Hydrogen Production using PEM Hydrogen Production using PEM Water ElectrolyzersWater Electrolyzers

Numerical and Experimental Investigations Numerical and Experimental Investigations

Abhijay Awasthi2005ch50101

Project SupervisorProf. Suddhasatwa Basu

Technology Overview

Advantages over conventional alkaline electrolyzers Higher production rate High power density at low temperatures Compact mass volume characteristics Greater safety Direct source of high purity hydrogen gas

Applications of PEM electrolyzers • Hydrogen supply for fuel cells• Regenerative fuel cell systems• On board generation of oxygen in space applications• Analytical chemistry (Gas chromatography)• Manufacture of semiconductors for electronic industry

Outline

Modeling of PEM electrolyzer unit

Analysis of PEMWE & PEMFC system with solar energy source

Experimental investigations of PEMWE

Multiphysics modeling of PEMWE

Part 1Modeling and Simulation of a PEMWE

Objective

To develop a model for PEMWE and study its performance by simulating it

Why model ??A good model can predict PEM water electrolyzer performance under a wide range of operating conditions eliminating the need to do experiments again and again.

Can be used to identify better materials and operating conditions for optimum performance.

Model

1. Anode Section

2. Cathode Section

3. PEM Section

4. Voltage Section

Simulations done on MATLAB/SIMULINKSimulations done on MATLAB/SIMULINK

Mass Transport

Anode Cathode

FH2Oin

FH2OoutFO2out

FH2Oeod

FH2Od

FH2out

O2g H2g

FH2OpeH2Ocons

PEM

Mass Transport

Species generation and consumption using Faraday’s law

nF

IO2g =

Mass transport through membrane

- Electro osmotic drag

- Diffusion

- Due to pressure difference

Interaction Interaction with anode & with anode &

cathode cathode sectionssections

Electro-osmotic Drag

H+ ions while conducting from anode to cathode side drag some water molecules with themselves.

FH2Oeod = nd . I / F

nd = electro osmotic drag coefficient (mol H2O/mol H+)

nd depends on the water content of the membrane.

Values vary significantly in literature. Most of the data is for fuel cell operation.

Transport due to diffusion and ΔP

• Transport due to diffusion using Fick’s law

• Darcy law used to find transport due to ΔP

Species concentration required to calculate diffusion

Species Concentration

Anode Cathode

CH2O,ch,an

CO2,ch

CO2,me

CH2O,me,an

CH2O,me,cat

CH2O,ch,cat

CH2,chCH2,me

A Detailed mathematical formulation can be found here:F.Marangio, M. Santarelli, M. Cali. Theoretical model and experimental analysis of a high pressure PEM water electrolyzer for hydrogen production

Voltage

Open Circuit Voltage

Activation Overvoltage

Ohmic Overvoltage

Overpotential

OCV – Calculated using Nernst equation using a temperature dependent value for reversible cell voltage

Activation Overvoltage

α = charge transfer coefficienti = current densityio = exchange current density

Ohmic Overvoltage

Vohm = I. Rohm

Model Validation

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2

Current Density, A/cm2

Vo

lata

ge,

Vo

lt

T=55 deg C, P=10 bar, exp

T=55 deg C, P=10 bar, model

T= 40 deg C, P=70 bar, exp

T=40 deg C, P=70 bar, model

Contribution of various overvoltages

i = 0.2 A/cm2 i = 1 A/cm2

Effect of Temperature and Pressure

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2

Current Density, A/cm2

Vo

lta

ge

, V

T=80 deg C T=40 deg C

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2

Current Density, A/cm2

Vo

lta

ge

, V

P = 1 Mpa P= 5 Mpa P = 10 Mpa

Effect of Temperature and Pressure

Transient response

0

5

10

15

20

25

30

35

40

45

0 100 200 300 400 500 600 700

time,sec

Po

wer

, W

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0 100 200 300 400 500 600 700

time, sec

Hyd

roge

n ou

tflo

w, m

ol/s

A high temperature operation favors the electrolysis.

High pressure operation increases the polarization voltage but it is cost effective in terms of storage of hydrogen.

Model is control oriented and can be used to model a prototype.

Model can capture the varying power conditions and can be used to couple electrolyzer with other renewable energy systems like solar energy, wind energy etc.

Conclusions

Part 2Analysis of PEMWE & PEMFC system with solar energy

source for residential power requirements

• Solar PV systems

- sun dependent power supply

- time dependent power demand of a residence• Use of energy storage device in conjunction with PV array

Rechargeable lead acid batteries – most commonly used

Another alternative – Regenerative fuel cells

Background

PV power supply Power Management

Residential power

demand

Utility gridBattery RFC

Gas Storage

Model

PV power supply

Residential power

demand

Battery

Electrolyzer

Utility grid

Excess power

Gas Storage

Power flow strategy

PV power supply

Residential power

demand

Battery

Fuel cell

Utility grid

Deficit power

Gas Storage

Power flow strategy

• Input data to the model – Time of the day• Output – Power supplied

• Time of the day gives solar irradiance (from look up table) which is fed to the PV array.

• Solar irradiance gets converted into the max. power supplied from the PV array based on a non linear Maximum power point tracking scheme (MPPT).

PV array model

I-V characteristics determined using a set of differential and algebraic equations with solar irradiance G (kW/m2) as the parameter

PV cell

Output of a 4.5 kW PV array

PV array – solar irradiance

• Data required - Battery voltage vs. state of charge (SOC)• Charging and discharging rates selected to be constant

•Continuous integration of SOC value of battery w.r.t. time• Based on SOC, discharge voltage of the battery is selected from the lookup table• Power supplied/consumed by battery is calculated using this discharge voltage value

Battery – empirical model

Fuel cell Electrolyzer

PEMFC and PEMWE model

• Rate of H2 addition = generation by electrolyzer – consumption by fuel cell

• Faradays law = I/nF• Integration w.r.t. to time

• Moles of H2 updated with time

Hydrogen storage

MATLAB/SIMULINK model

Battery

Fuel cell

Electrolyzer

PV Source

LoadGrid

• Grid parallel• Standalone• With/Without battery• Variable battery, fuel cell and electrolyzer capacities

Modes of operation

Ref. Jha A k, Saksena S. A. Novel analytical method for sizing of standalone PV system.2005, IE journal

Data for village homes in Jammu

Load/Demand Profile

Hydrogen generation in system

Battery : 500 A-HrPEMWE & PEMFC : 5 kw

In 24 hour run% load powered by individual components

Without battery With battery

Results - Standalone

System efficiency: η = kWh (load) + kWh(H2 produced) kWh (PV) + kWh(battery)

Moles of H2 converted into kWh values by using Gibb’s free energy value

System Efficiency - Standalone

In 24 hour run% load powered by individual components

Without battery With battery

Results – grid parallel

System efficiency: η = kWh (load + H2 produced + to grid) kWh (PV +battery + from grid)

System Efficiency – grid parallel

RFC system used before battery in the power flow strategy

RFC as primary energy storage device

Grid parallel

Standalone

Effect of varying RFC size

• When RFC is employed in a standalone scenario, it has to operate at very high power densities which may cause irreversible damage to the cells .

• In a standalone scenario, battery inclusion increases the system efficiency to a large extent . It also decreases the load on fuel cell system.

• Grid parallel scenario is much more beneficial.

• A large increase in H2 storage is also achieved.

• Use of battery does not have a significant effect on system efficiency.

Conclusions

Part 3 Experimental Investigations on PEMWE using Pt black catalyst

PEM electrolysis

Cathode catalyst: Pt blackAnode catalyst: Pt blackElectrolyte: Nafion 115/112

Current collector: Gold plated Ti mesh

Temperature range: 20-130 deg CPressure range: 0.5 – 3 bar (gauge)

Catalyst loading: 1 mg/cm2

Active area: 1 cm2

Experiment

Catalyst ink preparation

Pt black catalyst + Deionized water + solvent + Nafion solution

Solvent: 1.Ethanol2.Iso-propanol3.AcetoneGlycerol added as a thickening agent

Nafion solution: 10-40 % by wt

Catalyst ink sprayed directly over the membrane surface.

Catalyst Coated Membrane

Solvents for CCM

Solvent Boiling point deg C

Viscosity, cP Swelling ratio, %

Iso-propanol 82 2.1 199

Ethanol 78 1.07 238

Acetone 56 0.31 154

Acetone spraying leaves the membrane surface cracked Iso-propanol and ethanol give smooth surfaces A thickening agent like glycerol can be added

Membrane preparation Cleaned with 3% H2O2 solution to remove organic impurities Soaking in 0.5 M H2SO4 solution followed by washing in DI water All treatments at 70 deg C

Solvents for Catalyst ink

PEMWE Cell

Experimental setup

Results: Effect of temperature

Results: above 100 oC

Effect of pressure

Effect of membrane thickness

Effect of nafion content in catalyst ink

A high temperature operation is favorable

At lower temperature, a low membrane thickness gives high performance

At higher temperature, a higher membrane thickness is required

Increasing the pressure at higher temperature increases the performance

A higher nafion content decreases the cell performance

Best performance is obtained with nafion 115 at 100 deg C and 3 bar pressure

Conclusions

Part 4Multiphysics modeling of PEMWE unit

Model divides electrolyzer unit in 5 divisions:Model divides electrolyzer unit in 5 divisions: Anode flow channel Anode electro catalyst and diffusion layer Membrane Cathode electro catalyst and diffusion layer Cathode flow channel

Model incorporates:Model incorporates: Electronic and ionic charge balance Butler-Volmer charge transfer kinetics Flow distribution in channels (Navier stokes equation) Flow in porous media (Darcy’s law) Mass balance (Convection and diffusion) Membrane water transport: EOD, diffusion and pressure difference

Model Formulation

Model Geometry - Grid

Model Validation

Current Density distribution

Hydrogen production rate

Ohmic overvoltage distribution

Water flow inside the channels

Current density distribution is very skewed inside the cell leading to non uniform catalyst usage Gas generation rate over the surface of MEA is non uniform Low water flow rates lead to incomplete use of catalyst layer

Model can be used to study over-potential distribution inside the cell Model can successfully predict mass transfer inside various sections of electrolyzer cell Model can be used as the basis for optimum designing of flow fields of the electrolyzer cell Model needs improvement in terms of including operational problems while running the unit

Conclusion

Thank you