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Losses in Fuel Cells

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전체 글

(1)

Losses in Fuel Cells

(2)

Fuel Cell Charge Transport

(3)

Mass Transport

Convection dominant in flow channels Diffusion dominant in electrode

(4)

Concentration Drop

i i

v v

p p nF

E RT E

Reactants Product

0 ln

Π

Π

=

=

RT nF

P RT P

nF

R

R e

C e C

C j C j

η α η

α (1 )

0 0 0

(5)

Diffusion In Electrode

(6)

Limiting Current Density

- To increase jL

Q: Are 1 & 2 are easy?

Q: Does anode has larger jL?

(7)

Diffusivity & Effective Diffusivity

Diffusivity from kinetic theory

Effective diffusivity

(8)

Nernst Effect

(9)

Concentration Effect

B-V for high current density

(10)

Mass Transportation Loss

Adding two losses

(11)

Pictorial View

Theoretical EMF or ideal voltage

Cell Voltage (V)

Current Density (A/cm2)

j ηact

N’

ηconc,1

ηconc,1 A

V

A’

N

(12)

Pictorial View

Theoretical EMF or ideal voltage

Cell Voltage (V)

Current Density (A/cm2) j

ηact ηconc,1

ηconc,1

ηconc,2

V

N’

A

A’

N

A*

(13)

A Very3 Tricky Example

PEMFC with or without temperature control

0 50 100 150 200 250 300 350 400 450 0.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Voltage

Power density

Current density (mA/cm2) Voltage (V)

0 50 100 150 200 250

Power density (mW/cm

2)

(14)

Convection

(15)

Viscosity

Temperature effect or

Mixture

(16)

Viscosity: Example

(17)

Viscosity: Example

(18)

Viscosity: Example

In general, flow in fuel cell is laminar.

(19)

Pressure Drop In Flow Channels

(20)

Convective Mass Transport

(21)

Convective Mass Transport in PEMFC

5. Flow velocity in the channel is constant.

(22)

Convective Mass Transport in PEMFC

(23)

Convective Mass Transport in PEMFC

(24)

Convective Mass Transport in PEMFC

(25)

A More Realistic Alternative Solution

) ) (

(

) (

) (

x dx Bj

x dC

nF u

x j dx

d

nF dx x j u dx

u

in in in

=

=

=

+

ρ

ρ ρ ρ

ρuin

dx

(ρ+ρ/dx)uin j/nF

(26)

A More Realistic Alternative Solution

If constant Voltage instead of constant current is assumed

ax in

ax

RT nF

Ke x

j

A c

c

Ae x

c

x ac c e

x Bj c

x dx Bj

x dc

=

=

=

=

=

=

=

) (

) 0 (

) (

) ) (

) ( ) (

( /

0 0

0

η α

•Current drop Exponentially

•Concentration also drop exponentially

•HW 5.10

(27)

Flow Channel Design

(28)

PEMFC Flow Channel Design

(29)

PEMFC Flow Channel Design

(30)

Toyota Metal Mesh Flow Field

참조

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