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Contract No. DE-AC36-08GO28308

Opportunities and Challenges 

for Development of a Mature 

Concentrating Photovoltaic 

Power Industry

Sarah Kurtz

Technical Report

NREL/TP-5200-43208

Revised June 2011
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Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

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Contract No. DE-AC36-08GO28308

Opportunities and Challenges 

for Development of a Mature 

Concentrating Photovoltaic 

Power Industry 

Sarah Kurtz

Prepared under Task No. PVA7.4401

Technical Report

NREL/TP-5200-43208

Revised June 2011
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Opportunities and Challenges for Development of a Mature 

Concentrating Photovoltaic Power Industry

Introductory Note

This report attempts to summarize the status of the concentrating photovoltaic (CPV) industry and to 

identify problems that may be encountered as the industry matures, with the ultimate goal of increasing 

the growth rate of the CPV industry. This report strives to guide industry investments as well as to help 

set research agendas for the National Renewable Energy Laboratory (NREL) and other R&D 

organizations.

Recent progress in the CPV industry is impressive, and has recently drawn more attention from the 

mainstream PV community. Specific examples are summarized in the report. If you have suggestions 

about this report, especially to update the tables to show your company’s latest installations or add your 

company’s name, please e-mail Sarah.Kurtz@nrel.gov.

____________________________________________________________________________

Executive Summary of Recent Changes to the CPV Industry

The high-concentration PV industry has made great strides in the last year, including:

• Demonstration of full-scale products with high efficiency: ~31% for small module and 

27% AC for a full-scale system, as reported by individual companies.

• Multiple companies installed 1 MW in 2010 and are planning tens or hundreds of 

megawatts in 2011 and 2012.

• The CPV approach is attracting some big names, including companies such as RFMD 

and JDSU, both of which have expressed interest in the multijunction concentrator cell 

business.

• Dozens of companies are working on developing products or participating in the supply 

chain.

Si-based CPV approaches are also making significant strides:

• Solaria, Skyline, WS Energia, and others could show dramatic growth in coming years.

• Dozens of companies are working on developing products or participating in the supply 

chain.
2

The Promise of CPV

Today’s photovoltaic (PV) industry is growing at a rapid rate, but the industry would grow even 

faster if costs could be reduced for both the final products and the capital investment required 

for scale-up. For today’s risk-adverse investors, reduced capital expenditure translates to 

reduced risk. One strategy for reducing the module cost and the need for capital investment is 

to reduce the amount of semiconductor material needed. Many companies are thinning the 

silicon wafers to reduce costs incrementally; others use thin-film coatings on low-cost substrates 

(such as amorphous/microcrystalline silicon, cadmium telluride, or copper indium gallium 

diselenide on glass or other substrates). CPV follows a complementary approach and uses 

concentrating optics to focus the light onto small cells. The optics may be designed for low or 

high concentration. Low-concentration concepts use silicon or other low-cost cells; highconcentration optics may use more expensive, higher-efficiency cells. Higher-efficiency cells 

can reduce the cost per watt if the cost of the small cells is a small fraction of the total cost.

CPV approaches vary widely according to the type of cells used, the concentration ratio, type of 

optics (refractive or reflective), and the geometry. For this report, we have chosen to treat the 

types of systems in three parts as described in Table 1. Part I discusses CPV using 

multijunction (GaAs-based) concentrator cells, which, because of their high cost, require 

concentration ratios higher than ~400X. Part II discusses medium-concentration systems 

(typically 10X–20X) that require silicon or other types of concentrator cells; a wide range of 

approaches is included. Part III discusses the use of conventional silicon modules with 

enhanced performance from mirrors on either side of the modules. Appendix A summarizes a 

cost evaluation of all technologies.

Table 1. Description of Classes of CPV Treated in Parts I–III of This Report

Part Class of CPV Typical Concentration Ratio Type of Converter

I High-concentration, MJ cells >400X Multijunction

II Medium-concentration, cells ~3X–100X Silicon or other cells

III Enhanced concentration, 

modules <3X Silicon modules

The value of CPV within the PV portfolio can be summarized as:

• Lower capital investment because of the reduced use of semiconductor material 

compared with flat-plate silicon; this reduces risk for the investor and allows more rapid 

adjustment of plans based on changing markets.

• High energy yield (kWh/installed kW) associated with the use of tracking and small 

temperature coefficients; in areas with high direct-normal irradiance, this can be a 

significant effect, providing lower cost of electricity even for products with higher 

$/W cost.

• Higher efficiency, allowing smaller module area; in some cases, CPV requires less than 

half the module area to deliver the same power (note that this may not translate to 

higher energy for a given field if the systems are widely spaced to reduce shading).

• Lower product costs are being demonstrated because of a reduced use of 

semiconductor material and because of a steeper learning curve.

• Better match to load profile because of excellent performance in late afternoon (as a 

result of tracking and lower temperature coefficients).

• Installation costs can be low because of high efficiency of modules.
3

• Qualitatively different approach that complements low-efficiency approaches and 

contributes to a strong technology portfolio for solar, especially for the sunniest 

locations.

• Low environmental impact for pedestal-mounted systems.

CPV joins flat-plate PV in providing these benefits:

• Renewable electricity source with a cost that already competes with conventional 

electricity sources in some locations

• Modular: can be installed in sizes ranging from kilowatts to multiple megawatts

• Production profile that is fairly predictable and is a relatively good match to the load 

profile

• Low maintenance

• Low water use

• Can be installed with minimal environmental impact, sometimes in configurations that 

allow dual use of the land

• Low carbon intensity and energy payback that can be less than a year.

[1]

These will be discussed in greater detail throughout this report.
4

Part I. High-Concentration CPV (HCPV) Using High-Efficiency, 

Multijunction Solar Cells

Concentrator cells have achieved increasingly impressive efficiencies, inspiring interest in the 

high-efficiency, HCPV approach. There are currently seven multijunction cell architectures with 

reported efficiencies in the 40% range (Table 2). The exact structures could be further 

differentiated within each of these architectures. The current record efficiency is 43.5% by Solar 

Junction. This cell uses a dilute nitride alloy for the lowest junction. Spire achieved 42.3% [2]

efficiency with a bi-facial approach (GaInP/GaAs on the front and GaInAs on the back of a GaAs 

wafer). Other structures and measured efficiencies[3-7] are tabulated in Table 2. A historical 

summary of champion cell efficiencies is shown in Fig. 1. Multijunction concentrator cells have 

achieved much higher efficiencies than any other approach. This is not surprising for two 

reasons: (1) the highest theoretical efficiencies may be achieved if multiple semiconductor 

materials (with a range of bandgaps) are chosen to match the spectral distribution of the sun, 

and (2) the compound semiconductors used in these cells are mostly direct-gap materials and 

can be grown with near-perfect quality. The multijunction approach has been described 

extensively in the literature.[3-5,8-17]

Table 2. Summary of Champion Efficiencies for Multijunction Cells

Cell Architecture Champion 

Efficiency* Company Comments

Dilute nitride

43.5%

(NREL)

(>43% @ 400–

1000 suns)

Solar 

Junction

The exact structure has not been 

published, probably GaInP-GaAs dilute 

nitride, all lattice matched

GaInP-GaAs-waferGaInAs[2]

42.3% @ 406 

suns

(NREL)

Spire

Requires epi growth lattice matched on 

front and mismatched on back of GaAs 

wafer

GaInP-Ga(In)As-Ge[3]

41.6% @ 364 

suns

(NREL)

Spectrolab Commercially available; lattice matched

GaInP-GaInAs-Ge[4]

41.1% @ 454 

suns

(Fraunhofer 

ISE)

Fraunhofer

ISE

Mismatched; similar to design being 

launched by Spectrolab

GaInP/Ga(In)As/GaInAs[6] 40.8% @ 326 

suns

(NREL)

NREL Inverted metamorphic

GaInP-GaInAsQD-Ge[7] ~40% Cyrium Uses quantum dots in middle junction

GaInP-GaInAsQW-Ge ~40% Quantasol Uses quantum wells in middle junction

*Efficiencies were measured at the indicated accredited test laboratory.
5

Figure 1. Historic summary of champion cell efficiencies for various PV technologies. The highest 

efficiencies have been achieved for multijunction solar cells; these efficiencies are still increasing each 

year. Multijunction cell efficiencies have the potential to approach 50% in the coming years.

When compared with solar thermal approaches, CPV provides a qualitatively different 

approach, typically with lower water usage and greater flexibility in size of installation, but with 

greater sensitivity to cloud transients. The tracking used for CPV also implies relatively higher 

electricity production per installed kilowatt, compared with fixed flat plate (see below).

Ten years ago, there was little commercial interest in CPV for the following reasons:

• The PV market was dominated by building-integrated or rooftop applications, whereas 

most CPV products are better suited to solar farms.

• The champion concentrator cell was only ~30% efficient, compared with ~43% today.

• The total size of the industry was less than one-tenth of what it is today, making nearterm, high-volume CPV deployment unlikely (i.e., CPV achieves ultralow cost only when 

the volume of manufacturing is large).

In the last 10 years, the solar industry has grown exponentially, doubling about every two years, 

and the CPV industry has grown rapidly, with dozens of companies developing new products. 

Cumulative investment in CPV is >$1 billion. Solar fields, which often use tracked systems, are 

becoming more common, providing a potentially huge market for CPV products. With the overall 

PV market growing in the gigawatt range, CPV has an opportunity to enter the market with 

production of tens or hundreds of megawatts per year. This is significant because CPV is 

unlikely to achieve low costs when manufacturing at less than tens of megawatts per year. Ten 

years ago, it would have been difficult for companies to have confidence that they could find 

markets for the needed volume. The growth of the market, and especially growth of the market 
6

segment that uses trackers, is an important contributor to the increased interest in CPV. The 

potential for CPV industry growth has been widely discussed in recent years.[10-12,18] 

The most important current advantage of the CPV approach may be the reduced need for 

capital investment (scalability). The growth of the silicon PV industry has been challenged by 

the need for capital investment, especially in silicon purification facilities. By reducing the 

amount of semiconductor material, the capital investment need is also reduced. Although no 

CPV companies have demonstrated it, the relative ease of scale-up of CPV is logical and could 

be a significant advantage in a rapidly growing market. Amonix and Concentrix are now 

positioned to begin a ramp up in production, enabling the needed reduction in cost.

Some cost analyses have predicted that using lenses or mirrors to concentrate the light on small 

cells can lead to low costs for solar electricity.[11,12] These studies imply that there is a potential 

for cost-effective implementation of CPV systems even in locations such as Boston, 

Massachusetts.[12] The cost assumptions published in references [11,12] are out of date, but the 

fundamental conclusion that CPV has the potential for lower costs still stands.1 The uncertainty 

in the cost estimates is greater than the difference between the estimated costs, implying that it 

is too early to predict which technologies will achieve the lowest costs for each application. In a 

recent reexamination of his earlier cost analysis (presented as the opening talk at CPV6), 

Richard Swanson projected that the HCPV, thin-film PV, and low-concentration PV (LCPV)

approaches all have similar costs (within the uncertainty of the analysis). Maintaining a portfolio 

of technologies increases society’s chance of identifying the best options; CPV represents a 

qualitatively different approach from both silicon and thin-film PV and has a credible path to 

playing an important role in PV markets, especially in sunny locations. Demonstration that a 

low-cost structure can be achieved will require development of a reliable CPV product, followed 

by large-scale deployment. The CPV industry has made dramatic progress toward this in the 

last five years.

Installations of the first megawatts of products are often subsidized by venture capital. 

However, when production passes 10 MW (or 100 MW for the best-funded companies), the 

selling price and actual cost must quickly converge. In 2008, a number of CPV companies 

installed ~1 MW. Because of the global economic recession, 2009 was a slow year for the CPV 

industry, but 2010 showed a dramatic surge in growth, and large projects are planned for 2011 

and 2012. Amonix is populating a 30-MW field near Alamosa, Colorado, and has just opened a 

new fully automated production facility, giving the company ~100-MW/y manufacturing capacity.

Concentrix was recently purchased by Soitec and is planning to install 150 MW for San Diego 

Gas & Electric by 2015, constructing a 200-MW manufacturing facility in the San Diego, 

California, area. As the installation volume increases, the cost of CPV products will become 

increasingly clear. Once these baseline costs are established, some have predicted that the 

learning curve for CPV costs will be steeper than for flat-plate costs.

CPV, like all PV technologies, is most cost effective for sunny regions with clear skies. The 

benefit of clear skies is most obvious for CPV systems, because they use the direct beam and 

do not effectively capture diffuse light. This solar resource is often referred to as direct-normal 

irradiance (DNI). Although the diffuse light is not effectively captured by CPV, the DNI resource 

is often greater than the resource available to fixed flat-plate panels because of the value of 

tracking; the resource available to flat-plate PV increases if the flat-plate modules are tracked to 

 1 The energy payback of some CPV systems has also been studied.[1] Peharz G and Dimroth F, "Energy Payback 

Time of the High-concentration PV System FLATCON," Prog. Photovolt. 13, 627-734 (2005).
7

follow the sun. Large flat-plate PV systems today are often mounted on one-axis trackers.

Figure 2 shows the ratio of DNI to global irradiance on a one-axis tracked surface.

Figure 2. Ratio of DNI to global irradiance on a one-axis-tracked surface (no tilt) as a function of the 

average daily irradiance. Source of data: http://rredc.nrel.gov/solar/old_data/nsrdb/1961-

1990/redbook/sum2/state.html

Current Status of the CPV Industry

The year 2011 may be a turning point for the CPV industry as Amonix completes the first 

>10-MW field, and the manufacturing capacity of the CPV industry begins to grow in the 

hundreds of megawatts/year range for the first time.

Table 3 provides a list of more than three-dozen CPV companies pursuing designs with 

multijunction cells. Although many of these companies are just getting started, others have had 

prototypes on sun for multiple years and are ramping up production. Two key trends are seen in 

2011: a number of acquisitions and increased involvement in China. Past history of the growth 

of the CPV industry has been documented in previous versions of this report and by PHOTON 

International articles.

[19,20]

Table 3. Summary of CPV Companies

(This information changes rapidly. Companies described in gray appear to have moved away from this 

approach, but should not be discounted completely.)

Company Type of 

System Location On Sun in 

2009*

Installed 

in 2010*

Completed 

or in 

Progress in 

2011*

Manufacturing

Capacity*

Abengoa Solar Lens, 

pedestal Madrid, Spain 400 kW

Alitec Lens, 

pedestal

Navacchio, 

Italy
8

Company Type of 

System Location On Sun in 

2009*

Installed 

in 2010*

Completed 

or in 

Progress in 

2011*

Manufacturing

Capacity*

American CPV Orange, CA, 

USA

Amonix Lens, 

pedestal

Torrance, CA, 

USA

+240 kW 

(multijunction)

~14 MW (Si)

3 MW

>35 MW 

under 

construction

~100 MW/y

Angelantoni 

Industrie Lens Italy

Arima Ecoenergy Lens, 

pedestal

Taipei, 

Taiwan 330 kW 7 MW/y

Becar-Beghelli Reflective Italy prototypes

Boeing (recently 

sold to SES)

Mirror, 

Pedestal

Seal Beach, 

CA, USA

BSQ Solar Spain

CBF Engineering Refractive Vicentino, 

Italy

Chengdu Zsun Lens, 

pedestal

Chengdu, 

Sichuan, 

China

Circadian Solar Coventry, UK

CompSolar 

(Compound Solar 

Technology Co.)

Refractive & 

reflective

designs

Hsinchu 

Science Park, 

Taiwan

32 kW 30 MW/y

Concentracion 

Solar La Mancha

Lens, 

pedestal

Ciudad Real, 

Spain 11 MW/y

Concentrating 

Solar Systems

Bangalow, 

Australia

Concentrating 

Technologies

Small mirror, 

pedestal Alabama

Concentrix Solar

(recently 

purchased by 

Soitec)

Lens, 

pedestal

Freiburg, 

Germany 600 kW 2 MW

150 MW 

announced 

through 2015

25 MW/y

Cool Earth Solar Inflated 

mirrors

Livermore, 

CA, USA

Daido Steel Lens, 

pedestal

Nagoya, 

Japan 30 kW 100 kW

Delta Electronics Lens, 

pedestal Taiwan >2 MW/y

Edtek

Mirror, 

pedestal, 

hybrid

Kent, WA, 

USA

EMCORE Lens, tilt & 

roll

Albuquerque, 

NM, USA

1 MW, original 

design See Suncore 10 MW/y

ENEA Lens, Si cells, 

pedestal Portici, Italy

Energy 

Innovations

Lens, each 

module 

tracked

Pasadena, 

CA, USA 13 kW 300 kW ~ 20 MW/y

Enfocus 

Engineering

Lens, flat 

pivot

Sunnyvale, 

CA, USA

Entech Lens, 

pedestal

Keller, TX, 

USA

ESSYSTEM

Lens, 

pedestal 

(Green & 

Gold)

Gwangju-city, 

Korea
9

Company Type of 

System Location On Sun in 

2009*

Installed 

in 2010*

Completed 

or in 

Progress in 

2011*

Manufacturing

Capacity*

EverPhoton Lens, 

pedestal

Taipei, 

Taiwan

Green and Gold 

Energy

Lens, 

pedestal

South 

Australia 150 MW/y**

GreenVolts Small mirrors, 

carousel

San 

Francisco, 

CA, USA

Guascor Foton Lens, 

pedestal

Ortuella, 

Spain

12 MW 

(Si-based,

Amonix)

15 MW/y

Helios Solar CPV Lens (Green 

& Gold)

Denver, CO, 

USA

Heliotrop Lens, 

pedestal France

small module 

prototype in 

2009

30 kW 

planned 1 MW/y

Huanyin 

Electronic

Jiangsu, 

China

IBM Lens Armonk, NY

Isofoton Lens, 

pedestal

Malaga, 

Spain

400 kW 

Puertollano 30 kW 10 MW/y

Jiangsu White 

Rabbit Lens Jiangsu, China

Menova Energy Fresnel 

reflector

Markham, 

ON, Canada

Morgan Solar

Lateral 

photon 

collection

Toronto, ON, 

Canada

MST Lens, 

pedestal

Rehovot, 

Israel 50 kW Setting up manufacturing

OPEL 

International

Lens, 

pedestal

Shelton, CT, 

USA ~400 kW 0.3 MW 3 MW/y

Pirelli Labs Lens, 

pedestal Milan, Italy 7 kW

Pyron Solar Lens, 

carousel

San Diego, 

CA, USA 20 kW

Rehnu Dish Tucson, AZ 0.5 kW

Renovalia Madrid, Spain

SahajSolar Lens Gujarat, India

Scaled Solar Dish

San 

Francisco, 

CA, USA

Semprius Microlens Durham, NC, 

USA

small 

systems

Shanghai 

Solaryouth Lens Shanghai, China

Shap Reflective Rome, Italy

Sharp Lens, 

pedestal Japan

Sol3g Lens, 

pedestal

Cerdanyola, 

Spain 12 MW/y

Solar Systems

Dish, 

pedestal; 

developing 

central 

receiver 

(heliostat)

Victoria, 

Australia 1.3 MW 5 MW/y
10

Company Type of 

System Location On Sun in 

2009*

Installed 

in 2010*

Completed 

or in 

Progress in 

2011*

Manufacturing

Capacity*

SolarTech Lens, 

pedestal

Phoenix, AZ, 

USA

Solar*Tec AG Lens, 

pedestal

Munich, 

Germany

SolarTron Energy 

Systems Small dish Nova Scotia, Canada

Solergy Glass lens Piedmont, CA 100 kW

SolFocus Small mirror, 

pedestal

Mountain 

View, CA, 

USA

500 kW 1.2 MW 1.4 MW 50 MW/y

Soliant Energy

(purchased by 

EMCORE)

Lens, flat 

pivot

Pasadena, 

CA, USA 100 kW

Soltec Energias 

Renovables Reflective Spain

Spirox Lens, 

pedestal 

Hsinchu, 

Taiwan 6.5 kW

Square 

Engineering

Lens, side 

support Pune, India

Sun Synchrony Miniaturized 

reflectors

Alameda, CA, 

USA

Suncore 

Photovoltaic 

Technology

(Sanan 

Optoelectronics & 

EMCORE)

Lens, 

pedestal Fujian, China 3 MW

Plan 200 MW/y 

capability by 

end of 2011

SunCycle Rotating 

lens/mirror

Eindhoven, 

Netherlands 0.5 kW Plan product launch in 2011

SUNRGI Lens Hollywood, 

CA, USA

Suntrix Lens Shanghai, 

China

Xtreme 

Energetics

Two designs: 

central station 

and rooftop

Livermore, 

CA, USA

Zenith Solar Dish, hybrid Nes Tziona, 

Israel 70 kW

ZettaSun Lens, internal 

tracking Boulder, CO

Zytech Solar Reflective Zaragoza, 

Spain

Totals 14 MW (Si) 

5.5 MW (MJ) 10 MW

~30 MW

Plans for 

~200 MW

*Based mostly on public presentations or website announcements/press releases. Note that some companies refrain 

from posting information about their deployments, so the lack of a number may not mean that they have made no 

installations.

**Includes capacity of Green and Gold Energy technology through ES System, Energies AC Gava, Square 

Engineering, Solar Ace, and Zolar Distributors.

Most PV technologies have required years of development before showing success on a large 

scale. First Solar’s rapid expansion was based on years of development work. As noted above, 

the multijunction CPV industry may be preparing to emerge from the development phase. As the 

CPV companies transition from the prototyping phase of development to scaling up 
11

manufacturing, they will encounter the standard problems. The following discussion reflects the 

concerns that have been raised by industry participants during discussions related to this study. 

Prototype Development

CPV companies are exploring a wide range of CPV approaches. Each has done its own 

assessment of which designs will give the best performance, lowest cost, and longest reliability. 

The range of types of designs continues to expand. Primary considerations include:

• Performance: Optical efficiency, cell cooling, and performance losses associated with 

manufacturing imperfections, soiling, tracking errors, flexing in the wind, thermal 

expansion/contraction, or wind stow.

• Cost: Use of inexpensive components, ease/automation of assembly.

• Reliability: Degradation of optics, poor performance of tracker or other loss of alignment, 

loss of adhesion or breakdown of bonds between cell and the optics and heat sink, etc.

These considerations are often interlinked, with improvements in performance and reliability

also causing an increase in cost. Companies have demonstrated that each of these goals can 

be achieved separately; recently we have observed the demonstration that all three can be 

achieved simultaneously as CPV companies have begun to be awarded contracts for > 10 MW 

fields. 

Prototype Testing

Many of the companies have one or multiple prototypes in the field. Initial prototypes are usually 

on the order of 1 kW in size, with subsequent prototypes in the 2–30-kW range. Others are 

moving on to manufacturing and >1-MW installations.

After designing and assembling the prototypes, the most immediate need of many of the 

companies is testing. Testing needs may be broken into two parts: the first quantifies the 

performance and identifies opportunities for improving performance; the second assures that 

the performance is stable, preferably over decades of use. The initially measured performance 

is usually lower than is hoped for. Identification of the cause of the performance loss can be 

complicated. 

Some of the types of diagnostics include:

• Low short-circuit current

- Optical losses (may be caused by soiling of optics, condensation within the module, 

imperfect optical interfaces, manufacturing imperfections, misalignment)

- Mismatch of multijunction cell design with observed spectrum. This can be 

complicated to diagnose because it may vary with time of day and cell alignment. It is 

best diagnosed with a single lens-cell assembly by monitoring the fill factor 

throughout a sunny day.[21]

- Misalignment of cell with optics or poorly designed optics so that some of the light 

misses the cell, or misalignment of tracker.

• Low open-circuit voltage

- Poor heat-sink design can be detected quickly by measuring the heat-sink 

temperature

- Poor thermal contact between cell and heat sink.
12

• Low fill factor for string of cells

- This can result from inconsistencies in the alignment or from inconsistent component 

quality. The acceptance angle (measured at the maximum power point) of a singlelens cell assembly should be similar to that of a string of cells. If the acceptance 

angle for the string is larger, or if the operating temperature of the cells is not the 

same for all cells, there may be some variation in the alignment. A quick way to 

identify variations is to look for bypass diodes that are activated, and especially to 

see if different bypass diodes are activated as the alignment is changed or the 

spectrum varied.

- Variability of the optical transmission or the solar cell performance may also cause 

lower fill factors. Again, looking for the activated bypass diodes will help to identify 

the problematic lenses or cells.

- If the fill factor is low because of a series-resistance problem, this can quickly be 

distinguished from the above problems. Poor electrical connections, inappropriate 

cell design, or non-uniform illumination of the cells are common causes.

The above list is not meant to be an exhaustive guide to identifying causes of poor performance, 

but gives a sense of the many ways that the performance can be compromised. 

Most companies are testing prototypes and would like to accelerate reliability testing. Many of 

the stress tests are designed to run over several weeks. If these could be replaced by faster 

accelerated tests, testing cycles might be reduced to less than a week. For example, higher 

temperature and humidity could be applied in a slightly pressurized system. Unfortunately, the 

technical basis for this sort of acceleration has not been established. Some efforts to do this 

have concluded that the use of harsher conditions for a shorter time can expose failure modes 

that are not observed in the field, defeating the purpose of the tests. 

There is concern that failures in the field for even a single company could discredit the entire 

CPV industry. Sharing observations of failures can facilitate early detection of failures, reducing 

the probability of premature deployment, but companies are often reluctant to do so. In 2008, 

the Accelerated Aging Workshop, which was sponsored and organized by the U.S. Department 

of Energy (DOE) and the national laboratories, included a breakout session for the CPV industry

(see p. 46). It was suggested that the national laboratories should place the highest priority on 

the cells, bonding, and packaging, although a myriad of other concerns were also expressed.[22]

The Photovoltaic Reliability Workshops in Feb. 2010 and in Feb. 2011 also included break-out 

sessions on CPV that discussed spectral issues, quantitative predictions using the weather to 

predict lifetime of the cell attachment, revisions of the thermal cycling qualification test, etc.

Some testing standards are available, but the standards for CPV are behind those for flat-plate 

PV. Table 4 summarizes a few of the key IEC standards for PV and tabulates those that have 

CPV versions. Clearly, the CPV industry and customers must work together to establish CPV 

versions of the standards to form the foundation for the emerging CPV industry.

The international community has not developed a consensus about the irradiance condition for 

defining a power rating. The IEC TC82 WG7 is moving toward defining a standard test condition 

(1000 W/m2

, 25°C cell temperature) and a standard operating condition (900 W/m2, 20°C 

ambient temperature).
13

Table 4. Summary of Standards

Silicon PV Standard Corresponding CPV Standard

IEC 60904 – Photovoltaic devices. 

Part 1: Measurement of photovoltaic currentvoltage characteristics.

Part 2: Requirements for reference solar devices.

Part 3: Measurement principles for terrestrial 

photovoltaic (PV) solar devices with reference 

spectral irradiance data.

Part 5: Determination of the equivalent cell 

temperature (ECT) of photovoltaic (PV) devices by 

the open-circuit voltage method.

Part 7: Computation of spectral mismatch error 

introduced in the testing of a photovoltaic device.

Part 8: Measurement of spectral response of a 

photovoltaic (PV) device.

Part 9: Solar simulator performance requirements.

Part 10: Methods of linearity measurement.

Each of these building blocks is being 

addressed as the more complex 

standards are developed (see below).

IEC 61215 – Crystalline silicon terrestrial PV 

modules. Design qualification and type approval.

IEC 62108 – CPV modules and 

assemblies. Design qualification and type 

approval.

IEC 61853 – Photovoltaic (PV) module 

performance testing and energy rating. Part 1: 

Irradiance and temperature performance 

measurements and power rating (Committee draft 

is approved).

Draft under development: IEC 62670. 

Power rating for CPV. In addition, 

technical specifications for an acceptance 

test and for use of an average 

performance ratio to define an energy 

rating.

IEC 61730 – PV module safety qualification Draft under development: IEC 62688

UL 1703 – Flat-plate photovoltaic modules and 

panels

Draft under development: UL 8703 –

Concentrator photovoltaic modules and 

assemblies; STP formed in late 2009.

Manufacturing Scale-Up and Retesting

After reliable prototypes have been demonstrated, companies must automate the manufacturing 

and then retest the reliability to ensure that subtle changes in the design do not negatively 

impact reliability. Some of the companies have planned for high-volume manufacturing from the 

start, but all companies must include this step in their development plans at some stage.

The details of high-volume manufacturing will be key toward cost reduction. Automated 

manufacturing of complete systems under a single roof will take substantial effort to set up, but 

may show significant advantages in the long run. Most companies have found that preassembly 

can greatly reduce installation costs.

Some recent advances include:

• In May 2011, Amonix cut the ribbon on a new manufacturing plant in Las Vegas, 

Nevada, that can be ramped to 150-MW/y production.

• In May 2011, DOE announced a loan guarantee of $90 million to Cogentrix of Alamosa, 

Colorado, for construction of a 30-MW HCPV generation project using Amonix 

technology.
14

• In April 2011, Suncore held a symposium discussing CPV and describing the 200-MW/y 

manufacturing plant it is building in Huainan, China.

• In March 2011, Soitec announced that its Concentrix technology was chosen by 

Tenaska Solar Ventures for a 150-MW installation, and that it will be building a 

200-MW/y manufacturing plant in the San Diego, California, area.

• In November 2010, Southern California Edison signed a contract with Amonix to deliver 

28.5 MW of CPV by 2014.

Performance (Power) Rating

A power rating is traditionally used as a nameplate rating and is useful for sizing of inverters and 

other system parts as well as for verification of system delivery under some contracts. The IEC 

Technical Committee 82 Working Group 7 has elevated the power rating to the highest-priority 

need. Both indoor and outdoor measurement procedures are being defined.

Energy rating is most important for power-purchase agreements and utility applications. The 

methods for determining these ratings are still being debated. The methods used for predicting 

energy production for flat-plate systems are sufficiently documented to satisfy most investors, 

but investors have much less confidence in similar predictions for CPV systems. This puts CPV 

companies at a disadvantage for some applications. Pierre Verlinden of Amrock Pty Ltd has 

proposed that the electricity generated over a year’s time be measured and compared with the 

same year’s irradiance. This approach is related to the “performance ratio” measurement 

described in IEC 61724.

It is useful if the metrics used for CPV are relatively consistent with those used for flat-plate PV 

and that they are logical. For example, the peak-watt rating is generally assumed to imply the 

highest performance observed for a module or system. If the performance routinely exceeds the 

peak-power rating, the inverters and other aspects of the system must be appropriately sized. 

Some locations routinely experience DNI values of ~1000 W/m2

. 

For modeling of expected performance at a new location, a useful tool would be a model that 

could take readily available data and create a set of hourly data for the direct spectrum, 

temperature, and wind speed. If the model were created, such data could be generated to 

represent an average day for each month of the year for any site in the United States. Tools for 

estimating energy production (e.g., PVWatts) are available for flat-plate systems and might be 

extended to CPV systems. Efforts are under way to improve the modeling for CPV in NREL’s

Solar Advisor Model.

Some companies are interested in solar resource data for Spain and other locations outside the 

United States. Such data exist, but this information is not widely available. The direct solar 

resource is strong in southern Spain, but is significantly reduced toward the northern part of the 

country.

The National Solar Radiation Data Base and other solar resource data that include the direct 

resource usually include the circumsolar resource, which most high-concentration CPV systems 

cannot use. The importance of this effect has not been quantified, although anecdotal 

information implies that it can be significant in locations with pollution or other sources of haze 

that cause small-angle scattering. 
15

Cell Supply

The availability of concentrator cells has been a concern, but this has not been a problem in 

2011. Spectrolab, EMCORE, and Azur Space have been shipping concentrator cells to multiple 

CPV companies. A significant number of new companies have demonstrated the capability for 

epitaxial (single-crystal) growth of multijunction cells. They are summarized in Table 5. 

Table 5. Summary of Companies with Capability for Epitaxial Growth of Multijunction Cells

Company 

Name/Web Link Location Comment

Arima Taipei, Taiwan Reported achieving >40% cells.

Azur Space (RWE) Heilbronn, Germany Commercial product ~40%; champion 41.2%.

[23]

CESI Milano, Italy Datasheet reports 38% efficiency.

Compound Solar 

Technology

Hsinchu Science Park,

Taiwan

Website shows I-V curve with 33.4% efficiency.

Cyrium Ottawa, Canada Datasheet describes typical >39% cells.

EMCORE Albuquerque, NM, 

USA

Datasheet describes typical 39% cells and 

receivers at ~500 suns. 

Epistar Hsinchu, Taiwan Multijunction cells are in development.

IQE Cardiff, Wales, UK Has demonstrated state-of-the-art efficiencies.

JDSU Milpitas, CA, USA Advertises multijunction concentrator cells on 

website, claiming efficiencies approaching 40%.

Microlink Devices Niles, IL, USA Multijunction cells removed from substrate in 

development

Quantasol Kingston upon 

Thames, Surrey, UK Multijunction cells with quantum wells, claim ~40%

RFMD Greensboro, NC, USA Multijunction cells in development

Sharp Japan

Has demonstrated high efficiencies, but has not 

indicated plans for commercialization outside of 

supplying cells for its own CPV systems.

Solar Junction San Jose, CA, USA Announced 43.5%, NREL confirmed

Spectrolab Sylmar, CA, USA Is selling 40% product. Shipped ~35 MW in 2009, 

and ~100 MW in 2010 (@500X).

Spire (Bandwidth) Boston, MA, USA Announced 42.3% efficiency, NREL confirmed

VPEC Ping-jen city, Taiwan Multijunction cells in development

In April 2011, at CPV7 in Las Vegas, Nevada, USA, the following achievements were 

presented:

• Spectrolab and Azur both described commercial products with 40% specification.

• For the first time, a metamorphic cell has been released as a commercial product—this 

is Spectrolab’s most recent product release.

• Spire presented its 42.3% efficiency.

A quick review of the companies in Table 5 implies that the supply of cells could be expanded 

quickly. The entry of large companies such as RFMD and JDSU could bring the experience of 

the larger industry for making cheaper cells. Essentially all of the companies in Table 5 can 

fabricate cells with efficiencies greater than 30%; some have demonstrated efficiencies 

approaching or exceeding 40%. Although all of the companies on this list have some capability 

for growing multijunction cells, not all of them have demonstrated a capability for high-yield 

manufacturing.
16

The most immediate concern about the concentrator cells expressed by CPV representatives is 

whether the reliability testing is adequate. Both Spectrolab and EMCORE report that they have 

tested the cells and are confident of their stability and performance, but most CPV 

representatives were not satisfied with the detail of the test data. More than three years of field 

data is now available, and, so far, no degradation has been directly linked to the cells (though 

failures of the packaging of the cells are known.)

The injection of forward-bias current during thermal cycling is observed to damage some cells. 

Two studies presented at CPV-7 concluded that the cause of the damage could not be linked to 

defects in the cells, and that the cell failures appear to be caused by voids under the busbars 

leading to thermal runaway in the cells.[24]

The existing qualification standards may or may not identify all of the degradation modes. High 

solar fluxes may be more harmful to encapsulant materials than to the semiconductor material. 

Si modules are known to exhibit corrosion associated with moisture ingress near the Ag grid

lines. Thus, CPV cells with Ag grid lines could experience similar corrosion. Nevertheless, if 

CPV cells are operated in hot, dry climates, moisture ingress may be less of a problem. A 

technical basis has not yet linked the standard damp heat (85°C/85% relative humidity) with 

field performance for CPV systems. Until the correlation between accelerated testing and fieldtesting is established, most CPV companies are applying the standard damp heat test to identify 

potential failures.

The current cell production capacity exceeds the CPV installation rate by a factor of about 10, 

so cell availability is not an immediate concern. In the event of a rapid growth in demand for 

multijunction cells, the situation could quickly evolve into that which is currently observed for the 

silicon PV industry: Companies must plan on negotiating firm multiyear contracts so that the 

semiconductor suppliers can appropriately plan and finance their expansion. Expansion of the 

manufacturing volumes should allow reduction in cost because of economies of scale. 

Cell Efficiencies

Cell efficiencies have been increasing at a rate of about 0.5% to 1% per year in recent years. 

See Table 2 and Fig. 1 for summaries of champion efficiencies. Efficiencies are expected to 

continue to increase toward 45%–50%. 

The trade-off between cell cost and cell efficiency is highly dependent on the relative costs of 

the cells and the systems. A simplistic analysis is shown in Fig. 3. The cell cost in $/W is 

strongly dependent on concentration. EMCORE reported a sale to Green and Gold at 

$24 million for 105 MW, which translates to $0.23/W for a concentration ratio of 1100. The cell 

costs of $0.50/W and $0.10/W represent the high end of what EMCORE is currently delivering 

and lower costs that might be achieved, respectively. The $1,000/m2 area non-cell cost 

potentially includes not only the module costs, but also installation and land-use costs. Lower 

costs will need to be achieved to be competitive in the marketplace; the $100/m2 target is 

aggressive, but demonstrates how the role of cell efficiency changes when the system cost 

becomes dominated by the cell cost. Clearly, for $1,000/m2 systems, efficiency is a strong cost 

driver. But, if the balance-of-system cost can be reduced to $100/m2 without change in cell cost, 

then efficiency becomes less important. The evaluation of the importance of cell efficiency and 

cost is fairly straightforward once the system design (especially the concentration) is fixed and 

the relative costs are known. An example equation is included in the Fig. 3 caption. This 

analysis assumes that cell cost is fixed. In practice, more efficient cells tend to cost more, 

implying that the curves in Fig. 3 would be flatter in a specific scenario.
17

Figure 3. Total system cost as a function of cell cost (either $0.10/W or $0.50/W) and non-cell costs 

(taken in the range of $100–$1,000/m2

). The system power was decreased from 850 (standard reporting 

conditions) to 690 W/m2 to account for optical and thermal losses. The equation used to calculate these 

data was Cost ($/W) = Area cost ($/m2

)/Efficiency X 690 (W/m2)) + Cell cost ($/W). The definition of cell 

cost in $/W has 20%–35% uncertainty, because it may or may not account for optical and/or thermal 

losses.

Substrate Supply

The manufacture of multijunction space cells in the last decade has been based primarily on 

germanium wafers supplied by a single company: Umicore (Brussels, Belgium). Now, multiple 

companies are developing a germanium wafer capability, including AXT (Fremont, California); 

Sylarus (St. George, Utah); and PBT (Zurich, Switzerland). Umicore has completed a plant in 

Quapaw, Oklahoma, to help service this growing market. In addition, if the inverted method[17] of 

fabricating the multijunction cells or other approaches that make possible reuse of the wafers 

(e.g., Microlink, Semprius) become popular, the substrates may be reused or the material 

recycled. Some of these approaches use GaAs instead of Ge. Although it is possible that the 

industry could be so successful as to create a shortage of wafers, this is not currently on the 

horizon.

Germanium (Ge) metal is obtained principally as a by-product of zinc refining or coal-burning 

(recovered from the fly ash). In 2007, Ge suppliers produced about 100 metric tons, most of it in 

the form of germanium tetrachloride (GeCl4) and germanium dioxide (GeO2).[25] Canada and 

China are the world's largest Ge sources, each supplying more than one-third of world 

production. Mining companies indicate there is a 50-year known reserve at today's consumption 

rate, and that this reserve does not include vast new reserves available in Africa (especially the 

Democratic Republic of Congo). The major Ge consumers in 2007 were fiber optics (35%), 

infrared optics (30%), PET catalysts (15%), and electronics and solar applications (15%).[25]
18

Wafer-industry experts tell us there is sufficient Ge to support a CPV installation rate of 

~4 GW/yr. Industry experts also point out that a significant Ge consumer, PET plastics, is 

moving aggressively to replace Ge with lower-cost catalysts, and at least two Chinese PET 

manufacturers have reported using a titanium-based solution.[26] It is significant that the PET 

catalyst percentage of the Ge market has declined from 31% in 2005 to 15% in 2007.[27] As 

worldwide Ge production increases and PET demand diminishes, the experts contend that there 

will be ample Ge available to support even the most optimistic terrestrial III-V CPV market 

scenarios through 2030 and beyond.

Optics

The primary concerns expressed about the optics are related to the reliability. Yellowing or 

pitting of plastic lenses, the need for washing, etc., are all concerns. Some companies are using 

glass lenses to avoid the abrasion expected for plastic lenses. The availability of optics was not 

raised as a concern; however, the identification of the best suppliers for the optics part of the 

supply chain has continued to be a challenge for the community, especially as companies are 

still trying to define their choices of optical design. 

Most optical designs include both primary and secondary optics to increase tracking and 

alignment tolerance, although some companies have chosen to avoid the cost of an optical 

secondary by carefully maintaining alignment quality and sacrificing a few percent in 

performance under some circumstances.

For the primary optic, the majority of companies have chosen to use lenses rather than mirrors. 

In general, the direct-transmission approach simplifies the optical design and facilitates passive 

cooling, reducing design and maintenance complexity. Historically, companies have favored the 

use of acrylic in the lenses, with injection molding providing a cost benefit at the highest 

volumes (embossing provides a cost benefit at lower volumes). There is also strong interest in 

using glass to reduce abrasion and increase lifetime. Currently, there is increased discussion of 

the use of silicone-on-glass lenses, which provide the benefit of excellent durability with ease of 

manufacture, however require some special design to avoid loss of alignment at lower 

temperatures.

[28,29] All-glass lenses are more difficult to manufacture. Additional (beyond 

abrasion and ease of manufacture) considerations include: quality of manufacture, retention of 

alignment at all temperatures and humidities, chromatic aberrations (which may be avoided to 

some extent by using total internal reflection), absorption losses, adhesion (for silicone-on-glass 

lenses), and sensitivity to UV-induced degradation.

The fraction of companies using reflective designs is relatively small, but reflective designs can 

have the potential to be lower in cost if they use low-cost reflectors. If the control of the shape of 

the mirror is near perfect, reflective designs reach higher concentrations than some refractive 

designs because of the avoidance of chromatic aberrations. Thermal management designs 

associated with reflective optics are more likely to use active cooling. (Active cooling has the 

disadvantage of added maintenance and parasitic power consumption, but may have the 

advantage of being able to keep the cells cooler than passive designs on hot days.) Creativity 

can help to reduce shading losses associated with placement of the cooling systems.

The secondary optics are sometimes exposed to ~100 W/cm2 intensities, implying that any 

absorption can cause large increases in temperature (in some cases vaporizing polymeric 

materials). Even if the secondary optic is 100% transparent, it may run hot because of being 

attached to the cell, which may operate 40°C or more above ambient temperature. The 

secondary optics must be able to withstand both high temperatures and the potential stress 
19

from differential expansion if the temperature is non-uniform. If the secondary optic becomes 

soiled, the associated heating can lead to catastrophic failure. The secondary should also be 

designed to maintain the highest possible optical efficiency, even when the system is misaligned 

for some reason. Reflective secondaries that redirect off-target light may have no impact on the 

optical efficiency as long as the system alignment is maintained. Refractive secondaries 

typically cause a reduction in optical efficiency by a few percent, but usually increase the energy 

production enough to justify their use if their cost is acceptable. The expected UV stress on 

secondaries is especially problematic for designs using reflective primary optics. Most lenses 

absorb UV strongly, preventing these harmful rays from reaching the optical secondaries. 

Trackers

Although industry representatives did not describe trackers as a serious problem, trackers are 

known to require periodic maintenance, and glitches in performance or outright mechanical 

failure can decrease performance and increase maintenance costs substantially. The Institute of 

Concentration Photovoltaics (ISFOC) reports that trackers currently account for >50% of 

observed problems in the field.

[30]

Some companies expressed the desire for standardization and the associated reduced cost. As 

flat-plate companies have increased their use of trackers, the number of companies supplying 

trackers has also increased. A standard to specify the attributes of a tracker and how to 

measure these is being drafted by IEC TC82 WG7.

Trackers are also in demand for flat-plate and solar-thermal applications. In recent years, there 

is evidence that the community’s investment in trackers is improving performance and reducing 

costs. An interesting trend is a small movement toward smaller trackers, which leverage designs 

for concentrating solar thermal heliostats. An example is Energy Innovations’ 29% module that 

is designed for mounting on small trackers, leveraging heliostat experience from eSolar, a sister 

company.

Power Electronics

As DC-DC converters have become cheaper, more efficient, and with excellent reliability (e.g.,

DC-DC converters are used in laptops to convert the varying battery voltage to the voltage 

needed to run the computer), interest has grown in using them for PV modules. For CPV, there 

is special benefit to using them for two reasons. (1) It can be a challenge to create a dish with 

uniform irradiation on a central receiver; use of DC-DC converters could allow the image on the 

central receiver to be non-uniform without substantial loss of performance. (2) Whereas tracked 

flat-plate systems can use back tracking to avoid shading early and late in the day, highconcentration CPV systems must be 2-axis tracked and, thus, must experience shading when 

the sun is low in the sky. Use of DC-DC converters could avoid dramatic losses associated with 

this shading, so could enable a field with more closely spaced CPV pedestals.

Cell Bonding and Encapsulation

The bonds between the cell and heat sink and between the cell and the optics (or air) can be 

problematic. Many of the companies report degradation of these bonds during stress testing and 

have had to study multiple designs. One study reported subjecting five encapsulant materials to 

the equivalent of 20 years of UV exposure, and found only one that did not degrade.

[31] Optical 

coatings may, for example, darken over time or trap moisture and accelerate degradation. A 

wormlike bubble has been found at the interface between the cell and the secondary optics. The 

cell suppliers and system integrators need to work together to understand potential issues here, 
20

but concerns over competition and protecting proprietary processes inhibit the necessary 

disclosure and cooperation. 

Weathering from sunlight is well known; when the sunlight is concentrated 1000 times, or even 

higher locally, the associated weathering problems can be severe, although much of the UV 

light may be absorbed by the optics before reaching a sensitive component.

[32] Accelerated 

testing of the effect of concentrated light is especially challenging and has not been well 

defined.

Cell Assembly/Receiver Fabrication

The solar cells must be attached to a heat sink and electrical connections completed. In most 

cases, the resulting piece is called a receiver or cell assembly. Most of the cell companies have 

developed a couple of standard concentrator cell assembly/receiver designs. Ideally, cell 

assemblies can be tailored to match each CPV optical design. For each design, the assembly 

equipment must be automated and the final product carefully tested. Although more than a 

dozen companies are developing a cell capability and more than 30 companies are developing 

CPV systems, far fewer companies (in addition to the cell companies) are marketing 

multijunction CPV cell assemblies. ENVOLTEK is one of the few CPV receiver assembly 

suppliers.

The expertise needed to create these cell assemblies is fairly well established in the LED 

industry, which represents a business opportunity for such companies. In the long run, it is 

probable that entities with cell assembly capabilities will be targeted for acquisition, as the 

industry later moves toward vertical integration. It is not yet clear whether it is better for the cell 

mounting to be done by the cell companies or another company in the supply chain.

Enclosure Design

The system enclosure must be designed to avoid dirt burning onto the optics and moisture 

condensation that can either obscure the optics or “fry” the cells. Although this appears to be a 

mundane problem, it is quite challenging. If the enclosure is sealed, atmospheric pressure 

variations can cause the optics to deform like a balloon. If the enclosure does not breathe well, 

the optics may act as insulation, causing the cells to run hotter. 

The companies are experimenting with many approaches to this, including desiccant and active 

ventilation. One interesting approach is to use material that blocks transmission of liquid water, 

but allows water vapor to be transported, such as the membranes made by Gore.

Skilled Labor

The availability of appropriately skilled labor is a challenge for all of the CPV companies. 

Nevertheless, individuals with experience working with LEDs, optical design, reliability testing, 

etc., are making important contributions to developing CPV prototypes. This difficulty is shared 

across the board among renewable energy firms today. 
21

Utility Interactions

Electricity bills use a variety of algorithms for defining charges. An understanding of these is 

necessary to calculate payback times for installations in different billing areas. Some of the 

companies expressed a desire to have this information compiled for easy access.

Material Availability Limits

Projections of materials availability are always complicated by the potential development of new 

mining techniques driven by increased demand. Nevertheless, raw material costs have been 

rising lately. Here, we reference a study by Feltrin and Freundlich (Fig. 4).[33] Their use of 200X 

as the concentrating factor is conservative compared with what most companies are currently 

pursuing (500X–1000X). The first bar implies a fairly severe limitation regarding the availability 

of Ge, based on U.S. supplies. Compared with the first bar, the second bar implies 60 times 

higher availability, this time limited by Ga availability. The third bar in Fig. 4, labeled “EPI Liftoff,” is potentially relevant to the inverted, metamorphic approach,[17] with availability of indium 

as the limiting factor, allowing four times higher production than indicated by the second bar. 

More studies of this sort are needed to gain confidence in the conclusions, but these data imply 

that material availability will not prevent the success of CPV.

Figure 4. Material availability study from Ref.[33] (A. 

Feltrin and A. Freundlich, "Material Challenges for 

Terawatt Level Deployment of Photovoltaics," 

Conference Record of the 2006 IEEE 4th World 

Conference on Photovoltaic Energy Conversion, ©2006 

IEEE, Reproduced with permission.) The dotted box 

includes the supplies they estimated would be available 

worldwide.

Summary

The use of concentrated sunlight on very small, but highly efficient (~40%) solar cells has the 

potential to provide cost-effective, large-scale, solar-electricity generation, especially in sunny 

locations. More than a dozen companies have learned to fabricate multijunction concentrator 

cells, positioning themselves to respond to the growing demand for these cells. About three 

dozen companies are developing concentrator photovoltaic systems, and several have already 

deployed >1 MW in the field. This industry is showing signs of being poised for substantial 

growth in the next years as the world enthusiastically embraces solar energy. 
22

Part II. Medium-Concentration Approaches Using Silicon or Other 

Cells

The silicon PV industry has grown dramatically in recent years. The industry is working hard to 

cut costs for every step of the manufacturing and installation processes. Significant effort has 

focused on thinning the silicon wafers in order to reduce the usage of silicon material. A 

complementary approach is to reduce the area of silicon needed by using optics to redirect the 

light toward smaller cells. This provides the possibility of much more dramatic reduction in the 

use of silicon and also allows the possibility of decreased cost for the non-silicon costs 

associated with the cells. (The non-silicon costs can be half of the total cell cost and may 

actually increase rather than decrease as the silicon cell is thinned). Thin-film PV such as 

copper indium gallium diselenide (CIGS) or cadmium telluride (CdTe) may also be used in CPV, 

but will not be discussed in this report. 

The use of silicon, instead of III-V multijunction, cells leverages the huge investment already 

made in the silicon supply chain. Although the silicon cells must be able to handle the higher 

currents, most of the elements of the supply chain are unchanged. This reduces both the 

development time and cost for new products. 

Perhaps the more significant advantage of using the medium-concentration approach is the 

divorce it brings from the silicon supply chain. In 2007 and early 2008, PV industry growth was 

limited by the community’s inability to predict the need for purified silicon and to create the 

investment needed for the appropriate scale-up. In an uncertain, and risk-adverse, investment 

climate, investors are likely to be attracted to approaches that reduce the required capital 

expenditure and, especially, a capital expenditure that must be made for growth predicted far 

into the future. The capital expenditure may be reduced all along the supply chain for the Si 

poly, ingot, wafer, and cell manufacturing. The scalability of products depending primarily on 

glass, metal, and plastic (instead of cells) may enable growth of a silicon-based CPV industry, 

especially as there is evidence that this approach is getting attention by some mainstream 

companies in recent years.

In his opening talk to the CPV6 conference, Richard Swanson noted that one change in the last 

ten years is the availability of a low-cost, high-efficiency silicon cell that would be appropriate for 

medium-concentration designs. He noted that SunPower was first founded as a CPV company, 

studied microconcentrators in years past, and is continuing its interest in various CPV 

approaches as was described in a paper at the PVSEC, 2010.

[34]

Some investors see a medium-concentration, silicon-based product as less risky than highconcentration CPV. Using familiar cells and low-accuracy trackers may be perceived as more 

“bankable” than the high-efficiency, disruptive approach described in Part I. Higher risk 

translates directly to a need to demonstrate a lower cost in order to interest the investors. 

Although the primary semiconductor cost reduction is achieved with even a small concentration 

of light, a medium concentration allows use of slightly more expensive, but more efficient, cells. 

Just as efficiency can be leveraging for HCPV (see Fig. 3), the higher efficiency can be 

important for silicon-based CPV.

The possibility of increased performance must be balanced with the loss of solar resource that 

comes from a reduced use of diffuse light. The maximum acceptance angle is a function of the 

concentration and the index of refraction of the medium.[35] Specifically, for a linear 
23

concentration ratio, C, and index of refraction, n, the theoretical maximum acceptance angle, θ, 

can be found from 

C=n/(sin θ).

For point-focus systems, this concentration may be achieved in both dimensions, implying the 

square of the above concentration may be reached. For fixed systems, a small acceptance 

angle can dramatically reduce the available resource. For 2-axis tracked systems, and low 

concentration ratios, the reduction in the available resource may be less than 10%. The 

maximum acceptance angle that can be achieved theoretically is plotted as a function of the 

concentration ratio in Fig. 5. Most Si-based CPV systems are able to use the circumsolar solar 

resource (light that is outside of the direct beam, but within a couple of degrees of the direct 

beam). The circumsolar resource varies strongly with location, and can be significant in some 

locations.

Figure 5. The theoretical maximum for the acceptance angle (red curves; left axis) that can be achieved 

as a function of linear concentration and the fraction of diffuse light that can be collected theoretically 

(blue curves, right axis) assuming that the diffuse light is isotropic.

Tracking 

A few years ago, most systems were deployed on rooftops in a fixed configuration, but recently 

the number of systems deployed on trackers has increased. If a tracker is cost effective for flatplate modules, chances are that it can also be cost effective for concentrator modules. Thus, the 

increased use of trackers for flat-plate applications may be paving the way for concentrator 

systems.

A contradictory viewpoint is that trackers will not be used in the future because PV cost must be 

significantly reduced in order to compete with fossil fuels. As the PV module cost is reduced, if 

the tracker cost is not reduced by a similar amount, it may no longer be cost effective to use a 

tracker. Thus, we conclude that low-cost trackers are likely to be key to the success of lowconcentration systems. There is strong evidence that tracker cost is decreasing. At the CPV6 
24

meeting, Swanson estimated that 1-axis trackers (for flat-plate modules) currently cost about 

$80/m2 compared with ~$57/m2 for fixed mounting. 

The tracker accuracy requirement for low-concentration systems may be relaxed (compared 

with those for high-concentration systems), potentially reducing cost, increasing reliability, and 

increasing energy production.

Current Status – Companies Involved

In terms of the number of companies and total investment, the development of mediumconcentration systems currently lags that of high-concentration systems. But the approach has 

attracted significant interest in recent years as silicon PV companies look for creative ways to 

continue to reduce cost. NaREC reports that it has been contacted by ~70 groups in recent 

years, looking for silicon cells that can function well under concentrated light. The approach is 

not new: Entech developed a linear ~20X concentrator system using silicon cells in the 1980s. 

In the 1990s, Entech deployed hundreds of kilowatts of this medium-concentration 

technology.[36] The performance of these was well documented through the PVUSA project, 

demonstrating the highest efficiency of the systems studied. However, it appears that this was a 

technology before its time: The market for tracked systems was very small in the 1990s, and 

Entech needed high volume to achieve competitive costs. After several years of developing 

concentrators for space applications, Entech, in partnership with WorldWater, is now marketing

a modified version of these systems.[37] Although Entech's early efforts did not lead to a 

commercial success, today’s companies can learn much from Entech's early field experience.

BP Solar also developed a linear-focus, medium-concentration system using Si cells. Working 

with the Instituto de Energia Solar within the EUCLIDES project, BP Solar used a reflective 

trough, first demonstrating a single unit and then scaling up to 480 kW with multiple troughs.[38] 

Today’s companies may also learn from the EUCLIDES experience, which suffered from

inadequate design testing before scale-up.

The number of companies working on medium-concentration designs has increased 

significantly in recent years, as shown in Table 6 and elsewhere.

[19] The range of approaches 

extends from the types of systems just described to designs that can function much like flat 

plate, including holographic and luminescent concentrators. Although in the early developmental 

stages, many of these companies are making good progress and are receiving substantial 

public recognition. A number of other companies are not listed in Table 6 at their request.

Solaria has certified its low-concentration design to UL1703 and IEC61215. The company

estimates that it can achieve a cost that is 40% lower than conventional silicon.
25

Table 6. Summary of Companies Developing Low- or Medium-Concentration PV 

Products Using Silicon or Other Cells

(This information changes rapidly. Companies described in gray appear to have moved away 

from this approach, but should not be discounted completely.)

Company Type of 

System Location

On Sun 

in 

2009*

Installe

d in 

2010*

Completed 

or in 

Progress in 

2011*

Capacity*

Aavid 

Thermalloy

Refractive, 

10X Concord, NH

All Optronics 15X Tucson, AZ

Anhui 

Yingtian 

Renewable 

Energy

Reflective Anhui, China >50 kW

Absolicon

Solar 

Concentrator

Reflective 

trough, Si 

cells, thermal 

hybrid

Harnosand, 

Sweden

9 small 

systems are 

documented 

on website 

(since 2006)

Banyan 

Energy

Flat-plate 10X, 

total internal 

reflection, Si 

cells

Berkeley, CA

Cogenra 

Solar

Reflective, 

hybrid PVthermal

Mountain 

View, CA

50–100 

kW

Covalent 

Solar

Luminescent, 

multiple types 

of cells

Boston, MA, 

USA

CPower

Reflective, 

25X–30X 

(point focus), 

Si cells

Ferrara, Italy 9 kW

Entech

Linear Fresnel 

lens, Si cells; 

hybrid PVthermal

Fort Worth, 

TX, USA

Greenfield 

Solar

Reflective, 

edgeilluminated Si 

cells (not 

systems)

Cleveland, 

OH, USA

HyperSolar Optical 

coating

Hackensack, 

NJ, USA

KD Solar Co. Holographic 

3X

Kyunggi-Do, 

Korea

Maxxun Luminescent Eindhoven, 

Netherlands
26

Company Type of 

System Location

On Sun 

in 

2009*

Installe

d in 

2010*

Completed 

or in 

Progress in 

2011*

Capacity*

MegaWatt 

Solar

Reflective, 

linear, 20X, 

pedestal

Hillsborough, 

NC, USA 35 kW

Netcrystal Non-tracking, 

Si cells

San 

Francisco, 

CA, USA

Optoi Reflective, Si 

cells Trento, Italy

Optony Thin-film cells

Silicon 

valley, CA, 

USA

Pacific Solar 

Tech

Dome-shaped 

lens, Si cells

Fremont, CA, 

USA

Pirelli Labs 

(CIFE) Static Milan, Italy

Prism Solar 

Technologies

Holographic, 

Si cells

Lake Katrine, 

NY, USA

Pythagoras 

Solar

Building 

integrated

Hakfar 

Hayarok, 

Israel

QD Soleil Luminescent Palo Alto, CA

Silicon CPV

Fresnel (point 

focus, 120X) 

Si cells

Essex, UK

Skyline Solar Reflective, 

14X, Si cells

Mountain 

View, CA, 

USA

24 kW 83 kW 6 MW

100 MW/y 

be end of 

‘’11

Solaria

2X–3X, small 

strips of Si 

cells

Fremont, CA, 

USA 20 kW 1.2 MW >10 MW 40 MW/y

Solaris 

Synergy

15X linear 

reflective, Si 

cells floating 

on water

Jerusalem, 

Israel 1 kW 

Solbeam

Tracking 

optics in flat 

configuration

Laguna 

Niguel, CA, 

USA

Stellaris

Static, 3X 

“see-through,” 

Si cells

North 

Billerica, MA, 

USA

SV (Silicon 

Valley) Solar 

Flat-plate 

dimensions

Sunnyvale, 

CA, USA 2 MW/y

Sunengy

Fresnel (point 

focus), Si cells 

in water

Sydney, 

Australia
27

Company Type of 

System Location

On Sun 

in 

2009*

Installe

d in 

2010*

Completed 

or in 

Progress in 

2011*

Capacity*

SunPower Reflective, 7X, 

Si cells

San Jose, 

CA ~24 kW

Sunseeker 

Energy Lens Schindellegi, Switzerland

Thales 

Research

Static, 

reflective

Severna 

Park, MD, 

USA

Transform 

Solar

Low X, Sliver 

cells

Boise, Idaho, 

USA

Whitfield 

Solar

Fresnel lens, 

~40X, Si cells Reading, UK 9 kW

Zytech Solar

Reflective, Si 

modules; 4X–

150X

Zaragoza, 

Spain

Totals ~150

kW

~150 

kW ~ 1 MW

*Based on public presentations or website announcements/press releases. Note that some companies refrain from 

posting information about their deployments, so the lack of a number may not mean they have made zero

installations.

Cell Supply

Historically, a key challenge of the medium-concentration approach has been obtaining a 

consistent supply of solar cells that function well under the desired concentration. The primary 

difference between standard, one-sun solar cells and concentrator cells is the need for a 

reduced series resistance. In addition, the cells may need to be fabricated in different 

geometries and may benefit from improved thermal contact with a heat sink. As with the highconcentration approach, there is typically a benefit to purchasing higher-efficiency cells. Buriedgroove-contact cells and back-point-contact cells have been of special interest for mediumconcentration applications in the past. 

SunPower offered off-the-shelf silicon concentrator cells at one time, and now has the capability 

to make high-efficiency silicon cells appropriate for use anywhere between one and 250 suns. 

However, SunPower has chosen a vertically integrated business model and is no longer 

interested in selling silicon cells (either one-sun or concentrator). Making custom-designed 

concentrator cells for every company is a distraction for most cell companies. However, NaREC 

(Alex.Cole@NaREC.co.uk) has expressed an interest and willingness in making custom silicon 

concentrator cells. Q-cells AG and BP Solar have also made silicon concentrator cells on

occasion, and all silicon cell manufacturers could, potentially, be sources. There is also interest 

in the use of CIGS or CdTe. The concentrator version of the CIGS cell must be moved from a 

glass substrate to a metal or other thermally conducting substrate. Daystar planned in the 1990s 

to develop a low-concentration system using CIGS cells, but has now dropped the concentrator 

approach.

The medium-concentration approaches face many of the same challenges as prototype and 

tracker development and testing, as well as the need for development of appropriate standards. 

These are discussed in Part I and are not repeated here.
28

Novel Approaches

Luminescent concentrators have attracted substantial attention in recent years, proposing that 

light be absorbed and then reemitted within a sheet of glass or other material that acts as a 

waveguide. The glass (wave guide) directs the reemitted light to the edges, where it is 

converted to electricity by a concentrator cell. Two fundamental processes can lead to an 

enhancement of brightness. The first is dependent on the index of the material; a higher index of 

refraction can lead to a small enhancement. A more dramatic enhancement is achieved if a 

luminescent material absorbs high-energy light and reemits it at a lower energy. To understand 

how this works, consider a material in glass that absorbs light and luminesces at the same 

wavelength. If the luminescent-material is put into the glass at a concentration allowing light to 

be absorbed during one pass through the glass, then light reemitted for lateral transmission will 

be reabsorbed within a distance that is similar to the thickness of the glass. The light may be 

absorbed and reemitted many times before reaching the edge of the glass. Each time the light is 

reemitted, there is a chance that it will escape from the glass, and, because the direction is 

randomized with each reemission, the probability of this light reaching the edge of the glass is 

small, resulting in no increase in concentration.

Next, in contrast, consider a material that absorbs a high-energy photon and luminesces a lowenergy photon. If the absorption coefficients of the two photons differ dramatically, then it is 

possible to choose a concentration of luminescent material that absorbs the high-energy light in 

one pass, but allows the low-energy light to travel long distances within the glass before being 

reabsorbed. In this case, very high concentrations can be achieved, theoretically. This limits the 

ability of a luminescent concentrator to concentrate light with energy close to the reemission 

energy. Although a luminescent concentrator provides an elegant way to concentrate light, it 

relies on identification of stable materials with the appropriate luminescent properties. So far, 

this approach has not been successful at achieving the needed performance, but new 

nanomaterials could lead to breakthroughs in this area.

Summary

The use of optical concentration to reduce the amount of silicon needed per watt in solar 

systems has the potential to provide cost-effective, large-scale, solar-electricity generation that 

is less sensitive to market volatility. Almost two dozen companies are publicly developing 

products. The reduced need for silicon and associated capital expenditures could allow these 

companies to grow at a rate that significantly exceeds that of the rest of the industry.
29

Part III. Silicon Modules with Enhanced Concentration 

In 2007 and 2008, when silicon modules were in short supply, many companies devised 

creative methods for making their silicon modules generate more electricity. Specifically, adding

mirrors to enhance the irradiance on the modules was commonly used. The silicon modules can 

be incorporated directly into low-concentration designs without significant performance losses. 

Similarly, tracking systems from either high-concentration PV or flat-plate PV may be used in 

low-concentration systems. By leveraging the infrastructure used for these other products, the 

product development time for these enhanced-concentration products can be quite short. 

Table 7 summarizes some companies that have pursued this approach. We note that the 

oversupply of silicon modules decreased in 2009, almost eliminating interest in this approach for 

current markets, but a handful of companies are still pursuing it.

Table 7. Summary of Companies Developing Low-Concentration PV Products Using 

Conventional Silicon Modules

Company Type of System Location On Sun in 

2009*

Installed 

in 2010* Capacity*

Abengoa Solar Reflective, linear Madrid, Spain 1.2 MW

Archimedes Reflective, linear Stuttgart, 

Germany

Ehw Reflective, linear La Seyne sur 

Mer, France

EVERPHOTON 2X Taipei, Taiwan

JX Crystals Reflective, linear Issaquah, WA, 

USA >100 kW

Opel International Reflective, linear Shelton, CT, 

USA

WS Energia Reflective, linear, 

2X 

Oeiras, 

Portugal 263 kW

*Based on public presentations or website announcements/press releases.

Manufacturing

This low-concentration approach builds on the existing know-how of the flat-plate PV industry in 

providing high volumes of Si panels and relatively low-cost trackers. Because the lowconcentration approach is only incrementally different from flat-plate silicon, product 

development may be completed more quickly than with high-concentration approaches. Once 

low-concentration products are fully developed, the companies may scale up production rapidly, 

being less encumbered by the need for silicon feedstock. Although the silicon feedstock 

shortage disappeared in 2008, the possibility that it could recur forces companies to make longterm plans. Smaller capital investment translates to smaller risk, allowing the scale-up to 

happen more easily. For this reason, Si-based CPV may be attractive to risk-adverse investors.

Abengoa has already installed more than 1 MW. WS Energia installed 218 kW in 61 installations 

in 2008: 8 systems in Italy, 6 in Spain, and 47 in Portugal. WS Energia currently reports having 

completed 110 PV systems in 2008 and 2010, about half of which are low concentration. A 

company representative compared performance of flat-plate and low-concentration systems at 

CPV-7.[39]
30

Summary

The use of mirrors to boost the performance of conventional flat-plate modules attracted the 

highest interest during the time when silicon was in short supply. Interest in this approach has 

decreased since then, but could easily resume if a new shortage develops, or if one of the 

companies in Table 7 begins an expansion.

Acknowledgments

We extend our sincere gratitude to the many individuals who contributed to this report. Special 

thanks go to Richard Swanson, Bob Conner, Doug Rose, Matt Evans, Geoff Kinsey, Greg 

Peisert, Jerry Olson, Bob Cart, Brad Hines, Jeff Gordon, Craig Cornelius, Lori Greene, 

Gianfranco Sorasio, Susan Moon, and Carl Bingham. This work was supported by the U.S. 

Department of Energy under Contract No. DE-AC36-08GO28308 with the National Renewable 

Energy Laboratory.

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32

Appendix A. Cost Evaluations

When pursuing any new technology, it is essential to evaluate whether it will be cost 

competitive. However, it is also essential to recognize that cost estimates can have substantial 

uncertainty, and that placing emphasis on small cost differences could lead to unwise decisions 

in the long run. The cost of electricity from PV systems depends on the location and mounting 

details; the strongest cost driver is the cost of the money used to create the initial installation. 

In 2000, Swanson published a comprehensive study comparing the expected costs of electricity 

for multiple PV technologies (Fig. 6 and Table 8).[12] In April 2010, Swanson revisited this study 

in his plenary presentation at the CPV-6 conference. He noted that many of the projections 

made in 2000 were accurate within 10% or 20% of what is found today. However, the arearelated balance-of-system costs dropped more than projected for fixed mounting [projected to 

be $88/m2 (2010 $); now estimated to be $57/m2

] and 1-axis mounting [projected to be $113/m2

(2010 $); now estimated to be $80/m2

]. The costs of 2-axis trackers did not come down as much 

as projected, perhaps because this segment of the market has not grown as robustly as the 

others.

Inverter costs also dropped more than projected (projected to be 38 cents/W; now estimated to 

be 30 cents/W). Swanson’s conclusion (presented in April 2010) is that HCPV (multijunction IIIV), LCPV (silicon), and thin-film (CdTe) approaches are in a dead heat. Adjusting to current 

dollars, he reported that the relative costs of HCPV, LCPV, thin films, and crystalline silicon 

were projected to be 0.86, 1.35, 1.18, and 1.33, but are now found to be 1.0, 1.0, 1.0, and 1.13, 

respectively. He noted that the crystalline silicon costs dropped more than projected because of 

unexpectedly rapid market growth. SunPower’s high-efficiency, low-cost silicon cells have 

reduced the cost of the low-concentration approach, explaining much of the increased interest in 

this approach documented in Part II of this report. The HCPV approach has not yet increased in 

volume adequately to define its cost, so the uncertainty in this analysis is emphasized. Learning 

from Swanson’s comparison of 10-year-old predictions to today’s reality, we may expect that the 

uncertainty in the relative projections can be as much as 50%. It will be interesting to see how 

these costs evolve in the next ten years!
33

Figure A3 (from Ref.[12], color modified). For medium-sized plants in Boston, the GaAs dish surprisingly maintains its 

lead, despite the lower direct normal solar resources. (In other words, a dish based on 35% efficient cells is 

something of the ultimate technology.) The thin-film approach is a close second place. (R.M. Swanson, "The Promise 

of Concentrators," Prog. Photovolt. Res. Appl. 8, 93111, ©2000 John Wiley & Sons Limited. Reproduced with 

permission.)

Figure 6. Cost of electricity calculated for a set of technologies as presented in Ref.

[12]
34

Table 8. Cost Assumptions Used to Calculate the Cost of Electricity 

Presented in Figure 6

Table A1 (in Ref. [12]). Detailed assumptions for medium-sized PV plants. (R.M. Swanson, "The Promise of 

Concentrators," Prog. Photovolt. Res. Appl. 8, 93111, ©2000 John Wiley & Sons Limited. Reproduced with 

permission.)

MEDIUM PLANTALBUQUERQUEGaAs 

Dish

GaAs

2-Axis 

Fresnel

Si Dish 2-axis 

static

Si

2-Axis 

Fresnel

Thin 

Film

Static 

Conc

Central 

Rec.

Albedo 

FFP

2-axis 

FP

Si 1-

Axis 

Fresnel

1-axis 

FP

FFP

Desert 

(Albuquerque)

KWhr/

m2/day

6.566 6.566 6.566 8.624 6.566 6.336 6.336 5.025 6.336 8.624 6.08 7.41 6.336

Diffuse (Boston) KWhr/

m2/day

3.626 3.626 3.626 5.782 3.626 4.554 4.554 2.775 4.554 5.782 3.42 4.94 4.554

Albedo factor 1 1 1 1 1 1 1 1 1.3 1 1 1 1

BOS Area (low) $/m2 70 70 70 70 70 70 70 70 70 70 70 70 70

BOS Area (high) $/m2 140 140 140 140 140 140 140 140 140 140 140 140 140

BOS Power (low) $/W 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

BOS Power (high) $/W 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6

Tracking (low) $/m2 35 35 35 35 35 0 0 35 0 35 20 20 0

Tracking (high) $/m2 67 67 67 67 67 0 0 67 0 67 40 40 0

Module (low) $/m2 90 115 90 115 115 75 85 30 85 75 90 75 75

Module (high) $/m2 160 230 160 230 230 150 160 60 165 150 160 150 150

Cell (low) $/m2 30000 30000 15000 300 15000 0 300 20000 200 200 5000 200 200

Cell (high) $/m2 10000

0

100000 20000 1000 20000 30 1000 25000 400 400 15000 400 400

Cell Efficiency 

(high)

0.3325 0.35 0.26 0.21 0.27 0.12 0.21 0.26 0.2 0.2 0.24 0.2 0.2

Cell Efficiency 

(low)

0.285 0.3 0.23 0.17 0.24 0.08 0.17 0.23 0.15 0.15 0.2 0.15 0.15

Operating Temp. 65 65 65 60 65 55 60 65 60 55 65 55 55

deta/dteta 2.20E031.90E032.20E033.30E

-03

2.20E032.00E

-03

3.30E

-03

2.20E033.30E033.30E

-03

2.40E033.30E

-03

3.30E

-03

Concentration 1000 1000 400 4 400 1 4 400 1 1 50 1 1

Module 

Transmission

0.85 0.85 0.85 0.9 0.85 0.95 0.9 0.85 0.95 0.95 0.9 0.95 0.95

BOS eff 0.85 0.85 0.85 0.9 0.85 0.9 0.9 0.85 0.9 0.9 0.85 0.9 0.9

Conc premium 0 0 0 0 0 0 0 0 0 0 0 0 0

O&M cost (low) ¢/KWhr 0.8 0.8 0.8 0.8 0.8 0.2 0.2 0.8 0.2 0.8 0.8 0.8 0.2

O&M cost (high) ¢/KWhr 2.0 2.0 2.0 2.0 2.0 0.8 0.8 2.0 0.8 2.0 2.0 2.0 0.8

Cost-diff low ¢/KWhr 12.8 13.2 15.8 13.7 16.6 13.2 13.4 17.1 15.4 16.5 19.9 18.6 18.5

Cost-diff high ¢/KWhr 30.0 31.8 32.4 37.5 35.4 41.1 37.7 34.9 39.6 42.7 52.2 48.0 48.2

Cost-Desert low ¢/KWhr 7.4 7.7 9.1 9.4 9.5 9.6 9.7 9.8 11.1 11.3 11.5 12.6 13.4

Cost-Desert high ¢/KWhr 17.5 18.4 18.8 25.8 20.4 29.7 27.3 20.2 28.7 29.3 30.3 32.7 34.9

Cost-low $/W 1.59 1.64 1.99 2.71 2.10 2.16 2.19 1.66 3.18 3.32 2.38 3.20 3.05

Cost-high $/W 3.70 3.94 4.02 7.49 4.42 6.69 6.14 3.33 8.18 8.58 6.27 8.30 7.89