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NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
CHARACTERIZING SHADING LOSSES ON PARTIALLY SHADED PV SYSTEMS
Chris Deline
PV Performance Modeling Workshop
September 23, 2010
Albuquerque, NMNREL/PR-520-49504
Innovation for Our Energy Future
Overview
2
Introduction: Shading on PV systemsTheory: Shaded PV power lossPractical issues with modeling shaded PV
• Shade Estimation• IV curve analysis
Methods of implementing partially shaded PV modelingSome experimental resultsCurrent and future work
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Introduction – Shading on PV systems
3
Shading and mismatch occur on all types of PV installations.
• Nearby shade obstructions like trees and telephone poles
• Horizon shading from faraway structures
• Self-shading from adjacent rows
• Imp mismatch from orientation, manufacturing tolerance, differential aging or soiling
Some types of shading are easier to quantify and model than others.
1
2
31: Lakewood, CO. 2: Maumee, OH. 3: Arlington, VA
Credit: NREL
Credit: NREL PIX 15617
Credit: NREL PIX 08558
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Introduction – Impact of Shade
4
Shade impact depends on e.g. module type (fill factor, bypass diode placement), severity of shade, and string configuration.Power loss occurs from shade, also current mismatch within a PV string and voltage mismatch between parallel strings.Power lost is greater than proportional to the amount of shade on the system
‘Shade Impact Factor’ (ratio of power lost to area of shade) for a single module in a single string PV system [1]
[1] C. Deline, IEEE PVSC, 2009
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Bypass diode operation in most modules
5
+-+-
-
+
+-
I+V-
NN-12
1
+
D1
Shade
Bypass diodes typically protectsubstrings of 15-20 cells.Shade on one of these cells cancause the diode to turn on,removing those cells electricallyfrom the string.Current is continuous in the PVstring; a small amount of shadecan greatly reduce outputpower.On typical Si modules, reducing1 cell’s irradiance by 25% canlead to bypass diode turn-on.
1 substring
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I
V
Imp
Shaded cell
Vrev
Unshaded cell
Theory – Partially shaded substring of cells
6
A shaded cell has reduced Isc. Inorder to pass the string current Impthe cell will operate in reversebias. The total substring voltageis a sum of the various operatingvoltages including the reversebiased cell.
If the total substring voltage < 0, the bypass diode turns on and the shaded cell will operate near Vrev.
Variability exists in the reverse-bias characteristics of differentcells – the same shading couldresult in different outcomes.
Full I-V curve of a partially shaded cell. Current continuity requires the shaded cell to operate in reverse bias to pass the Imp current of the rest of the substring.
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0
10
20
VoltsC
urre
nt0 50 100 150
0
500
1000
Pow
er
Unshaded IV + Shaded
IV
Theory – system level IV curve
7
System IV curve is built from individual substring IV curves in series and parallel.
+ =
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0
10
20
VoltsC
urre
nt0 50 100 150
0
500
1000
Pow
er
Global max [A]Local max [B]
Theory – system level IV curve
8
System IV curve is built from individual substring IV curves in series and parallel.Partial shading can lead to Local [B] and Global [A] maxima.Bypass diode turn-on depends on the peak power point chosen. For instance, operating at point [A] requires shaded bypass diode turn-on while point [B] does not.
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Practical matters – shade estimation
9
Shading trees
Proposed array
Rooftop survey
Shading site survey typically relies on aerial imagery andfisheye shade analysis e.g. SunEyeTM, or Solar PathfinderTM.Some issues include: foliage changes throughout the year,spatial resolution requires multiple pictures, shading objectsare considered 100% opaque, nearby objects have moreposition uncertainty, 3D CAD modeling is time intensive.
Credit: Chris Deline / NREL
Credit: Chris Deline / NREL
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Methods of modeling substring IV curves
10
Full 5-parameter IV curve• High accuracy, but slow (when
calculated 1000’s of times)
Simplified IV curve (3-parameter)• Computationally less intense,
reduced accuracy for V < Vmp.
• I = C1 – C2 exp(C3 * V)
Empirical ‘Shade impact factor’• System-specific lookup table, based
on shade % and diffuse / global ratio.
0 5 10 15 20 25 30 350
2
4
6
8
Volts [V]
Cur
rent
[A]
5-parameter3-parameter
Comparison of full 5-parameter IV curve with a simplified 3-parameter IV curve for an Evergreen ES-200 PV module. Accuracy is better for V > Vmp .
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Real-world application of shade modeling
11
Site survey conducted on a ‘typical residential installation’[3]
• Most shade from 6-10am, 2-6pm• ~21% annual irradiance loss• 2 strings of 7 mSi modules @ 3kWSite survey picture taken at each PV module substring• 3 images per module = 42 total• Provides # of shaded substrings
for a given hour and date
‘Typical residential installation’. 2x7 mSi panels
Site survey:~20% irradiance loss due to shade[3] R. Levinson, Solar Energy 83, 2009
Credit: Brent Nelson / NREL
Credit: Chris Deline / NREL
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Numerical shade simulation
12
Simulation uses PVWatts with additional shade derating[4]
• Derating based on empirical relationships between shade extent and power loss, as determined in a scale experiment at NREL.• TMY3 weather data and the default PVWatts AC to DC factor (0.77)
Two shade conditions are simulated: 1) both strings are shaded as per the survey, and 2) one string is entirely unshaded• Two-string shading is more realistic, but some installations may have
more limited shading.
[4] C. Deline, IEEE PVSC, 2010
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Σ
Simulation method - overview
13
Site survey: one image for each substring TMY3 database
Experimental Shade Impact Factor
Beam/Global Irradiance
PVWatts modelX
DC Derating (hourly)
Annual shaded power production
# Shaded Substrings
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Simulation results
14
Annual power produced Power lost to shade
Unshaded baseline 4.4 MWh 0Site survey estimate -21%2 strings shaded 3.5 MWh -22%1 string shaded 3.7 MWh -17%PVWatts simulation results using site survey data for a two-string PV system.
5 10 15 200
500
1000
1500
2000
2500
Time
DC
pow
er (W
)
March modeledMarch actualDec. modeledDec. actual
Modeled results (dots) and measured data (lines) for two representative sunny dates
Modeled results compare favorably with measured data on representative sunny days.Annual results show close agreement with site survey’s ‘solar resource fraction’ (but this is not always the case)
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Large commercial installation
15
0 5 10 15 20 250
0.2
0.4
0.6
0.8
1
Percent of string shaded
Stri
ng p
ower
FF = 0.65FF = 0.73Observed
6 8 10 12 14 16 18-0.5
0
0.5
1
1.5
2
2.5
3
Time
Stri
ng p
ower
(kW
)
AM shading
30% loss from shade
Morning power loss is monitored with DC current transducers. 30% loss is coincident with 12.5% string shading
Modeled shade impact for large parallel systems. Note that higher FF is more sensitive to shade.
This 1MW installation has 16 PV modules per string. Periodic shading occurs from nearby light poles.
Credit: Chris Deline / NREL
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Current / Future work at NREL
16
• Shade simulation feature going into Solar Advisor Model, specifically for inter-row shading of large utility-scale systems.• Further work on developing and validating shaded PV models• Test & Evaluation of DC-DC converter devices and micro-inverters to determine the performance improvement
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Questions / Comments?
17
AcknowledgmentsThis work was supported by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory.
Chris [email protected]: (303) 384-6359
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0 10 20 30 40
0
0.2
0.4
0.6
0.8
1
Cell shade %
Rel
ativ
e su
bstri
ng p
ower
No DC-DC
Bypass diode turn-on
0 10 20 30 40
0
0.2
0.4
0.6
0.8
1
Cell shade %
Rel
ativ
e su
bstri
ng p
ower
Bypass diode turn-on
With DC-DC
No DC-DC
1 2 30
50
100
150
200
250
300
# of M10's substrings shaded
Sys
tem
pow
er lo
ss (W
)
2126
60
4122
117
78
18
175
M1-M5M6-M9M10
Mismatch
(No DC-DC)(With DC-DC, 50% shade)
1 2 30
50
100
150
200
250
300
# of M10's substrings shaded
Sys
tem
pow
er lo
ss (W
)
M1-M5M6-M9M10
Results – single module shading
19
Without DC-DC devices:• Single-cell shading of 25%
causes bypass diode turn-on• Mismatch loss accounts for
~40% of the total shade lossWith DC-DC devices:• Bypass diode turn-on can be
delayed• Mismatch losses reduced• Shaded module output
proportional to shade opacity
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0 10 20 30 40 50 60 7050
60
70
80
90
100
Single PV string shade(%)
Sys
tem
pow
er(%
)
>60% shadeSIF = 1.63
Results – Shade Impact Factor
20
Shade Impact Factor without DC-DC= 1.63With DC-DC, SIF = shade opacity
With DC-DC
Shade %
Power loss
m = SIF
0 10 20 30 40 50 60 7050
60
70
80
90
100
Single PV string shade(%)
Sys
tem
pow
er(%
)
75% shadeSIF = 0.73
100% shadeSIF = 0.97
50% shadeSIF = 0.48
No DC-DC
SIF = 1.63