CONTENTS

articles

18The Story of ADAM With their Advanced Automotive Manikin 

(ADAM), NREL researchers have created a tool 

that will help in the testing and design of ancillary automotive systems, making cars and light 

trucks more comfortable and energy effi cient.

22 Harvesting the 

Ocean Wind Winds coursing 

the “deep” waters off the shores of New 

England could provide that region with 

100 MW or more of clean wind-generated electric capacity. NREL engineers 

are tackling the technical challenges 

to designing and building deepwater wind turbines.

24 Research Update In other 

research areas, NREL scientists are the fi rst 

to achieve negative refraction of light using 

natural materials, help to develop concepts 

that will enhance the production of 

geothermal power, and perfect new ways 

to measure minority-carrier lifetimes in 

photovoltaic and semiconductor materials.

feature

departments

1 Perspective Meeting our mission 

requires an R&D management philosophy that 

carefully shepherds research from inception 

to the development of technologies that will 

benefi t the nation.

2 NREL in Focus 2004 news 

stories include the retirement of one director 

and the arrival of another, breaking ground 

for a new research facility, NREL 

becoming a hydrogen 

center of excellence, 

ISO certifi cation for 

calibrating reference 

cells, and recognition of an 

NREL research fellow for highcitation impact.

4 Awards & Honors During 

2004, NREL was honored with two R&D 100 

Awards, three regional technology 

transfer awards, a solar energy 

award, a Secretary of Energy Gold 

Award, and the naming of a new 

science society fellow. 

The NREL Research Review. The National Renewable Energy Laboratory is operated by Midwest Research Institute and Battelle for the Department of Energy’s Offi ce of Energy Effi ciency and 

Renewable Energy. NREL is the nation’s primary national laboratory for renewable energy and energy 

effi ciency research and development. The Research Review is published yearly and describes the Laboratory’s accomplishments in science and technology to a wide audience. The purpose is not simply to 

describe the progress being made, but also to show the promise and value of NREL’s R&D to people, 

industry, the nation, and the world. 

8 NREL Nanoscience & 

Nanotechnology

Aligning Quantum Dots. Quantum dots will 

be important for a variety of renewable energy 

technologies in the 21st century, especially for 

a new generation of photovoltaic technologies. 

Properly aligned quantum dots could result 

in highly effi cient, low-cost devices. The cover 

shows the use of cellulosome proteins for this 

alignment—a unique technique being explored 

by NREL scientists and described in the feature 

article. 

Page i

March 2005

ISSUE 3

a publication about NREL R&D highlights
Dr. Stanley R. Bull, Associate 

Director, Science & Technology

PPeerrssppeeccttivivee

Science and Technology:

From Innovation and Knowledge to Implementation

Our mission at NREL is not simply to perform R&D on renewable 

energy and energy effi ciency technologies, but also to make sure that 

the innovation and knowledge we 

generate is taken through implementation to address the nation’s 

energy, environmental, and economic goals. The path from idea to 

fruition is challenging, and our R&D 

can face many obstacles on the way.

To help ensure success, we have a 

management paradigm that takes 

science and technology from initial concept through a process that 

enables the concept to reach its fi nal 

goal. This process includes strategy 

and plans, hypothesis and verifi cation, delivering R&D accomplishments in technology development, 

and forming and nurturing partnerships—all ending in the development of a technology that is ready to 

benefi t the nation and the world.

One of our principal measures of 

success is to win R&D 100 awards, 

which are presented by R&D Magazine to the 100 most technologically 

signifi cant products and processes 

of the year. The awards represent an 

affi rmation that our research ideas 

are being used in the real world to 

help meet America’s needs and that 

we are meeting our mission. Since 

1979, NREL has won 37 of these 

awards, 31 since 1991.

In 2004, NREL won two more R&D 

100 awards (see stories on pages 

4–6)—one for lightweight, fl exible, copper indium gallium diselenide PV modules; and the other for 

enzymatic hydrolysis of biomass 

cellulose to sugars. 

Both technologies 

are representative 

of our R&D management paradigm. 

Consider the enzymatic hydrolysis 

technology, which 

has a long history 

at NREL. One of 

the Laboratory’s original programs 

involved the development of technologies to produce alternative fuels 

from biomass, to relieve America’s 

growing dependence on imported 

oil. From the beginning, NREL realized that, in the long term, cellulosic biomass presented the largest 

resource for such alternative fuels. 

Moreover, enzymatic hydrolysis of 

cellulose represented the preferred 

pathway for lowering the cost to 

where fuels derived from cellulosic 

biomass could not only be competitive with other alternative fuels but 

also with petroleum.

NREL subsequently laid out a longterm strategy to develop the enzymatic technology base and meet the 

cost goals, emphasizing the emerging fi elds of genetic and protein 

engineering and enzyme production. The R&D approach under this 

strategy involved basic research to 

discover appropriate microorganisms, engineer the variations, and 

produce those variations. It also 

involved a parallel effort in applied 

research and engineering to integrate the microorganisms into 

hydrolysis and fermentation processes. Once the technology was 

suffi ciently proven, NREL sought 

partners who could better engineer 

enzymes and who could effi ciently 

produce the enzymes in quantity. NREL reached a partnership 

agreement with 

two of the best in 

the world—Genencor International 

and Novozymes 

Biotech Inc. This 

NREL/Genencor/

Novozymes partnership quickly resulted 

in technological 

advances that cut by 20-fold the cost 

of enzymatic hydrolysis. This is now 

becoming a “real-world” technology 

that will help open America’s vast 

biomass resources for the economic 

production of not only alternative 

fuels, but also chemicals, plastics, 

and many other products—while 

offsetting the use of petroleum. 

Today, with its research in nanoscience and nanotechnology (see story 

on pages 8–17), NREL is facing a 

situation analogous to that which 

it faced with enzymatic hydrolysis 

a decade or so ago. Some of our 

R&D is still in the basic phase, with 

hypotheses being formed and tested. 

But in other avenues of exploration, 

we are performing applied research 

and are entering into collaborations 

and partnerships with universities, 

institutes, and companies. Nanotechnology is an extremely promising area for energy research and 

development. If the R&D is managed following our established paradigm, it will lead to a new frontier of 

energy solutions, which is so important for the nation and the world. 

As with past and present successes, 

we are confi dent that the nanoscience and nanotechnology being 

researched at the Laboratory today 

will become the real-world technologies of tomorrow. 

This is not just our belief—it is our 

mission. 

Page 1
NREL appointment, Arvizu joins Midwest Research Institute (MRI) executive 

management and assumes a senior vice 

president position. NREL is operated 

for the Department of Energy by MRI 

and Battelle.

Prior to joining NREL, Arvizu held 

prominent positions at CH2M Hill 

from 1998 to 2004. Most recently, he 

served as senior vice president and 

chief technology offi cer overseeing 

technology development and acquisitions for seven major business groups, 

with more than $2.5 billion in annual 

revenue. Arvizu spent more than 20 

years managing energy research programs at Sandia National Laboratories, 

including supervising the Photovoltaic 

Cell Research and Concentrator Division. Arvizu was credited with expanding the laboratory’s technology transfer 

efforts, fi rst as director of the Technology Commercialization Center and 

later as director of the Advanced Energy 

Technology and Policy Center. 

In 2004, Arvizu was appointed by President George W. Bush to serve on the 

National Science Board, which is the 

governing board of the National Science Foundation. He also serves on 

several university advisory boards, 

on the board of directors of the Hispanic Engineers National Achievement 

Awards Corp., and on the board of 

advisors for the Greater Denver Metro 

Area Salvation Army.

Arvizu has served on numerous other 

boards and advisory committees, 

including the National Coal Council, 

the Army Science Board, the National 

Academies of Sciences and Engineering, the G8 International Renewable 

Energy Task Force, and the Council on 

Competitiveness. 

Breaking Ground for New 

Facility

NREL broke ground in July to signal 

the start of construction of the new Science and Technology Facility (S&TF). 

The 71,000 square foot, energy-effi -

cient building will include laboratory 

space, offi ces, and a lobby. Situated 

at the foot of South Table Mesa on 

NREL’s main campus, the S&TF will be 

connected by an elevated bridge with 

the Solar Energy Research Facility. The 

new building will expand NREL’s capabilities in photovoltaics, hydrogen, 

solid-state lighting, basic sciences, and 

nanotechnology and will reduce barriers and time delays associated with 

transferring technology from R&D to 

industry.

National leaders present at the groundbreaking included U.S. Senator Wayne 

Allard and Congressman Bob Beauprez, who extolled NREL as an asset to 

Colorado and the nation, and praised 

the project as a benefi cial new resource 

for ensuring U.S. energy security.

“This new facility will . . . hasten the 

day when we reach our goal of providing . . . clean, affordable energy 

solutions that can be used by all Americans,” said David Garman, DOE’s acting under secretary for energy, science, 

and the environment and assistant secretary for energy effi ciency and renewable energy.

NREL In Focus

Passing the Torch

NREL welcomed new director Dr. Dan 

E. Arvizu January 15, 2005, and honored the accomplishments of retiring 

director Richard H. Truly.

In his seven years at NREL’s helm, Truly 

established the National Bioenergy 

Center at NREL; brought the hydrogen systems integration function to 

the Laboratory; secured a key for NREL 

in the Center of Excellence for exploratory research in hydrogen storage; 

began construction of the Science and 

Technology Facility, NREL’s fi rst major 

new building in more than a decade; 

and created Sustainable NREL, a campus-wide effort to incorporate sustainable energy practices throughout 

the workplace. Streamlining management systems was another priority that 

bore signifi cant benefi ts and resulted 

in more funds directed toward laboratory R&D.

“Science and engineering have a magical quality about them, in that they are 

all about imagining a future that gets 

created by those lucky enough to have 

their ladle in the broth,” Truly said 

in announcing his retirement. “I feel 

a deep privilege to have been a small 

part of NREL’s successes over these 

years.”

Dan Arvizu is NREL’s eighth director 

in its 27-year history. In addition to his 

Admiral Richard H. Truly Dr. Dan E.Arvizu

NREL’s Science and Technology Facility, 

which is scheduled for completion in 2006, 

will be NREL’s first major new facility in a 

decade. The S&TF will be used for R&D in 

photovoltaics, hydrogen, solid-state lighting, basic sciences, and nanotechnology.

Page 2 
Construction began in earnest in early 

2005, with completion expected in late 

2006.

The value of the construction subcontract is nearly $18 million. The total 

value of the project, including equipment and furnishings, is approximately 

$28 million. The M.A. Mortenson 

Company signed the contract to build 

NREL’s fi rst major new facility in a 

decade. 

NREL Named to Hydrogen 

Center of Excellence 

NREL was named by DOE to play a 

key role in one of three “Centers of 

Excellence” for exploratory research 

in hydrogen storage and nanostructured carbon-containing materials. The 

other centers—to study chemical and 

metal hydrides for hydrogen storage—

have Los Alamos National Laboratory 

and Sandia National Laboratories in 

key roles. Each center will lead R&D on 

different material systems to address 

major technical barriers to storing 

hydrogen on-board cars, vans, and 

light trucks. The goal is to develop systems that will store enough hydrogen 

to enable a driving range greater than 

300 miles without impacting cargo or 

passenger space. 

NREL is in the Center of Excellence 

for carbon-based materials and will 

lead research on carbon nanotubes, 

nanofi bers, and aerogels, alkali 

metal intercalated carbons; and other 

novel carbon-based materials. In this 

endeavor, it will partner with industry 

(Air Products and Chemicals Inc.); universities (California Institute of 

Technology, Duke University, 

Penn State University, Rice 

University, University of Michigan, 

University of North Carolina, and 

University of Pennsylvania); and other 

national laboratories (Lawrence Livermore National Laboratory, National 

Institutes of Standards and Technology, 

and Oak Ridge National Laboratory).

NREL Wins ISO 

Certifi cation

NREL’s National Center for Photovoltaics received ISO (International 

Organization for Standardization) 

17025 accreditation in recognition of 

the Center’s stringent standards in calibrating secondary photovoltaic reference cells. These reference cells are 

used by calibration and testing laboratories and by PV manufacturers, who 

can now be assured that their calibrations and measurements are traceable, 

via NREL’s certifi ed reference cells, to 

national and international standards. 

The ISO 17025 accreditation was 

granted by the American Association 

for Laboratory Accreditation. Only one 

other laboratory in the world—the 

European Solar Test Installation—has 

the same accreditation, according to 

NREL scientist Keith Emery. Other labs 

have accreditation in similar types of 

calibration. 

ISO 17025 accreditation is awarded 

in many industries, each with its own 

standards. It is a recognition sought 

by testing and calibration laboratories, inspection bodies, profi ciency testing providers, and reference material 

producers. To win such accreditation, 

a laboratory must have a quality system at three levels of staff: top management, supervisors, and operating 

personnel. Quality procedures must be 

described in a manual. The laboratory 

must have a policy and procedures for 

corrective actions in the event of nonconforming data results.

NREL Theorist Recognized 

for High-Citation Impact

Dr. Alex Zunger, NREL research fellow, is the co-author of a paper that has 

had the fi fth-highest citation impact in 

the Physical Review during the past 110 

years, according to a recent analysis of 

papers appearing in the journal.

The analysis, performed by Sydney 

Redner of Boston University, includes 

more than 3 million citations from 

more than 329,000 manuscripts 

appearing in the prestigious physics journal from 1893 through 2003. 

NREL research fellow Alex Zunger co-authored one of the highest-ranked papers 

(in terms of citation impact) ever published 

in Physical Review.

Page 3 

 Citation impact is based on the number of citations to a publication times 

the average age of these citations. Such 

a measure emphasizes publications 

that have infl uence over an extended 

period of time.

Zunger’s achievement is based on 

his work with John Perdew on “Selfinteraction Correction to the Density 

Functional Approximation for ManyElectron Systems “published in Phys. 

Rev. B 23, 5048 (1981). Their paper 

established a fundamental treatment 

of Walter Kohn’s and Lu Sham’s celebrated “Density Functional Theory” 

(ranked number one in Redner’s analysis). This opened up the fi eld of using 

the quantum theory of Kohn and 

Sham to predict optical, structural, and 

energetic properties of many molecules 

and solids.

Zunger’s Solid State Theory group at 

NREL has used this method during the 

past 20 years to predict and analyze the 

properties of PV materials and to study 

spontaneous ordering, alloy bowing, 

and, more recently, the properties of 

nanostructures.

Dr. Zunger has been with NREL for 

25 years and has been a research fellow at the Laboratory since 1991. He 

has authored and co-authored more 

than 400 journal articles. According to 

a recent study by the Institute of Scientifi c Information, other papers by 

Zunger place him in 39th place (of 

50,000 physicists reviewed) in terms of 

citations by all physics journals in the 

years 1987–1997.

An important concept for hydrogen storage that NREL is exploring is a single-wall 

carbon nanotubes (blue), shown here with 

hydrogen (yellow) diffusing to the lattice 

of the nanotube in the presence of a catalyst (silver).
R&D 100 Awards

Every year, R&D Magazine holds an 

international competition to select 

the 100 most signifi cant technological advances of the year. Winning one 

of these hotly contested awards—often 

referred to as the Academy Awards of 

technology—has become a widely recognized honor for the winning technology and the R&D that went into its 

development. Since NREL fi rst opened 

its doors as the Solar Energy Research 

Institute in 1979, the Laboratory—

often in collaboration with research 

partners—has won 37 of these awards, 

31 of them since 1991. In 2004, NREL 

was honored with two more R&D 100 

awards.

Making Fuels and 

Chemicals from Sugar 

NREL and its partners—Genencor 

International and Novozymes Biotech 

Inc.—developed a technology that uses 

enzymatic hydrolysis to inexpensively 

break down the cellulose fraction of 

lignocellulosic biomass to sugars. 

The sugars can then serve as platform 

chemicals from which to derive ethanol fuel, other fuels, chemicals, and 

many other products—all sustainably. 

Such a concept, known as biorefi ning, is analogous to petroleum refi ning 

except it is based on renewable feedstocks and could help America offset 

its use of petroleum-derived fuels and 

chemicals.

Before the advent of our technology, 

the process for breaking down cellulose to sugars was too expensive to 

compete with the technology used to 

break down starch in corn kernels to 

sugars; or to compete with breaking 

down hydrocarbons in petroleum for 

the production of the fuels, chemicals, 

and products that help run our modern economy. 

Lignocellulosic biomass consists of cellulose, hemicellulose, and lignin, with 

cellulose constituting up to 50% of 

the total mass. Like starch and sugar, 

hemicellulose and cellulose are carbohydrates (compounds of carbon, 

hydrogen, and oxygen). The sugars 

of which they are made are linked 

together in long chains called polysaccharides, which form the structural 

portion of plant cell walls. Unraveling 

these complex structures is the key to 

economic biorefi ning. 

Cellulose microfi brils consist of a 

crystalline structure of thousands of 

strands, each of which contains hundreds of glucose sugar molecules. 

These microfi brils are wrapped in a 

sheath of hemicellulose and lignin, 

which protects the cellulose from 

microbial attack. Hemicellulose is relatively easy to break down into its component sugars using heat, pressure, and 

dilute acid. This “dilute acid hydrolysis” pretreatment step also disrupts the 

hemicellulose/lignin sheath around 

the cellulose, making the cellulose 

accessible to further hydrolysis. 

To hydrolyze the cellulose, NREL and 

its partners developed a technology 

that employs a cocktail of three types 

of cellulase enzymes—endoglucanase, 

exoglucanase, and betaglucosidase. In 

consort, these enzymes break cellulose chains in two, exposing the ends 

of the chains, and then break down 

these chains into units of cellobiose 

(a dimeric sugar comprised of two 

Awards & Honors

Page 4 
NREL researchers (left) collaborated with researchers from Genencor International 

(middle) and Novozymes Biotech Inc. (right) to develop an award-winning technology 

that enzymatically reduces cellulose to sugars. Only a few members and leaders of these 

large research teams are present in the photos. 

 glucose molecules). They then split 

the cellobiose molecule into two separate glucose molecules, making them 

available for processing into fuels or 

 chemicals.

Until the advent of our breakthrough 

technology, other methods of hydrolyzing cellulose to sugars—including strong acid hydrolysis and older 

versions of enzymatic hydrolysis—

were ineffi cient, expensive, and had 

low sugar yields. Enzymatic hydrolysis was also too expensive, but it had 

great potential. The strategy to take 

advantage of this potential was twofold: improve the pretreatment (i.e., 

the dilute acid hydrolysis) process and 

reduce the cost of the enzymes. Reducing the cost of the enzymes also followed a twofold strategy: engineer 

better enzymes, so that less enzyme is 

needed per gallon unit of sugar output; and optimize the enzyme-production system. The task of improving the 

pretreatment fell to NREL, while reducing the cost of the enzymes became 

the task of Genencor and Novozymes 

(working separately on their respective proprietary strains to produce 

more effi cient cellulase systems and to 

improve the economics of their proprietary production methods). NREL 

also validated and verifi ed technical 

improvements at each stage of development.

As a result, NREL and its partners developed a technology that is effi cient, 

has high sugar yields, and has already 

dropped the cost of cellulose hydrolysis by 20-fold. Moreover, this technology holds the promise of decreasing 

the cost by as much as another order of 

magnitude, and maybe more. 

One of the reasons for developing this 

technology is to produce inexpensive ethanol fuel from lignocellulosic 

 biomass. The United States has enough 

of this resource to sustainably make 

as much as 30 to 45 billion gallons of 

ethanol per year. This would not only 

signifi cantly offset the nation’s consumption of gasoline (which stands 

at about 135 billion gallons today), 

but it would also lead to large offsets of greenhouse-gas emissions as 

the ethanol displaces our use of petroleum. Ethanol derived from lignocellulosic biomass can use the lignin 

portion to produce the heat and electricity required to run the process to 

make it. The use of lignin thus offsets 

energy that would otherwise be conventionally obtained from fossil fuels. 

This, along with the fact that burning 

ethanol produces less net CO2 emissions as a result of re-assimilation of 

CO2 into plant matter via the carbon 

cycle (growing biomass absorbs CO2), 

enables the entire “plant to wheels” 

process of lignocellulosic production 

of fuel ethanol to actually have negative greenhouse gas emissions relative 

to the “oil well to wheels” process for 

gasoline. The end result is a net offset 

of up to 600 g of CO2 per mile when 

lignocellulosic ethanol is used for 

transportation in place of petroleum. 

But producing ethanol fuel is just the 

beginning. Using different microbes 

and processing techniques, we can 

build biorefi neries with which to 

derive many other products, including plastics, fi bers, textiles, polymers, 

fi lms, lubricants, adhesives, cements, 

solvents, chemical intermediates, photographic chemicals, cleaning agents, 

coatings and paints, fi llers and insulation, cosmetics and personal-care 

products, pharmaceuticals and medical 

and dental products, other alcohols, 

antifreeze, softening agents, sweeteners, dyes and perfumes, insecticides, 

food additives, and much more. Our 

technology, therefore, could help biorefi neries begin to play a major role in 

America’s economy.

Portable Solar Electricity 

Thanks to Global Solar and its partners 

at ITN Energy Systems and NREL, the 

world now has its fi rst portable, fl exible 

PV systems made from copper indium 

gallium selenide (CIGS)—for use in 

mobile applications. These PV systems 

can provide from 12 W to 56 W of 

power, be folded into sizes as small as 

a 9 x 12 envelope, and be easily stowed 

in a small backpack. In addition, they 

are light enough to be carried over 

long distances, and can be connected 

together, when needed, to provide as 

much as 2.8 kW of power. One line of 

products using this technology is fl exible enough to be rolled up to a size 

comparable to modern, lightweight, 

self-infl ating mattresses used in backpacking. 

The U.S. Army is already using these 

new systems in applications where 

both weight and space are at a premium—such as for scouts, forward 

fi eld offi cers, special operations units, 

and forward communication units. For 

the military, the fl exible CIGS modules 

can also provide portable power in the 

fi eld without the drawback of the heat 

signature of diesel or gasoline generators.

Many of the features that make this 

technology so attractive to the military also make it appealing to a wide 

range of other applications. This 

Page 5 
CIGS technology can provide reliable 

power for hikers, campers, and boaters; police; portable electronics, such 

as cell phones, satellite phones, GPS 

units, computers, radios, and MP3 

players; survival kits; battery chargers; 

and more. 

The CIGS systems are not the fi rst portable PV systems available. Systems 

based on amorphous silicon technology have been on the market for a few 

years. But the new CIGS portable PV 

technology has many advantages over 

these systems. Compared to them, for 

example, the CIGS systems:

• Are lighter, more fl exible, and portable

• Are more effi cient and reliable

• Have two to three times the powerto-weight ratio 

• Have more than fi ve times the 

power-to-volume ratio

• Cost less

• Are inherently self-repairing due 

to the natural tendency of copper 

atoms in the CIGS material to spread 

into damaged areas, thereby repairing the crystal structure. In fact, CIGS 

modules can even take a bullet hole 

and continue to operate. 

Another advantage of the CIGS technology is that it is extremely versatile. 

Modules can be fabricated on a variety 

of substrates—fl exible, rigid, or substrates that can conform to many surfaces. As such, the CIGS systems can 

be included on all kinds of structures 

such as signs, bus shelters, sun roofs, 

or awnings; or they can be integrated 

into building applications and be used 

on metal roofs, as roof shingles, or in architectural fabrics 

or facades.

From 1994 through 2004 

worldwide PV module sales 

increased more than 15-fold—

from less than 70 MW per year 

to more than 1.1 GW per year. And the 

annual market for the sale and installation of PV systems is fast approaching $10 billion. The great majority of 

this market belongs to silicon PV technology, especially crystalline silicon. 

The advent of fl exible CIGS technology 

is signifi cant for at least two reasons. 

First, this technology is opening up a 

whole new category of applications 

for photovoltaics. Second, but equally 

important is the arrival of this valuable 

thin-fi lm material on the commercial 

scene. It has a great potential, especially as it gains a larger share of the 

market, to reduce the cost of PV electricity and thus augment the phenomenal growth of the PV market. 

Other Awards & 

Honors

Technology Transfer 

Awards 

NREL was honored with three awards 

from the Mid-Continent Region of the 

Federal Laboratory Consortium (FLC) 

for Technology Transfer. The awards 

were presented at a September banquet 

on South Padre Island, Texas.

Two of the awards were for Notable 

Technology Development. The fi rst of 

these recognized the work that the Vehicle Ancillary Loads Reduction Team 

performed for the Thermal Comfort 

Project. Team members Rob Farrington, 

John Rugh, Jason Lustbader, and Charlie King developed tools to analyze and 

improve the effi ciency of climate control in vehicles. Their work has resulted 

in the development of the ADvanced 

Automotive Manikin (ADAM), the 

world’s most advanced thermal comfort manikin. Using computer models 

that simulate human physical and psychological responses, it mimics human 

responses such as sweating and breathing. ADAM is designed not only to 

determine how hot or cold an occupant 

is, but also to predict how the occupant 

will feel.

The second award recognized the 

Advanced Petroleum-Based Fuels 

(APBF) Project, whose team members 

include Wendy Clark, Shawn Whitacre, 

Theresa Alleman, and Matt Thornton. 

The award commended their efforts in 

forming industrial partnerships, with 

the ultimate aim of reducing transportation-related petrochemical use and 

emissions. Project partners include 

vehicle manufacturers, equipment suppliers, and fuel providers. The APBF 

team brings value in the form of stateof-the-art laboratories and research, 

and patented technologies available for 

licensing.

Both projects are managed by the Laboratory’s Center for Transportation 

Technologies and Systems.

Also honored by the FLC was Lawrence 

“Marty” Murphy, manager of NREL’s 

Enterprise Development Programs. 

Murphy received a Distinguished Service Award for his leadership during 

the past nine years in building and 

promoting NREL’s Enterprise Development Program and Industry Growth 

Venture Forums, which encourage the 

creation and growth of clean, effi cient, 

and renewable energy companies. In 

the past few years, these 

activities have resulted in 

at least $5 million in private capital and $5 million in state and local 

funds for clean-energy 

commercialization, and 

more than 1,000 jobs created in successful cleanenergy companies. 

A Global Solar team (right), 

worked with teams from NREL 

(left) and ITN Energy Systems 

to develop lightweight, flexible, portable PV modules 

(above).

Page 6 
Page 7 

Steve Kelley Elected 

IAWS Fellow 

In recognition of his valuable contributions to the fi elds of wood chemistry and technology, Principal Scientist 

Steve Kelley was elected a fellow of the 

International Academy of Wood Science 

(IAWS).

Since he joined NREL in 1992, Kelley’s 

research has primarily focused on two 

areas: developing value-added products from biomass, and applying novel 

analytical tools for characterizing biomass and natural polymers. 

Kelley has worked on developing phenolic resins from wood and bark, to 

replace petrochemical adhesives used 

to make wood products such as plywood. He has also worked to develop 

technology for making chemicals, 

monomers, and plastics from biomass 

rather than from petrochemicals.

“Cellulose, hemicellulose, and lignin 

in biomass are polymers to begin 

with,” Kelley explains. “In the biorefi nery approach that NREL is developing, 

we use chemical or biological techniques to break down the polymers to 

building blocks that can be refi ned and 

used for value-added applications. We 

have also looked at isolating and using 

the polymers in their original form.” 

Kelley was a key player in developing 

NREL’s rapid biomass analysis technology, for which he and fellow researchers earned a prestigious R&D 100 

Award in 2000. Combining near-infrared spectrometry with sophisticated 

multivariate analysis, the technology 

allows virtually instantaneous, nondestructive analysis of biomass, eliminating the need for expensive laboratory 

analysis that might take weeks. Kelley 

has taken this technology beyond its 

initial application for chemical analysis, extending its capabilities to determine mechanical properties and wood 

decay, and to evaluate wood composites. For example, it can be used to 

assess the dry strength of lumber that 

could be made from a tree while the 

tree is still standing in the forest. 

Kelley has been the NREL lead on a 

series of projects designed to demonstrate the utility of these rapid analysis tools for understanding the effects 

of process variables on the chemical 

and physical properties of bio-based 

materials. He holds a B.S. in wood science from Oregon State University; an 

M.S. in forestry and wood science from 

the University of Wisconsin, Madison; 

and a Ph.D. in polymer chemistry from 

Virginia Tech. He has co-authored 65 

publications and has 15 patents and 

submitted applications.

Women in Solar Energy 

Award

Cecile Warner, an NREL staff member 

for more than 25 years, received the 

2004 Women in Solar Energy Award 

from the American Solar Energy Society. She was praised for outstanding 

and sustained contributions to solar 

photovoltaics, particularly for her public outreach efforts.

In 2002, Warner led the Laboratory in 

developing and managing the highly 

acclaimed U.S. Solar Decathlon for the 

U.S. Department of 

Energy. The 

Solar Decathlon is a competition in 

which teams 

of college and 

university students compete 

to design the 

most attractive, 

effective, and 

effi cient solarpowered house. 

She also served 

as director of 

the Laboratory’s 

Center for Renewable Energy Resources 

from 1996–2000; and was project 

director of Sunrayce 93, a university 

competition of solar cars. Warner was 

elected to the Board of Directors of the 

American Solar Energy Society in 2000. 

She holds a B.S. and M.S. in mechanical engineering.

Truly Receives Energy 

Secretary’s Gold Award

Former Secretary of Energy Spencer 

Abraham presented the Secretary’s Gold 

Award to NREL Director Richard Truly 

and seven other current and former 

directors of DOE national laboratories. 

The award is DOE’s highest honorary 

award and includes a plaque with citation, a medallion, and a rosette. 

“I’m proud to recognize the people 

whose hard work and dedication contribute so much to the Department 

of Energy’s vital missions,” Abraham 

said. “Our world-class laboratories are 

a marvelous resource and have made 

far-reaching contributions—not only 

to the Department of Energy, but to 

our nation and, indeed, the world. The 

incredible work done in the laboratories is made possible by the strong, 

steady, and responsible leadership of 

these directors.” 

Truly was recognized for his superior 

leadership as director of NREL and for 

his vision and perseverance in helping transform NREL into the premier 

national institution in its fi eld. 

Secretary Abraham presented the 

awards at an October 25 luncheon at 

the Woodrow Wilson International Center for Scholars in Washington, D.C. 

Cecile Warner stands on the mall in the nation’s capitol, where crowds of people flock 

to see solar-powered homes designed by 

teams of college students for the U.S. Solar 

Decathlon.

The Secretary’s Gold Award plaque was 

presented to NREL director Richard Truly by 

former DOE Secretary Spencer Abraham. 
NREL 

Nanoscience & 

Nanotechnology

Meeting 21st Century 

Energy Challenges

Many futurists, scientists, and technologists believe that nanotechnology in the 21st century could prove to be as socially 

transformative as running water, antibiotics, plastics, or semiconductors have been. Nanotechnology will have a crucial impact on everything from 

cars, health, and security to computers and materials science. Serious projections for it include 

building materials that can sense weather and 

change properties accordingly, lightweight materials far more durable than steel and hundreds of 

times stronger, computers that are orders of magnitude faster than today’s, and nanoscale biosensors that can be injected into blood streams to 

diagnose and treat illnesses. 

At NREL, we see the 21st century also as a century of new energy concepts and nanotechnology as a technology that will enable sustainable, 

benign, effi cient, and inexpensive energy. In 

fact, when applied to energy, the impact of nanotechnology may turn out to be as great as that 

brought about by civilization’s turn to fossil fuels, 

which resulted in the industrial revolution. To 

put this in perspective, today the world consumes 

about 13 terawatt-years (TW-yrs) of energy per 

year. (A terawatt is 1012 W.) But according to the 

DOE’s Basic Energy Sciences Advisory Committee, the world’s energy consumption will more 

Page 8
quantization effects, they 

have several advantages 

over other starting 

materials. They can 

be easily and inexpensively prepared 

using colloidal chemistry. They have lower 

melting points than 

their macroscopic counterparts, which allows the 

use of low-temperature, 

inexpensive processing techniques with which to prepare 

macroscopic devices. And they 

allow versatile formulations of 

solutions and inks that cannot be 

obtained with larger starter material.

One example is NREL’s novel 

organometallic inks, which can be 

used to produce silver and copper contacts and metallic grids for solar cells, 

contacts for touch screens and fl at-panel 

displays, busbars for integrated circuits, 

and metallic grids for conjugated polymer solar cells and light-emitting diodes. The 

grids and contacts can be printed directly on 

the device using inkjet technology—a rapid, 

atmospheric, non-contact technique that not 

only produces high-resolution grid lines but that 

also can be used to print on fl exible, curved, 3-

D surfaces. This technology has the potential to 

supplant the more expensive and slower vacuumbased technology currently in use.

Nanoparticle precursors are also being used to 

make fi lms of CdTe, CuIn(Ga)Se2, and other 

materials for thin-fi lm solar cells. The application of the precursors enables the use of spray 

deposition techniques, which are low-cost, lowtemperature alternatives for obtaining vacuumquality materials.

Carbon Nanotubes. Like the name suggests, carbon nanotubes are hollow tubes of pure carbon, 

in which the carbon atoms arrange themselves 

in hexagonal rings, like very small chicken wire 

rolled into a tube. There are two types of carbon 

nanotubes: single-wall nanotubes (SWNTs), comprised of a single shell of carbon atoms, approximately 1–2 nm in diameter and several microns 

long; and multi-wall nanotubes (MWNTs), comprised of multiple concentric nested tubes, with 

an outside diameter of several tens of nanometers.

NREL is one of the few labs to use all four of the 

known major methods for synthesizing nanotubes. The fi rst one used at NREL was the same 

method used to discover SWNTs: arc-discharge. 

Subsequently, NREL developed a laser vaporization technique in which a graphite target is irradiated with laser pulses. The pulses evaporate the 

graphite target to produce a variety of reactive carbon intermediate structures that, when quenched 

Carbon nanotubes, such as the 

single-wall type 

depicted above, 

exhibit properties 

that make them attractive for a wide variety 

of applications. On the 

 opposite page are quantum dots being aligned 

using a cellulosome polymer; the De-Broglie wavelength equation for matter waves; a π-conjugated 

polymer structure; and a 

C60 “buckyball.” 

than double by 2050. And papers from Hoffert et 

al. published in the journal Nature show that, if 

we are to meet this demand while stabilizing the 

CO2 in the atmosphere at a reasonable level—

such as 550 parts per million, or twice the preindustrial level—at least 100% of this new energy 

would have to come from non-CO2 generating 

sources. This is a daunting challenge that will take 

a major effort and important breakthroughs in 

energy technology and science. Nanotechnology 

will help meet this challenge. 

Nanotechnology deals with materials and phenomena on the scale of 1 billionth (10-9) to several tens of billionths of a meter. This is larger 

than the atomic scale, smaller than the bulk scale, 

and about the size of some molecules, including 

DNA and proteins. But the nanoscale is not simply a move toward miniaturization. It is a scale at 

which properties are qualitatively different from 

those of bulk materials. This includes electrical, 

 material, and optical properties. Because of these 

properties and because of new techniques, we can 

design and construct new materials and systems 

with attributes and capabilities not found in bulk 

 material or in nature. 

NREL’s Early Start

It is because of these qualitatively different properties and their applications to renewable energy 

and energy effi ciency that NREL has long been 

studying nanotechnology. In its early efforts, 

NREL explored specifi c nanoscale components 

and materials, how to make them, their characteristics and properties, the mechanisms underlying 

those properties, and how to understand those 

properties and predict others. 

For example, in 1983, shortly after the Laboratory 

fi rst opened its doors, NREL began making and 

studying colloidal quantum dots (QDs). NREL 

researchers were among the fi rst to report on the 

quantization effects of nanosized semiconductor 

particles—a phenomenon whereby emission and 

absorption spectra vary dramatically with the size 

of the nanoparticle (see sidebar “Quantum Confi nement: Shifting Toward the Blue”). 

Starting in 1991, NREL’s Solid-State Theory 

Group began fundamental explorations of QDs, 

developing a theory that enables them to model 

optical and electronic properties of neutral and 

charged QDs and QD arrays. NREL also began its 

study of single-wall carbon nanotubes in 1993, 

the same year their existence was discovered, and 

of nanoparticle precursors in 1995.

Nanoparticle Precursors. Nanoparticle precursors 

are typically metal or semiconductor material 

crystalline structures less than 20 nm in diameter 

and that are used to form a precursor state for a 

macroscopic device. In the process of forming the 

macroscopic device, the nanoscale nature of the 

starting material is altered or destroyed. Although 

nanoparticle precursors do not generally exhibit 

Page 9
Page 10 

in the presence of the appropriate catalyst, form 

the nanotubes. The diameter of the SWNTs can 

be manipulated by controlling the pulse power of 

the laser—the higher the pulse power, the smaller 

the carbon fragments and the smaller the diameter of the tubes. NREL has also developed chemical vapor deposition methods to form SWNTs 

 using methane as a feed gas. This process, though 

not as controllable as the laser-based one, promises to be amenable to low-cost, large-scale manufacture of nanotubes. Recently, NREL developed a 

hot-wire chemical vapor deposition technique, in 

which a hot wire fi lament in a vacuum chamber is 

heated in a carbon-bearing gas, such as methane. 

As the gas decomposes, it frees the carbon atoms, 

which recombine to form multi-wall nanotubes. 

Carbon nanotubes have interesting properties 

that make them valuable for a variety of applications. For example, they have extremely high tensile strength and could be used in composites for 

making armor, space crafts, or lightweight, resilient cars (important for energy-effi cient transportation). Multi-wall nanotubes have interesting 

fi eld emission characteristics, prodigiously emitting electrons at low voltage from their tips; this 

gives them great potential for use in solid-state 

lighting and fl at-plate displays.

Because SWNTs have small diameters, typically 

less than 2–3 nm, quantum effects are readily 

apparent. For example, electrons in a SWNT can 

maintain their quantum state and travel ballistically along the nanotube, electrons and phonons are strongly coupled, and exciton binding 

energies exceeding 1 eV have been predicted. In addition, SWNTs can be 

made with either semiconductor 

or metallic properties, depending 

on how the two- dimensional 

sheets of graphite are rolled 

up. The semiconductor variants exhib it strong photoluminescence. These properties 

make SWNTs ideal candidates for wires that carry 

high current and for applications in which highly conductive and high-surface materials 

are required, such as in fuel cells, ultracapacitors, 

and organic photovoltaics. They are also strong 

candidates for supports for other nanoscale catalysts. Nanotubes can also be used as interconnects 

in highly effi cient electronic switches, for photoactive media in transistors or solar cells, and as 

conduits for transferring charge carriers between 

quantum dots.

One characteristic of great interest to NREL is the 

ability of SWNTs to reversibly store lithium ions 

and hydrogen gas. This capability is particularly 

important for use in lithium batteries which, for 

energy storage, have the advantages of high energy 

density, low maintenance, long life, light weight, 

and design fl exibility. For hydrogen, nanotubes 

show promise of safe, lightweight, high-density 

storage, which may prove to be particularly valuable for the future hydrogen economy—in which 

abundant, benign hydrogen, along with hydrogen 

fuel cells, will supply energy for heating, cooling, 

electricity, industrial processes, and transportation. Carbon nanotubes and related carbon-based 

materials are among our best bets for meeting 

the DOE goal to develop systems that will store 

enough hydrogen for our cars and trucks to be 

driven more than 300 miles between fi ll-ups without affecting cargo or passenger space. (For more 

on hydrogen storage with SWNTs, see the article 

“New Horizons for Hydrogen” in the National 

 Renewable Energy Laboratory 2003 Research Review.)

Quantum Dots. Quantum dots are small semiconductor nanocrystals with diameters that range 

from about 2 to 10 nm and that typically contain 

from hundreds to thousands of atoms. Because 

of their small size, quantization effects become 

 important. Quantum dots confi ne electrons to 

discrete, widely separated energy levels, which 

gives the dots optical and electronic properties 

that are dependent on the size of the dots and the 

material out of which they are made (see sidebar “Quantum Confi nement: Shifting toward the 

Blue”). This makes them particularly valuable for 

solar cells, as a pathway toward high effi ciency 

and low cost. 

NREL makes several types of QDs, including 

CdSe, CdS, CdTe, InP, InAs, PbSe, and PbS. We 

A salient feature of QDs is that they 

can be tailored to absorb or emit specific 

wavelengths of light simply by changing the 

size of the dot. Compared with the bulk material, the 

light spectra emitted or absorbed by QDs will shift to the 

blue (greater energy or shorter wavelength); the smaller the 

dot, the greater the shift. The shift is dependent on the size 

of the dot and the type of material. 

Why does such quantized absorption or emission happen? 

Fundamental to quantum physics is the concept of the 

wave/particle duality of matter. For example, an electron 

can be thought of either as a particle or as a wave moving 

through its space. In bulk material, electrons exist at many 

energy levels—in fact, a continuum of energy levels in 

 valence and conduction bands—because of the numerous 

 atoms in the material. (No electron, however, can exist in 

the forbidden region between the top of the valence band 

and the bottom of the conduction band.) But, as the size of 

a material approaches the nanoscale, it starts to confine the 

energy levels at which an electron may exist. 

To see why, think of an electron as a wave and the QD as a 

sphere with a diameter approximately the size of the Bohr 

radius, which confines the electron (see also sidebar “Excitons”). In such a confinement, the electron has to travel 

within the inside circumference of the sphere so that its 

wavelength does not destructively interfere with itself. This 

means that the confining circumference must be equal to the 

electron wavelength or some whole number multiple of 

To exist, an electron in a 

confined space (such as in 

a spherical quantum dot) 

must have an associated 

wavelength that is equal 

to the inside circumference of the confining 

space or a whole number 

fraction thereof (1⁄2, 1⁄3, 

1⁄4, etc.). The smaller the 

dot, the shorter the wavelength that fits, the higher the energy of the electron. Hence, the absorption and emission spectra shift to the blue (higher energy) for 

smaller 

dots.

Quantum Confinement: Shifting toward the Blue 
Page 11

Imaging 

with atomic 

force microscopy 

shows QDs (conical 

shaped) attached to an 

individual SWNT (thin wire 

shape), demonstrating the 

ability nanotubes have to 

organize quantum 

dots. 

to move along the length of the rod. Nanorods 

of some materials can be grown using colloidal 

chemistry in a similar manner to QDs, except that 

growth is promoted along one axis and retarded 

along the other two axes via appropriate control of 

the growth solution, growth kinetics, and surfactants. There are other methods being explored to 

grow nanorods, such as via nucleation in a supersaturated solution. Nanorods of particular interest 

to NREL include CdS, InP, PbSe, PbS and several 

oxides, such as ZnO and TiO2. Like QDs, nanorods 

are particularly promising for use in solar cells 

(also see sidebar “Printing Polymer Solar Cells”).

Nanostructured Solar Cells 

NREL’s early explorations are paying off. 

They have given us the ability to make 

and understand nanoscale components. In addition, our use of 

sophisticated 

measurement 

techniques—

such as atomic 

force 

 microscopy, 

scanning tunneling microscopy, 

transmission electron 

microscopy, near-fi eld scanning optical microscopy, and 

more—have enabled us to image what 

is occurring on the nanoscale and to measure the material, optical, and electronic properties of these nanocomponents. This ability to 

make, measure, image, and model nanoscale components and systems has given us the tools with 

which to proceed to the next important step—

to build composite devices and systems with new 

properties that will allow us to do new things.

Among these new composite systems is nanostructured solar cells. Solar PV cells represent an 

extremely promising technology for supplying 

large amounts of non-carbon energy in the future. 

However, to meet that promise—to be used on 

the massive scales needed—PV electricity has to be 

competitive with the least-costly alternative. This 

means the present cost of PV electricity has to be 

decreased by at least three- to fi vefold 

to 6¢/kWh, or less. Plus, if PV electricity is to be used to competitively produce hydrogen (via the electrolysis of 

water) on a central-station scale for 

the future hydrogen economy, the cost 

has to be reduced by at least another 

factor of two to three.

make dots in two ways (both of which can be selforganizing): epitaxial growth and colloidal chemistry. Epitaxial growth is the growth of crystals 

of one semiconductor material on the surface of 

 another, in which both materials have the same 

structural (lattice) orientation. NREL researchers 

are exploring a particular version of epitaxial 

growth known as Stranski-Krastanow growth, in 

which a mismatch of the lattices produces strains 

between the two materials causing islands to 

form. Most of the deposited material will accumulate on the islands, thus forming a matrix of 

QDs, whose size can be directed by controlling 

the growth conditions. To make these QDs, 

researchers use either of two well-known vacuum 

technologies: molecular beam epitaxy or metalorganic chemical vapor deposition.

Colloidal chemistry, however, is NREL’s primary 

method of making quantum dots. This is an inexpensive way to grow the dots, requiring only the 

right chemicals, low-cost equipment, and roomtemperature processing. Colloids are ultra-fi ne 

particles suspended in a liquid solution—colloids are formed by not having the solute molecules break up into atoms and thus dissolve. 

The formation of QDs involves a chemical reaction between a metal ion (such as indium) with 

a molecule that donates a different ion (such as 

phosphide). The size of a QD is manipulated by 

controlling the concentration of the reactants, the 

medium in which they react, the temperature of 

the reaction, and the length of the reaction.

The trick is to keep QDs from agglomerating 

when they have reached the desired size. This is 

done generally by having the reaction take place 

in the presence of organic molecules that act as 

surfactants and coat the surface of the nanoparticles—the organic molecules stop the crystals from 

clumping together.

Nanorods. These are semiconductor rods that are 

typically on the order of 2–10 nm in diameter 

and tens to several hundreds of nanometers long. 

It is the small diameter that promotes quantization effects and gives them similar characteristics 

to quantum dots. Nanorods, however, are unconfi ned in one direction, and charge carriers are free 

the wavelength. If it is other than this, the wavelength would 

 interfere with itself and destroy its possibility of existing. 

Thus, only certain (quantized) wavelengths can exist in 

such a confinement. The shorter the diameter of the confinement, the shorter the wavelength of the electron. And 

 because energy is inversely proportional to wavelength 

 (directly proportional to frequency), the smaller the QD, 

the smaller the allowable wavelength, the greater the 

 frequency, and the bluer the associated photon emission or 

absorption spectra.

The same phenomenon holds for nanorods. But in this case 

the rod is to be likened to a cylinder and the confinement 

space dictated by the diameter of the cylinder.
Excitons 

In the typical PV cell using bulk semiconductor material, 

 incident photons dislodge electrons from their bound state 

in the valence band into the conduction band. A dislodged 

electron leaves behind a hole of opposite charge. This electron-hole pair is bound together by the coulomb force as 

a neutral excited state. The distance between the electron 

and the hole is known as the Bohr radius and is typically on 

the order of a few nanometers.

In bulk PV semiconductors, the freed electrons and holes 

can move freely in all directions. And because there are an 

immense number of atoms connected via crystalline lattice networks, the coulomb attraction between an electron and its hole can be weakened because of screening by 

other atoms and the lattice, and because of the mass of the 

constituents of the material. As a result of this weak coupling, electron-hole pairs readily dissociate at room temperature and can be swept to opposite sides of the PV cell 

Today, PV electricity costs about 

18−30¢/kWh, depending on the 

application and its scale. The technology that has dominated the PV 

market for the past two or three 

 decades—the so-called fi rst-generation PV technology—is based on 

thick (150–300 μm) cells of crystalline silicon. Although a mature 

technology, further innovations 

and economies of scale may be 

able to cut the cost of silicon PV 

electricity by another factor of two or so.

Emerging PV systems based on thin-fi lm layers 

(1–2 μm) of semiconductor materials—known 

as the second-generation PV technology—are currently competitive with crystalline silicon. Even 

though modules from these materials are not 

generally as effi cient as those of crystalline silicon, because of the thinness of the material and 

the ability to use mass production, thin-fi lm PV 

has the potential to drop PV costs to the aforementioned 6¢/kWh.

Even if thin-fi lm PV meets its long-term potential, according to a report by the Basic Energy Sciences Advisory Committee we would still have 

to drop costs another three to six times to make 

it economically attractive enough to provide the 

scale of applications the world will need. But to 

achieve such cost reductions we need a quantum leap to a third-generation PV technology. This 

is where nanotechnology comes into the picture. 

By using nanotechnology, polymer technology, 

and innovative production techniques, there are 

two paths whereby nanostructured solar cells can 

 reduce costs suffi ciently: by making the solar cells 

extremely inexpensive, and by making them very 

effi cient at converting sunlight to electricity.

An Inexpensive Route—the 

Dye-Sensitized Solar Cell

Titanium dioxide (TiO2) is an inexpensive material that exhibits semiconductor properties. But as 

a solar cell material, it has a major drawback—its 

band gap is greater than 3 eV. The solar spectrum, 

on the other hand, is comprised of photons with 

energies ranging from 0.3 eV to 3.5 eV. So TiO2 by 

itself can absorb and convert only that small portion of the solar spectrum whose photons have 

an energy equal to or greater than 3 eV. However, 

scientists from NREL and elsewhere who have 

worked on this concept have long recognized that 

TiO2 has the potential to produce inexpensive 

electricity by using it in a photoelectrochemical 

cell with an appropriate dye that can absorb 

much of the solar spectrum. 

The basic design of the cell consists of TiO2, the 

dye, and electrolyte sandwiched between a transparent conducting oxide (TCO) electrode and 

a counter-electrode (see fi gure). The sunlight 

 excites and oxidizes the dye molecules, which 

produce electrons that are injected into the conduction band of the titanium dioxide. Charge 

separation occurs at the interface between the dye 

molecules and the titanium dioxide. The electrons are transmitted by the TiO2 to the electrode 

to provide current for a circuit to do work and 

 return to the cell via the counter-electrode. The 

oxidized dye molecules are reduced by electrons 

transferred by redox (oxidation-reduction) couples in the electrolyte.

This concept did not work well until used with 

TiO2 nanocrystals of approximately 30 nm diameter. Although these nanocrystals do not exhibit 

quantization effects, their use greatly increases the 

contact area between the dye, the TiO2, and the 

electrolyte to enhance the effi ciency of electron 

injection from the dye. 

One drawback to the concept is that it uses a liquid electrolyte. But this can be circumvented by 

using a polymer-based electrolyte or a hole-transporting polymer; either alternative leads to longer life and lower cost and provides the ability to 

make fl exible solar cells. 

Other alternatives being explored include the 

use of QDs in place of the dye, or even turning 

to a different approach altogether—such as a 

polymer/metal oxide confi guration (see the sidebar “Printing Polymer Solar Cells”)—to achieve a 

low-cost solar cell. 

Although this approach has not resulted in higheffi ciency solar cells—with the best dye-sensitized 

cells exhibiting effi ciencies around 11% and modules having typical effi ciencies of about 5%—it 

has a high potential for low cost because of the 

low cost of the TiO2, the dye or the QD alternative, and the polymers and because of inexpensive 

Page 12
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by an induced electric field (typically via doping semiconductor layers with extra electrons or holes) to be collected 

by a transparent conducting oxide layer or a metallic grid. 

 Alternatively, electrons and holes could typically recombine within a few microseconds before they have a chance 

to be further separated for collection. 

In QDs (or nanorods or conjugated polymers), the situation 

is different. The electron and its associated hole are close 

together, are not screened from each other, and the coulomb attraction between the two is quite strong (especially 

in conjugated polymers)—a situation that leads many in 

the field to label such an electron-hole pair as an “exciton.” 

Under such conditions, the electron-hole pairs in excitons 

can quickly recombine—typically on a sub-nanosecond 

time scale. Moreover, excitons retain a neutral charge, so 

you cannot use an electric field to dissociate them. Rather, 

excitons in QDs (and conjugated polymers) are dissociated 

at interfaces with other materials that have high affinities 

for electrons or holes, with electrons taken in one direction 

at the interface and holes in another. Movement of excitons is initiated via diffusion; but with a high concentration of excitons and interfaces, this diffusion readily results 

in dissociation. The dissociation, in turn, sets up a chemical 

 potential gradient that drives the PV effect.

So what can you use to scavenge the excitons and dissociate 

the charge carriers to produce the chemical gradient and 

a photocurrent? Both fullerenes (C60) and single-wall carbon nanotubes have high electron affinities that can overcome an exciton’s binding energy to siphon freed electrons. 

Conjugated polymers, in which polymer matrices can be a 

combination of hole-conducting and electron-conducting 

can also be used to dissociate and conduct electrons in one 

 direction and holes in another.

The dye-sensitized solar cell 

represents a pathway toward low-cost solar electricity. It uses dye molecules 

to absorb a large portion of 

the solar spectrum and produce electrons, which are 

injected into the conduction 

band of the TiO2 nanocrystals, and then transferred 

to the TCO electrode for collection. A variation on this 

concept is to replace the 

dye molecules with quantum dots for harvesting the 

sunlight and producing the 

charge carriers.

processing techniques available for making 

the cell.

This is also a versatile approach because it will 

 enable us to explore a variety of different substrates on which the cell can be deposited, it 

could allow production of fl exible solar cells 

that could fi t on any surface, and it would allow 

 other layers, such as WO3, to be added to the 

device, which would merge the solar-cell function with an electrochromic function. Such a 

device employing other layers could be used for 

windows, for example, that would not only generate electricity but would also lighten or darken 

as desired in accordance with the amount of sunshine.

The Road to High Effi ciency 

Today’s typical solar cell relies on a semiconductor 

material with a single junction and a single band 

gap. Such a cell has limited effi ciency because 

photons with less energy than the band gap will 

not be absorbed, while photons with energy 

greater than the band gap will dislodge electrons 

into the conduction band, but with too much 

energy. The excess energy absorbed by an electron 

will push it into a high and unstable energy level. 

The electron will quickly relax to a 

more stable level, emitting the 

excess energy as heat. This not 

only wastes the excess photon 

energy, but the heat it produces 

further decreases the effi ciency 

of the cell.

One strategy to overcome this 

limitation is to use several layers of semiconductor materials 

with different energy band gaps, 

stacking the layers in decreasing band-gap order. That way, 

the higher energy photons of the 

solar spectrum will be absorbed 

by the higher band gap material and lower energy photons by lower 

band gap material. Thus, the photon energy will 

be used more effi ciently because of a closer match 

Page 13

with the energy band gaps of the absorbing materials, and not so much heat will be produced. By 

stacking an increasingly large number of materials with different band gaps, as the number of 

layers approaches infi nity you can approach the 

theoretical effi ciency limits of 68% under one 

sun and 86% under concentration. The problem 

is that the engineering and physical challenges 

become increasingly diffi cult the more layers that 

are stacked in this manner; and costs rise accordingly. Thus far, the best cell using this approach 

has been an expensive three-junction device of 

high-quality material that attained 37.3% effi -

ciency under high solar concentration.

Quantum dots potentially offer a way around 

this conundrum. Quantum dots can easily be 

made with different band gaps simply by controlling the size of the dots. By using differentsize dots, a many-junction device can be devised 

to capture and convert the entire solar spectrum. 

By using a suffi cient number of dots with differing sizes, the theoretical limits of effi ciency may 

be approached. And it may be done inexpensively 

because of the use of colloidal chemistry to produce the QDs and a low-cost matrix in which to 

imbed them.
Carbon Nanotubes. The use of carbon nanotubes 

for organizing and connecting QDs combines the 

depth of experience that NREL researchers have 

with QDs with their expertise with single-wall 

carbon nanotubes. In this vein of research, scientists are exploring both chemical and physical 

methods to bring SWNTs and QDs into intimate 

electronic contact. 

With one alternative, NREL researchers have 

 developed a simple way to form 

one-dimensional closepacked arrays of semiconductor quantum dots. By 

combining size-selected 

InP and CdSe QDs and 

highly pure nanotubes 

Page 14 

The Right Medium/

The Right Confi guration

With QDs, there are immediate barriers to overcome. The 

fi rst is to dissociate an exciton’s freed electron from its 

paired hole before they recombine. This is done at interfaces 

with other materials, which 

will siphon the charge carrier, taking the hole in one 

 direction and the electron in 

the other to separate grids or 

collection points (see the sidebar “Excitons”).

One way to do this is to 

 deposit the QDs into a 

medium consisting of a mixture of conjugated polymers 

(polymers that exhibit semiconductor properties), one of 

which attracts and conducts 

holes and the other of which 

 attracts and conducts electrons. This is a highly desired approach because it 

has the potential to be a very inexpensive alternative, using plastics and colloidal QDs, and it may 

even allow for high effi ciency. 

Thus far, however, cell conversion effi ciencies 

 using this approach have generally been less 

than 3%–4%. Diffi culties arise with conjugated 

polymers because they contain many sites that 

 engender the recombination of electrons and 

holes; the electrons and holes of the induced 

 excitons are strongly attracted to each other and 

so will quickly recombine; and the charge carriers do not have great mobility and are not quickly 

dissociated from one another, which increases the 

probability of recombination. Moreover, the conjugated polymer matrix would have to be blended 

correctly to ensure that most QDs were in contact 

with both hole- and electron-conducting polymers, to ensure a high level of exciton dissociation and conduction of charge carriers. 

Connecting the Dots

To achieve high-effi ciency cells, therefore, it is better not to mix QDs randomly in a polymer blend, 

but to align the dots in arrays so that they communicate with one another effi ciently. This could 

be done by arraying the dots within a few angstroms of one another to enable electronic coupling among the dots so that electrons can be 

transported long distances. How you align the 

dots into arrays becomes the hot question. NREL 

researchers are exploring a couple of innovative 

ways to accomplish this: carbon nanotubes and 

proteins. 

Other Uses for NREL 

Nanotechnology • • •

There are many other possible uses inherent in NREL’s 

nanotechnology research.

• Nanocatalysis. Catalytic 

 reactions occur on catalyst 

surfaces. Because smaller 

particles increase surface 

area, nanocatalysts have 

the potential for a great 

amount of surface area 

to perform catalytic reactions. Other nanostructures (such as nanofibers) 

can also provide a large 

 active surface area for catalyst support. The modification of electronic and 

material properties at the 

nanoscale may also play a 

beneficial role in catalysis. 

Nanocatalysis has particular promise for the production of fuels from biomass, 

producing hydrogen from 

biogas during the watergas-shift reaction, and for 

use with membrane technology for the separation 

of hydrogen gas.

• Gas-Separation Membranes. Gas-separation 

membranes are used to 

separate types of gases 

from one another. This 

can be accomplished 

 because of the porosity 

of the membrane materials or because of their 

ion- or electron-affinity. 

In either case, nanomaterials, especially those of 

carbon nanotubes, which 

have high electron affinity, may offer advantages. 

Also, because nanostructures offer advantages for 

catalysis, they can be used 

to help drive the process 

of gas separation. Gas separation membranes could 

find applications in the 

separation of hydrogen 

from biogas and in fuelcell membranes to separate hydrogen protons 

from the electrons of a 

 hydrogen atom. 

Cellulosome polymers can 

be used to align quantum 

dots. Here, CdSe quantum dots 

(blue-green) are aligned via attachment to a streptactin protein (yellow), which, in turn, binds to the 

cellulosome via cohesin domains 

(red). Surface binding domains 

(blue) are shown binding to the surface of a cellulose microfibril.

In solution, SWNTs exhibit a photoluminescence spectra whose characteristics differ in accordance with the diameter of the nanotubes and their chirality. Photoluminescence occurs because of the recombination of electrons 

and holes induced by incident light. Such a spectra is indicative that SWNTs 

can be used to harvest light to generate electricity. 
 • • • Even More Uses 

• Filtration of Microbial 

Pathogens. NREL has developed an R&D 100-winning technology based on 

nanoscale ceramic fibers. 

Because of an extremely 

high surface area and a 

chemical affinity, these 

 fibers are particularly useful for filtering microbial 

pathogens and viruses 

from water and for purifying blood. The fibers are 

also useful for eliminating 

heavy metals from water, 

biosynthesis and bone 

growth, catalyst support, 

and filtration of other liquids and gases.

• Air Filtration. Nanomaterials can be used in combination with desiccant 

 materials for conditioning 

and dehumidifying air. In 

the process, nanomaterials enhance the absorption 

of a wide variety of particles from the air, including 

pathogenic bacterial and 

fungal spores. 

• Ultracapacitors. Like batteries, ultracapacitors store 

electricity. But they do not 

do it chemically; they do 

it physically by separating positive and negative 

charges. Compared with 

batteries, they can store 

large amounts of electricity in a very small volume, 

and can deliver orders of 

magnitude more charge 

and discharge cycles. 

Nanomaterials, in particular carbon nanotubes, 

can be used in the development of ultracapacitors 

that will have important 

uses for all walks of life—

from communication to 

transportation. 

• QD OLEDs. Organic lightemitting diodes (OLEDs) 

have the great promise of 

 emitting tunable wavelengths of light inexpensively and with great 

energy efficiency. Quantum dots are particularly suitable for use with 

OLEDs because they can 

be easily tuned to emit 

the desired wavelengths, 

which can be combined to 

yield white light.

Page 15

organic polymer on the planet. Through the Laboratory’s work with biofuels, in which alcohol 

fuels are derived from biomass cellulose, NREL 

biochemists have plenty of experience with cellulosomes. Now they are working with NREL physicists to use cellulosomes to align quantum dots.

A cellulosome is a conglomerate of functional 

subunits. The backbone of this conglomerate is 

known as the scaffoldin, which organizes and 

 integrates the subunits into a multi-enzyme complex (see fi gure). The scaffoldin unit contains 

a site known as the surface-binding domain, 

which enables the cellulosome to bind to a cellulose substrate. It also holds a series of cohesin 

 domains, to which enzymes will bind via sites on 

the enzymes known as dockerin domains.

This is the normal order of things. But the size 

of the cohesin domains where the enzymes normally dock is approximately equal to the size of 

quantum dots. The trick, therefore, is to remove 

the enzymes and encourage the cohesin to 

 instead bind to quantum dots or, alternatively, to 

bind to another protein, which then could bind 

to quantum dots. Once this is done, and because 

these units can self-organize into long polymer 

chains, this approach can be used to produce a 

chain of quantum dots. Consequently, using an 

appropriate substrate to which scaffoldin units 

can adhere, QDs can be maneuvered into arrays 

with close enough spacing to promote electronic 

coupling among the dots. 

This is breakthrough, state-of-the-art research. 

Nonetheless, NREL scientists have shown it can 

be done using a genetically modifi ed cohesin/

dockerin polymer to align (CdSe)ZnS QDs 

on cellulose fi brils. The next challenge for the 

 researchers is to reduce the spacing between dots 

that have low defect densities, while gently refl uxing both in organic solvents, linear arrays of QDs 

are self-assembled by van der Waals forces into 

the grooves between adjacent nanotubes.

In another alternative, SWNTs can be diced into 

short segments and solubilized in solution using 

surfactants, much in the same way that semiconductor colloids are typically solubilized. Once 

in solution, the soluble SWNTs can be treated 

as reagents and reacted with QDs in solution, to 

promote coupling among the quantum dots 

One interesting characteristic of SWNTs is that 

they exhibit emission and absorption spectra similar to those shown by QDs of different sizes. 

In particular, the photoluminescence intensity 

of the peaks in the spectra is dependent on the 

 diameter of the nanotubes and their chirality (i.e., 

their helical pitch, or how the graphite sheets roll 

up). This presents quite a fl exibility when using 

SWNTs with quantum dots. Depending on the 

type of SWNT used to provide the coupling, the 

interconnection can either provide simple electronic communication among the dots (when the 

SWNT is a wire), or it can be used to help harvest 

the solar spectrum when the SWNT is semiconducting and photoactive. Here, a long-term goal 

is the construction of hybrid nanoscale systems 

in which sunlight can be effi ciently harvested and 

converted to generate electricity or fuels, such as 

hydrogen.

Proteins. The use of proteins to align dots is a 

method unique to NREL researchers. To be more 

specifi c, researchers genetically modify a cellulosome protein polymer so that it will attach 

to quantum dots. A cellulosome is an intricate 

multi-enzyme complex that effi ciently degrades 

cellulose, which is 

the most abundant 
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from a minimum of 50 Å to about 30–40 Å and 

to make the spacing more homogeneous.

Hot Carrier Solar Cells 

But other breakthrough results may be required 

to approach the high-effi ciency limits of solar 

cells. One possibility is to develop so-called 

“hot electron” solar cells. When a freed electron 

is bumped high into the conduction band by a 

too-energetic photon, its electronic temperature 

 becomes quite hot (as high as 3,000 K). The hot 

electron will relax to the bottom of the conduction band, typically within a few hundred femtoseconds, imparting heat to the lattice as it does so.

This can be avoided by removing the hot electron 

for use in a circuit before it relaxes. Doing so will 

achieve two things: 1) use of the high-energy electron will increase the photovoltage of the device 

as well as its effi ciency; and 2) the excess energy 

will be prevented from heating the device and 

from lowering its effi ciency. 

The key to removing the hot electron is to retard 

its relaxation. And the key to this is to confi ne 

the exciton to a quantum dot. A hot electron will 

cool quickly, especially in the presence of a hole. 

Remove the hole promptly and, due to quantization effects of a QD, the energy levels to which 

the electron can readily relax will be limited. The 

hole can be siphoned in a matter of a few femtoseconds by imbedding the QD in the right material, and perhaps even the right polymer material. 

This could extend the hot electron relaxation time 

by one or two orders of magnitude. Combine this 

with a properly ordered array of QDs, and the hot 

electrons could be transported along the array via 

resonant tunneling. 

Another way to utilize hot or highly excited electron-hole pairs (excitons) in QDs is to use the 

Page 16

Next, the nanorods are coated with a thin layer (less 

than 20 nm) of conjugated polymer, which absorbs photons to produce excitons. Finally, a silver electrode is 

deposited on the polymer layer. 

Although this is a state-of-the-art concept, NREL 

 researchers have not only successfully fabricated such 

a device, they are already exploring ways to improve it. 

One way is to coat the nanorods with a monolayer of a 

material that has high electron affinity, such as fullerenes (C60), before the conjugated polymer is applied. 

In conjugated polymers, photon-induced excitons have 

low mobility and the electron and hole can typically 

 recombine on a sub-nanosecond time scale. That is why 

C60 (or some other material with high electron affinity) 

would be valuable in such a device; it could quickly dissociate excitons and extract electrons from the polymer 

and transfer them to the ZnO nanorods, which would 

conduct the electrons to the TCO. This would enable the 

conjugated polymer to conduct the dissociated holes to 

the silver contacts.

The device can be designed so that excitons will travel only 5–20 nm before encountering an interface. This 

Printing Polymer Solar Cells

NREL scientists are combining the use of nano-, polymer, and low-cost deposition technologies in an 

 approach that promises inexpensive solar cells. The cell 

concept is innovative. It starts with an inexpensive substrate on top of which is layered a transparent conducting oxide (TCO) that lets light through to the cell and 

acts as a cell electrode. This TCO layer can be made with 

nanoparticle precursors to allow constituent materials 

to be incorporated into an ink that can be written onto 

the substrate via piezoelectric inkjet printing. 

A nucleation layer is then deposited on the TCO. This 

layer is chemically prepared to enable nanorods to 

grow vertically (via nucleation in a supersaturated solution) and at close intervals from the surface. The nanorods, which can be of ZnO or other oxides, grow a few 

nanometers thick and 200 or more nanometers long. 

This gives the rods a high aspect ratio (i.e., ratio of the 

rod’s length to its width), which, in turn, promotes a 

great amount of surface area per given mass of material—up to 600 m2/gm. This gives the nanorods a great 

ability to scavenge and transport freed electrons from 

the surrounding material. 
Page 17

 excess energy to create multiple electron-hole 

pairs. NREL researchers have found that up to 

three excitons per photon can be created in small 

band gap semiconductor QDs when the photon energy is four times the QD band gap. This 

 approach is very promising for creating much 

more effi cient nanocrystalline solar cells.

The exploitation of hot electrons is just one of 

the cutting-edge nanotechnology concepts being 

 explored by NREL researchers. But breakthroughs 

in this area, along with continued advances in 

QDs, nanorods, carbon nanotubes, conjugated 

polymers, and other related fi elds discussed here 

have the potential not only to present the world 

with inexpensive renewable electricity, but also to 

provide it with plentiful non-carbon energy in all 

usable forms—for transportation; building heating, cooling, and lighting; and industrial processing needs.

This solar cell concept uses conjugated polymer to harvest light and 

produce excitons, C60 to dissociate electrons from the excitons, 

ZnO nanorods to transport the 

electrons to the TCO electrode, 

and hole-conducting polymer 

to carry holes to the silver 

electrode. With the potential 

for tuning the cell to absorb 

and convert much of the solar 

spectrum and to inkjet print 

much of the cell, this approach 

could result in very inexpensive 

electricity.

 enhances the ability of electrons and holes to be separated more quickly than they may recombine. By 

 designing a device in which electrons and holes are separated at interfaces in a few tens of femtoseconds (10-15

seconds) or less, charge carriers are efficiently collected 

to produce an electric current. 

Such a concept has great potential. Researchers believe, 

for example, that there may be the possibility that 

much of the device—the TCO, the fullerenes, the polymer, and the silver electrode—could be deposited via 

inkjet printing. Also, because conjugated polymers may 

be chemically altered to exhibit different absorption 

characteristics, there is the potential that they could be 

tuned to absorb the entire solar spectrum to produce 

carriers. One way of doing this is to chemically tune the 

band gap by modifying the conjugated polymer backbone. Another way is to attach side chemical groups to 

the polymer backbone. 

Effectively, each nanorod/C60/polymer combination can be considered a minicell, connected to the silver electrode on one end and to the TCO electrode on 

the other. This opens the door for tuning each minicell 

 combination to different wavelengths of light. Thus, a 

cell like this could become a many-junction device that 

could efficiently absorb and convert the entire solar 

spectrum to excitons.

Such a solar cell would have many advantages. The 

 entire cell, for example, would be approximately 200 

nm thick and would thus use very little material. It 

could be made on an inexpensive and flexible substrate 

to fit surfaces as needed. There is the possibility that 

 almost the entire structure could be made using solution chemistry, nanoprecursor inks, and inkjet printing. 

The entire device could be processed at temperatures 

below 400 ̊C, and probably at less than 200 ̊C—both 

of which are far lower than the temperatures at which 

 solar cell are currently processed. The device could be 

made without using vacuum technology, using much 

less expensive equipment. And it could be made translucent, for use on such spaces as windows to allow lighting and to produce electricity. All of this could add up 

to the potential for an extremely inexpensive solar cell 

and affordable electricity. 

For More Information

Brabec, C. J.; Nann, T.; Shaheen, S. E. (2004). “Nanostructured p-n Junctions for Printable Photovoltaics.” MRS Bulletin. Vol. 29(1), January 2004; 

pp. 43–47.

Ding, S. Y.; Rumbles, G.; Jones, M.; Tucker, M. P.; Nedeljkovic, J.; Simon, 

M. N.; Wall, J. S.; Himmel, M. E. (2004). “Bioconjugation of (CdSe)ZnS 

Quantum Dots Using a Genetically Engineered Multiple Polyhistidine 

Tagged Cohesin/Dockerin Protein Polymer.” Macromolecular Materials and 

Engineering. Vol. 289(7), 2004; pp. 622–628.

Green, M.A. “Third Generation Photovoltaics: Concepts for High Effi -

ciency at Low Cost.” Proceedings for the Symposium on Photovoltaics for the 

21st Century, II; March 25–30, 2001, Washington, DC. ISBN 1-56677-316-

4. Pennington, NJ: The Electrochemical Society, Inc.; pp. 3–20.

Micic, O. I.; Cheong, H. M.; Fu, H.; Zunger, A.; Sprague, J. R.; Mascarenhas, A.; Nozik, A. J. (1997). “Size-Dependent Spectroscopy of InP 

Quantum Dots.” Journal of Physical Chemistry B. Vol. 101(25), 1997; pp. 

4904–4912.

Nozik, A. J. (2002). “Quantum dot solar cells.” Physica E. Vol. 14(1-2), 

April 2002; pp. 115–120.
The

Story

of

ADAM

Genesis of a More Effi cient,

More Comfortable Car

will, for example, see a dramatic rise in the use of 

HEVs (hybrid electric vehicles) in which an internal combustion engine (ICE) is used in combination with batteries, an electric motor, and 

generator to power our cars. Today, HEVs are 

some of the most fuel-effi cient vehicles on the 

road, and the concept is starting to be offered 

not just in compact cars but also in luxury cars, 

SUVs, and other vehicles. The components that 

constitute a car’s propulsion system will become 

more compact, effi cient, and lightweight. Vehicles 

will get double or triple their fuel economy and 

greatly increase their range. 

We will also see the beginning of the hydrogen economy, in which hydrogen will be used 

to power our vehicles, using fuel-cell technology. Fuel cells are not only far more effi cient than 

ICEs, but they produce no pollution or greenhouse gases—the only product of a hydrogen fuel 

cell (other than energy) is water vapor. 

Our next generation of cars will be sleeker and 

more comfortable. The hump that runs down 

the middle of the fl oors of many cars could 

become an historical curiosity, as could foot 

pedals and mechanical devices used to control 

our cars. 

In America, the personal car (or light truck) is not 

only a right we expect, or a necessity for our way 

of life but also a symbol of our freedom. So with 

the twin threats of rising energy prices and increasing worldwide tailpipe emissions, are we in danger 

of tarnishing or losing our symbol? No. With the 

potential of new materials and upcoming technologies, we may be re-inventing the car and the freedom it represents.

The next generation of vehicles, for example, will 

bring a greater freedom of choice. We will be able 

to select the chassis we want and the body type to 

fi t on that chassis; or even several body types to fi t 

a single chassis.

It will bring the freedom of new materials out of 

which to make car bodies—new fi bers or composites that weigh far less than today’s materials, 

are stronger, longer-lasting, do not rust, can better 

stand impacts, and provide greater safety. The new 

materials also will bring a new way to manufacture 

cars. Car bodies could be molded and stamped 

out, in contrast to meticulously riveting and welding them together. This, in turn, will bring greater 

liberty to design car bodies.

The next generation of cars will also provide 

greater energy and environmental freedom. We 

Page 18 
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Running the A/C has a 

relatively larger impact 

on a high-mileage HEV 

than it does on a more 

typical car. Still, the HEV will get 

greater absolute gas mileage. 

(Chart is based on test data taken 

from SC03 driving cycles.)

And they will have A/C (air conditioning) components, systems, and strategies that will make 

you more comfortable on a hot day and do it 

more effi ciently, saving considerable energy. 

ADAM—Helping to Design 

 Effi cient Ancillary Systems 

This is where ADAM (NREL’s ADvanced Automotive Manikin) comes in, to help us design systems 

and strategies to make you more comfortable in 

your car. ADAM may be a manikin, but he’s more 

than a crash-test dummy. 

Crash-test dummies are made of materials to simulate humans during crash tests. They have sensors in the head, neck, legs, torso, and feet that 

collect and relay information about results of the 

crash tests—acceleration and deceleration, forces 

on different parts of the body, and the defl ection 

of parts of the body during a crash. This information has helped auto makers design and build 

cars that are far safer than those of decades ago. 

Today, you would be hard put to fi nd a new car 

without seat belts, air bags, a collapsible steering 

column, and more, all as standard features.

ADAM is analogous to crash-test dummies except 

that he breathes, heats, sweats, and tells us how 

comfortable he is. But we don’t crash him into 

walls or ram things into him. Rather, we put him 

into hot (or cold) cars to see how he feels and to 

see how we can make him comfortable. ADAM 

will do for effi ciency and comfort what crash-test 

dummies did for automobile safety. The information he generates will help us design cars that will 

cut the energy a car uses for ancillary loads—like 

climate control, power steering, lighting, water 

and oil pumping, all of which require fuel but are 

not used to propel the vehicle. 

When operating, the most energy-intensive of 

these is the A/C load. To see why, consider that 

your body generates 100–150 W of heat. But to 

cool you down, your car uses up to 6,000 W of 

power. This is because the A/C system is designed 

not simply to cool you, but also to cool the entire 

car for the worst-case scenario—in Phoenix, Arizona, in the height of summer after the car has 

been soaking in the hot sun.

Using this kind of vehicle-cooling strategy, the 

United States today consumes about 7 billion gallons of gasoline per year just to cool its cars. This 

is equivalent to nearly 10% of the oil we import, 

which costs the nation about $13 billion per year. 

It also adds considerably to air pollution, increasing the tailpipe emissions of nitrogen oxides, carbon monoxide, and hydrocarbons. 

This need not be. We can cool our cars far more 

effi ciently, more quickly, less expensively, using 

far less fuel, and be more comfortable in the bargain. To accomplish this, we have to design new 

components and systems and test the results of 

incorporating them into our vehicles. But to test 

a wide variety of people with different comfort zones and in different conditions would be 

time-consuming and expensive. This is where 

ADAM comes in, as a surrogate for this testing, which will help streamline the process for 

design and implementation. 

Three-Part Harmony

To be a reliable proxy for how we sense and 

respond to heat and cold and how comfortable we feel, ADAM the manikin cannot stand 

alone. Rather, he is simply the physical, sensing component of a three-part system that also 

has a regulatory component (a physiologicalresponse model), and a feeling component 

(a thermal-comfort model). 

ADAM the Sensor. As a manikin, ADAM can 

sense and respond to the environment, but his 

response is directed and controlled by the physiological response model. ADAM is designed to 

represent the 50th percentile American male. 

He is 5 ft. 9 in. tall (175 cm) and weighs about 

136 lbs. (61 kg). He is fl exible at the waist, 

and has adjustable pre-tensioned joints at the 

knees, feet, wrists, elbows, and shoulders that 

allow him to be posed in various positions, 

most notably sitting behind the steering wheel 

of a car.

ADAM has two major layers—a skeleton and 

a surface layer (skin). The skeleton is made of 

laminated carbon fi ber, making it light and 

strong. It supports the structure and houses 

internal components: energy storage in the 

thighs and torso; communication modules that 

allow the manikin to remotely communicate 

with the physiological model (ADAM also can 

be plugged into a power supply for continuous, 

non-remote communication); water storage, 

from which water is sent to the skin to enable 

sweating; and “lungs” that simulate breathing 

with exhalation of warm humid air. Through its 

joints, the skeleton also allows passage of wiring harnesses and sweat tubes, to enable communication with the skin. 

The surface layer is composed of 126 

individual segmented sections, each 

of which is a standalone unit of intricate design. Each 

section has several 

layers—an outer, 

low-porosity metal 

layer through which 

water is distributed 

for sweating; a highporosity metal layer 

that distributes the 

water to the outer 

layer; and a carbon fi ber structural 

backing, in

Driving Cool in Your HEV

So you finally bought an 

HEV—a model that emits 

about one-third the greenhouse gases and is rated at 63 

mpg, nearly three times the 

gas mileage of a typical car. 

It’s a comfortable, sleek car 

with snappy acceleration. And 

it’s so quiet that you sometimes don’t know that it’s on. 

To take it on its inaugural 

spin, you get on the interstate, accelerate to an easily 

maintained speed of 70 mph, 

switch on the A/C, and cruise 

down the highway for 410 

miles, at which point you exit 

the interstate to fill the gas 

tank. Then comes the surprise. 

You used 10 gallons of gasoline. That’s only 41 miles per 

gallon! What gives? 

The culprit is your A/C system. Your car’s engine powers 

the same type of 4-kW, inefficient, vapor-compression A/

C system powered by the average American car. The problem is, your HEV engine is a 

1-liter, 70-hp engine, whereas 

typical sedan engines range 

from 2–3.5 liters and 160–250 

hp. It takes about the same 

power to drive the A/C systems for both cars. But that 

power has a relatively higher 

impact on the smaller engine. 

So your car will experience a 

greater drop in gas mileage 

than will the typical, non-HEV 

vehicle. (Nonetheless, your 

HEV will still get more than 

twice the mileage than will 

the normal car driven with the 

A/C system on.)

What can be done? On your 

part, cut back on the use of 

the A/C when possible. In the long run, 

ADAM will play a part 

by testing and helping optimize the 

design of better ancillary systems to reduce 

the cooling load, 

including the design 

of more efficient A/C 

technologies. (See the 

sidebar “A Sound Idea 

for a Cool Car.”) 

Page 19
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which is imbedded a grid of heater 

wires to heat the segment. 

Each section also has an array 

of imbedded thermistors to 

sense skin temperature, a heat 

fl ux gauge to measure the transfer 

of heat between the backside surface 

and the internal body cavity of the manikin, and 

electronics to control the segment and to transmit 

data to and from the communication system.

Because of this segmentation, ADAM can cool 

or heat different sections of his body at different rates, depending on the environmental conditions to which parts of the body are exposed.

Central Control. Although ADAM is a sophisticated surface sensor that measures rates of heat 

loss in each zone, he does not respond on his 

own. Instead, he sends 120 sets of data on surface 

heat fl uxes to the computer-based physiologicalresponse model (120 sets because 6 of the 126 

surface-layer segments are controlled in pairs). 

Using these data, the model predicts breathing 

rates, local skin temperatures, and sweat rates, 

and transmits the set points to ADAM. Based on 

the set points, ADAM adjusts his heating systems 

and sweat and breathing rates accordingly. Then, 

as ADAM interacts with the environment, he 

transmits the resultant changes in heat fl ux back 

to the model to continue the cycle. 

The physiological-response model contains a 

detailed simulation of human internal physiological systems and thermoregulatory responses. 

The model consists of a human tissue system and 

a thermoregulatory system. The human tissue system represents the human body, including the 

physiological and thermal properties of the tissues—the bone, muscle, fat, and skin in arms, 

legs, and torso. It also includes the lung, abdominal, and brain tissues. The model calculates 

 conduction heat transfer based on temperature 

gradients between tissue nodes. 

The thermoregulatory system simulates the 

circulatory system and the respiratory system, 

the fl ow rates of these systems, and the heat 

transfer due to conduction within the fl uids, mass transfer of fl uid, and convective 

heat transfer to tissues. It also controls physiological response, such as vasomotor control (how nerves and muscles cause blood vessels 

to constrict or dilate), sweating, and shivering. 

Vessel constriction and dilation varies with skin 

and core temperatures and with each body zone 

because of vessel diameters. Sweating response is 

a function of skin and core temperatures and the 

number of sweat glands in each zone. Shivering 

is a function of skin and core temperatures and 

the amount of muscle in each zone. Blood fl ow is 

a function of skin and core temperatures and the 

metabolic rate.

The physiological-response model consists of 

about 40,000 nodes and elements. Because the 

model is so detailed, it presents a fairly complete 

picture of temperature distribution. As a result, it 

accurately simulates temperatures, heat transfers, 

and physiological responses to heat transfer. But 

it cannot by itself tell ADAM how comfortable he 

ought to feel. That part belongs to the thermalcomfort model. 

Feelin’ Good. By themselves, sensing and physiological responses have little value for designing systems to help passengers be comfortable in 

their cars. That responsibility falls to the thermacomfort model. This empirical model is based 

on work done at the University of California at 

Berkeley, in which human subjects were tested 

in transient and steady-state thermal conditions 

to determine their perceptions of local and overall thermal comfort. Using this testing data, the 

model predicts the local sensation of 19 body 

parts, the comfort of those parts, the overall sensation (given the input on local sensations), and 

the overall comfort. Plus, the model predicts how 

quickly a body or parts of the body will become 

comfortable under transient thermal conditions. 

ADAM the manikin does not directly “talk” to 

the thermal-comfort model. Instead, the “conversation” is between the two models. The 

A three-dimensional detail 

of a surface segment 

shows its sophisticated 

construction and design, 

which enables it to sense 

and respond to heat and 

to control sweating. 

simple . . . the waste heat from your vehicle can be used to 

set up a temperature difference across a pile of plates or 

“stack.” During periodic fluctuations in gas pressure, the 

gas passing through the stack is heated at the proper phase 

in the acoustic cycle to amplify the oscillations—much like 

light waves in an optical laser. The imperfect thermal contact in the stack’s pores provides the phasing between compressions, expansions, and acoustic displacement necessary 

to lead the gas through the desired thermoacoustic cycle. 

Furthermore, our thermoacoustic device pumps heat using 

standing sound waves to take the working fluid (helium) 

through a thermodynamic cycle. We rely on the heating and 

A Sound Idea for a Cool Car

NREL engineers are working on an idea that could make 

good use of the waste heat from your car’s exhaust system. 

They are researching a device that would use that heat to 

produce sound and then use the sound to cool your car—a 

concept known as thermoacoustic cooling.

Thermoacoustic effects have been understood for more 

than 100 years. However, only over the past two decades 

has substantial improvement been made to the design of 

thermoacoustic engines and refrigeration cycles. Recent 

thermoacoustic refrigerators have flown on the space shuttle and cooled electronics in a Navy destroyer. The idea is 

Page 20 
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vehicle—see sidebar “A Sound Idea 

for a Cool Car”).

Something ostensibly as simple as 

seats can save signifi cant energy 

while making you 

more comfortable. For example, lightweight 

mesh seats allow greater circulation for quicker 

cooling. Even with current A/C systems, such a 

seat can reduce the energy used by the A/C system by 3%–5%. But we can do far better than 

this. We could design seats such that each seat 

will conform to its user and will have its own 

individual A/C system in which low-velocity 

cool air circulates along your body on demand to 

cool you, without having to cool the entire car. 

Moreover, we could combine such a system with 

the use of new approaches in A/C systems, such 

as thermoacoustic cooling.

By using ADAM to test and prove such new concepts, we will be able to help the automotive 

industry incorporate a wide variety of advanced 

ancillary systems in our cars and light trucks. This 

could decrease our use of energy for ancillary systems by 75% or more. If we project that kind of 

reduction to a national level, our next generation 

of cars and trucks could reduce the nation’s consumption of gasoline by 5 billion gallons per year, 

just for our A/C systems. 

The three-part system: 1) 

ADAM the manikin, which 

senses temperature and 

sends data on heat fl ux to 

the 2) physiological-response 

model, which predicts the 

local skin temperatures and 

sweat rates, and transmits the 

set points to ADAM to control 

sweat and breathing rates, 

and 3) the thermal-comfort 

model, which determines the 

comfort of the manikin and 

its various segments based on 

surface and core temperature 

data received from the 

physiological model.

 physiological model sends sets of surface temperature data and core temperature data to the 

thermal comfort model, which uses that data to 

determine local and global sensations. It then 

uses the local and global sensations to determine the local and global comfort of ADAM the 

passenger. 

A More Comfortable Car

Given this, we are now ready to dress ADAM, put 

him in a car, subject him to different environmental conditions, and test ancillary systems to 

see how effective they are in making him comfortable, while trying to save energy at the same time. 

Dressing him is important because the clothes he 

wears affect the transient thermal response. 

What kind of ancillary systems will our research 

help make possible in our next generation of 

cars because of ADAM? There are many. We can, 

for example, design better windshields and windows that will substantially reduce solar radiation 

entering the vehicle, while allowing visibility that 

is as good as typical windshields. Especially in 

hot, sunny climates, solar radiation will quickly 

heat a car and necessitate the use of substantial 

A/C energy to cool the car down. Advanced windshields and windows could also be designed with 

coatings that could quickly de-ice them in cold 

weather without having to rely on defrosters. 

We could also design cars that are ventilated 

when parked, to dissipate heat when a car is left 

to soak in the sun. This would save A/C energy 

that may otherwise have to be used to cool the car 

from a high temperature.

A major improvement would be advanced A/C 

systems. Rather than rely on engine-driven vapor 

compression, we could use the heat from exhaust 

systems to cool our vehicles. There are several concepts being explored for this, including 

absorptive cooling (which uses an absorption 

refrigeration cycle rather than a vapor compression cycle), metal hydride cooling (in which the 

endothermic reaction—a reaction that pulls heat 

from the environment—of desorbing hydrogen from a metal hydride can cool the vehicle); 

or thermoacoustic cooling (in which waste heat 

is used to produce sound waves, which generate compression and expansion cycles to cool the 

For More Information

Farrington, R.B.; Rugh, J.P.; Bharathan, D.; Burke, R. (2004). “Use of a 

Thermal Manikin to Evaluate Human Thermoregulatory Responses in 

Transient, Non-Uniform, Thermal Environments.” SAE document number: 2004-01-2345. http://www.sae.org/servlets/ productDetail?PROD_

TYP=PAPER&PROD_CD=2004-01-2345. Accessed December 30, 2004.

Farrington, R.; Rugh J. (2000). “Impact of Vehicle Air-Conditioning on Fuel 

Economy, Tailpipe Emissions, and Electric Vehicle Range.” Preprint. 12 pp.; 

NREL Report No. CP-540-28960. 

Farrington, R. B.; Anderson, R.; Blake, D. M.; Burch, S. D.; Cuddy, M. R.; Keyser, M. A.; Rugh, J. P. (1999). “Challenges and Potential Solutions for Reducing Climate Control Loads in Conventional and Hybrid Electric Vehicles.” 

15 pp.; NREL Report No. CP-540-25975.

“Thermal Comfort Project: A Cool Solution to the Nation’s Energy Security 

Challenges.” Center for Transportation Technologies and Systems (CTTS) 

Fact Sheet. (2002). 2 pp.; NREL Report No. FS-540-32285.

cooling that accompany the compression and expansion of 

a gas in a sound wave to produce the cooling for the interior 

of the vehicle. 

This concept has many potential advantages over a conventional A/C system. It uses waste heat, is reliable and inexpensive, does not entail the use of an extra energy load on 

the engine, relies on gases that are environmentally benign, 

has no sliding parts (and thus should have a long lifetime), 

and requires no lubrication. The down side is, because of 

its low energy density, the device could take up a lot of volume. If we can overcome that barrier, it could be one of the 

cool technologies in your next-generation car. 

In a thermoacoustic device, the heating and cooling that accompany 

the compression and expansion of a gas in a sound wave produces 

cooling for the interior of the vehicle. 

Page 21
T hroughout history, Europeans have relied 

on the sea for sustenance, and once again 

they are turning to the oceans to harvest 

energy from the wind. The fi rst to do so was Denmark when, in 1991, it placed 11 turbines off 

the Atlantic coast near the port of Vindeby. Since 

then, the concept of offshore wind energy has 

been gathering momentum, with 17 projects currently providing 600 MW of generating capacity 

off the shores of northern Europe. 

For Europe, this is just the beginning. According to the European Wind Energy Association, 

another 150 wind projects are being planned, targets that call for an additional 70,000 MW of offshore capacity by the year 2020. In contrast, the 

United States does not yet have offshore wind 

farms, but there are some on the drawing boards.

Why the discrepancy between continents? Europeans have long enjoyed greater fi nancial incentives for wind development. For example, 

Germany offers low-interest loans for wind development. Ireland forgives taxation on a portion 

of utility company profi ts reinvested in renewable energy. However, geography plays a major 

role as well—in much of northern Europe there is 

a scarcity of land-based wind sites with adequate 

resource. This is compelling the wind industry in 

nations such as Denmark, Germany, and Great 

Britain to move offshore where winds are stronger, steadier, and abundant. 

Harvesting

the

Ocean

Wind

Generating Electricity with 

Deep-Water Wind Turbines

Page 22 
Page 23

Shown above is a concept 

for anchoring large wind 

turbines in deep water. On 

the opposite page, a 3.6-MW 

General Electric wind turbine 

stands in shallow water 

in an area off the coast of 

Ireland known as the Arklow 

Bank (opposite page). The 

first seven turbines in this 

proposed 200-turbine, 

520-MW wind field started 

producing power in 2003. 

(Photo courtesy of GE 

Energy,  2004, General 

Electric Company.)

In the United States, where the land-based wind 

resource in the Midwest is abundant, the incentive 

to go offshore is not as obvious until one compares the proximity of the major East Coast load 

centers with the wind resources. Projects located 

in the ocean can avoid long-distance electric transmission bottle necks and take advantage of the 

local high winds. The most publicized offshore 

undertaking in this country has been the Cape 

Wind project, proposed to stand in waters several miles from Martha’s Vineyard, Massachusetts. 

The prospect of a wind farm has raised objections 

from some residents who think the turbines could 

ruin their view. Others back the project, including 

environmental groups such as Greenpeace and the 

Union of Concerned Scientists. 

In New York, the Long Island Power Authority has 

proposed another offshore wind farm that could 

soon add 140 MW to the electric grid—a proposal 

that thus far appears to meet the approval of some 

Long Island organizations, the Governor, and several environmental groups. 

Ideas that have incubated for years in the labs of 

NREL’s National Wind Technology Center could 

create technologies to enable wind farms to generate electricity much farther away from the 

shore—in deep water, where the winds are stronger, windy sites are an order of magnitude more 

abundant, and land-use confl icts are easier to mitigate. With an aggressive R&D program to push it 

forward, researchers believe this technology could 

be economically feasible in 10–15 years.

How deep is deep water? “There is no universal 

defi nition,” says scientist Walt Musial. “For purposes of discussion, let’s say it’s the depth where 

we would need substantial new technology.” Currently that depth is 20 or perhaps 30 meters, he 

says. Beyond that, it is no longer practical to use 

the monopole construction of today’s turbines, in 

which a steel pole 5 meters or more in diameter is 

driven into the seabed fl oor. 

Floating platforms of the kind used in offshore 

oil and natural gas drilling may be replacing the 

monopole. Variations of this theme are being 

computer-modeled by NREL’s wind scientists, 

working with researchers from the Massachusetts 

Institute of Technology (MIT). Using wind turbine codes such as FAST or ADAMS with MIT’s 

fl oating structures dynamics code, SML, they 

strive to simulate the inevitable rocking motion 

of a fl oating platform with the workings of a 

wind turbine. By approximating the load environments in which offshore wind turbines operate, their work will foster more reliable turbine 

design tools, and better techniques for reducing 

the uncertainty of offshore wind turbines.

A challenge inherent to farther-out wind farms is 

transmitting the power to shore. From today’s turbines, electricity is transferred via submarine electric cables. Greater distances will add cost, but it 

won’t necessitate any radical new science, Musial 

believes.

One idea for transferring energy is tied to the 

vision of a future “hydrogen economy,” in which 

hydrogen would be a major fuel and energy carrier for the nation. At remote wind sites (on land 

as well as at sea), electricity could be used to produce hydrogen from water. This approach would 

not only obviate the need for investing in electric 

cables and infrastructure, it would also provide 

a high-value dispatchable energy source whose 

production and use would emit no carbon dioxide. The hydrogen would be transported to shore 

and used to supply fuel and power for transportation, industry, and other applications. 

Aside from technological advances, large-scale 

deep-water offshore wind development would 

require new thinking, says Musial. “We’ve learned 

a lot about the use of fl oating platforms from the 

oil and gas industry, but we couldn’t do business 

the same way. Our intent would be to leapfrog 

the status quo.” For example, wind developers 

would seek economies of scale by manufacturing platforms by the hundreds, rather than one or 

two at a time as in the petroleum industry.

New thinking would also evolve from management practices at today’s wind installations, both 

offshore and land-based. Operation and maintenance costs for offshore installations might be 

triple those of land-based systems. Turbines off 

the European coasts currently experience a lot of 

downtime because they are often inaccessible in 

severe weather. NREL researchers are developing 

ways to make future turbines more reliable, tolerant of faults, and capable of diagnosing themselves. 

The U.S. electric grid carries about 920 GW of 

capacity. The wind off the coasts of New England theoretically could yield a sizable fraction 

of that amount—perhaps 100 GW of capacity—said David Garman, DOE’s assistant secretary for Energy Effi ciency and Renewable Energy. 

Researchers estimate that this capacity could provide enough electricity to run about 40 million 

U.S. homes. With so much energy so near, NREL 

scientists are exploring new ways to exploit that 

resource, applying ideas that were unimagined 

a decade ago. “We can’t let the projects drive the 

research,” says Musial. “We want to get ahead of 

what’s being done today, so new technology is 

available when the industry is ready.”

For More Information

Musial, W.; Butterfi eld, S. (2004). “Future for Offshore Wind Energy in 

the United States.” Preprint. 16 pp.; NREL Report No. CP-500-36-313.

Web site for the European Wind Energy Association, accessed 

December 30, 2004. http://www.ewea.org/.

“Wind Farms off East Coast Could Yield 100 GW of Energy; DOE 

Offi cial.” Inside Energy; August 30, 2004; p. 15.
Page 24 

In EGS, water is injected into a geothermal reservoir to promote fracturing. It is 

then circulated through the hot rock to heat the water and returned via a production well to produce geothermal electricity.

Enhancing Geothermal 

Power

Producing power from geothermal 

energy resources is, in some ways, a 

remarkably successful endeavor. It 

is a clean source of power that produces very little carbon dioxide and 

almost no nitrous oxide or sulfur-bearing gases. Geothermal power plants 

have high capacity factors and average availabilities of 90% or more. And 

geothermal power production is a billion-dollar-a-year industry, producing 

nearly 20% of the nation’s non-hydro 

renewable electric energy. Since 1970, 

geothermal power plant capacity has 

grown from 500 MW to more than 

2,600 MW today.

But geothermal power production is 

currently limited to sites where geothermal energy reservoirs are relatively 

close to the surface. Because the power 

plant technology relies on extracting 

hot water from these reservoirs, they 

must also consist of fractured rock permeated with water, a geographic coincidence that occurs in limited locations 

in the West.

Given these limitations, how can the 

industry grow? The answer is to drill 

deeper to reach the geothermal heat 

and, when necessary, to add water and 

create new fractures. Such “enhanced 

geothermal systems,” or EGS, can 

potentially expand the use of geothermal energy to any part of the country.

“It becomes almost an unlimited 

resource,” says Gerry Nix, NREL’s technology manager for Geothermal Technologies.

The main thrust of EGS is to inject 

high-pressure water into geothermal 

hot spots to open fi ssures that may be 

blocked with mineral deposits. It’s a 

technique often used today by the oil 

and gas industry. At the Coso geothermal plant, about 100 miles north of 

Negative Refraction 

of Light

NREL physicists Yong Zhang, Brian 

Fluegel, and Angelo Mascarenhas have 

bent light the “wrong way,” the fi rst to 

do so using natural materials. In physics this is a pretty big deal. Why? Consider a stick in water. It appears bent 

where it enters the water. This is due to 

the phenomenon known as refraction. 

When light goes from one medium 

into another, it changes speed, causing 

it to alter direction. In refraction, the 

incident light and the refracted light 

are always on opposite sides of the normal, a line drawn perpendicular to the 

interface between the two materials. 

Thus it has always been. 

The ancient dictum that says light must 

be refracted to the opposite side of the 

normal was challenged in 1968 by Russian physicist V.G. Vaselago, who suggested that under special circumstances 

light could be “negatively refracted,” 

or bent to remain on the same side of 

the normal. In the past couple of years, 

teams of scientists have begun to show 

that Vaselago may have been right. 

They have designed special, 

elaborate confi gurations of 

“metamaterials” made of copper rings and wires and Tefl on, 

which they have used to negatively refract narrow bands 

of microwaves. (A metamaterial is an artifi cially fabricated 

structure that exhibits properties not found in nature.) 

NREL physicists have taken 

the demonstration a few steps 

further. Using “real” material —twinned bicrystals of 

a III-V semiconductor alloy 

(a twinned bicrystal is one in 

which two crystals intergrow 

and are mirror symmetrical 

about their interface)—they 

were able to attain negative 

refraction for visible light. 

More than that, they showed 

that, depending on the angle 

of incidence, they could get 

positive or negative refraction. What’s more, they demonstrated that they could do the same 

thing for ballistic electrons as well as 

for light. And, fi nally, they showed that 

light could be refracted without losing 

energy or intensity (due to refl ection), 

which is noteworthy in itself. 

Can we harness this new capability? 

Perhaps. Although this is a new and 

fundamental discovery, we can already 

see possible uses in imaging, lens technology, data storage, semiconductor 

technology, and more. But uses inherent in the ability to steer and bend 

light, and to collimate electrons, will 

have to wait on more fundamental 

research and experimentation.

Research Update

Using a III-V bicrystal, NREL researchers can refract 

light normally (positive refraction—bottom) or abnormally (negative refraction—top), depending on 

the angle of the incident light.
In the background is an early prototype of NREL’s RC-PCD, a non-contact 

method that quickly and accurately measures minority-carrier lifetimes 

of photovoltaic and semiconductor materials. The latest version of the 

instrument (not shown) is far more compact and self-contained. Page 25 

Los Angeles, a project now under way 

is fracturing the existing reservoir to 

increase its size. The effort is expected 

to boost power production at Coso by 

20 MW, to 260 MW total.

EGS also includes efforts to drill 

deeper at less cost and to convert the 

hot water into electricity more effi -

ciently. As such, EGS is a graded 

approach to expanding the geothermal 

industry. By developing a “toolbox” of 

new techniques, the industry can gradually increase power production from 

existing geothermal reservoirs while 

expanding into areas that are currently 

beyond its reach. NREL is part of a virtual team spanning the DOE national 

laboratories that is developing this 

toolbox.

“NREL is helping to defi ne the next 

generation of power plants, so we 

can effi ciently convert the heat into 

power,” says Nix. “We have assembled 

a group of experts to provide the planning, analysis, and integration needed 

for the scale of this research program.” 

By successfully developing and employing the EGS toolbox of techniques, 

DOE expects that the United States 

could increase its geothermal electric 

capacity to 20,000 MW by 2020.

Measuring MinorityCarrier Lifetimes

An analysis technique developed at 

NREL offers a simple, effective way 

to test the performance of PV materials used to build solar cells. The 

technique—the Resonant-Coupled 

Photoconductive Decay System, or 

RC-PCD—offers a hands-off method 

to measure the lifetime of so-called 

“minority carriers” in PV materials. 

A solar cell is made from semiconductor layers. One layer is engineered to be 

n-type, to have an excess of unbonded 

electrons as potential negative charge 

carriers. The other layer is prepared as 

p-type, to have an excess of free “holes” 

as potential positive charge carriers. 

Sandwiching the layers together forms 

a p/n junction with the free electrons 

and holes traveling across the junction 

to set up an electric fi eld 

across the cell. 

When light strikes either PV layer, the 

light energy frees negatively charged 

electrons within the material, which 

leaves holes where electrons use to 

be. Electrons freed by sunlight in ptype material are minority carriers; 

they tend to be propelled by the electric fi eld across the junction into the 

n-type material where they migrate 

toward that end of the cell. Likewise, 

holes freed by sunlight in n-type material are minority carriers in that material; they are propelled by the electric 

fi eld across the junction to the p-type 

material to migrate toward that end of 

the cell. It is this migration of minority 

carriers that creates the electric current 

and power.

Unfortunately, in many parts of a solar 

cell, the electrical fi eld is too weak to 

whisk minority carriers off to their 

designated ends. Instead, the electrons 

and holes may wander randomly, possibly recombining with each other and 

wasting their energy as heat instead 

of producing power. Minority carriers 

that “live” longer have a greater chance 

of producing power, so PV materials 

should ideally have long minority-carrier lifetimes. Defects in the materials 

create stepping-stones that facilitate 

recombination, so minority-carrier 

lifetimes are also an indirect way to 

measure material defects.

The RC-PCD system, created by an 

NREL team led by Dick Ahrenkiel, 

measures minority-carrier lifetimes 

using a coil of wire. This coil, placed 

near the material, acts as an antenna, 

and a high-frequency signal pumped 

into it generates a low-energy microwave signal. Using electronic circuitry, 

the antenna can be tuned to perfectly 

transfer its energy to the material—a 

technique called resonance coupling.

The system uses laser pulses to generate electron-hole pairs 

in the material. 

The pairs 

change the material’s conductivity 

and cause it to respond differently to 

the antenna’s signal, refl ecting part of 

the signal back to the antenna. Ahrenkiel’s team can detect that refl ected signal and watch it drop off as the pairs 

recombine—a process called “photoconductive decay.” By pulsing the laser, 

the system causes the photoconductive 

decay to occur repetitively. An oscilloscope averages the signals to create a 

smooth decay curve that provides an 

accurate measurement of the minoritycarrier lifetime.

“We can do things here that nobody 

else can do,” says Ahrenkiel. “It’s 

become a standard in the PV program; we’ve run hundreds of samples 

through here. It has a lot of advantages 

over commercially available systems.” 

Those systems, mostly invented for 

the semiconductor industry, operate 

at higher frequencies that cannot penetrate into the depth of the material 

as the RC-PCD system can. That’s fi ne 

for semiconductors, but unsatisfactory for thick solar cells, which depend 

on the entire thickness of the material to generate a current. The RC-PCD 

also offers other advantages. It has a 

greater dynamic range, is far more sensitive and accurate over that range, can 

measure lifetimes for a wider variety 

of materials and sample sizes, and has 

a greater ability to detect defects and 

their effect on lifetimes. 

The system does have a drawback. It 

is limited to measuring lifetimes of 

50 nanoseconds or longer, a limitation that some commercial devices 

can beat. To overcome that limitation, 

graduate student Jamiyana Dashdorj is 

inventing a new, better antenna. Ironically, software and electronics developed for the burgeoning cell phone 

industry are proving ideal for the task, 

so the cell phone technologies of 

today may help the solar cell industry 

invent the ideal energy source 

for tomorrow.