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TEA
Solar Photovoltaic Cell Thermal Measurement Issues
Bernie SiegalThermal Engineering Associates, Inc.
2915 Copper Road
Santa Clara, CA 95051
USA
P: 650-961-5900
| F: 650-227-3814E: [email protected]
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Topics
Introduction
SPVC Type Diversity
SPVC Equivalent Circuit
SPVC
Performance Limiters
SPVC Temperature Characteristic
Unique Power Dissipation
Junction Temperature Calibration
Thermal Measurement Setup
Junction Cooling
Charge Storage
Thermal Transient Heating Curve
Summary
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Introduction
Success of the Solar Photovoltaic Cell (SPVC) as a carbon-less source of global energy is dependent on:
the amount of energy that it can generate,
the efficiency of incident-to-output energy conversion,
the capital investment required for SPVC installations, and
the continuing output energy cost. All of these dependencies are based on the SPVC junction temperature.
While much effort is being expended to address these depen-
dencies, most of this effort is not specifically targeted to controlling the junction temperature. Thus, not much effort is being put into either the direct measurement of junction temperature or determination of the heat flow conditions from the cells to its environment.
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SPVC Type Diversity
SJ units typically subjected to 110 “Suns”
MJ units typically subjected to 1001000 “Suns”
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SPVC Maximum Conversion
Maximum theoretical conversion efficiency –(≈
30% and 68%, for SJ and MJ types, respectively)
–
is determined by the Shockley–Queisser
limit that deals with the amount of photon energy that can actually be converted into electrical energy
Reference: http://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit
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SPVC Equivalent Circuit
Diode converts the solar energy into electricity
All the resistive ele- ments
cause losses
RConductor
and RSeries
are generally the same order of magnitude
Single Junction SPVC
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SPVC Equivalent Circuit
Diodes convert different segments of the solar energy into electricity.
All the resistive elements cause losses
RConductor
and RSeries
are generally the same order of magnitude.
Multi-Junction SPVC
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SPVC Performance Limiters
A key measure of SPVC performance is the cells ability to convert incident solar energy (i.e., light) into electrical energy. Commercially available SPVC modules currently offer conversion efficiencies in the range of 12% and 28% from single junction and multi-junction units, respectively. Some of the most important performance limiters include: Semiconductor processing limitations Conductor series resistance Conductor shadowing Junction temperature
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SPVC Performance Limiters
Semiconductor processing limitations– Large area junctions are more prone to wafer material
defects, and/or processing defects and/or inter-cell variations
Conductor series resistance– Need to minimize RS
to maximize deliverable energy and minimize internal heating
Conductor shadowing– Increasing conductor width to minimize RS will reduce
incident light area– Increasing conductor thickness leads to potential
manufacturing issues Junction temperature
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SPVC Temperature Characteristic
Shown for a SJ-SPVC
Conversion efficiency falls by about 20% for a TJ
rise of 35ºC
http://en.wikipedia.org/wiki/Solar_cell#Cell_temperature)
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Unique Power Dissipation
1 Sun = 1,000W/m2
= 0.1W/cm2
Pincident
= 22.5W (150mmX150mm SJ-SPVC, 1 Sun)
Pincident
= 62.5W (25mmX25mm MJ-SPVC, 100 Suns)
Pdissipation
= Pincident
– Pconversion
+ Plosses
1
10
100
1000
10000
1 10 100 1000Concentration Ratio
Pow
er (W
atts
)
Thermal Pow er
Thermal Pow er
Incident Pow er
Incident Pow er
150mm X 150mmSingle Junction
25mm X 25mmMulti-Junction
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Junction Temperature Calibration
Classic diode Electrical Test Method diode TJcalibration setup, consistent with Mil-Std 750 and JEDEC JESD51 standards
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Thermal Measurement Setup
Classic diode Electrical Test Method setup, consistent with Mil-Std 750 and JEDEC JESD51 standards
FfFiF VVV
FJ VKT
JiJ TTT
HH
F
H
JJX IV
VKPT
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Junction Cooling
Junction starts to cool as soon as the power is removed.
Must define when the VFf
measurement is actually made
May need to correct for junction cooling
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t0 is the instant the Heating Power is removed
Initial region is due to non- thermal switching effects – equipment switching speed limits, inductance between equipment and test sample, capacitance of test sample, etc.
Linear portion on semi-log graph is due to actual junction cooling
Linear portion is projected back to the Y-axis value “a”
Actual measurement is made in linear portion value “b”
Correction is the ratio of “a”/”b”
Correction Factor CF = a/b
Junction Cooling Correction
Junction Temperature Cooling Curve
10
100
0 10 20 30 40 50 60 70 80 90 100
Measurement Delay time (tMD) [µs]
delta
VF [
mV]
RawReg
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Applying Correction Factor
baVKΔT FJ
JiJ TTT
HH
F
H
JJX IV
baVK
PT
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Charge Storage
High forward current applied to the diode junction(s) will cause charge to be stored in the diode junctions
Charge must be dissipated either through recombination or reverse current in order to quickly measure VFf
Usually not too much of a problem in MJ-
SPVC measurements.
Usually is a problem in SJ-SPVC measurements
Charge Dump is the process of reverse biasing the junction for a short time to remove the charge
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Other Measurement Issues
Incident light energy must be minimized during the calibration and measurement activities
Typically, very high Heating Currents are required to generate sufficient ΔVF
to improve signal-to-noise ratio
Current crowding hot spots may occur during measurement and create hot spots
These hot spots will distort measured junction temperature value
Some of the applied heating power will be converted into optical energy (i.e., junctions “glow”
during measurement
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MJ-SPVC Assembly Stackup
MJ-SPVC usually mounted on a heat spreader to better transfer heat into the package
Each sequential bulk element in the heat flow path has an increased thermal time constant
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Thermal Transient Heating Curve
The difference in time constants produces a thermal transient heating curve that can be used to analyze the thermal interfaces
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Summary
The purpose of this presentation was to provide an over-
view of the issues associated with the thermal per-
formance
and thermal measurement of diode-type solar photovoltaic cells. The general measurement approach follows the procedure
and circuitry for standard diodes. The specific requirements of SPVC have been mentioned
to insure better thermal measurement results.The SPV industry needs a thermal measurement standard
to generate comparable measurement results