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Opportunities and Challenges for Power 

Electronics in PV Modules

National Center

for 

Photovoltaics

ARPA E Workshop

February 8, 2011

Arlington, VA

Sarah Kurtz

Chris Deline

John Wohlgemuth

Bill Marion

Jennifer Granata

NREL/PR-5200-50826
Outline 

What are we talking about?

– Power electronics: DC-DC converters, smart bypass diodes, microinverters

– Imperfect solar cells

– Ways to implement power electronics in module

Opportunities: Enables new technologies/approaches

– Non-uniform thin film and thin silicon wafers that sometimes crack

– High-shading configurations

– Potential to mitigate safety issues

– System-level savings

Challenges: Daunting for some implementations; easier for others

– Efficiency – parasitic losses can be greater than benefits

– Cost – goal is to reduce cost of power electronics by factor of two, while 

increasing performance

– Reliability – could be a nightmare

Conclusions 
Power electronics: Example: DC-DC converter 

• What is a DC-DC converter? Just as a transformer can step up or 

step down the voltage for AC power, a DC-DC converter can 

efficiently adjust the DC voltage in a circuit. 

• Have been used for decades

• What’s new??: lower cost and increased efficiency

• DC-DC converter is probably used in your laptop or cell phone to 

stabilize output voltage of the battery. 

• Efficiencies of 96%-99% are available.
Imperfect Solar Cells

• A solar cell may produce less 

electricity because of reduced 

photocurrent. 

• Today’s thinner Si cells sometimes 

crack.

• Could power electronics help?

• Solar cells may fail in lots of ways. 

• Usually the fill factor (squareness

of the curve) is reduced

• Increased series resistance may be 

most common, but not always

• Usually, all parameters are affected

• Could power electronics help?
Examples of imperfections

• Silicon – low photocurrent because of cracked cell

• CdTe – high series resistance because of back 

contact 

• CIGS – high series resistance because of 

hydrolysis of ZnO transparent conductor

• Amorphous silicon – reduced photocurrent from 

light-induced degradation

• All technologies see some degradation of 

photocurrent, photovoltage, and fill factor

• A full list would cover many pages
Wiring in modules – examples incorporating power electronics

+ Bypass diodes -

Conventional wiring

+ - Power electronics

String-level electronics

+ -

Cell-level electronics
Value of power electronics

Simulation of 50% 

loss of current 

from 1 cell; 

power electronics 

100% efficient; 

Three strings of 24 

cells each, only 

one string has bad 

cell.

Cell-level electronics recover more of lost power, but add complexity
Opportunities – manufacturing variability

Crystalline silicon cells 

are measured and binned 

– manufacturing 

variations < 1%

• For crystalline silicon, the 

binning process is highly 

automated, so adds minimal 

cost

• Binning is useful to test for 

highly defective cells so 

manufacturers want to keep it

• The binned cells may vary in 

current by only ~1%

• Adding power electronics is 

unlikely to be cost effective for 

this application
Opportunities – manufacturing variability

Most thin-film cells cannot 

be binned; variability can 

be large, especially for 

new products, providing 

possible opportunity for 

power electronics

• Strips in thin-film modules cannot be 

binned

• Variability across thin-film module 

can be high

• Opportunity for power electronics, 

but challenging to implement

• Adding power electronics to each 

cell will require redesign of module

Current flow with no power electronics
Opportunities – manufacturing variability

Most thin-film cells cannot 

be binned; variability can 

be large, especially for 

new products, providing 

possible opportunity for 

power electronics

• Strips in thin-film modules cannot be 

binned

• Variability across thin-film module 

can be high

• Opportunity for power electronics, 

but challenging to implement

• Adding power electronics to each 

cell will require redesign of module

Current flow with power electronics at ends

Lateral current flow encounters resistance 

of transparent conductor
Opportunities – degradation variability

Immature thin-film PV designs show variable 

degradation – opportunity for power electronics?

Thin-film modules deployed in Florida
Opportunities – enables new technologies

Thinner silicon cells have lower cost 

but are observed to crack 

sometimes

+ DC-DC converters can mitigate 

issues if thin cells break 

- May be a temporary problem: 

broken cells are undesirable –

need to avoid breakage
Opportunities – improve safety - arcing

Arcs may appear if high voltage (~100 V) can appear across a gap

String inverter

Conventional series Electricity Grid -connected PV panels

DC

AC

PV panels connected to AC micro-inverters

Electricity Grid

AC

Micro-inverters

Arc

No Arc Power electronics can limit voltages 

within system and isolate arcing areas 
Opportunities – improve safety – hot spots

Hot spots can be caused by shading

1. Shading blocks flow of current

2. Shaded cell goes into reverse bias

3. Bypass diode protects cell

4. If bypass diode fails, heating may be severe

5. Smart electronics may be more effective

Shaded cell is forced 

into reverse bias

+ Bypass diodes -

Shaded cell is forced into 

reverse bias; power 

dissipation can be severe; 

can cause thermal runaway 

or accelerated degradation

“Hot-spot test” is part of qual test
Opportunities – enables new technologies

Creative designs may be enabled (e.g. shade tolerant)

+ tenKsolar uses DC-DC converters to enable higher 

packing density on roof, boosting power with mirrors

DC-DC converters may enable designs that would otherwise be dismissed

Shade tolerant 

designs can be 

attractive when 

space is 

constrained or, 

when output can 

be enhanced, as 

in this case 

using mirrors; 

System-level 

advantage 
Challenges – Parasitic losses 

Parasitic losses may affect every kWh

Stated efficiency may not represent average efficiency

“96% efficient converter” (as 

long as conditions don’t vary 

too much)

Higher voltage converters tend 

to have higher efficiencies and 

wider windows 

Low efficiency could limit celllevel implementation
Challenges – Benefits may be transient

If there are losses from inconsistent 

construction or degradation, these 

must be reduced, reducing the 

benefit of power electronics

• CdTe & CIGS modules 

installed before 2000 showed 

higher degradation rates; after 

2000 show improved stability. 

• New designs may benefit 

from power electronics, then 

the benefit may decrease

Thanks to Dirk Jordan for this graph, 

published in SolarPro, Dec 2010

Most benefit, but degradation 

is unacceptable – community 

expects < 1%/yr degradation
Challenges – Cost

At cell level, any increase in cost needs to be offset by 

increased performance

If used to replace bypass diodes, then cost target is equal 

or less than diode cost (~0.1 cent/W)

If power electronics can replace the inverter, then cost goal 

is half of current cost.

Two strategies:

- Look for ways to avoid cost (materials or processing)

- Look for ways to increase power so that higher cost can 

be tolerated
Challenges – Reliability

At cell level, power electronics that fail will be a nightmare

Today’s inverters are weakest part of the system, but are 

tolerated because replacement can be simple. 

Elecctronics embedded in the module must match the 

reliability of the module 

At string level, power electronics could be in junction box, 

making them replaceable, but replacement for a field 

would still be a nightmare

At module level, reliability is still important, but replacement 

becomes more feasible, especially if devices are self 

diagnostic
Targets

• Performance:

- Benefit (reduce losses from shading and other differences) 

- Parasitic losses (may affect total output)

- Net benefit = Loss from imperfection X Fraction recovered - Parasitic losses

- Example: If shading losses are 20% with 80% of that recovered and parasitic 

losses are 6 %, then Net benefit = 0.2 X 0.8 – 0.06 = 10% 

• Cost

- Would like to reduce total cost of power electronics by factor of two

- When considering energy yield, acceptable cost increase depends on 

level of shading or other losses (application dependent)

• Reliability

- 25 yr life is expected for modules

- If replacement is inexpensive and convenient, life needs to 

comparable to inverter life

National Renewable Energy Laboratory Innovation for Our Energy Future
Conclusions

• Opportunities – innovation opens doors

Enable new technologies, until they achieve consistent 

performance 

Compensate for shading (see advantage at system level)

Improve safety

• Challenges – daunting?

Implement while decreasing cost

Implement with excellent reliability

• Easiest place in a module to use smart electronics may be 

to replace the current bypass diodes – other opportunities 

at system level

National Renewable Energy Laboratory Innovation for Our Energy Future
Resources

National Renewable Energy Laboratory Innovation for Our Energy Future

http://www.nrel.gov/docs/fy11osti/50003.pdf

Sarah.Kurtz@nrel.gov

Chris.Deline@nrel.gov

Bill.Marion@nrel.gov

JEGrana@sandia.gov