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: bsiegal@thermengr.com

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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