Compound Semiconductor Based
Micro-Thermophotovoltaic Power Generation Technologies
Francis M. O’Sullivan
May 12th, 2003
Courtesy of Francis M. O'Sullivan. Used with permission.
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What is Thermophotovoltaic Power Generation?
The conversion of thermal radiation to electrical energy directly!
The same principle by which solar power is generated.
Lower bandgap compound semiconductor are used to manufacture tpv cells.
The lower bandgap material better matches the available spectra.
TPV Block Diagram
Emitter
Heat
filter
PV Cell
P N
Waste
Heat
+
-
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DC power
6.772: Compound Semiconductor Devices
The emitter is heated and emits radiation. Typically with black-body like
sprectrum
SiC is commonly used for this purpose: (Operated at temperatures in the
1500-1800K region)
Power Density (W/cm )
10
2
9
8
7
6
5
4
3
2
1
0
1
0
GaSb (Eg=0.7 eV)
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2
3
4
5
6
7
8
Wavelength ( ? m)
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Ideal Filter for TPV Application
10
9
Power Density (W/cm2)
8
7
6
5
4
3
2
1
0
0
1
GaSb (Eg=0.7 eV)
2
3
4
5
6
7
8
Wavelength (mm)
1
transmittance
Wavelength (mm)
1
reflectance
Wavelength (mm)
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What has tpv to offer in the real world?
Static power conversion, a major reliability issue!
Relatively good power density (In the 1.5 – 2.5 W/cm^2 range)
Problems do exits…
The technology does not scale upwards in size very well.
High power , high temperature engineering is exceptionally challenging.
The solution…
Increase the output power density.
Reduce the operating temperatures.
But how?
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Consider again the situation…
Thermophotovoltaics
SiC Emitter
PV Cell
P N
Heat
large
~10 mm
Waste
Heat
Due to the difference in
permitivities only a fraction of the
energy in the emitter can be
delivered to the PV cell.
+
DC power
-
The idea is to increase this
transfer and use more of the
emitters potential
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The solution to the problem lies in reducing the distance
between the emitter and PV cell to a sub-micron scale…
SiC Emitter
PV Cell
P N
Heat
0.5 um
Waste
Heat
When the distance is scaled
down some evanescent modes
in the emitter couple to the
PV cell and begin to propagate.
+
DC power
-
This is described as: Micro-Thermophotovoltaics
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The amount of power that is transferred in the micro-tpv
case is huge compared to the standard situation!
Micro-Thermophotovoltaic
Power Transfer Characteristic
Power Density (W/cm2)
25
20
15
Power transfer at 1500K
10
5
0
0.5
1
1.5
2
2.5
Wavelength (um)
3
3.5
4
-6
x 10
Macro-Thermophotovoltaic
Power Transfer Characteristic
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So how can this increased transfer be utilized?
The massive increase in power transfer allows for significantly higher power
density.
Alternatively the emitter temperature can be significantly reduced whilst
maintaining a power density figure comparable to standard tpv.
The micro-tpv technology offers the possibility of static power generation for
use with MEMS devices.
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Significant challenges remain…
Although micro-tpv systems can operate at lower temperatures thermal
management is still a huge problem.
Spectral control for micro-tpv systems is more challenging than for the
conventional tpv systems.
Fabrication of a constant gap is extremely difficult.
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Thank you kindly for your attention.
5/12/2003
6.772: Compound Semiconductor Devices