40

60

80 100 Current [A]

120

140

160

Fig. 4. Polarization curve of the PEMFC.

b) Hybrid powertrain ICE/FC: the conventional thermal engine is supported by an EMG, whose operating mode depends on the torque demand. Under positive demand, it operates as a motor, whereas, when there is a negative load, it operates as a generator, actuating a regenerative braking. Since this feature alone is not enough to keep the SoC constant along the driving cycle, battery recharging can occur in two different modes: (A) using the ICE, which moves the EMG, or (B) using the FCS, which feeds the battery directly and limits the use of the thermal engine especially at low load, like in the urban cycle. About the auxiliaries, in this case the alternator keeps working when the ICE is on to recharge the 12V battery.

Fig. 5. Efficiency curve of the FCS. Y-axis values are omitted for the sake of confidentiality. Supposing that the system works at the maximum efficiency point ηFCS = 0.457, its useful power is determined, and so are cell voltage VFC and current IFC from the polarization curve; known the maximum power demand, it is easy to evaluate the number of cells in the stack. The sizing of the fuel cell stack is carried out assuming constant power operation and, therefore, as a function of the energy ENEDC needed to ensure chargesustaining operation of the battery over the entire driving cycle:

Pstack

E  NEDC t NEDC

3. WELL-TO-WHEEL ANALYSIS The performed simulations provide the amount of both gasoline and hydrogen consumptions along a homologation driving cycle, such as the New European Driving Cycle (NEDC). The results have been also extended to a Well-toWheel (WTW) CO2 emissions analysis. 3.1 Tank-to-Wheel

(8)

First, a Tank-to-Wheel (TTW) analysis has been carried out to evaluate the consumption of both gasoline and H2. Hydrogen consumption needs to be converted into an equivalent gasoline consumption in order to assess the effective energetic advantage of the three configurations against the baseline:

Table 3 indicates the values of power, numbers of cells, voltage and current obtained for each considered configuration, illustrated in the following section. Finally, to obtain the global system efficiency, it is also necessary to consider the presence of a DC/DC boost converter (see Figure 1), which adjusts the fuel cell output direct current to the voltage of the battery; it has been considered sufficiently accurate for the purpose of the analysis to estimate its efficiency as a constant ηconv = 0.9.

mH 2 , gas ,eq  mH 2

Ncell Vcell Istack Pstack

Unit [/] [V] [A] [W]

a) 4 0.8 85.94 275

LHVgas

mgas,eq  mH2 , gas,eq  mgas

(9) (10)

Obviously, hydrogen consumption is zero for the conventional vehicle. TTW CO2 emissions are evaluated via eq. (6).

Table 3. Fuel cell system operating parameters. Parameter

HHVH 2

Configuration b) - A b) - B 15 22 0.8 0.8 75.00 76.71 900 1350

3.2 Well-to-Tank It is worth remarking that fuel cells are generally considered zero emissions devices (when powered with pure hydrogen). Nevertheless, production of hydrogen is a very energy intensive process and leads to CO2 and other pollutant emissions, especially if fossil or, more generally, nonrenewable energy sources are used: an example is the reforming of fossil fuels, such as methane, which is the most

2.3 Configurations

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widespread pathway nowadays. Even “clean” pathways, which involve for example solar or wind energy, entail indirect GHG (Green House Gases) emissions due to compression, transport, storage, etc., which, in this case, are considered using grid electric power. The environmental impact of the different pathways is expressed by the Global Warming Potential (GWP) index, measured as gCO2,eq/gH2 to consider also the effect of other pollutant species, mainly CH4 and N2O. Table 4 lists the pathways analyzed and their GWP.

313

the SoC value. Again, eq. (8) allows estimating a necessary power to be installed for the FCS of 1350 W for a vehicle with a mass of 1560 kg.

Table 4. GWP (gCO2,eq/gH2) of different H2 production pathways (Edwards et al., 2014). H2 production pathway

GWP index 12.52 5.23

Steam reforming Steam reforming + Carbon Capture & Storage Wood Gasification Grid Electrolysis (EU-mix) Wind Electrolysis

Fig. 6. SoC of 12 and 48 V batteries during a NEDC cycle using the A strategy.

2.11 27.14 1.56

5. RESULTS OUTCOMES

2

4

4

2

APPLICATION-ORIENTED

The TTW analysis showed that, by gradually replacing the use of the thermal engine with fuel cells, from feeding the auxiliaries to supply part of the energy for traction, it is possible to achieve a considerable energy saving, in terms of equivalent fuel consumption. Moreover, reducing the fraction of energy coming from gasoline with respect to the global value requested by the vehicle, it directly translates into lower TTW CO2 emissions, as shown in Figure 8.

A GWP index is also defined for chemical species, indicating how many grams of CO2 have the same impact of one gram of the considered substance. In this way, the equivalent amount of carbon dioxide is computed as: (11) gCO ,eq  gCO  gCH GWPCH  gN OGWPN O 2

AND

2

where it has been considered GWPCH4 = 25 and GWPN2O = 298. In order to evaluate the greatest advantages achievable, 700 bar compressed hydrogen produced by means of wind electrolysis has been taken into account among all systems: in this case, according to the JRC (Edwards et al., 2014), the equivalent emission of carbon dioxide per kg of hydrogen is only of 1.56 kg. On the other hand, for a complete comparison, also production and distribution of gasoline have been considered, with a carbon footprint of 0.60 gCO2,eq/ggas. Finally, the results achievable with the different production systems have been compared. 4. ENERGY MANAGEMENT STRATEGIES About the energy management, for the b) configuration, two alternative strategies have been adopted, here called A and B (see section 2.3). The A strategy is the most conservative one: the ICE is turned on when the SoC of the 48V battery falls below an imposed threshold and during the whole extra-urban part of the NEDC, while the EMG is activated only when SoC > 0.4 and the vehicle speed is lower than 40 km/h. This results in a SoC with a limited variation along the cycle (SoC = 0.75÷0.85), as shown in Fig. 6. This is mainly due to the continuous recharge performed by the FCS, which in this case has been designed using eq. (8) for a power of 900 W, considering a new vehicle mass of 1548 kg.

Fig. 7. SoC of 12 and 48 V batteries during a NEDC cycle using the B strategy. The maximum benefits achievable vary from a 3.42 % CO 2 reduction in the a) configuration to a 28.33 % and 42.24 % for the A and B energy management strategies in the b) hybrid powertrain, respectively. It is worth remarking that these values can be also interpreted as a decrease in gasoline consumption, which is in part due to an effective energy saving and in part to a replacement with “clean” energy from H2. On the other hand, the total energy saving, due to the increased efficiency of the powertrain, goes from a 0.57 % for the first scenario, to 20.0 % and 28.4 % for hybrid A and B respectively.

In the B strategy, the ICE is activated only in high load conditions and for traction purpose while the EMG behaves as a motor in urban route: this means that the SoC is depleted during urban phases and restored during the extra-urban (SoC = 0.70÷0.85) by ICE (as shown in Figure 7). The continuous recharge operated by the fuel cells avoids a steep decrease in 313

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close to the cleanest production process, especially if coupled with carbon capture and storage. Moreover, as stated above, it can be observed that grid electrolysis almost annihilates the benefits achieved in the TTW analysis, at the point of making the emissions even worse in the first configuration.

Fig. 8. CO2 emissions level of the three configurations compared with other vehicles and with targets; the label APU refers to the strategy a) whereas the labels HYBRID A and HYBRID B refer to the strategies b)-A and b)-B, respectively.

Fig. 9. Comparison between different H2 pathways on WTW CO2,eq emissions for the three considered strategies..

The WTT analysis provides the amount of CO2,eq emitted during the production process of fuels: the estimated emissions follow the TTW trend, but clearly with lower values. As it can be easily inferred, the gasoline contribution to WTT GHG emissions decreases from the baseline configuration to the most advanced hybrid configuration, while the opposite trend can be observed for the hydrogen contribution. Again, to remark the maximum benefits achievable, Table 4 refers to wind electrolysis pathway for hydrogen production: in this scenario, GHG saving ranges from a negligible 1.15 % for the simplest configuration to 43.45 % for the “B” hybrid, almost halving the reference value.

6. CONCLUSIONS In this paper, model-based scenario analyses of micro and mild tri-hybrid vehicle configurations have been presented. The analyses refer to a medium passenger car, in which the conventional powertrain, composed of a turbocharged SI engine, is integrated with small power proton exchange membrane (PEM) fuel cell (FC) systems and, eventually, additional battery pack. The simulations have been performed for two powertrain configurations corresponding to conventional ICE with fuel cell Auxiliary Power Unit (APU) and hybrid ICE/FC. In this latter case, two energy management strategies have been investigated. The results allow comparing the gasoline and hydrogen consumptions along a standard driving cycle and have been also extended to a WTW CO2 emissions analysis. The Tank-to-Wheel TTW analysis shows that gradually replacing the use of the thermal engine with fuel cells it is possible to achieve a considerable CO2 reduction up to 42%. The WTW analysis reproduces the TTW trend, but with a worsening in the percentage CO2 saving, with CO2 reduction up to 40%. Future work will focus on refining the energy management strategy, carry-on advanced optimization analyses of powertrain design and related control strategies and extending the application to other driving styles, in such a way as to pursue both reduced CO2 emissions and costeffectiveness targets.

The WTW analysis is reported in Figure 9: to obtain the WTW results, TTW CO2 emissions have been added to the WTT one. The WTW analysis reproduces the TTW trend, but with a worsening in the percentage CO2 saving. Going from TTW to WTW involves a reduction in the values shown above to 3.05, 27.25 and 40.44 against the baseline, for the APU, hybrid A and B strategies respectively. Table 4. GHG savings H2 production through wind electrolysis pathway. TTW [%] Strateg y a) b) A b) B

CO2,eq 3,42 28,33 42,24

Energy saving 0,57 19,97 28,38

WTT [%] CO2,eq

WTW [%] CO2,eq

1,15 26,64 43,45

3,05 27,25 40,44

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Nevertheless, hydrogen production from renewable sources must be considered as a concrete alternative only for long term and electrolysis is still a very energy intensive method. For this reason, WTW benefits have been assessed also for industrially consolidated pathways, such as methane steam reforming, which is employed today for around 48 % percent of global production, and compared with the “ideal” case to obtain a less optimistic and more plausible estimation for the short term. In the latter case, WTW results are still interesting, being quite 314

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