THERMOPHOTOVOLTAICS:
direct conversion of heat into electricity
Alex Freundlich
Photovoltaic and Nanostructure Group
Texas Center for Superconductivity and Advanced Materials
University of Houston
1
Thermophotovoltaics (TPV)
 Photons emanating from a hot emitter are
converted by a PV cell (IR sensitive solar cell)
Emitter temperature :~900-1500oC
2
Solar PV /TPV
Solar PV
emitter (sun)-PV cell distance 150 millions km
Sun= black body at 5800 K, output power 0.01-0.03 W/cm2
TPV :
emitter
- PVcell distance few cm
TPV emitters1200<T<1800 K,
1<PTPV<30 W/cm2
3
As for solar PV
•higher bandgap favor high operating voltage/low current
•Lower bandgap favor high current output but a low voltage operating voltage
Ideal bandgap for TPV cell is a function of the emitter spectrum
4
Efficiency/0utput power density
Assuming loss of transparency loss photons
Efficiency vs bandgap
Output power vs bandgap
5
Converter Materials
 Suitable band gap
(~0.4-0.6eV)
 Existing converter
materials: InGaAs/InP
InGaAsSb/GaSb
InAs
6
Applications/ advantages
7
TPV for military applications
Advantages :
•Low IR and acoustic signature
(favors stealth operation)
•High specific power
(increased troop mobility)
•TPV device degrade graciously
(mitigates catastrophic failures)
Applications:
•Diesel/propane fuel operated TPV
(0.5-1.5 KW compact portable power
generation units )
•Electric power for naval applications
8
TPV for automotive applications
“Viking 29 “concept car developed
by Univ. of Washington and JX
Crystals
•High power density
•Silent (no moving parts)
9
Co-generation of electricity
residential/industrial waste heat recovery
•Example use of TPV for self supported
/self regulated heating system
(pilot system test in Switzerland)
10
Co-generation for remote and leisure markets
JX Crystals, Inc.
the fuel is already burned for heat and the electricity to charge batteries is "free".
11
TPV for Deep space exploration
Radioisotope TPV
• PuO2 GPHS heat source
• Solid selective emitter
• MIM PV array
• Target h of 20%
Advantages:
•No moving parts
•Degradation if any gradual
•compact
•Three time more efficient
than existing TE technology
12
Recycling unused IR back to the emitter
Theoretical efficiency is increased to about 40-50% for 0.5-0.6 eV TPV
13
Front spectral control (Filters)
Filter
Conductive substrate)
After H.Sai et al, 2003
14
Converter Materials for Rear Spectral Control
 Suitable band gap
(~0.4-0.6eV)
 Lattice- matched to
existing semiinsulating substrates
GaAs or InP
 Existing converter
materials: InGaAs,
InGaAsSb/GaSb
 Problems: Lattice
relaxation, degradation
InAs
15
Defect filtering in 0.5-0.6 eV InGaAs on InP
Cross-Sectional TEM of Full 0.52 eV
Structure
 Misfit dislocation networks
confined to InAsyP1-y graded
region
 Low density of threading
dislocations (<1x107 cm-2) into
active layers
Data courtesy of. R.WehrerBechtel/Betis (2004)
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Rear surface spectral control
Low-Bandgap MIM Photovoltaic Devices
Data courtesy of Dr. D. Wilt
Glenn Research Center
at Lewis Field
Drawing of monolithic interconnected module (MIM) showing interconnect and two cells
Photo of 1cm2 15 cell MIM
Reflectance of MIM structure providing spectral control
features as well as energy conversion functionality
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Heat to electric conversion efficiency > 20%
Lattice-mismatched 0.6 eV, epitaxially grown
InGaAs diodes form the power-producing
element. A power conversion efficiency of
20.6&percent; and a power density of 0.90 W/cm2
with a silicon carbide radiator operating at 1058°C
is achieved for a 4 cm2 TPV cell operating at
26.7°C.
Sergei et al, AIP, 2003
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Novel Converter Approaches
Other Solution: tandems, strained QW new III-V-N materials
19
InGaAs Tandem TPV Devices
Inverted (epi-down) Structure
Epitaxial
surface
Metal on epitaxial surface
InP Substrate
Transparent LCL
n/p “High” Eg Cell
Isolation
Trench
Tunnel Junction
Buffer Structure
60 mm
V=Sumof(Vi)
Micrograph
processed
n/p “Low” Eg Cell
Contact / BSR
tandem TPV device
D. Wilt et al, Proc. TPV 6 conf 2004, ibid
M. Wanlass et al
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Pseudomorphically strained (defect free) MQW TPV cells on InP
Conventional p/n
Proposed
Multi Quantum Well
1.0
InP (100)
InGaAs cell
1.00E+021
-2 )
E
EgInP
E
Conventional
0.75 eV InGaAs
TPV MQW
(A) capture
47% Ga
InGaAs cell
thermal
escape
10% Ga in well
- 0.5
EgInP
absorption
EgQW
EFP
1500 K
5.00E+020
EFN
recombination
Flux (photon/s cm
In0.53Ga0.47As
Normailized Spectral Response
TPV cell
In0.53Ga0.47As
1000 K
(B)
InP (100)
0.00E+000
1.5
2.0
2.5
3.0
Wavelength ( micron)
A. Freundlich, US patent Nov 2001
Increased IR sensitivity
21
New IR nanostructured semiconductor
lattice matched to InP
Absorption threshold (eV)
 Superlattice of alternate
layers of GaAsN and InAs
(N), lattice matched to InP.
0.75
Absorption threshold for
|3/2,3/2> band to electron bands
|3/2,1/2> band to electron bands
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0
T=300K
1
2
3
4
Nitrogen (%)
5
6
A. Freundlich et al, Phys Stat Solidii 2004
L. Bhusal et al, Nanotechnology, 2004
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Concluding remarks
•TPV is still in its infancy
•Capable of providing efficient (>20% demonstrated)
heat to electricity conversion and high power density
•Excellent candidate for waste heat recovery and cogeneration
•Enabling technology for space and military applications
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Absorption threshold
10
10
10
T=300K
0
Transmission probability
Transmission probability
0.528eV
|3/2,3/2 VB
-50
-100
100xGaAs0.95N0.05/InAs SL
Thickness of GaAsN/InAs
layer=15 /23 Å
-0.5
-0.4
-0.3
-0.2
Energy (eV)
10
10
-0.1
0.0
T=300K
0
-50
-100
10
0.00
0.08
0.16 0.24
Energy (eV)
0.32
0.40
L. Bhusal, et.al PRB, 2002, submitted
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Absorption threshold
0
10
Transmission probability
Transmission probability
0.463eV*
1 1100
|3/2,1/2 VB
-50
10
-100
10
-0.6
-0.5
T=300K
-0.4 -0.3 -0.2
Energy (eV)
* ~300meV
-0.1
-50
10
100xGaAs0.95N0.05/InAs SL;
Thickness of GaAsN/InAs
-100
10
0.0
0.00
layer=15/23 Å
0.08
0.16 0.24
Energy (eV)
T=300K
0.32
0.40
smaller than the InGaAs lattice matched to InP
L. Bhusal, et.al., PRB, 2004, submitted
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