2014 pv performance modeling workshop: toward reliable module temperature measurements:...

May

5, 2

01

4

MONALI JOSHI, BLACK & VEATCH RAJEEV SINGH, PV EVOLUTION LABS

TOWARD RELIABLE MODULE TEMPERATURE MEASUREMENT: CONSIDERATIONS FOR INDOOR PERFORMANCE TESTING

• Black & Veatch is a leading global engineering and consulting company specializing in infrastructure development in energy, water, telecommunications, management consulting, federal and environmental markets

• Founded in 1915

• $3.4 Billion in revenue in 2013

• Over 9000 employees worldwide

• Over 100 offices worldwide

• Completed projects in over 100 countries on six continents

WHO IS BLACK & VEATCH?

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• Involved in 45% of operating PV projects in North America and 20% of projects in advanced development (on a MW basis)

• Engaged as Independent Engineer, Owner's Engineer, Power Purchaser rep.

• Advisor to equity investors, lenders, venture capital firms, major utilities, and government agencies

• Leader in PV third party technical due diligence and extended engineering services

SOLAR PV QUALIFICATIONS & EXPERIENCE

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• Feasibility Studies • Resource Assessment • Interconnection Planning and Design • Power Purchase Agreement Support • Energy Production Estimating

• Technology Due Diligence • EPC Specification Development • Owner’s Engineer Support • Construction Monitoring • Performance Monitoring

• Generation of PVsyst module characterization files (“PAN files”) from laboratory measurements of module performance

TEMPERATURE-DEPENDENCE OF MODULE POWER OUTPUT

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Operating temperature is dependent upon many factors:

• Solar irradiance • Ambient temperature • Wind speed • Wind direction • Panel material composition • Mounting structure

PV module operating temperature impacts energy production


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Significant Heat Loss Mechanisms 1) Conduction through encapsulation 2) Convection from surfaces 3) Radiation to surroundings

Significant Heat Sources 1) IR radiation from solar spectrum 2) PV conversion “inefficiency”

Cell Operating Temperature (Tcell) is result of thermal balance


6 Thermal gradients within the module should be considered when assessing Tcell

Layer Thickness (mm)

Thermal Conductivity (W*m-1*K-1)

Glass 3.0 1-2

EVA 0.5 0.2-0.3

Si 0.250 148

Al back contact

0.01 237

EVA 0.5 0.2-0.3

Tedlar 0.1 0.2-0.3

PV module operating temperature impacts energy production

U · (Tcell-Tamb) = α· Ginc ·(1-η) (1)

U = Uc + Uv · V (2)

7 PVsyst computes module efficiency based on irradiance and modeled Tcell

PV system energy simulation relies on estimation of Tcell Within PVsyst :

MODELING TCELL FOR ENERGY PREDICTION

1) Tcell calculated from incident irradiance (Ginc), ambient temperature (Tamb), and wind speed

• Optical absorption • User-defined thermal loss factors

2) Estimate module IV curve characteristics at calculated Tcell

• PAN file defines η surface, function of Ginc, Tcell

INDOOR MODULE PERFORMANCE CHARACTERIZATION CONSIDERATIONS

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Goal of customized PAN files: High fidelity representation of measured module performance a function of incident irradiance and cell temperature

IEC 61853-1: PV Module Performance Testing and Energy Rating • Describes “requirements for

evaluating PV module performance”

• Specifies measurement of back-of module temp (Tmod)

• Not prescriptive on method of temperature control

Because temperature control methodologies can vary, 1) Possible Tmod ≠Tcell

2) Tmod /Tcell relationship may not be fixed or predictable


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Factors impacting accurate and repeatable temperature measurements: • Directionality of heat source

vs.


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Factors impacting accurate and repeatable temperature measurements: • Directionality of heat source • Uniformity of heat source

vs.


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Factors impacting accurate and repeatable temperature measurements: • Directionality of heat source • Uniformity of heat source • Hold time at temperature


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Factors impacting accurate and repeatable temperature measurements: • Directionality of heat source • Uniformity of heat source • Hold time at temperature • Number, type, location of sensors

x

x

x x x x x x

vs.


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Factors impacting accurate and repeatable temperature measurements: • Directionality of heat source • Uniformity of heat source • Hold time at temperature • Number, type, location of sensors • Calibration

Lack of specificity in many of these factors in 61853-1 leaves room for lab-to-lab variation

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“Oven”

• Module heated on all sides by laminar flow of hot gas

• In-situ IV curve measurement

• Uniform temperature profiles possible

• Equilibrium possible

“Hot Potato”

• Module heated in thermal chamber; placed in ambient

• IV curves assessed while cooling (no temp control)

• Non-uniform temperature profiles possible

• Non-steady state

“Back-side Toaster”

• Constant, adjustable heat source at back surface

• Uniform x-y thermal profile possible

• Non-uniform thermal profile in z

• ~ Steady state possible

MODULE PERFORMANCE CHARACTERIZATION FOR ENERGY PREDICTION

Variety of indoor temperature control methodologies currently in use , all of which may be consistent with 61853 guidelines, but differences can lead to largely different results

x

y

z

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Each temperature control methodology leads to a distinctive relationship between Tback/mod and Tcell thus measured power output and thermal coefficients may vary if measured as a function of Tmod

Tfront

Tback

Tcell

=

=

Toven/ambient

=

Tfront

Tback

Tcell

≠

<

Tambient <

Theat source

<

Tfront

Tback

Tcell

≠

≠

Tambient ≠

MODULE PERFORMANCE CHARACTERIZATION FOR ENERGY PREDICTION

“Oven”

“Hot Potato” “Back-side Toaster”

CASE STUDY—IMPACT OF MEASUREMENT LOCATION USING “TOASTER” HEATING

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In conjunction with PV Evolution Labs

“Backside Toaster” heating methodology, two temperature measurement methodologies:

Temperature controlled such that CP temperature is within ± 0.5° of 61853 temperature targets (15°C, 25°C, 50°C, 75°C)

1. Cell probes (CP): hypodermic thermocouple needles inserted underneath backsheet and contacting cell busbar

2. Backsheet (BP) probes : standard thermocouples adhered to backsheet using Kapton (polyimide) tape

1

2

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60 cell p-Si Module Type 1

No systematic correlation between Tmod and Tcell even among same footprint, same manufacturer

For this test set-up, Tcell cannot be reliably predicted from Tmod

60 cell p-Si Module Type 2

CASE STUDY RESULTS—TCELL VS TMOD

0

50

100

150

200

250

300

0.0 20.0 40.0 60.0 80.0 100.0

Po

we

r O

utp

ut

Temperature

Cell ProbeBacksheet Probe

CASE STUDY RESULTS—IMPACT ON NORMALIZED POWER CURVES

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Power output curves (normalized to nominal output at STC) developed for each dataset

For this case, referring to back of module temp as proxy for operating temp leads to overestimation of module performance

1100 W/m2

1000 W/m2

800 W/m2

600 W/m2

CASE STUDY RESULTS – IMPACT ON PAN FILE OPTIMIZATION AND ENERGY PREDICTION

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Optimized PAN File Parameters Using Tmod

Optimized PAN File Parameters Using Tcell

Parameter Value

Isc (A) 9.20

Voc (V) 37.9

Imp (A) 8.61

Vmp (V) 30.2

T. Coeff. Isc (mA/°C)

3.57

Rshunt (Ω) 333

Rseries (Ω) 0.390

Rshunt at G = 0 (Ω) 830

Rshunt exp 5.5

T. Coeff. Pmp (%/°C)

-0.37

Using B&V’s iterative optimization process to replicate the measured

efficiencies within PVsyst, different optimized PAN file

parameters result

In this case, all thermal

coefficients found to differ by 15%

Parameter Value

Isc (A) 9.20

Voc (V) 37.9

Imp (A) 8.61

Vmp (V) 30.2


4.18

Rshunt (Ω) 333

Rseries (Ω) 0.400


Rshunt exp 5.5


-0.44

CASE STUDY RESULTS – IMPACT ON PAN FILE OPTIMIZATION AND ENERGY PREDICTION

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Optimized PAN File Parameters Using Tmod

Location Difference in Predicted Energy*

Arizona + 1.4%

Central California

+ 1.1%

Ontario +0.01%

*with respect to PAN file based on Tcell

Parameter Value

Isc (A) 9.20

Voc (V) 37.9

Imp (A) 8.61

Vmp (V) 30.2


3.57

Rshunt (Ω) 333

Rseries (Ω) 0.390


Rshunt exp 5.5


-0.37

Parameter Value

Isc (A) 9.20

Voc (V) 37.9

Imp (A) 8.61

Vmp (V) 30.2


4.18

Rshunt (Ω) 333

Rseries (Ω) 0.400


Rshunt exp 5.5


-0.44

Optimized PAN File Parameters Using Tcell

CASE STUDY RESULTS—HIGHER RELIABILITY, REPEATABILITY WITH CELL TEMP PROBES

21 For this configuration, temperature coefficients derived using cell probe method are more repeatable

Sample 1 Sample 2 Sample 1 (repeat) Sample 2 (repeat) Datasheet Value

INDOOR MODULE CHARACTERIZATION FOR DEVELOPMENT MUST GO BEYOND 61853

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• Direct measurement of Tcell or demonstration of front/backside equilibrium

• Modify procedure for data quality and consistency

Demonstrated temperature control repeatability

Demonstrated irradiance control repeatability

Ideally, IV curves measured at steady state

• Incorporate additional, more granular measurements for more assessment of temperature coefficients

BLACK &VEATCH PAN FILE DATA MEASUREMENT SPECIFICATION

The B&V specification requires:

In addition to methods outlined in 61853

Measurement of Tcell or demonstration of frontside/backside equilibrium

IV curve assessment under 61853-1 defined

conditions

Redundant measurements to demonstrate

repeatability of irradiance and temperature control

Linearity of Isc to validate irradiance control

Temp. Coeff. Measurement

• 61215 measurement range: 5° increments over 30°

• Reference Tcell

15°C 25°C 50°C 75°C

1100 W/m2

1000 W/m2

800 W/m2

600 W/m2

400 W/m2

200 W/m2

100 W/m2

30°C 35°C 40°C 45°C

1000 W/m2

CONCLUSIONS

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• Module performance and simulation of module performance within PVsyst rely on Tcell

• Lab methodologies may be consistent with 61853-1 requirements, however differences in temperature control process may lead to different results

• For indoor characterization procedures, relationship between Tmod and Tcell may vary with temperature control methodology, leading to differences in measured performance

• Unreliable characterization of performance leads to non-trival accuracy errors in energy forecasting

• Modification of 61853-1 procedure necessary to produce reliable, repeatable indoor performance data for generating PAN files

FUTURE WORK

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• B&V Thin Film Module Specification for Indoor Characterization

• Considerations: light stabilization; response time/pulse length;

• B&V Outdoor Characterization Specification

• Considerations: Placement of temperature sensors, Backside insulation for temp. coeff measurements, Irradiance measurement/spectral correction

• Characterization of Thermal Loss constants in PVsyst

• Considerations: PV technology/BOM/mounting method

THANK YOU

PRESENTER NAME OPTIONAL TITLE COMPANY DIVISION

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2014 pv performance modeling workshop: toward reliable module temperature measurements:...

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