EFFECT OF HYDROGEN FEEDING SUBSYSTEM ON EFFICIENCY AND DURABILITY OF PEM FUEL CELL SYSTEMS

Transcription

1 Piero Lunghi Conference December 11-13, 2013, Rome, Italy EFFECT OF HYDROGEN FEEDING SUBSYSTEM ON EFFICIENCY AND DURABILITY OF PEM FUEL CELL SYSTEMS F.Migliardini, C. Capasso, P.Corbo National Research Council of Italy Via G. Marconi, Naples - ITALY

2 Presentation outline Brief overview of Istituto Motori previous activities in the field of PEFC fuel cell systems. Analysis of performance of a new generation 6 kw PEM fuel cell system in both steady state and dynamic conditions, for application on hybrid bus. Effect of the hydrogen feeding subsystem (deadend or flow-through with 100% recirculation) on cell efficiency and performance regularity

3 Previous activities on direct H2 fuel cell systems Stacks and systems characterization (500 W, 2,4 kw and 16 kw) : effect of pressure, temperature and air stoichiometric ratio Effect of air sub-system on stack efficiency and dynamic operation Effect of humidification strategy on Fuel Cell System performance for automotive application

4 The Fuel Cell System H2 storage FCS Air Fuel supply system FC stack Air supply system heat Residual Air system Thermal/Water system Electric energy H2O Cooling Humidification Water neutrality

5 The Fuel Cell System: the efficiency Energy losses due to auxiliary components Efficiency of stack and FC-System Vs Load Air compressor has the highest impact In optimized steady-state conditions air supply system consumed at least 6-7% of stack power 44% H 2 Electric Power Thermal Power Air Compressor Cooling Pump Other Auxiliaries 47% 3% 1% 5%

6 BOP component energy consumptions Stoich. Ratio is related to the excess of air, as it defined 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

7 The 16 kw Laboratory Apparatus Air Management and Humidification System Water Cooling System THD 888 C/R.H. Air inlet AC PT 888 m3/h CV FD FR FT BV SV BV M BV THT BV Humidification Water Tank Bubbler De-ionized Water Inlet TC WL TR PT 888 C BV BV TT STACK TD BV EV CD BV BV Air outlet Cooling Water In TT PT FD 888 m3/h WF FR FT STACK TT Cooling Water Out SV M Cooling Water Tank De-ionized Water Inlet TC WL 888 C TD AF BV N2 inlet AF EH Shell Water outlet HE Shell Water inlet EV BV BV CV WL AL WF WP Hydrogen Feeding System EV PR M SV EV EV H2 inlet H2 Purge BV PT BV N2 inlet STACK

8 The Fuel Cell System Rotary Vane or Side Channel Membrane humidifier

9 The Fuel Cell System: the polarization curves Operative Conditions of a 16 kw FCS: P= 1.3 kpa T = 346 K Effect of stoichiometric ratio R = air flow rate/air stoich

10 The Fuel Cell System: the air subsystem Power losses associated with two different low pressure air supply systems 3,5 Power consumption, kw 3 2,5 2 1,5 1 0,5 0 blower vane Air flow rate, Nm3/h

11 The Fuel Cell System: the air subsystem Study of the interaction between stack and air supply system in Fuel Cell Systems for automotive application The air management strategy optimized for steady state conditions of low pressure plant (minimum energy consumption) results not fully satisfactory in terms of stack dynamic performance A strategy characterized by excess of cathode air slightly higher would permit better cell uniformity during faster dynamics The best system efficiency is reached with the air supply system based on side channel blower The higher pressures of rotary vane compressor improve membrane humidifier performance permitting higher humidification rates for air stream entering the fuel cell stack

12 Cell humidification issues (2.5 kw stack) B: Drying out of membranes A: Flooding of anode and cathode compartments

13 Cell humidification issues (2.5 kw stack) Effect of flooding, intervention of hydrogen purge (2.5 kw stack) Dynamic ramp up to 1.8 kw, at 150 W/s

14 Cell humidification issues (2.5 kw stack) Effect of drying out, intervention of external humidification by water injection (2.5 kw stack) Stack power: 700 W T: 60 C 0,82 0,81 0,8 0,79 Cell Voltage, V 0,78 0,77 0,76 0,75 0,74 0,73 0,72 0,71 before injection after injection

15 Stack reliability in automotive conditions : Characterization by eletrochemical techniques EIS and CV were used to characterize an aged 500 W PEM stack, after about 500 h of powertrain tests in dynamic and steady state conditions ECSA : Electrochemical Surface Area EIS main results evidenced that: Charge Transfer Resistance (R ct ), associated with O 2 reduction reaction, can be correlated to some cell irregularity, a not satisfactory membrane hydration and consequent sintering of Pt in the MEA ECSA, m 2 /g Cell Number

16 Aim of the presentation

17 MAIN TECHNICAL SPECIFICATIONS OF THE FUEL CELL SYSTEM Parameter Value Stack power 6.2 kw Cell number 96 Stack voltage range Hydrogen pressure Air pressure External humidification Water cooling V kpa < 40 kpa Absent kpa K 20 l/min

18 Stack polarization curve, Power and T Voltage, V 2 Temperature, C 1 Power, kw Stack current, A reliable zone

19 Hydrogen feeding sub-system in flow-through mode (without ejectors or pumps the scheme refers to dead-end) : On-off valve 2: Pressure regulator 3: Proportional valve 4: Pressure transducer 5: Stack 6: Ejectors at high and.low flow 7: Purge valve H 2 tank 6 6 Anode Cathode 5 7

20 Cell voltage acquisition stationary condition: 323 K, 3.3 kw, stoichiometric ratio: 1.7 The cell voltages refer to average values calculated on groups of 3 except the first and last three ones, which were individually acquired dead end flow through Cell Voltage [mv] Cell Group Number No significant difference was observed for steady state runs

21 Cell voltage acquisition acceleration phase from 1.2 to 3.8 kw in 12 s: 313 K, 3.6 kw, stoichiometric ratio: flow through dead end Cell Voltage [mv] Cell Group Number slightly higher individual values of cell voltages and more uniform distribution were detected for flow-through mode

22 Cell voltage acquisition during warm up acceleration phase from 1.2 to 3.1 kw in 16 s 288 K, 2.6 kw, stoichiometric ratio: 2.2 Cell Voltage [mv] flow through dead end Cell Group Number dead-end procedure required a higher hydrogen purge frequency with respect to the flow-through (about 2.5 times higher) and determined a not optimal cell voltage distribution. a general diminution was observed for the last 20 cells

23 Conclusions The experimental tests carried out on a 6 kw PEM FCS in dynamic conditions evidenced the effect of hydrogen feeding subsystem on cell performance. Higher cell efficiency and better voltage uniformity can be obtained in transient phases with hydrogen recirculation, especially at operative conditions that favor stack flooding phenomena (low T and high load). The flow-through mode allowed the stack regular working to be obtained with lower purge frequency. At least in dynamic conditions here investigated, the fuel recirculation favors a higher cell efficiency and voltage uniformity. This last aspect is very critical as far as durability is concerned, as a scarce cell voltage uniformity can accelerate MEA degradation phenomena.

24 Thank you for your attention!

25 Effect of compressor on membrane humidifier performance Membrane humidifier is a passive device able to increase enthalpy and water content of streams fed to the stack (Tstack-Tci) C vane compressor (Tstack-Tci) C side channel blower vane-water flow rate g/min blower-water flow rate g/min ,3 kw -34 C 0,6 kw - 36 C 0,9 kw - 38 C 1,2 kw - 40 C 1,5 kw - 42 C 1,8 kw -44 C 2,1 kw - 46 C 2,4 kw - 48 C 0 0,3 kw - 34 C 0,6 kw - 36 C 0,9 kw - 38 C 1,2 kw - 40 C 1,5 kw - 42 C 1,8 kw - 44 C 2,1 kw - 46 C 2,4 kw - 48 C The rotary vane compressor permit higher T of humidified air streams at the cathode inlet (U.R. about 100%) and water exchange rates to be obtained