NREL/PR-5J00-64525 

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.

Opportunities and Challenges in the 

Development of Smart PV Systems

Trina State Key Laboratory 

2015, Changzhou, China

Chris Deline

June 25-26, 2015
2

Roadmap

• About NREL

• Overview of Distributed MPPT (DMPPT) in PV

• Performance benefits of DMPPT

• Maxim smart modules - performance
3

NREL is located in Golden, Colorado, and is: 

• Part of the Department of Energy’s laboratory system

• Focused on renewable energy, including Solar, Wind, Bioenergy and Grid 

Integration

• Operated as a government research facility, and also as a user facility for 

industry testing and R&D
4

Operations began in 1977 as the Solar Energy Research institute, but quickly 

expanded to include other renewables including wind, hydro and biomass.

Current NREL statistics include:

• 1720 full employees and 678 part-time researchers, interns, and contractors

• $371M budget (2013)

• 57 R&D 100 awards (top 100 technologies of the year)
5

The PV performance and reliability group at NREL focuses on outdoor field 

performance / reliability and indoor accelerated test and characterization:

• Accurate PV performance characterization and modeling

• PV degradation rate and failure mode assessment

• US and international standards for reliability and quality 
6

Literature data show statistical distribution of degradation rates: 

Majority meet performance warranty

Statistical analysis of degradation rates

Jordan, Prog. Photovolt: Res. Appl. 2013; 21:12–29. DOI: 10.1002/pip.1182
7

Three types of failures in the laminate

• Broken ribbons

o Testing is much like testing a paper clip

o Cyclic mechanical stress gives very high acceleration

• Broken solder bonds

o Time-dependent response to stress increases time 

needed to induce damage

o Acceleration factors are smaller and harder to 

quantify, but testing is effective; 200-2000 cycles?

• Cracked cells

o Power loss occurs if cracked piece becomes 

disconnected

o Problems are uncovered by mechanical load followed 

by thermal cycling

We know how to detect design flaws that cause these failures!

But, we’d like to detect the problems faster and nondestructively!
8

Encapsulant Discoloration

• Discoloration may be 

primary source of 

degradation (Isc loss)

• Competing reactions 

cause discoloration and 

bleaching

• What if bleaching is 

accelerated more than 

discoloration? High 

acceleration factors may 

result in “wrong” answer Chemistry of polymers can be complex

What to do?

Be relevant by using high acceleration

BUT, be scientific by defining degradation 

kinetics and doing parallel tests with low 

acceleration factors! 
9

Roadmap

• About NREL

• Overview of Distributed MPPT (DMPPT) in PV

• Performance benefits of DMPPT

• Maxim smart modules - performance
10

Examples of Distributed MPPT products

Frame – attached

(Enphase Energy)

Credit: Enphase Energy, Tigo Energy, Maxim Integrated

J-box embedded

(Tigo Energy)

Laminate embedded

(Maxim Integrated)

2.15 cm
11

Examples of Distributed MPPT products

Frame – attached

(Enphase Energy)

Credit: Enphase Energy, Tigo Energy, Maxim Integrated

J-box embedded

(Tigo Energy)

Laminate embedded

(Maxim Integrated)

2.15 cm

DMPPT = MLPE, microinverter, power optimizer...
12

US residential market (1H 2014)

DMPPT market 

share: 56%

Source: GTM Research U.S. PV Leaderboard, Q2 2014
13

US residential market – 3rd party ownership

3rd-party-owned 

in 2014 : 68%

Source: GTM Research U.S. Residential Solar Financing 2014-2018
14

DMPPT pro/con

Pro:

• Flexibility of design

• NEC 690.12 safety

• Data monitoring

• Shade performance

• Warranty

Con:

• Cost

• Efficiency (maybe)

• More potential 

points of failure

• O&M concerns
15

NEC 690.12 (2014)

Code requires rooftop PV conductors greater 

than 10’ from the array to be de-energized in 

event of emergency

Compliance options:

Remote relays Roof-mounted inverters

Credit: Midnite Solar, SMA, Tigo

Microinverters and Power 

Optimizers
16

Other emerging inverter trends:

• California Rule 21 (grid support capabilities for 

PV inverters)

• Storage capabilities for utilities without net 

metering, or with time-of-day pricing

• NEC 690.12 (2017): proposed module-level 

shutdown
17

Roadmap

• About NREL

• Overview of Distributed MPPT (DMPPT) in PV

• Performance benefits of DMPPT

• Maxim smart modules - performance
18

Recoverable vs. Non-recoverable loss in PV

0

10

20

Volts

Current

0 50 100 150

0

500

1000 Power 

Global max [A]

Local max [B]

Partial shading leads to 

power loss from reduced 

irradiance (non-recoverable) 

and panel mismatch 

(recoverable)

The better the peak-power 

tracking (MPPT) granularity, 

the higher the recoverable 

power

Mismatched panels in a series string. 

Either bypass diodes are turned on in 

the shaded panels (point [A]) or all 

panels operate at a low current 

(point [B]) 

C. Deline, B. Marion, J. Granata, S. Gonzalez. A Performance and Economic Analysis of Distributed Power Electronics in 

Photovoltaic Systems. NREL Report No. TP-5200-50003. Golden, CO: National Renewable Energy Laboratory, December 

2010. 
19

Overview of Distributed MPPT

S.M. MacAlpine, M.J.Brandemuehl, R.W.Erickson, “Potential for power recovery: simulated use of distributed 

power converters at various levels in partially shaded photovoltaic arrays,” in Proc. PVSEC 2011. 

+2% +8% +9% +12%

Cell

Annual performance improvement 

under heavy shade

Finer peak-power tracking granularity 

leads to reduced mismatch

String Module Submodule

MPPT Granularity
20

Sub-module power conversion

1,200

1,300

1,400

1,500

1,600

1,700

0.00 0.20 0.40 0.60 0.80 1.00

PRODUCTION (KWH/KWP)

GROUND COVERAGE RATIO

ANNUAL ENERGY - COLORADO RTC SITE

Conventional

Maxim

Sub-module DC-DC converters:

• Enable 10-20% tighter row pitch, -or-

• 1-3% more energy / panel at the same row pitch

C. Deline et al., “Evaluation of Maxim Module-Integrated Electronics at the DOE Regional Test Centers”, 40th IEEE 

Photovoltaic Specialists Conference, Denver, CO, 2014

*Mismatch occurs within

the module
21

Shading Survey gives real-life conditions

Survey of >60 residential 

installations provides 

distribution of shade 

extent

Credit: Solmetric

Imaging tool shows shade 

obstructions for residential rooftops Deline, C., Meydbray, J., Donovan, M. (2012) NREL Report No. 

TP-5200-54876

M
22

Validation: before and after Power Optimizers 

0.60

0.70

0.80

0.90

1.00

1 3 5 7 9 11

Temperature corrected PR

Month of year

Initial Results

With Tigo DC-DC

Hanson A, Deline C, MacAlpine S, Stauth J, Sullivan C, “Partial-Shading Assessment of Photovoltaic Installations via 

Module-Level Monitoring”, IEEE Journal of Photovoltaics, 4 1618-1624, 2014

Monthly performance for 3 years

*5.8% more energy with modulelevel MPPT. 30% shade recovery
23

Roadmap

• About NREL

• Overview of Distributed MPPT (DMPPT) in PV

• Performance benefits of DMPPT

• Maxim smart modules - performance
24

Maxim installation at FSEC

• Florida site uses 

VT8024 modules

o 3 converters / 

module

• Performance data 

still coming on-line 

Maxim VT8024 module –for landscape 

deployment. One chip per 24 cells

Credit: Maxim Integrated
25

FSEC installation particulars

• Side by side 

Maxim panels 

(blue) with 

Conventional 

(yellow)

• 3 row 

spacings

GCR 0.58

GCR 0.68

R1

R2

R3

V5

R4

V4

V6

V3

V2

V1

GCR 0.68

GCR 0.78

N
26

FSEC system: performance model

• PVsyst models 

predict performance 

gain up to 8% at 

tight spacing

• Or increase packing 

density 10-20% for 

the same shade loss

1,200

1,300

1,400

1,500

1,600

0.20 0.40 0.60 0.80 1.00

PRODUCTION (KWH/KWP)

GROUND COVERAGE RATIO

PVSYST ANNUAL ENERGY - FLORIDA RTC

Conventional

Maxim
27

Summary

• Embedded power electronics promise to 

reduce cost and increase MPPT granularity
28

Summary

• Embedded power electronics promise to 

reduce cost and increase MPPT granularity

• US rooftop PV is moving to DMPPT for ease of 

design and increased safety

o Rapid DMPPT acceptance, string inverter decline
29

Summary

• Embedded power electronics promise to 

reduce cost and increase MPPT granularity

• US rooftop PV is moving to DMPPT for ease of 

design and increased safety

o Rapid DMPPT acceptance, string inverter decline

• Sub-module MPPT (Maxim) can improve 

performance and increase packing density for 

large un-shaded PV systems using low-cost 

converters
 NATIONAL RENEWABLE ENERGY LABORATORY

Questions?

30

Acknowledgments

This work was supported by the U.S. Department of Energy under 

Contract No. DOE-AC36-08GO28308 with the National Renewable 

Energy Laboratory

Chris Deline

chris.deline@nrel.gov

Ph: (303) 384-6359