Hydrogen and proton exchange membrane fuel cells for clean road transportation

Hydrogen and proton exchange membrane fuel cells for clean road transportation

Journal of Industrial and Engineering Chemistry 17 (2011) 633–641 Contents lists available at ScienceDirect Journal of Industrial and Engineering Ch...

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Journal of Industrial and Engineering Chemistry 17 (2011) 633–641

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Hydrogen and proton exchange membrane fuel cells for clean road transportation Fortunato Migliardini, Ottorino Veneri, Pasquale Corbo * Istituto Motori, Italian National Research Council, Via G.Marconi, 8, 80125 Naples, Italy

A R T I C L E I N F O

Article history: Received 5 June 2010 Accepted 19 November 2010 Available online 13 May 2011 Key words: Hydrogen PEM fuel cell system Hybrid electric vehicles Clean transportation

A B S T R A C T

In this paper the main issues related to the working of hydrogen fuel cells and their utilization in the transportation field are presented and discussed, starting from the experimental analysis of dynamic performance of a laboratory fuel cell system based on a 20 kWe H2/air proton exchange membrane stack. The experiments were carried out on successive load steps characterized by low reactant relative pressures (15–50 kPa) and stack temperature of 310–350 K. The effect of air flow rate, hydrogen purge and membrane humidification strategy on stack output voltage was evaluated evidencing the role of different sub-systems in stack management (for reactant feeding, stack cooling, membrane humidification) and their effect on efficiency losses of the overall system. The results presented in this paper provide experimental indications about potentialities of hydrogen fuel cell systems as clean electric energy generators for automotive applications. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction The so-called ‘‘hydrogen economy’’ is considered a possible approach to overcome the incoming fossil resources scarcity without compromising the desired levels of global economic growth. The utilization of hydrogen as energy carrier would permit to develop new energy strategies based on a mixed utilization of different primary sources, including renewable (hydro, solar and wind) and nuclear, with consequent positive effects also on the reduction of global CO2 emissions [1,2]. The question of primary energy reserves is of fundamental importance for the transportation sector, which is strongly dependent on fossil fuels availability. Both spark and compression ignition engines are currently fuelled with carbon-containing fuels deriving from fossil sources (oil or natural gas), and are continuously improved in order to meet the always more stringent legislative limits on pollutant emissions. However, the significant contribution to the greenhouse effect due to CO2 emissions from transportation means, pollution issues in urban areas, and the foreseeable future scarcity of fossil fuels, drive the interests of private and governmental research centers towards the study of carbon-free fuels for clean road transportation. Compared to fossil fuels the main advantages of hydrogen are the highest energy density (35.7 kWh/kg) and the absence of carbon atoms in its molecule, with consequent absence of CO2 emissions when the fuel is oxidized. However hydrogen is not present in nature, at least in a form directly utilizable for energy

* Corresponding author. Tel.: +39 0817177180; fax: +39 0812396097. E-mail address: [email protected] (P. Corbo).

production, but it is abundant in its compounds. The different technologies for hydrogen production utilize as raw materials any substance containing hydrogen atoms, such as hydrocarbons derived from fossil fuels, biofuels, water (by thermal splitting or electrolysis), synthesized hydrogen carriers such as methanol, ammonia and synthetic fuels [3,4]. The so-called carbon capture and sequestration (CCS) technology represents the key step for an environmental friendly diffusion of H2 production methods based on fossil feedstocks, but the its economical feasibility needs to be demonstrated [5]. A critical aspect for the application of hydrogen in the automotive sector is the availability of this fuel on the vehicle, with related safety and infrastructure implications. Two options are currently considered for hydrogen transport and distribution: liquid or compressed gas delivered by tracks, or gaseous pipelines. A centralised management of the world-wide hydrogen production and distribution, coherently with the existent realty, would favour a pipeline based distribution, while a decentralized production and utilization would be more compatible with hydrogen delivery by tracks. Regarding on board storage issues the US Department of Energy (DOE) has established some targets which should be met by hydrogen storage tanks, in terms of volumetric and gravimetric storage system performance (at least 1.5 kWh/l and 2 kWh/kg, respectively). These targets imply for hydrogen tanks values of at least 45 gH2 =l as volumetric density and 6 wt% as gravimetric density [6]. In addition to conventional storage technologies (cryogenic liquid and high pressure compressed gas) in recent years the adsorption of H2 on solid state materials has been proposed as a very promising option [7]. The idea is to develop materials able to soak up and release hydrogen thank to absorption and/or

1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.05.011

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adsorption processes [8]. Lightweight metal hydrides, such as lithium and sodium alanates (LiAlH4, NaAlH4), have been widely studied for their characteristics of high hydrogen content and low release temperatures, but reversibility and kinetic of the hydrogen adsorption/desorption cycle need to be further improved. Interesting storage capacities have been claimed also for carbon nano-structures and zeolites at 77 K [9,10], while recent research activities have been addressed towards a new class of structured nanoporous materials, constituted by metal organic frameworks (MOFs), whose potentialities are related to their low solid density and very high specific area, ranging from 1000 to up 6000 m2/g [11]. The interest towards MOF compounds is related to the theoretical possibility to optimize the hydrogen storage exploiting both selective binding energy and high specific area effects. Regarding its utilization in internal combustion engines for road vehicles, hydrogen presents some beneficial characteristics, such as a very higher octane number and wider flammability range than gasoline, however its oxidation process implies NOx emissions, together with traces of hydrocarbons and CO deriving from engine lubricant. While low ignition energy and high flame speed can cause self-ignition phenomena or flame flash-back during mixture preparation, the wider ignition range permits very lean air/fuel mixtures to be burnt, providing a way of partially controlling the NOx emissions. The hydrogen fuel cell technology appears to be an attractive solution to develop zero emission vehicles performing an overall efficiency significantly higher than those powered by internal combustion engines. In fact, when hydrogen is oxidized in an electrochemical way, water is the unique product of the reaction, and a very high conversion efficiency can be reached. Furthermore, the use of fuel cells permits the adoption of the electric traction with improved driving range with respect to battery powered electric vehicles [12,13]. Proton exchange membrane (PEM) fuel cells fed by hydrogen are the most suitable for automotive application, because of their high power density, low operative temperature, fast start-up procedure, good dynamic performance [14]. The current status of fuel cell technology demonstrates its high potential in terms of efficiency, especially for application in passenger cars [15], even if many advances are required to reduce costs and improve the reliability in dynamic conditions. The main operative characteristics of PEM stacks can be summarized in the following points:  Hydrogen is the best reducing agent for an efficient and reliable operation.  Oxygen is the ideal oxidant agent, but air can also be used (in this case an excess of oxidant is required).  Pressure and temperature increase the individual cell performance.  The electrolytic membrane needs to be properly humidified in all operative conditions, since the proton conductivity of Nafion-like membranes is possible only in the presence of water molecules [16,17].  Heat is a by-product of fuel cell reaction and progressively increases the stack temperature.  The stack temperature must not exceed 90 8C, to preserve membrane integrity. For these reasons a fuel cell stack cannot work by itself, but its operation requires the utilization of several auxiliary sub-systems for an efficient and reliable electric power production. A fundamental issue in the realization of a fuel cell power train is the interaction between stack and auxiliary components of the fuel cell system (FCS), especially if the electrochemical generator has to

support the dynamic demands of vehicular applications. When the FCS is used as a energy supplier in an electric vehicle, it is preferably coupled with electric energy storage systems by an electric parallel connection, in order to limit power and costs of the stack and simultaneously to distribute the dynamic stress on the two power sources. As a consequence the actual dynamic requirements that the FCS has to satisfy depend on the energy management within the selected hybrid configuration [18–21]. In this paper the main management issues of a hydrogen PEM fuel cell stack for automotive applications are discussed starting from the experimental analysis of a fuel cell system based on a 20 kWe stack. The investigation has been focused on FCS performance in dynamic conditions, evidencing the relation between stack efficiency and operative conditions of auxiliary sub-systems (reactant feeding, stack cooling and humidification), and evaluating their parasitic losses. 2. Experimental The optimization of PEMFC performance in terms of efficiency and reliability requires a proper design and management of the reactant feeding sections, as well as cooling and humidification sub-systems. The selection and sizing of auxiliary devices, also called ‘balance of plant’ (BOP) components, depends on both their interactions with the stack and all other possible inter-connections inside the overall system. A fuel cell system was specifically designed and realized to characterize a 20 kWe PEM stack, and investigate the interaction between stack and auxiliary components in steady state and dynamic conditions. A scheme of this plant is shown in Fig. 1, while a detailed description is reported elsewhere [22], and briefly recalled here. High purity hydrogen (>99.5%) was fed by high pressure cylinders, while air was supplied by a side channel blower connected to the electric network. This type of compressor was adopted to feed the oxidant at low pressures, in order to limit its own power consumption. This choice did not maximize cell performance [14] but avoided the use of other equipments, such as turbines, generally necessary for partial energy recovery in high pressure plants. The air flow rate was regulated acting on the compressor motor inverter, and measured by a variable-area flowmeter. The instantaneous stoichiometric ratio was calculated as R = Reff/Rstoich where Reff is the ratio between the air and hydrogen mass flow rates, while Rstoich is the same ratio as required by the stoichiometric equation of H2 oxidation (H2 + O2 = H2O). The R regulation determined values ranging from 8 at low load to 2 at full load, as an excess of air with respect to stoichiometric requirement was always necessary, due to mass transport limitations on cathode side. As membrane hydration control and water balance are critical issues for a durable operation of both stack and FCS, a sub-system for external humidification and a condenser at cathode outlet have been included in the plant, whose design is strictly connected to thermal management and reactant sub-system components. The humidification system adopted for this work was realized by flowing air through a bubbler filled with de-ionized water, which was heated at different temperatures by electric resistances before feeding the cathode side. This type of apparatus, realized for laboratory studies, could be replaced in automotive realizations by alternative low consumption devices, such as membrane or liquid injection humidifiers. Humidification and temperature sensors at cathode inlet permitted air relative humidity to be controlled. Stack temperature was measured by a thermocouple at the outlet of cathode side. Two pressure transducers were located upstream the stack to monitor anode and cathode pressure during the experimental runs.

[(Fig._1)TD$IG]

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Fig. 1. Scheme of a fuel cell system for laboratory tests.

tions to be realized with pre-programmed profiles of stack current. A d-Space board was used for acquisition and control of all signals associated to the FCS system components (valves, sensors, transducers). During all experimental runs the stack operated in dead-end configuration (hydrogen is fed to the stack as pressurized gas with anodic compartment normally closed), inlet hydrogen relative pressure was always lower than 50 kPa, and a purge electric valve was inserted at hydrogen line outlet to periodically drain the excess of water and nitrogen diffusing from cathode to anode side through the polymeric membrane. The hydrogen purge valve section was chosen to allow during its opening a minimum relative pressure at the anode inlet not lower than 25 kPa. The inlet hydrogen pressure was maintained slightly higher with respect to inlet air pressure during all experimental tests (10–30 kPa). The purge valve control was realized by selecting opening time and frequency values in order to discharge hydrogen volumes proportional to the stack current. This purge strategy implied inevitable energy losses, calculated as ratio between mass of fuel unreacted in the stack and mass of fuel entering the stack. A value of this parameter equal to about 0.10 resulted to be the best compromise between anode flooding and unused fuel discharge for all experiments. Regarding stack humidification the inlet air was saturated in the temperature range 300–340 K, while the stack temperature

Stack thermal management system is necessary since the reaction heat gradually increases the membrane temperature, putting at risk its integrity. This aspect, related also to humidification issues, requires the development of a specific sub-system able to control the stack temperature. The cooling system in Fig. 1 was based on a de-ionized water circuit equipped with pump and sensors for pressure, temperatures and flow rate measurements. A spiral heat exchanger fed with external water was used to control the internal water temperature. The stack was connected to an electric load realized by means of a DC–DC converter and 20 kW electric resistances. This way it was possible to control electronically the value of stack current and the electric load power to be discharged during the tests, allowing dynamic operative condiTable 1 Main technical specifications of the PEM fuel cell system. PEM fuel cell stack Number of cells Maximum power Stack voltage range Stack maximum current Auxiliary components Air supply compressor

Water pump

Heat exchanger

Acquisition and control system d-Space board Sensors Air mass flow meter Water flow meter Temperature sensors Pressure transducers Humidity transducer Current transducer Voltage transducer Electric load Type Rated power Input current Input voltage Remote control of stack current Electrical resistances

80 cells 20 kWe 50–80 V 360 A Side channel compressor Maximum flow rate 95 Nm3/h Maximum pressure 30 kPa Centrifugal pump Maximum pressure 400 kPa Maximum flow rate 100 l/min Spiral heat exchanger, 8 tubes (size 9.5 mm), shell diameter 273 mm  264 mm

[(Fig._2)TD$IG]

Sample time of 0.01 s Variable area flow meter, 0–60 Nm3/h Variable area flow meter, 0–5 m3/h Resistance thermometer, 0–100 8C Pressure range 0–500 kPa 0–100% RH Closed loop, Hall effect, 300 A Closed loop, Hall effect, 100 V IGBT electronic converter connected to electrical resistances 20 kW 0  300 ADC 0  100 VDC 0  10 VDC 4 kW modules

Fig. 2. Stack efficiency and power as function of stack current for the 20 kWe PEM stack.

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Table 2 Power consumptions of BOPs and FCS efficiency as function of stack power.

a

Stack power (kW)

Air compressor (kW)

Water pump (kW)

Others devicesa (kW)

FCS efficiency (%)

0 2.0 3.9 5.6 7.3 8.9 10.4 11.9 13.2 14.5 15.7

0.27 0.27 0.30 0.32 0.41 0.52 0.65 0.96 1.20 1.30 1.48

0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31

0.13 0.13 0.15 0.18 0.22 0.27 0.32 0.37 0.43 0.50 0.58

0 39 46 48 47 46 45 43 42 41 39

Sensors, cables, electrovalves.

is confirmed by data of Table 2, where the details of power consumptions of main BOP components are reported as function of load, together with the efficiency values of the overall fuel cell system. The consumption of humidification system is not reported in Table 2 since it was associated with the electric consumption of resistances used to heat the water inside the bubbler. As specified in experimental this system was chosen for laboratory experiments, and was not suitable for automotive real applications. The power consumption of a system to be installed on a vehicle, such as liquid injection or membrane humidifier, would imply not significant consumptions. Data of Table 2 evidence that taking into account the energy consumption of the air compressor and of other auxiliary components, the net FCS efficiency resulted higher than 45% in a wide load range. A more detailed analysis of air compressor influence on FCS global efficiency is shown in Fig. 3, where the power consumption of the air compressor used in this work is reported in terms of percentage ratio between power consumption of the compressor electric motor and the stack power, whose values are also reported in the abscissa of Fig. 3. The maximum influence of air compressor on the net efficiency (about 13%) was observed at low loads (up to 2 kW), as in these conditions air feeding consumes energy but the produced power is very low. For higher loads not negligible percentage consumptions were observed, but always lower than 9%. The characteristic curves of the air compressor are detailed in Fig. 4, that shows the power consumption values associated with the different air flow rates and pressures adopted in all experiments described in this paper. A power consumption of 1500 W was measured at stack power of

was maintained under 350 K, in order to avoid membrane integrity and de-hydration. Table 1 reports stack specifications and main technical characteristics of auxiliary components, with details on mechanical and electric devices necessary for stack correct operation.

The stack characteristic curve is reported in Fig. 2 in terms of efficiency and power versus current. It was obtained at R variable in the range 2–8 and 340 K, with the purge strategy above described and humidification temperature slightly lower than stack temperature, in order to maintain a relative humidity of cathode stream close to 100% for all experimental points. The stack efficiency was calculated as percentage ratio between the measured output stack voltage and the reversible open circuit stack voltage (1.23 V for each cell), while the stack power was measured at stack terminals. Data of Fig. 2 evidence the expectable decrease of efficiency when load increases, due to different types of voltage losses in a fuel cell (activation polarization, Ohmic and concentration polarization losses) [16], however also at low reactant pressure the stack efficiency results higher than 50% in the entire range of loads. In a previous paper a detailed analysis of parasitic energy losses associated with the individual sub-systems has been presented [22], and the predominant role of the air feeding system in determining the efficiency loss of the FCS has been evidenced. This

15

Air power consumption, %

[(Fig._3)TD$IG]

3. Results and discussion

12

9

6

3

0

2

4

6

7

9

10

12

13

15

16

Stack Power, kW Fig. 3. Air power consumption percentage as function of stack power.

1500

50

1200

40

900

30

600

20

300

10

0

637

Pressure, kPa

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Power consumption, W

[(Fig._4)TD$IG]

0 10

20

30

40

50

Air flow rate, Nm3/h Fig. 4. Characteristic curves of the side channel blower adopted in the plant of Fig. 1.

16 kW, using an air flow rate of 44 Nm3/h, corresponding to R = 2 and air pressure in the cathodic compartment slightly higher than 30 kPa. During dynamic experiments all the main parameters characteristic of stack operation were continuously monitored, in particular: (i) stack current and voltage, (ii) reactant pressure, (iii) air flow rate, (iv) stack temperature, (v) pressure, flow rate and temperature of cooling water, (vi) temperature and humidification level of air entering the stack. Fig. 5a–e reports the results of an experiment conducted starting from open circuit voltage up to 150 A, which is about half the maximum power of the stack. As shown in Fig. 5a the load variation up to 150 A was realized in about 300 s in two steps, with an intermediate steady state phase of about 200 s at 100 A. The state at 150 A was then maintained for 1100s. The voltage profile reported in the same figure follows the values corresponding to the stack efficiency shown in Fig. 2, in both dynamic and stationary phases. The slight voltage increase observed during steady state operations is associated with the stack temperature increase from 320 to 335 K (Fig. 5d). The thermal management is shown in Fig. 5d, where temperature profiles of cathode outlet stream (mixture of unreacted oxygen, nitrogen and water vapour), representative of stack temperature, and of cooling water (at inlet and outlet of the stack) are reported. The stack temperature was regulated by on/off acting of the water external circuit through the spiral heat exchanger, and was permitted to increase from 320 up to 335 K during the test. The profiles of cooling water temperatures show that cooling circuit was designed to maintain a temperature difference between inlet and outlet stack not higher than 5 K in all operative conditions (see also Figs. 6d and 7d), in order to guarantee a satisfactory temperature uniformity inside the overall stack, and consequent uniformity of cell performance. In Fig. 5b the profiles of air pressure and flow rate as measured at the compressor outlet (and before bubbler) are reported. The choice of maintaining constant the speed of compressor motor should imply constant values of both air pressure and flow rate, therefore the slight variations observed in Fig. 5b are associated with pressure drops increasing through the cells during stack operation, mainly due to stack temperature increase. The external humidification strategy, based on air saturation by a bubbler, is evidenced in Fig. 5e, where air relative humidity and temperature, as measured at the bubbler outlet (and before the stack) are reported. The electric resistance inserted inside the bubbler was controlled in order to realize temperatures of the saturated air stream close to those occurring inside the stack (Fig. 5d), in this way membrane drying out phenomena were strongly limited. The hydrogen purge intervention is evidenced in Fig. 5c, where instantaneous relative hydrogen pressure at the inlet anode compartment is shown. Each activation of the purge valve determined a fast hydrogen pressure

diminution of about 5 kPa, with immediate pressure recovery just after the valve closing. At 150 A an opening frequency corresponding to one valve activation every 40 s permitted the realization of the purge strategy described in Section 2, in order to limit both flooding and drying out of the anodic compartment [23]. Fig. 6a–e shows the acquisition of FCS operative parameters during two steps of current increase from 150 to 250 A, in particular a first step from 150 to 200 A was followed by a steady state phase of about 520 s, a second step up to 250 A and a second stationary phase of about 300 s (Fig. 6a). Air flow rate (consequently air pressure) was increased during the first dynamic phase to assure R values always higher than 2 (Fig. 6b), while hydrogen purge frequency was augmented during the first steady state condition at 200 A passing from a valve activation every 40 s to every 30 s (Fig. 6c). These regulations of operative parameters, together with a stack temperature increase up to 348 K, permitted a regular voltage profile to be observed up to 650 s (before the second dynamic step). Regarding the humidification strategy Fig. 6e shows that the bubbler temperature was maintained about 10 K lower than stack temperature and was not increased before 400 s, as the larger water quantity produced by the electrochemical reaction, correlated to stack current increase, was considered suitable to control membrane drying out. In order to maintain a regular behaviour of stack voltage also during the second part of the test (from 200 to 250 A plus second stationary phase), air flow rate (and pressure), purge frequency, hydrogen feeding pressure and stack and bubbler temperature were all increased (Figs. 6b–e). However some not negligible oscillations were detected for stack voltage, in particular from 700 to 1000s. This behaviour can be correlated to air flow rate and pressure profiles (Fig. 6b), where the short variations observed after 700 s can be explained as a consequence of pressure drops changes through the stack due to incipient flooding phenomena. The positive effect of a further load increase on voltage regularity is evidenced by the test of Fig. 7a–e, which reports the results of an experiment conducted increasing the stack current from 250 to 280 A in two steps in the time window 200– 470 s, followed by a steady state phase at 250 A up to the end of the test. During this experiment air flow rate (and pressure), purge frequency, hydrogen feeding pressure and stack and bubbler temperatures were maintained approximately constant and a stable stack voltage was observed in the range 200–1100s. In particular stack and bubbler temperatures were regulated at about 340 K, a value slightly lower with respect to those adopted in Fig. 6d, in order to prevent possible membrane drying out phenomena. The larger water production at higher loads went in the same direction, while the higher reactant drain rate favoured the removal of water droplets inside the feeding channels, limiting membrane flooding.

[(Fig._5)TD$IG]

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Fig. 5. (a) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of stack voltage and current versus time during two consecutive steps from zero to 100 and 150 A. (b) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of air pressure and flow rate versus time during two consecutive steps from zero to 100 and 150 A. (c) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of inlet hydrogen pressure versus time during two consecutive steps from zero to 100 and 150 A. (d) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of cathode outlet, cooling water stack inlet and outlet temperatures versus time during two consecutive steps from zero to 100 and 150 A. (e) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of stack inlet air temperature and humidity grade versus time during two consecutive steps from zero to 100 and 150 A.

[(Fig._6)TD$IG]

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Fig. 6. (a) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of stack voltage and current versus time during two consecutive steps from 150 to 200 and 250 A. (b) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of air pressure and flow rate versus time during two consecutive steps from 150 to 200 and 250 A. (c) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of inlet hydrogen pressure versus time during two consecutive steps from 150 to 200 and 250 A. (d) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of cathode outlet, cooling water stack inlet and outlet temperatures versus time during two consecutive steps from 150 to 200 and 250 A. (e) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of stack inlet air temperature and humidity grade versus time during two consecutive steps from 150 to 200 and 250 A.

[(Fig._7)TD$IG]

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Fig. 7. (a) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of stack voltage and current versus time during two consecutive steps from 250 to 270 and 280 A, followed by a final steady state phase at 250 A. (b) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of air pressure and flow rate versus time during two consecutive steps from 250 to 270 and 280 A, followed by a final steady state phase at 250 A. (c) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of inlet hydrogen pressure versus time during two consecutive steps from 250 to 270 and 280 A, followed by a final steady state phase at 250 A. (d) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of cathode outlet, cooling water stack inlet and outlet temperatures versus time during two consecutive steps from 250 to 270 and 280 A, followed by a final steady state phase at 250 A. (e) Dynamic test effected on the FCS based on the 20 kWe PEM stack. Acquisition of stack inlet air temperature and humidity grade versus time during two consecutive steps from 250 to 270 and 280 A, followed by a final steady state phase at 250 A.

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4. Conclusions The performance of a fuel cell system based on a 20 kWe PEM stack have been experimentally investigated. As the utilization of a FCS as a power source in transportation sector requires the maximization of electric energy produced by the system, an evaluation of the electric consumption of the auxiliary components has been effected, evidencing the impact of the air compressor as the main cause of parasitic losses. The experimental results of dynamic tests characterized by different consecutive load levels have been used as support for a discussion about the optimal management of the operative parameters, finalized at the reliable and efficient stack working, in the view of its application as a power generator in hybrid electric vehicles. The operational issues of the different sub-systems have been analyzed, with particular reference to oxidant and fuel feeding, stack cooling and membrane humidification. References [1] J.E. Mason, Energy Policy 35 (2007) 1315–1329. [2] E.I. Zoulias, N. Lymberopoulos, Renew. Energy 32 (2007) 680–696. [3] T. Rostrup-Nielsen, Catal. Today 106 (2005) 293–296.

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