2014 PV Performance Modeling Workshop: Toward Reliable Module Temperature Measurements: Considerations for Indoor Performance Testing

1. May5,2014
MONALI JOSHI, BLACK & VEATCH
RAJEEV SINGH, PV EVOLUTION LABS
TOWARD RELIABLE MODULE TEMPERATURE
MEASUREMENT: CONSIDERATIONS FOR
INDOOR PERFORMANCE TESTING

2. • 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?
2

3. • 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
3
• 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

4. TEMPERATURE-DEPENDENCE OF MODULE
POWER OUTPUT
4
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

5. TEMPERATURE-DEPENDENCE OF MODULE
POWER OUTPUT
5
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. TEMPERATURE-DEPENDENCE OF MODULE
POWER OUTPUT
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

7. 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

8. INDOOR MODULE PERFORMANCE
CHARACTERIZATION CONSIDERATIONS
8
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

9. INDOOR MODULE PERFORMANCE
CHARACTERIZATION CONSIDERATIONS
9
Factors impacting accurate and repeatable temperature
measurements:
• Directionality of heat source
vs.

10. INDOOR MODULE PERFORMANCE
CHARACTERIZATION CONSIDERATIONS
10
Factors impacting accurate and repeatable temperature
measurements:
• Directionality of heat source
• Uniformity of heat source
vs.

11. INDOOR MODULE PERFORMANCE
CHARACTERIZATION CONSIDERATIONS
11
Factors impacting accurate and repeatable temperature
measurements:
• Directionality of heat source
• Uniformity of heat source
• Hold time at temperature

12. INDOOR MODULE PERFORMANCE
CHARACTERIZATION CONSIDERATIONS
12
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.

13. INDOOR MODULE PERFORMANCE
CHARACTERIZATION CONSIDERATIONS
13
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

14. 14
“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

15. 15
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”

16. CASE STUDY—IMPACT OF MEASUREMENT
LOCATION USING “TOASTER” HEATING
16
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

17. 17
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

18. 0
50
100
150
200
250
300
0.0 20.0 40.0 60.0 80.0 100.0
PowerOutput
Temperature
Cell Probe
Backsheet Probe
CASE STUDY RESULTS—IMPACT ON
NORMALIZED POWER CURVES
18
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

19. CASE STUDY RESULTS – IMPACT ON PAN FILE
OPTIMIZATION AND ENERGY PREDICTION
19
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
T. Coeff. Isc
(mA/°C)
4.18
Rshunt (Ω) 333
Rseries (Ω) 0.400
Rshunt at G = 0 (Ω) 830
Rshunt exp 5.5
T. Coeff. Pmp
(%/°C)
-0.44

20. CASE STUDY RESULTS – IMPACT ON PAN FILE
OPTIMIZATION AND ENERGY PREDICTION
20
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
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
Parameter Value
Isc (A) 9.20
Voc (V) 37.9
Imp (A) 8.61
Vmp (V) 30.2
T. Coeff. Isc
(mA/°C)
4.18
Rshunt (Ω) 333
Rseries (Ω) 0.400
Rshunt at G = 0 (Ω) 830
Rshunt exp 5.5
T. Coeff. Pmp
(%/°C)
-0.44
Optimized PAN File
Parameters Using Tcell

21. 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

22. INDOOR MODULE CHARACTERIZATION FOR
DEVELOPMENT MUST GO BEYOND 61853
22
• 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

23. 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
   

24. CONCLUSIONS
24
• 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

25. FUTURE WORK
25
• 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

26. THANK YOU
PRESENTER NAME OPTIONAL TITLE
COMPANY DIVISION
26

27. www.bv.com
© Black & Veatch Holding Company 2012. All Rights Reserved. The Black & Veatch name and logo are registered trademarks of Black & Veatch Holding Company.