Thermoelectric Effect & Thermoelectric · PDF file · 2014-05-29AME$60634$$...
Transcript of Thermoelectric Effect & Thermoelectric · PDF file · 2014-05-29AME$60634$$...
AME 60634 Int. Heat Trans.
D. B. Go Slide 1
Thermoelectric Effect & Thermoelectric Devices
*borrowed heavily from presentation by G. Chen, MIT
AME 60634 Int. Heat Trans.
D. B. Go Slide 2
Seebeck Effect
cold
hot
Seebeck Effect: Temperature difference generates a voltage between two different materials
Thomas Johann Seebeck 1821, Germany
Conductor 1 Conductor 2
AME 60634 Int. Heat Trans.
D. B. Go Slide 3
Seebeck Effect
Thot Tcold
electrons diffuse from hot to cold
electric potential builds up that resists diffusion
S = −ΔVΔT
= −Vhot −VcoldThot −Tcold
Seebeck coefficient: S [V/K]
AME 60634 Int. Heat Trans.
D. B. Go Slide 4
Peltier Effect
Peltier Effect: Current flow can induce a temperature gradient depending on direction of current flow
hot
Conductor 1 Conductor 2
A
Jean Charles Athanase Peltier 1834, France
AME 60634 Int. Heat Trans.
D. B. Go Slide 5
Current and Heat Flow
€
F = −q
E − m* v
τ= m* d v
dt
Newton’s 2nd Law
Coulombic force drag due to collisions
The steady-state solution gives the average electron “drift” velocity
€
v = − qτm*
E
€
µe =qτm* ≡ electron mobility
The current density is the rate of charge transport per unit area (like heat flux)
jelec = −nelecqv = nelecq
2τm*
E =σ
∇Φ compare to Ohm’s law!
But the electrons carry heat with them!
jheat = nelecuv = u
qjelec =Πjelec
Peltier coefficient: Π [J/A]
AME 60634 Int. Heat Trans.
D. B. Go Slide 6
Peltier Effect
q q 1
2
jelec, jheat
jelec, jheat
q (Peltier): (Π1-Π2)×j
- Induced heating and cooling at the two junctions due to mismatch - Reversible by reversing the direction of current flow - A refrigerator! (current is “work” to drive “heat”)
AME 60634 Int. Heat Trans.
D. B. Go Slide 7
Thomson Effect
William Thomson, Lord Kelvin 1855, Ireland
Thot Tcold
current
heat release/adsorption
Thomson Effect: Current flow through a temperature gradient will generate/absorb heat because thermoelectric properties are temperature dependant
AME 60634 Int. Heat Trans.
D. B. Go Slide 8
Thomson Effect
Thot Tcold
electrons diffuse from hot to cold
current
heat release/absorption needed for energy balance
Thomson coefficient: τ = (1/i)×(dq/dx)/(dT/dx)
q(x)
i
Kelvin Relations:
Π = ST; τ = T dSdT
AME 60634 Int. Heat Trans.
D. B. Go Slide 9
Thermocouples & The Thermoelectric Effect
Thermocouples operate under the principle that a circuit made by connecting two dissimilar metals produces a measurable voltage when a temperature gradient is imposed between one end and the other.
AME 60634 Int. Heat Trans.
D. B. Go Slide 10
Thermoelectric Devices
http://www.energybandgap.com
AME 60634 Int. Heat Trans.
D. B. Go Slide 11
Peltier Coolers
Ideal Device: • No conduction (hot to cold) • No Joule heating
Tc, qc
Th, qh
qc = Π p −Πn( )× i
Real Device: • conduction (hot to cold) • Joule heating
qc = Π p −Πn( )i− i2R 2−σ cond Th −Tc( )
electrical resistance thermal conductance
R =Lp
Apσ p
+LnAnσ n
σ cond =ApkpLp
+AnknLn
AME 60634 Int. Heat Trans.
D. B. Go Slide 12
Peltier Coolers: Refrigeration Performance Voltage Drop:
V = iR+Sp − SnTh −Tc
Real Device: • conduction (hot to cold) • Joule heating
Tm =12Th +Tc( )
Coefficient of Performance:
COP = qcW
=Sp − Sn( )iTc − i2R 2−σ cond Th −Tc( )
Sp − Sn( )i Th −Tc( )+ i2R
Optimal Current to Maximize COP:
COPmax =Tc
Th −Tc( )1+ ZTM −Th Tc1+ ZTM +1
AME 60634 Int. Heat Trans.
D. B. Go Slide 13
The Z Parameter
Z =Sp − Sn( )
2
Rσ cond
=Sp − Sn( )
2
Lp
Apσ p
+LnAnσ n
"
#$$
%
&''ApkpLp
+AnknLn
"
#$$
%
&''
To Maximize Z:
Rσ cond( )min =kpσ p
+kpσ p
!
"##
$
%&&
2LnAp
LpAn=
σ nknσ pkp
!
"##
$
%&&
12
when
Leading to Z:
Zmax =Sp − Sn( )
2
kp σ p + kn σ n( )2
AME 60634 Int. Heat Trans.
D. B. Go Slide 14
Figure of Merit: ZT
ZT = σS2Tk
For a Single Material:
increase electrical conductivity decrease thermal losses (conduction)
http://chemgroups.northwestern.edu/kanatzidis/greatthermo.html
AME 60634 Int. Heat Trans.
D. B. Go Slide 15
Superlattice
ZT = σS2Tk
increase electrical conductivity decrease thermal losses (conduction)
kσT
=π 2kB
2
3q2constrained by
Widemann-Franz
ZT = σS2Tkelec + kphonon
http://lucidthoughts.com.au/
AME 60634 Int. Heat Trans.
D. B. Go Slide 16
http://www.kickstarter.com/projects/flamestower/flamestower-charge-your-gear-with-fire
http://www.customthermoelectric.com/
http://energyblog.nationalgeographic.com/2013/09/24/google-science-fair-winner-makes-flashlight-powered-by-body-heat/