National Renewable Energy Laboratory Innovation for Our Energy Future

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

NREL

Sarah Kurtz

2.3.2009

CPV Today CPV Summit 

2009

San Diego, CA

NREL/PR-520-45011

A Bright Future for CPV
Outline

• Solar

• a huge success, but still a long way to go

• can CPV help PV grow to significance?

• CPV is positioned to go far

• The challenge of product development in CPV –

why is it so hard?

• Infrastructure not there

• Many interconnected design details:

• Design with the bird’s eye view

• Troubleshoot from the worm’s eye view

• Some detailed suggestions

• Are we at the turning point for CPV?

National Renewable Energy Laboratory 2 Innovation for Our Energy Future
National Renewable Energy Laboratory Innovation for Our Energy Future

Growth of photovoltaic (PV) industry

0.01%-0.1% of electricity now comes from PV – want to be closer to 10%

Key to creating the needed business climate may be lower capital requirements

3

CPV has lower capital requirements
CPV progress

Multijunction cells 41% in lab; 37-39% in production

About a dozen multijunction cell companies

About three dozen companies putting these cells into 

systems

Complementary approach uses silicon cells under lower 

concentration – not the topic of this talk, but a strong 

partner

Some companies working on 1 MW installations

Why does it take so long???

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Why so hard? – need infrastructure

In 1990s, PV community decided that buildingintegrated, customer-owned, customer-sited would 

be the future: little interest in CPV, so little funding 

Investment in CPV came later than for other 

technologies, so CPV infrastructure development 

lags

Work together to define useful reliability tests, 

component suppliers, definition of new markets, etc.

Balance between competitive edge and partnership

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There’s enough market for all companies if we can get the cost down;

Let’s work together to develop the infrastructure!
Why so hard? – many tradeoffs

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Performance

Cost Reliability
Sarah’s advice – use two views

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Design

Bird’s eye view

Diagnose

Worm’s eye view
Bird’s eye view – concentration ratio

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Cell cost: > 1000X Mechanical: < 400X

Can cell cost decrease? Smart tracker isn’t enough

- thermal expansion

- wind

- cost of rigid structure 

Sarah’s advice: If you’re 

designing for over 500X, ask 

yourself why you’ll be successful
Bird’s eye view – cell size

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Large cells and optics

Reduced part count

Rigid structure

Can use active cooling

Modularity can be advantage

Sarah’s advice: Think ahead to automation of 

manufacturing, alignment tolerances, performance

Small cells and optics

Reduced materials cost

Aesthetic appeal

Heat is distributed

Smaller currents
Bird’s eye view – f number

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Higher f number

Alignment tolerance is wider 

(bigger depth of fields)

Sarah’s advice: If you use low f number, analyze the effects of 

imperfect optics and alignment

Lower f number

Reduces thickness

Innovative designs may 

have aesthetic appeal
Bird’s eye view – factory vs installation

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Build at the factory

Reduces installation costs

Sarah’s advice: Consider tradeoffs from beginning to end –

“cradle to grave”; “design for reliability”

Build in the field

Reduces transport costs
Bird’s eye view – many tradeoffs

There are dozens of design tradeoffs/choices with no 

clear winners and optimum may change

New ideas/technologies will affect optimal design

Optimal design is very dependent on application

Each company reaches a different conclusion

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Sarah’s advice: Examine each choice in the context of how it 

may affect other design choices; hire engineers to be a team

What will CPV systems look 

like 100 years from now?
Worm’s eye view – what happened?

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“Fast” optics 

Reduces part count for 

thickness

Innovative designs may have 

aesthetic appeal

Many losses, what happened?

Ideal performance for solar resource - 100%

Electricity out – 64%
Worm’s eye view – what happened?

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“Fast” optics 

Reduces part count for 

thickness

Innovative designs may have 

aesthetic appeal

So many potential losses, how do we identify solutions?

Ideal performance for solar resource - 100%

First reflection loss - 96%

Imperfect optics - 93%

Second reflection loss - 89%

Secondary optics loss – 84%

Cell nonuniform illumination - 82%

Cell temperature - 75%

Cell spectrum - 73%

Cell stringing - 70%

Resistance of wiring - 69%

Tracker misalignment - 65%

Electricity out - 64%
Worm’s eye view – Sarah’s advice

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Sarah’s advice: demand matched reference cells from your cell 

supplier

Why use matched reference cells?

- Quantify optical efficiency for each junction

- Depth of field and acceptance angle may be different for each 

junction (Use special mount that allows you to move each cell)

- Evaluate current matching of multijunction cell for optical design 

(may vary as a function of alignment)

- Reference cells quantify variation in spectrum

- Thorough characterization before start stringing cells

MJ top middle bottom MJ top mid bottom

Collimating tubes
Worm’s eye view – Use all parameters

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Short-circuit current - optical efficiency

Open-circuit voltage – cooling (adjust for concentration 

using transient)

Fill factor for reference cells - electrical resistance or 

shorts; non-uniform illumination

McMahon – PIP 2008

Fill factor for multijunction cell –

spectral effects
Worm’s eye view – Sorting out a module

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Be creative; cover the optics; use thin-film filters with partial 

transmission

Characterize module at maximum power point – short-circuit will 

miss many problems

Module should have same acceptance angle as single cell/optic

If not, measure cell temperature or use filter to see which cell is 

limiting the current; bypass diodes should not be hot; fill factor of 

module should be similar to ff of single cell without showing 

evidence of bypass diodes turning on

Forward bias should be consistent
Reliability - bond to heat sink

National Renewable Energy Laboratory Innovation for Our Energy Future

Cell

Heat sink

Optic

Electrical 

contact

Small ∆T

Electrical isolation

No voids

T cycle OK

• Borrowing experience from power electronics and DBC (direct 

bonded copper) makes this a smaller issue

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Work together to define improved tests
Concentrators – reliability challenges

• Wide variety of designs

• Qualification test is not well established

• Companies spend time developing their own 

accelerated tests to speed product development 

cycles

• Very few companies have heritage with field testing

• Everyone wants to bring a product to market 

immediately

• However, modularity of CPV may be an advantage

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NREL has multiple capabilities

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Solar resource assessment 

http://www.nrel.gov/solar_radiation/

Cell measurements 

www.nrel.gov/pv/measurements/device_performance.html

Indoor and outdoor module testing 

www.nrel.gov/pv/facilities_otf.html

Multijunction cell development 

www.nrel.gov/pv/electronic_materials_devices.html
Turning point for industry?

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PHOTON International 

predicts 50 MW in 2009

60

50

40

30

20

10

0

CPV installations (MW)

2008 2009

Year

Predicted

Is the CPV industry 

ready to ramp 

production?

PHOTON International
Summary

• Solar is growing rapidly and could become a 

significant source of electricity within our lifetimes

• CPV may be able to play a significant role because of 

scalability

• CPV product development is slowed because of the 

interactions of different design parameters

• Take a bird’s eye view for the design and a worm’s 

eye view for diagnosis

• Ask cell suppliers for matched reference cells

• Bright future – 50 MW in 2009?

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National Renewable Energy Laboratory Innovation for Our Energy Future

What will CPV look like 

100 years from now?

Olson: “Many options are a 

curse and a blessing”

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Let’s work together 

to make CPV a 

success

Thank you for your attention and thank you for sharing your wisdom!