2005 Research Review

National Renewable Energy Laboratory
i

feature

departments

1 Perspective

The President has told the nation that we are 

“...addicted to oil...[and] the best way to break 

this addiction is through technology.” NREL’s 

R&D in sustainable, domestic energy technologies is leading the way to a long-term solution 

to this problem.

2 NREL in Focus

NREL has a new deputy director/chief operating officer, has partnered with others to form a 

fuel-cell research center, has expanded its computing capabilities beyond the 1-teraflop 

milestone, and is about to open a major 

new facility. The Science and Technology 

Facility will greatly facilitate the transfer of 

renewable energy technology to industry.

4 Awards & Honors

During 2005, NREL received two R&D 100 

Awards and two technology transfer awards. 

In addition, several of our scientists were 

recognized for their special service and expertise.

The National Renewable Energy Laboratory is operated by Midwest Research Institute and Battelle for the Department of 

Energy’s Office of Energy Efficiency and Renewable Energy. NREL is the nation’s primary national laboratory for renewable 

energy and energy efficiency 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.

This year’s NREL Research Review staff includes Howard Brown, Don Gwinner, Molly Miller, and Paula Pitchford, writers, and 

Ray David and Al Hicks, graphic designers.

6 On the Road to 

Future Fuels & Vehicles

America’s dependence on imported petroleum 

can be reduced by using clean, domestic transportation fuels and advanced vehicles. NREL is 

developing the science and technology to allow 

us to move from corn ethanol, biodiesel, and 

flexible-fuel and hybrid-electric vehicles in the 

near term to cellulosic ethanol, green diesel, 

and plug-in hybrid-electric vehicles in the mid 

term to renewable-source hydrogen and fuelcell vehicles in the long term.

June 2006

ISSUE 4

a publication about NREL R&D highlights CONTENTS

articles

14 HighPerforming 

Commercial 

Buildings

NREL’s R&D shows that it is possible to design 

and construct attractive residential and commercial buildings that are much more energy efficient 

than current standard practice. Take a look at six 

such “high-performing” buildings.

18 Closing the Gaps

High-efficiency solar 

cells are one of many 

areas of solar research 

at NREL. Read about 

the basic rationale of 

multijunction cells and 

some innovative design 

twists that are boosting 

their efficiencies.
1

Securing Energy for the 

Future Through Science and 

Technology

Science and technology. They are at 

the heart of everything we do at the 

National Renewable Energy Laboratory, 

as we pursue innovative, robust, and 

sustainable ways to produce energy—

and as we seek to understand and illuminate the physics, chemistry, biology, 

and engineering behind alternative 

energy technologies.

Although our nation is learning to use 

energy more wisely—for example, by 

producing and buying more energyefficient vehicles and appliances—the 

demand is still great. Americans consumed 99.7 quadrillion Btu (quads) of 

energy in 2004; the Energy Information 

Administration projects that this will 

increase to 127 quads by 2025.

In his 2006 State of the Union address, 

President George W. Bush noted the 

seriousness of the situation and pointed 

the way to a remedy. The President said, 

“Keeping America competitive requires 

affordable energy. And here we have a 

serious problem: America is addicted to 

oil, which is often imported from unstable parts of the world. The best way 

to break this addiction is through 

technology.”

With our emphasis on research and 

development in market-relevant, 

domestic energy technologies, NREL is 

working toward a long-term solution to 

this problem. In this issue of the NREL 

Research Review, we present three st­ories 

that highlight recent advances and 

future directions in 

areas the President 

particularly noted in 

his Address: biofuels 

and ve­hicles, energyefficient buildings, 

and solar energy.

Biofuels and Vehicles—The alternative 

fuels industry is helping to reduce our 

dependence on imports by introducing 

more domestic ethanol and bi­odiesel 

options into the transportation fuels 

market. This issue describes NREL’s contributions, which include expanding 

feedstock sources, developing methods for breaking down feedstock for further processing, and making conversion 

methods for fuels and other products 

more efficient and cost effective.

The President proposes to “fund additional research in cutting-edge methods of producing ethanol, not just from 

corn, but from wood chips and stalks, 

or switchgrass. Our goal is to make this 

new kind of ethanol practical and competitive within six years.” Through the 

Advanced Energy Initiative, we will 

continue working toward this important goal in feedstock development and 

conversion methods.

Buildings—Our buildings consume 

nearly 40% of the energy Americans use 

each year. NREL’s buildings research 

and development demonstrate that it is 

possible to design and construct attractive residential and commercial buildings that are much more energy efficient 

than the current standard practice. 

Our work also develops good ways to 

i­ntegrate renewable 

energy technologies 

into these buildings.

The President’s 

Advanced Energy 

I­nitiative will invest 

in renewable energy technologies that 

“change how we power our homes 

and offices.” In this way, we can continue to assist the U.S. building industry in constructing energy-smart, 

“high-performing” buildings, like the 

ones described in this issue.

Solar Energy—As part of NREL’s 

research and development in photovoltaics, our scientists work closely with 

other expert groups to investigate and 

improve the way in which solar cells 

directly convert sunlight to electricity. 

The President’s Solar America Initiative 

will support the development of solar 

cells that convert the sunlight much 

more efficiently than they do now.

Work in high-efficiency solar cells is one 

of many relevant areas of solar research 

conducted at NREL. In the article in this 

issue, we discuss the basic rationale of 

multijunction cells and some innovative twists on their design that can boost 

efficiencies to higher levels.

From NREL’s inception in 1977 to the 

present, the science and technology of 

alternative energy has been the core of 

our mission. And this will continue in 

the decades to come. The advances we 

make in all areas of renewable energy 

and energy efficiency will play a vital 

role in assuring the security, strength, 

and competitiveness of the nation.

Dr. Stanley R. Bull, 

Associate Director, 

Science & Technology

PIX006943/Warren Gretz

Perspective
2

vision. He will also serve as a vice president for Midwest Research Institute, 

which has operated and managed 

NREL for DOE since NREL opened as 

the Solar Energy Research Institute in 

1977.

New Colorado Research 

Partnership Formed to 

Develop Fuel Cells 

Power generation in the 21st Century 

faces some tough challenges: it has 

to score very high in energy efficiency 

and, at the same time, very low in environmental impacts. Right now, the fuel 

cell is one of the few technologies that 

appears to do both these things at once.

So, to help bring advanced fuel cell 

technologies to the marketplace, 

NREL has formed a partnership with 

the Gas Technology Institute, the 

C­olorado School of Mines (CSM), and 

Versa Power Systems. Their joint venture, known as the Colorado Fuel Cell 

C­enter (CFCC), is focusing on fuel 

cell research, development, testing, 

and commercialization in a state that 

is home to several related high-tech 

industries.

The new center is housed in the General Research Laboratory at the Colorado School of Mines in Golden. The 

Colorado Governor’s Office of Energy 

Management and Conservation has 

provided $2 million in start-up funding, and the partners are contributing 

$1 million. The CFCC aims to provide 

state leadership in the development 

and application of fuel cell technologies, increase the public’s understanding of the technologies through 

education and outreach activities, and 

foster economic development through 

partnerships with local businesses.

The CFCC will also coordinate the 

development of fuel cell R&D programs, facilitate access to experts in 

NREL and CSM research labs, assist in 

forming strategic alliances among local 

developers and university research centers, and serve as an independent testing agency. In addition, the center will 

promote the development of fuel cell 

curriculums in colleges, universities, 

and trade schools in order to build a 

technically viable labor force for this 

young industry. 

The tremendous growth 

expected in the fuel cell 

industry in the coming 

decade will mean new 

high-tech jobs in R&D 

and in manufacturing that 

will eventually extend well 

beyond Colorado and the 

industry itself. 

At the end of two years, 

the CFCC should be 

self-sustaining through 

research and development contracts, consulting 

agreements, and other 

sources of funding. The 

center will then emphasize the development 

of the Colorado fuel cell industry as 

it responds to national solicitations. 

For more information, contact CFCC 

Executive Director Robert J. Remick, 

rremick@mines.edu.

NREL Computing Power 

Surpasses Teraflop

NREL’s high-performance computing 

capability recently surpassed the peakperformance mark of 1 teraflop—or 

one trillion floating point operations 

per second. “We’ve gone from 96 gigaflops of power to 1.2 teraflops in less 

than four years,” said Steve Hammond, 

director of NREL’s Computational Sciences Center (CSC), highlighting this 

significant milestone in building highperformance computing expertise and 

capabilities at the Laboratory.

NREL In Focus

Bill Glover Is New NREL 

Deputy Director and COO

On February 

1, 2006,NREL 

welcomed 

William (Bill) 

Glover as the 

Laboratory’s 

new Deputy 

Director and 

Chief Operating Officer. 

“I’m glad to 

be here,” he 

said. “I’m very 

excited about the Lab’s mission and I 

am looking forward to working with 

everyone at NREL.”

Bill has had more than 35 years of 

executive-level experience. His extensive experience with the Department 

of Energy (DOE) has included senior 

management responsibilities in standards identification and management, 

self-assessment and independent oversight, quality assurance, performance 

measurement and evaluation, and 

readiness reviews. He was on a headquarters level group overseeing the 

rollout of DOE’s Standards Program, 

and he led a facilities maintenance program assessment team at Los Alamos 

National Laboratory.

Before joining NREL, Bill was an independent consultant, President/General 

Manager of TENERA Federal Services 

LLC, and Director of Performance 

Assurance with EG&G, Rocky Flats. He 

served with distinction for 26 years in 

the United States Navy, including managing a nuclear submarine squadron 

with more than 3,500 personnel.

Bill has a B.A. in chemistry from the 

University of Rochester and the equivalent of an M.S. in nuclear engineering 

(from the Navy Senior Officer Nuclear 

Power Training).

As Chief Operating Officer, Bill will 

oversee Laboratory operations. As Deputy Director, he will support the development and implementation of the 

Laboratory’s operational str­ategic 

Deputy Director and Chief 

Operating Officer Bill Glover PIX14712/Mike Linenberger

PIX14679/Ray David

The new joint-venture Colorado Fuel Cell Center is housed 

at the Colorado School of Mines a few miles away from the 

NREL campus.
PIX14651/Mike Linenberger

Energy-Efficient Science and Technology Facility Set to Open

NREL is nearly ready to roll out its newest research facility on the South Table 

Mountain campus in Golden, Colorado. The Science and Technology Facility, or 

S&TF, is a 71,000-ft2 (6,596-m2), multilevel facility dedicated to research and 

development in solar cells, thin films, nanostructures, hydrogen energy, and fuel 

cells, which are several of the most important renewable energy technologies 

highlighted in the Solar America and Hydrogen Initiatives.

The building is designed to increase collaboration among researchers and 

shorten the time it takes new technologies to move from the laboratory bench to 

commercial manufacturing—and ultimately, to consumers.

Laboratories and office space are on the facility’s ground level. The second level 

houses additional labs and a large, open Process Development and Integration 

Laboratory (PDIL). The third level contains mechanical equipment, such as 

exhaust fans. An elevated bridge connects the S&TF to the Solar Energy Research 

Facility next door.

The S&TF was especially designed to reduce technical and manufacturing 

barriers and accelerate the transfer of technology from R&D to industry. The 

centerpiece of the building will be the PDIL, an 11,400-ft2 laboratory space 

specifically designed to accommodate a new class of tools for thin-film deposition, processing, and characterization. Six research bays will be available for 

industry partners as they scale up processes for early application in manufacturing devices such as PV modules.

Brent Nelson, who is leading the process integration project, says, “The PDIL will 

give us integrated tools, integrated data, and the added bonus of people with 

diverse skills working together.”

The S&TF also includes laboratories for advanced materials, surface analysis, 

and novel deposition techniques. A flexible laboratory model guided the design, 

allowing smaller labs to be combined to form larger, more open ones, as needed. 

And the building’s “interaction spaces” encourage the kind of informal discussions that often lead to R&D breakthroughs and other innovations. Each interaction area features a white board and computer access.

“Going into this project, our emphasis was on designing functional and flexible 

laboratory space that will meet our research needs both today and in the future,” 

technical project manager Pete Sheldon says.

Like other NREL laboratory facilities, the S&TF is also a model of sustainable 

design and efficient energy use. It is expected to consume as much as 40% less 

energy than would a more conventional laboratory building. Daylighting, for 

example, which is the abundant use of natural light in a building, helps to reduce 

energy use by minimizing the use of artificial lights—and keeping the facility 

that much cooler—in the daytime. In the S&TF, natural light through numerous 

windows and clerestories illuminates the open office areas and even some of the 

labs. “Our goal was to have 100% daylighting in all office spaces,” says Otto Van 

Geet, an NREL senior project leader.

In addition, energy recovery technologies use exhaust air to preheat or cool 

supply air. This allows smaller heating and cooling systems to be used. In line 

with Colorado’s dry climate, evaporative cooling is also featured, as well as a 

raised-floor air distribution system. Overall, the facility has been designed to 

achieve a “Gold” rating as a U.S. Green Building Council Leadership in Energy 

and Environmental Design (LEED) building.

Construction of the S&TF began in fall 2004 after designs were completed and 

contractors selected. Because of the excellent progress made, project managers 

expect occupancy to begin in July 2006.

This increase in highperformance computing capability greatly 

aids scientific pursuits. 

For example, researchers in the Buildings & 

Thermal Systems Center have run computational fluid dynamics 

models of their novel 

patent-pending tab-fin 

heat exchanger without building expensive, 

time-consuming prototypes. Simulations help 

optimize the geometry by considering 

numerous different tab sizes and placements, leading to a design with the 

best performance.

Scientists in the National Bioenergy 

Center have likewise benefited. By 

using CSC’s quantum mechanical 

modeling, they have investigated the 

loss of fermentable sugars during biomass pretreatment. The increased computing capability has been an essential 

tool for understanding the kinetics of 

these chemical reactions, which can 

lead to higher ethanol yields and lower 

process costs.

Computational science—the process 

of using computers to model and simulate real-world processes and phenomena—is an indispensable partner 

with theory and experiment to advance 

scientific knowledge and engineering 

practice. The NREL CSC helps researchers study complex systems and natural 

phenomena that would be too expensive, dangerous, or even impossible to 

study by direct experiment. 

NREL’s Computational Sciences capabilities 

include cutting-edge tools and techniques for 

stereo 3D analysis and visualization of complex 

scientific data.

PIX14532/Jack Dempsey

3
PIX13344/Sinton Consulting, Inc.

Awards & Honors

R&D 100 Awards

NREL won two R&D100 awards from 

Research & Development (R&D) Magazine in 2005. These awards recognize 

what the magazine editors consider to 

be the most significant products introduced into the marketplace over the 

past year. NREL won the awards for its 

work in advancing residential energy 

analysis software and improving the 

testing of silicon used in solar cell 

production. 

Advances in Residential Energy 

Analysis The first R&D100 award recognizes the software application 

called TREAT—Targeted Residential 

Energy Analysis Tools—created by 

NREL and the New York State Energy 

Research and Development Authority (NYSERDA) and its partners. 

This software is a comprehensive 

energy analysis tool that models building energy consumption and identifies the most 

cost-effective energy-efficiency 

upgrades for both single and 

multifamily buildings. TREAT 

is based on SUNREL, a building 

energy simulation tool that NREL 

developed in the mid-1980s. Several 

innovations make TREAT particularly 

outstanding; for example, the software 

predicts occupant behavior, reports 

health and safety issues, and links users 

to outside data resources.

Advances in Silicon Testing The second R&D100 award highlights NREL’s 

novel method of detecting impurities 

and defects in silicon boules, the material from which most commercial solar 

cells are made. Designed by subcontractor Ron Sinton of Sinton Consulting, the instrument is called the Sinton 

Quasi-Steady-State Minority-Carrier 

Lifetime Analyzer. It provides valuable information to manufacturers, identifying substandard silicon 

before it is fabricated into cells, 

and thereby increases the number of high-quality cells produced, 

while boosting manufacturing 

yields and reducing manufacturing 

costs. The evaluation system helps 

the photovoltaics industry be competitive by allowing manufacturers 

to produce consistently better silicon at the lowest possible price.

Technology Transfer 

Awards

The Federal Laboratory Consortium 

for Technology Transfer (FLC) recognized NREL with two “Notable Technology Development” awards in 2005 

for technology transfer activities. The 

purpose of the FLC awards is to recognize achievements in moving technologies from the laboratory into the 

marketplace. 

The first award recognized NREL’s 

Thermal Comfort Project, which has 

resulted in several tools to analyze the 

efficiency of climate control in vehicles. The tools include the Advanced 

Automotive Manikin (ADAM), 

which mimics human physiological 

responses to hot and cold to predict 

how comfortable a vehicle occupant 

might feel. The project is managed 

by NREL’s Center for Transportation 

Technologies and Systems. 

The FLC also recognized this center’s 

Advanced Vehicles and Fuels Project. 

The project created a partnership with 

vehicle manufacturers, fuel providers, and others to incorporate innovative technologies into the industry to 

reduce fuel use and emissions.

The FLC also awarded Lawrence 

“Marty” Murphy, manager of NREL’s 

Enterprise Development Programs, 

a Distinguished Service Award. The 

award recognizes his leadership during the last 

nine years 

in building and promoting the 

program. In 

addition, he 

has worked 

tirelessly on 

the Industry 

Growth Venture Forums, 

which 

encourage the 

creation and 

growth of companies that promote 

clean, efficient, and renewable energy. 

The FLC is a nationwide network of 

federal laboratories that strives to link 

developed technologies and expertise 

with the private sector.

PIX14684/Warren Gretz

Marty Murphy

4
Academy of Engineering 

Elects Kazmerski

Lawrence 

Kazmerski 

was elected 

a member of 

the National 

Academy of 

Engineering in 2005. 

Membership in the 

Academy is 

bestowed 

on engineers who 

have made outstanding contributions to engineering research, practice, or education, and to the pioneers 

of new and developing fields of technology. Kazmerski, who is the director of NREL’s National Center for 

Photovoltaics, has worked at NREL 

since its inception. He has published 

more than 290 journal papers in the 

areas of solar cells, thin films, semiconductor materials and devices, surface and interface analysis, scanning 

probe microscopy, nanoscale technology, high-temperature superconductivity, and semiconductor defects. 

Kazmerski is also a Fellow of the Institute of Electrical and Electronics Engineers, the American Physical Society, 

and the American Vacuum Society. 

Under Secretary of Energy David Garman, invoking Kazmerski’s oft-used 

nickname, said, “Kaz is known around 

the world for his leadership in solar 

energy research. NREL, DOE, and the 

nation are fortunate to have such a 

talented and tireless researcher who 

has devoted his life to this 

important work.”

Ghirardi Among Nation’s 

Brightest

The Hispanic Engineers National 

Achievement Awards Corporation 

(HENAAC) named NREL’s Maria 

Ghirardi as one of the nation’s best 

and brightest engineers and scientists. Ghirardi is a senior scientist at 

NREL and a Research Associate Professor at the Colorado School of Mines. 

Her research at NREL involves photobiological hydrogen production and 

includes developing a system for producing large quantities of active algal 

hydrogenase enzyme, a major breakthrough in research. She has authored 

45 publications, and her research 

was the subject of a Discovery Channel program that aired in 2003. NREL 

Director Dan Arvizu adds, “Not only 

is her research on alternative fuels 

helping set the stage for our nation’s 

energy future, she is a role model for 

students who want to pursue careers 

in engineering, science, technology, 

and math.”

Coutts Wins IEEE Award

Timothy J. Coutts received the 2005 

William R. Cherry award from the 

Institute of Electrical and Electronics 

Engineers (IEEE) for his long-term 

contributions to the science 

and technology of 

photovoltaic energy 

conversion, including 

the dissemination of 

substantial publications and 

presentations. Coutts leads 

NREL’s device 

development 

work, within 

the National 

Center for 

Photovoltaics. 

This involves 

developing 

and optimizing both 

single- and 

polycrystalline solar 

cells. The 

annual award honors photovoltaics 

pioneer William R. Cherry and 

is bestowed each year at the IEEE 

Photovoltaics Specialists Conference.

Lawson Appointed to 

Air Quality Commission

Colorado Governor Bill Owens 

appointed Douglas Lawson to serve 

on the state’s Air Quality Control 

Commission. Lawson, principal scientist in NREL’s Center for Transportation 

Technologies and Systems, manages 

the Environmental Science and Health 

Impacts program for the Department 

of Energy. He has completed the first 

year of his three-year appointment to 

the commission, which consists of 

nine members representing different 

disciplines and interests. 

Lawson was appointed because of his 

strong technical background. “I bring 

the air pollution science background 

to the commission,” Lawson said. The 

commission 

is a rulemaking body 

that reviews 

rules and 

regulations 

proposed by 

the state and 

the U.S. Environmental 

Protection Agency. 

Lawson has 

published 

more than 

70 journal articles on various aspects 

of air pollution and served on two 

National Research Council committees 

on air quality. 

.

PIX14683/Warren Gretz PIX14716/Mike Linenberger

Maria Ghirardi

Timothy Coutts

Douglas Lawson

Lawrence Kazmerski

PIX13524/Warren Gretz

5
6

T he total price that Americans pay for gasoline and diesel fuel isn’t the one shown on the pump. Because we import about 60% 

of our crude oil, we are vulnerable to price fluctuations, disruptions in supplies, and political 

instabilities that affect our entire economy. We 

also pay a price in terms of our environment and 

health. For example, more than 30% of all U.S. 

air emissions come from burning traditional 

fossil fuels for transportation.

At least 240 million U.S. vehicles—one for 80% 

of all the men, women, and children in the country—were registered in 2004, according to Department of Transportation statistics. It takes a lot of 

fuel to keep all those wheels moving. The Department of Energy reports that the average American 

household uses more than 1,000 gallons of gasoline each year. Gasoline makes up nearly half of 

the 20 million barrels of petroleum products that 

we consume each day, to cover more than 7 billion daily miles.

Addressing these issues, in February 2006, President George W. Bush announced the Advanced 

Energy Initiative. This national initiative calls 

for greater reliance on domestic energy sources, 

including solar and wind energy, to power our 

homes and offices. And it increases funds for 

research and development (R&D) in advanced 

batteries for hybrid vehicles, electric cars, and 

On the Road to 

Future Fuels and 

Vehicles

NREL’s research helps move 

the nation toward energy 

independence
7

Hybrid-Electric &

Flexible-Fuel Vehicles

Plug-in Electric Vehicles

Fuel Cell Vehicles

Hydrogen

Cellulosic 

Ethanol

Grain 

Ethanol

Increased Transportation Use of Fuels Based on Renewables

Near-Term

Mid-Term

Long-Term

The Alternative Fuels Data 

Center. NREL is helping 

to expand the market for 

alternative fuels and vehicles through the Alternative Fuels Data Center, or 

AFDC. Supported by the 

Department of Energy’s 

Office of Energy Efficiency 

and Renewable Energy and 

housed at NREL for more 

than a decade, the AFDC is a 

searchable electronic library 

of technical data, publications, and information on 

advanced and alternative 

fuels and vehicles (see www.

eere.energy.gov/afdc). It also 

contains information about 

financial incentives, regulations, fuel performance, emissions, funding and 

training resources, industry contacts, and updates 

on advanced vehicles.

In addition, the AFDC Vehicle Buyer’s Guide 

helps both fleet managers and consumers evaluate various fuel and vehicle options. An Alternative Fuel Station Locator provides maps showing 

U.S. fueling stations. And additional databases 

provide useful technical data—for example, on 

vehicle emissions and performance.

Hybrid-electric vehicles. Though they are still 

new kids on the road, hybrid-electric vehicles— 

HEVs—are becoming increasingly popular; about 

1.3% of light vehicles sales in 2005 were HEVs. To 

improve fuel efficiency and performance, HEVs 

combine a smaller, but more efficient, gasoline 

engine with an electric motor. Estimates indicate that if everyone drove an HEV, 

we could reduce our petroleum 

consumption right now by at 

least 20% and perhaps as 

much as 40%.

Options for reducing our 

use of imported oil are 

available to consumers 

now. Diesel vehicles can use 

20% biodiesel. Flexiblefuel vehicles can use 85% 

ethanol. And 10% ethanol 

is widely used for fuelefficient hybrid-electric or 

any regular vehicle. NREL 

is working to improve the 

competitiveness of these 

current alternatives and to 

develop new technologies 

for the future. 

hydrogen-powered vehicles, as well as alternative 

transportation fuels, particularly ethanol.

With the support of the Department of Energy’s 

Office of Energy Efficiency and Renewable Energy, 

and in partnership with both public and private 

organizations, NREL’s scientists and engineers 

are assisting in this effort by conducting R&D in 

innovative fuel and vehicle technologies. These 

technologies will help to reduce our nation’s 

dependence on imports, enhance our energy 

security, and improve the quality of our air.

Here, we review where we are now, in terms of 

R&D and market readiness, as well as where we 

plan to be in 5 to 10 years and where we want 

to be in 20 years. This work has been, and will 

be, key to developing tomorrow’s fuel and vehicle technologies. These technologies include 

advanced hybrid electric vehicles, plug-in hybrids, 

biodiesel and other biofuels, ethanol produced 

from cellulosic biomass, advanced batteries, 

hydrogen fuels, fuel cells, and more.

Where We Are Now

Today, ethanol, biodiesel, flexible-fuel vehicles, and hybrid-electric vehicles are all available 

in some form, but in limited quantities, in the 

United States. Since the Energy Policy Act of 2005 

establishes a renewable-fuels standard requiring total U.S. transportation fuel sales in 2012 to 

include 7.5 billion gallons of renewable fuel, we 

should see steady increases in the use of biofuels 

and the vehicles that run on them.

Today, more than 30% of U.S. gasoline is E10—

gasoline blended with 10% ethanol. About 90% 

of the U.S.-produced ethanol in this blend is 

made from the starch and simple sugars in corn. 

The availability of E10 depends on local prices 

and air-quality regulations, however.

Flexible-fuel vehicles. In some parts of the country, particularly the Midwest, consumers can purchase E85—an ethanol blend containing 15% 

gasoline. But E85 can be used only in “FFVs”—

flexible-fuel vehicles that run on either gasoline 

or ethanol blends. At present, U.S. automakers 

manufacture flexible-fuel versions of several 

popular models, such as the Ford F150 truck 

and the Chevrolet Suburban sport utility vehicle. About 41⁄2 million to 5 million FFVs are 

estimated to be on the road today.

However, many FFV owners are unaware that 

they have one. A recent survey of FFV owners by 

an ethanol producer found that 70% didn’t know 

they owned a vehicle that can run on E85. Those 

who do know might wonder where they can purchase this fuel; less than 600 service stations 

offered E85 in January 2006. A tax credit in the 

Energy Policy Act of 2005 provides for developing 

an alternative fuel infrastructure, and this could 

help increase that number. And at least one automaker, GM, has stepped up public information 

efforts regarding FFVs and E85.

PIX13531/Charles Bensinger, and Renewable Energy Partners of New Mexico
8

Millions of Barrels Per Day

1973 1977 1981 1985 1989 1993 1997 2001 2005 2009 2013 2017 2021 2025 2029

1985

Freeze of

CAFE

Standards

1978

Adoption

of CAFE

Standards

0

5

10

15

20

25

30

Actual Projected

U.S. Dependence on Foreign Oil Where We Should Be in 

5 to 10 Years

Mid-term options such as cellulose-based ethanol 

and “plug-in” hybrid-electric vehicles are technologies in the latter stages of development, but 

they are still too expensive to compete in the marketplace. Therefore, NREL’s researchers and engineers are working to improve these technologies 

so they can be competitive and widely available.

Cellulosic ethanol. The starchy material in corn 

kernels now used to produce most of our ethanol 

is only a small fraction of the biomass—the 

plant-based materials and waste products—

that could be used. Two other components of 

plants, cellulose and hemicellulose, are also made 

of sugars, but those sugars are linked in long 

polymer chains that are not easy to convert to 

ethanol. Advanced biomass conversion technologies break down the polymer chains into their 

component sugars and then ferment them into 

alcohol to produce cellulosic ethanol.

This technology thus turns ordinary, low-value 

plant materials—such as corn stalks, sawdust, 

wood chips, wastepaper, and fast-growing trees 

and grasses—into ethanol and other valuable 

fuels and chemicals. Cellulosic ethanol could do 

much to reduce our dependence on imported 

oil and curb U.S. greenhouse gas emissions. The 

technology works—but it’s still too expensive. So, 

national laboratories and private-sector groups 

alike are developing more cost-effective production methods.

For many years, NREL has been developing technologies that produce ethanol and other valuable 

fuels and chemicals from cellulosic biomass. Our 

researchers have conducted much of the basic 

research underpinning a process in which a dilute 

acid is used to break down hemicellulose. In a 

step known as enzymatic hydrolysis, enzymes 

break cellulose down into its component sugars. While we focused on the process, our research 

partners made great strides in reducing the cost of 

the enzymes. Four years ago, these enzymes were 

too expensive to be used in a cellulosic ethanol 

process. Today, enzymatic hydrolysis is the lowest 

cost option for hydrolyzing cellulose to glucose.

NREL researchers are also leading the development of better pretreatment methods, which use 

dilute acid at elevated temperatures and pressures 

to break down hemicellulose into component 

sugars. And they engineered one of only three 

organisms to date that can co-ferment cellulosic 

and hemicellulosic sugars.

In NREL’s one-ton-per-day feedstock pilot plant, 

researchers continue to partner with companies 

in the vanguard of this emerging industry to validate new biomass-to-ethanol technologies. They 

still need to be simpler and less expensive; for 

example, the capital equipment and sophisticated 

processing steps required are costly. So NREL is 

Working with major U.S. automakers, NREL 

researchers have been contributing to the development of HEVs for more than a decade. Today, 

they are still investigating ways to improve the 

performance of HEV components and working 

with industry to put those improvements into 

practice.

One key to making HEVs more practical is to 

incorporate low-cost, integrated power electronics. NREL, other national laboratories, DOE, and 

industry are working on advanced components 

that condition the electrical signal between the 

power generation unit (a battery or fuel cell) and 

the electric motor.

Heavy-duty vehicles are prime subjects for R&D, 

because they consume much more fuel per 

vehicle than passenger cars do. Working with 

industry, NREL is developing advanced hybrid 

components and systems that could increase the 

fuel efficiency of heavy trucks as much as 100%.

Biodiesel. For diesel-powered vehicles, one 

renewable fuel option is B20—a blend consisting 

of 20% fatty acid methyl ester (known as 

biodiesel) and 80% petroleum diesel fuel. 

Biodiesel can be made from any animal or vegetable fat or oil and used in just about any diesel 

vehicle. It can reduce environmental emissions 

dramatically, depending on the blend. U.S. 

biodiesel is largely produced from soybean oil 

and recycled restaurant cooking oil.

Staff at NREL have been working to reduce the 

technical barriers that stand in the way of producing biodiesel and using it more widely. They 

also test biodiesel products in engines supplied 

by various industry partners. Their work will help 

to make biodiesel more cost-competitive, reliable, and plentiful.

Source: Historic data from Energy Information Administration (EIA), Monthly 

Energy Review 2006, Table 3.1A; projections from EIA Annual Energy Outlook 

2006, Table 11.
Enzymes

Feedstock

Conversion

 2001 2005 2010 2012 2015 2020

Actual Future Goals

$2.26/gal

DOE 2020

Target

$0.59/gal

DOE 2012 

Target

$1.07/gal Yield 65 gal/ton

Feed $45/ton

Yield 75 gal/ton

Feed $30/ton

Yield 94 gal/ton Feed $53/ton

Minimum Ethanol Selling Price ($/gal)

1.00

2.00

3.00

4.00

5.00

6.00

Cost of Ethanol from Cellulosic Biomass

(Costs in 2002 Dollars)

0.00

9

NREL continues to investigate 

technologies that allow us to produce 

ethanol economically from corn stover—

the husks, leaves, and stalks that today 

are left behind in the field after the corn is 

harvested. This feedstock is both high in 

volume and low in cost. However, capital 

equipment and ethanol processing costs 

are still too high for the technology to 

compete with gasoline and with ethanol 

made from corn kernels. Though NREL 

has made great progress in reducing the 

cost of cellulosic ethanol technology, 

some key hurdles remain to be overcome. 

NREL’s analysts calculate that a price 

of $2.26 per gallon is needed to allow 

industry to build a profitable plant today. 

And a price of $1.07 per gallon is needed 

for large-scale market penetration—

a goal we hope to reach by 2012.

Critics of corn-based ethanol often say that it takes more energy to 

produce the ethanol than we get from the resulting fuel. But official 

studies demonstrate just the opposite. The efficiency of both corn 

farming and ethanol production has dramatically increased over the 

years. The latest U.S. Department of Agriculture (USDA) study calculates a net energy balance of 1.67 for ethanol production; that is, for 

every unit of energy that went into growing the corn and turning it 

into ethanol in 2001, we got back about two-thirds more energy in 

the form of automotive fuel and animal feed. 

Even if that net energy balance were less than one, ethanol production and use still displaces oil imports with domestic nonpetroleum 

energy, which is a major plus in terms of reducing our dependence 

on imported fuel. On the basis of liquid fuels alone, the USDA calculated a net balance of 6.34. It is important to note that the energy in 

ethanol totally replaces gasoline energy, and relatively little petroleum product is used to produce it.

It is true that modern corn farming and ethanol production are both 

energy-intensive, and that the “net energy” gain of some early ethanol plants was relatively modest. But allegations about a negative 

net energy balance seem to be based either on using old data or failing to take into account the considerable value of the animal feed 

co-products of ethanol production. 

Animal feed co-products are a critical part of any effectiveness analysis because they are a major part of the economics of ethanol production. Corn, the largest U.S. crop, is used primarily for high-starch 

animal feed. Ethanol production uses up most of the starch from the 

kernels, but leaves the protein and fiber, so ethanol co-products are 

very high in protein and therefore are high-value animal feeds.

Ethanol from Corn Kernels: Is It an Effective Use of the Resource?

working to change this. When all process design 

targets are met and ethanol can be produced for a 

little more than $1.00 per gallon, cellulosic ethanol will be competitive with ethanol from starch 

and probably even with gasoline.

NREL researchers and their partners have received 

several technology awards for novel biomass conversion methods. And they are working to enable 

industry to produce fuels, chemicals, and other 

products in biorefineries that would manufacture 

a variety of products from biomass—much as 

today’s oil refineries and petrochemical plants do 

from petroleum.

Other mid-term biofuels. NREL researchers 

have helped to develop processes for gasifying 

or liquefying biomass by heating it with little or 

no oxygen. Using the Fischer-Tropsch conversion process and others, they can catalytically 

convert the resulting synthesis gas to diesel 

substitutes and other fuels. NREL has also 

partnered with industry to explore whether 

a bio-oil produced through the pyrolysis 

of biomass could be a useful intermediate 

for producing clean biofuels. Several small 

biomass gasification and pyrolysis plants are 

already up and running.

In the biorefineries of the future, selected sugars and intermediates would be made into 

high-value products for various markets. The 

remaining sugars would be fermented to ethanol fuel. The lignin in plants might also be 

gasified or pyrolyzed and converted to fuels 

and chemicals.

NREL scientists and their industrial partners are also evaluating an option known as 

“green diesel.” Biodiesel contains molecules 

Source: U.S. Department of Energy, Biomass Program data
10

produced by trans-esterifying triglycerides with 

methanol. Green diesel consists of paraffin molecules produced by hydrogenating triglycerides by 

means of a conventional petroleum refining process. Green diesel has a very high cetane number, 

so it ignites fairly quickly after injection, and a 

low pour point—the lowest temperature at which 

a fuel will pour. Thus, it is a high-quality diesel 

fuel and is totally compatible with petroleum 

di­esel.

Plug-in hybrids. In addition to producing transportation fuels from biomass, we can also put 

electricity to work to reduce our petroleum usage. 

Electric vehicles are clean and quiet, and there are 

already some vehicle recharging stations around 

the country. This vehicle technology could be 

fully renewable if the recharging stations provided electricity generated by wind, solar, and 

other renewable energy technologies.

Renewable

Fuels

Standard

Ethanol Production

Year

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012

Billions of Gallons

0

1.0

2.0

5.0

4.0

3.0

6.0

7.0

8.0

Ethanol Production

Millions of Gallons

0

10.0

20.0

50.0

40.0

30.0

60.0

70.0

80.0

Biodiesel Production

Year

1999 2000 2001 2002 2003 2004 2005

Billions of Gallons (Gasoline Equivalent) Per Year* 20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

THIS SALE

GALLONS

UNLEADED

THIS SALE

GALLONS

UNLEADED

Projected Gasoline 

Demand in 2015

159 billion gal/yr

34.7 billion gal/yr

104 billion gal/yr

Potential Grain Ethanol 

Production Potential Cellulosic Ethanol

or Other Biomass Energy

10

0

Billions of Gallons of Ethanol Per Year*

30

45

60

75

90

105

15

0

THIS SALE

GALLONS

UNLEADED

Entire 2015 Corn Crop

4.9 billion gal/yr

2012 Renewable

Fuel Standard

Standard 10% 

Ethanol Blend

*The right- and left-hand scales are different because ethanol has only two-thirds as much 

energy per gallon as gasoline. For example, 90 billion gallons of ethanol is equivalent--in 

energy terms--to 60 billion gallons of gasoline.

Projected 2015 Production

Can Biofuels Displace Gasoline?

Although the amount of ethanol produced 

from corn grain is increasing rapidly, 

it will probably displace only a small 

portion of the very large U.S. market for 

gasoline. However, cellulosic ethanol—

and the power, chemicals, and other fuels 

that can be produced from cellulosic 

biomass—could displace petroleum usage 

substantially. The Energy Information 

Administration’s 2006 Annual Energy 

Outlook projects annual gasoline demand 

will be almost 20 quads, or about 160 

billion gallons, in 2015. And the USDA 

projects that corn grain production for 

2014–2015 will be more than 12,000 

million bushels; of that, about 14% will be 

used for ethanol production (Agriculture 

Baseline Projections to 2015, February 

2005). DOE and the USDA estimate that 

cellulosic ethanol and other biomass 

energy supplies have potential to displace 

30% to 40% of current petroleum usage; 

see Biomass as 

Feedstock for a 

Bioenergy and 

Bioproducts Industry: 

The Technical 

Feasibility of a BillionTon Annual Supply,

April 2005.

Today’s HEVs do not depend on an external 

means of recharging their batteries. NREL is working to move HEV technology a step further by 

developing hybrids that can be plugged in to store 

electricity for later use. Adding extra batteries to 

an HEV and a means to plug them in, we would 

drive most of a typical day on domestic electricity 

and still have fuel in the tank for longer trips.

Today’s batteries are heavy and expensive, however. So, NREL is researching advanced drive 

trains and other vehicle components—such as 

batteries—that can be used in conventional, 

hybrid-electric, and plug-in hybrid vehicles. 

Advances in batteries and other energy-storage 

technologies are essential to the success of plugin hybrids. NREL is also exploring ways to make 

plug-ins reversible, so that excess power stored in 

the vehicle would go back into the utility grid, to 

the owner’s credit.
11

Where We Need to Be in 20 Years

Taking all this another step further, NREL foresees the emergence of “renewable communities” 

that would feature plug-in hybrids, zero-energy 

homes, and various renewable energy technologies. They might not be a reality now, but they are 

a part of our vision for the future.

We support the Department of Energy’s “30 by 

30” goal, which means that ethanol will make up 

at least 30% of our nation’s transportation fuels 

by 2030. Research in other long-term technology options, such as renewable-based hydrogen 

fuel and fuel cells, is also important, especially in 

terms of the basic science underlying those technologies. Hydrogen fuel cells for transportation 

and hydrogen vehicles are so promising that California and some states on the Atlantic Coast are 

setting up prototype hydrogen fueling stations to 

test new technologies as they develop.

Hydrogen production. Producing hydrogen by 

steam-reforming natural gas, today’s most economical method, would increase our reliance 

on an increasingly scarce fossil fuel. So, NREL is 

pursuing a renewable option: gasifying biomass 

and reforming the resulting syngas to hydrogen 

through a water-gas shift reaction.

NREL researchers are also exploring the use 

of cost-effective solar, wind, and other renewable technologies to electrolyze water to produce hydrogen. And they are pursuing both 

photoelectrochemical and photobiological technologies that could produce hydrogen directly.

The photoelectrochemical approach integrates 

elements of a photovoltaic cell with elements of 

an electrolyzer, so that the absorption of light 

energy triggers the splitting of a water molecule in an aqueous electrolyte. Because aqueous photovoltaic cells and electrolysis share 

anode, cathode, and electrolyte components, this 

approach is potentially much more efficient than 

the separate steps of electricity generation and 

electrolysis.

The photobiological approach takes advantage 

of the fact that certain microorganisms—such 

as green algae—naturally split water to produce 

hydrogen as a way to dissipate the energy they 

do not need in certain circumstances. Researchers have been creating new genetic forms of the 

microorganisms that can sustain hydrogen production in the presence of oxygen. One system 

uses a metabolic switch (sulfur deprivation) to 

cycle algal cells between a photosynthetic growth 

phase and a hydrogen production phase.

Fuel-cell research. Fuel cells could revolutionize the way we power our nation, providing 

clean, more efficient alternatives to burning fossil fuels. However, many challenges must be overcome before fuel cells will be competitive in the 

marketplace. Therefore, NREL’s work focuses on 

Where Will the Biomass Come From?

We are not likely to be able to replace all our petroleum-based fuels 

and chemicals with biomass-derived products. However, it is a good 

idea to set some goals for the contribution biomass can make to meet 

future petroleum demand.

The Biomass R&D Technical Advisory Committee to the Secretaries of Energy and Agriculture has stated that biomass could supply 

5% of current U.S. power needs, 20% of transportation fuel requirements, and 25% of chemical needs by 2030 (Vision for Bioenergy and 

Biobased Products in the United States, October 2002). This would displace 30% of our petroleum use and require about a billion tons of 

biomass per year to meet current U.S. consumption levels.

A subsequent joint DOE-USDA study headed by Oak Ridge National Laboratory that included contributions from NREL and others was 

conducted to determine whether that goal is feasible. The study 

found that forestlands and agricultural lands—the two largest sources 

of biomass—could produce the following annual tonnages by 2030:

Forestlands 

Fuelwood harvest from forests 52

Residues from wood processing & pulp & paper mills 145

Urban wood residues 47

Residues from logging and site-clearing operations 64

Forest thinnings to reduce fire hazards 60

Forestlands subtotal 368

Agricultural Lands 

Annual crop residues 428

Perennial dedicated energy crops 377

Crop grains used for biofuels 87

Manures and miscellaneous 106

Agricultural lands subtotal 998

Total production in 2030 (in million dry tons of biomass) 1,366

The study concludes that enough biomass would be available in 2030 

to meet about 40% of our current petroleum use. Business-asusual projections for petroleum use predict a substantial increase. 

But much more efficient vehicles and the use of electricity rather than 

liquid fuels (for example, for plug-in electric hybrids) could reduce 

the need for petroleum dramatically. Key assumptions in the study 

included these: crop yields will increase by 50% by 2030; cropland 

will switch to no-till management; 55 million acres will be converted to dedicated energy crops; crop and residue harvesting technology 

will improve; and environmentally sensitive lands and forestlands 

not accessible by road would be excluded. (See Biomass as Feedstock 

for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a 

B­illion-Ton Annual Supply, April 2005).

3.7

Billion Gallons/Year 10

20

30

40

50

60

Required Growth of Advanced Renewable Fuels to Supply 

30% of Current U.S. Gasoline Demand by 2030

0

Year

2005 2010 2015 2020 2025 2030

9.4

Cellulosic Ethanol 

and “Green” Diesel

Grain Ethanol and 

Conventional Biodiesel

12.8

44.8
12

Using NREL-created models such as ADVISOR 

(Advanced Vehicle Simulator), which is now 

commercially available, we can simulate 

potential vehicle technologies and combinations of technologies in days or weeks, rather 

than the months or years it takes to build and 

test prototypes. Using these computer tools, 

we can advise automakers, equipment providers, other government programs, and our 

own researchers on the most promising avenues to pursue to reduce the environmental 

impacts of vehicles and improve their efficiency and performance.

NREL has also developed a unique thermal comfort manikin known as ADAM (the 

Advanced Automotive Manikin), which is 

designed to help the automotive industry 

design smaller, more efficient climate-control 

systems in vehicles.

The ultimate goal of all this research and 

development is to enable U.S. industry to 

produce advanced, low-emission, economically competitive fuels and vehicles that will 

meet our future transportation and environmental needs. Pursuing all options, we are on 

the way to greater energy independence. 

New NREL Facilities Provide

Key Tools for Studying

Surfaces and Heat Flow

Two new NREL laboratories provide 

researchers with important resources 

they can use to improve fuel and 

vehicle technologies. The state-of-theart electron microscopes in the recently 

opened Biomass Surface Characterization Laboratory (BSCL) are powerful 

tools for understanding the core mechanisms underlying biomass conversion 

processes. The new Electrical Systems 

Laboratory (ESL) provides the essential capabilities needed to improve the 

efficiency and thermal management of 

power electronics for hybrid-electric, 

fuel-cell, and other advanced vehicles.

Surface analysis is key to understanding both biological and chemical reactions. These reactions are basically a function of the way a catalyst 

interacts with a substrate (for example, 

acid and water acting on hemicellulose 

or enzymatic proteins acting on cellulose). It takes sophisticated equipment to determine the actual shape 

of a chemical or protein and how it 

interacts with biomass. The new BSCL 

provides NREL biomass researchers 

with some of the best equipment available, including scanning-electron, 

atomic-force, transmission-electron, 

near-field-scanning-optical, and 

confocal microscopes.

For vehicle efficiency, it is important 

to understand heat flow and performance under anticipated uses. 

The ESL’s capabilities start with a 

125-kilowatt bidirectional power supply. This allows researchers to simulate 

the way electricity flows from a hybrid 

vehicle’s battery to its motor and back, 

both to assess performance under 

various drive cycles and to represent 

actual heat-flow situations. A highheat-flux test loop allows heat flow 

to be assessed within various power 

electronics components. Researchers are already using it to develop and 

test innovative cooling systems for 

the power inverters needed to convert the DC current of a battery to the 

AC required by the motor. Ultracapacitors are also being studied; they 

have less total power storage capability than batteries, but can deliver much 

more power for a short time and could 

boost the efficiency of certain types of 

vehicles.

NREL’s Bill Kramer shows a power 

inverter and a new cooling system that 

NREL developed and is testing in its 

new Electrical Systems Laboratory. 

PIX14682/Dean Armstrong

improving the performance and cost effectiveness of fuel cell systems, subsystems, and 

components. Specific research areas include 

system analysis and component research.

Fuel cells are rather complex, so innovative 

analysis tools are needed to design them. 

NREL is using a variety of tools to identify 

critical design issues for fuel cell vehicles and 

systems. We work with industry to share and 

apply robust design tools and techniques to 

address such issues as durability, cost, and 

efficiency.

Simulation tools. Much of NREL’s research 

in advanced vehicles makes use of computer 

simulations of vehicle technologies to model, 

evaluate, and optimize them. A basic problem underlying our dependence on imported 

oil is that today’s cars and trucks are not very 

efficient. Even a hybrid-electric vehicle uses 

less than one-fifth of the energy—fossil or 

renewable—that goes into it. Because several 

different configurations for hybrid-electric 

and fuel-cell vehicles are possible, and 

numerous alternative technologies could be 

used in key vehicle components, computer 

modeling is critically important.

For More Information

A. Aden, M. Ruth, K. Ibsen, J. Jechura, K. Neeves, J. Sheehan, B. Wallace, L. Montague, A. Slayton, J. Lukas.

Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis 

and Enzymatic Hydrolysis for Corn Stover. June 2002. National Renewable Energy Laboratory. NREL/TP-510-

32438 (www.nrel.gov/docs/fy02osti/32438.pdf). 

Biomass Technical Advisory Committee. Vision for Bioenergy and Biobased Products in the United States, 

October 2002, and Roadmap for Biomass Technologies in the United States, December 2002 (www.biomass.

govtools.us/pdfs/BioVision_03_Web.pdf; www.biomass.govtools.us/pdfs/FinalBiomassRoadmap.pdf).

R.D. Perlack, L.L. Wright, A.F. Turhollow, R.L. Graham, B.J. Stokes, D.C. Erbach. Biomass as Feedstock for a 

Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply. April 2005. Oak 

Ridge National Laboratory for the U.S. Department of Energy and U.S. Department of Agriculture (www.

eere.energy.gov/biomass/pdfs/final_billionton_vision_report2.pdf).

T. Penney, J. Elling. “The Race to Connect Cars, Communities and Renewables.” Geotimes. August 2005. 

American Geological Institute.

Renewable Fuels Association. From Niche to Nation: Ethanol Industry Outlook 2006 (www.ethanolrfa.org/

industry/outlook/). 

H. Shapouri, J. Duffield, A. McAloon, M. Wang. The Energy Balance of Corn Ethanol: An Update, 2002, Agricultural Economic Report No. 814, and The 2001 Net Energy Balance of Corn-Ethanol, presented at the Corn Utilization and Technology Conference, June 7-9, 2004, Indianapolis, IN. U.S. Department of Agriculture, Office 

of the Chief Economist, Office of Energy Policy and New Uses (www.usda.gov/oce/reports/energy/aer-814.

pdf; www.usda.gov/oce/reports/energy/net_energy_balance.pdf).

J. Sheehan, A. Aden, K. Paustian, K. Killian, J. Brenner, M. Walsh, R. Nelson. “Energy and Environmental 

Aspects of Using Corn Stover for Fuel Ethanol.” 2004. Journal of Industrial Ecology. Volume 7, Number 3-4 

(http://devafdc.nrel.gov/pdfs/8427.pdf).

Web sites:

• National Renewable Energy Laboratory Biomass Research, www.nrel.gov/biomass/

• National Renewable Energy Laboratory Advanced Vehicles and Fuels Research, www.nrel.gov/

vehiclesandfuels/

• U.S. Department of Energy, Energy Efficiency and Renewable Energy Alternative Fuels Data Center, 

www.eere.energy.gov/afdc/

• U.S. Department of Energy, Energy Efficiency and Renewable Energy Biomass Program, www.eere.

energy.gov/biomass/index.html, www.eere.energy.gov/biomass/publications.html 

• U.S. Department of Energy, Energy Efficiency and Renewable Energy FreedomCAR and Vehicle Technologies Program, www.eere.energy.gov/vehiclesandfuels/
13

Thousands of Vehicles

0

20

40

100

80

60

120

140

180

160

Year

2000 2001 2002 2003 2004 2005

Thousands of Vehicles

0

600

400

200

800

1000

1200

1400

Year

 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 2022 2024 

Projected Sales of 

Flexible-Fuel Vehicles Sales of

Flexible-Fuel Vehicles

Projected Sales of 

Hybrid-Electric Vehicles

Sales of Hybrid-Electric Vehicles

100%

63.4%

Engine

Losses

17.2%

Standby/Idle

Losses

2.9%

Accessories

Losses

3.5%

Driveline

Losses

energy for 

Remaining

13.0%

propulsion after 

subtracting all 

energy losses.

characteristics 

for a particular type 

of vehicle. For example, 

sometimes two or more design 

objectives—such as power and 

fuel economy—compete with each 

other; optimizing involves evaluating and comparing options to 

achieve the best overall effect. And analysts can tap into the considerable computational capability of NREL’s distributed computing system, making the entire process more efficient.

NREL has also developed a tool to evaluate technical targets and 

R&D goals, estimate market penetration, and project the impacts 

of advanced vehicles on U.S. oil usage. For more information, and 

a list of vehicle simulation and analysis tools currently used at 

NREL, please see www.nrel.gov/vehiclesandfuels/vsa/

simulation_tools.html.

Sophisticated Modeling Capabilities for Advanced Vehicles R&D

In developing a new vehicle, engineers 

can shave years off the process by using 

simulations that clearly show the designs 

and components that will (or won’t) achieve 

their performance goals. At NREL, vehicle 

systems analyses help to guide the development of the 

most promising advanced vehicle and component technologies, such as plug-in hybrids, fuel cell vehicles, and 

advanced storage technologies.

NREL’s expert analysts have considerable experience in evaluating advanced vehicle technologies. Using computer simulation and analysis tools, 

they evaluate new technologies and their impacts on fuel 

economy, vehicle performance, exhaust emissions, costs, and 

market potential.

For example, they use high-level, exploratory, and detailed modeling tools, as well as robust design methods, to determine 

optimum designs and components. High-level models, such as 

VISION, are used in design studies and analyses of cycle-related 

requirements. These flexible models are particularly useful in the 

preliminary design stage, when assumptions and requirements 

can change quickly.

Exploratory models, such as ADVISOR, allow analysts to investigate energy management strategies, interactions among components of various types and sizes, and the performance and energy 

implications of different powertrain designs. These are important in developing future plug-in and heavy-duty hybrid vehicles.

More detailed models, such as the Powertrain System Analysis 

Toolkit, provide in-depth analyses of components, events, and 

transients. And robust design integrates the latest computeraided engineering and design tools with advanced design techniques to solve technical barriers and accelerate development.

The Laboratory’s optimization tools and distributed computing 

capabilities complement this modeling work. Optimization tools 

help analysts find the best combinations of components and 

Hybrid-electric vehicle (HEV) 

sales are still small, but highefficiency vehicles are proving 

to be very popular. So, sales 

are expected to increase 

substantially. They also set the 

stage for “plug-in” HEVs, which 

could shift transportation 

energy use from largely 

imported oil to domestically 

generated electricity. There 

are already about 5 million 

flexible-fuel vehicles (FFVs) on 

the road that can use E85 fuel, 

a high-percentage blend of 

ethanol. The number of service 

stations selling E85 needs to 

grow proportionately to take 

advantage of the capabilities 

of this growing FFV fleet. 
14

High-Performing 

Commercial

Buildings

Designed for maximum energy 

efficiency, they are easy on the

eyes as well as the budget T he buildings we live and work in consume

about 39% of all the primary energy the 

nation uses annually—and 70% of the 

electricity. With energy costs on the rise, home 

and business owners need to know how to keep 

indoor areas comfortable without sending their 

energy bills through the roof.

To help meet that need, the engineers, architects, 

analysts, and programmers on the Commercial 

Building Integration (CBI) team in NREL’s Cen-

ter for Buildings and Thermal Systems study and 

design energy-efficient, high-performing com-

mercial buildings. High-performing buildings 

are designed to provide superior levels of indoor 

comfort and lighting while consuming signifi-

cantly less energy than those meeting minimum 

standards for energy efficiency.

The team monitors, analyzes, and documents 

how buildings actually perform. They make use 

of sophisticated building design software to sim-

ulate and analyze performance. And they sup-

port the U.S. Department of Energy (DOE) goal 

to have marketable zero-energy building designs 

ready by 2025. Zero-energy buildings produce at 

least as much energy as they consume. Thus, in 

addition to energy-efficient elements, these build-

ings incorporate energy-generating technologies 

such as photovoltaic (PV) solar electric systems.

The team’s work reflects a system-integrated, 

“whole-building” approach to design. This means 

that they look carefully at how each element of a 

building will affect other elements, and the build-

ing as a whole, in terms of energy efficiency.

Six Good (Energy) Performers

CBI team leader Paul Torcellini says, “The best 

time to start thinking about a low- or zero-energy 

building is when you’re designing the building 

envelope.”

The envelope is the building’s “skin”—walls, 

roofing, and other elements that together form 

a thermal barrier between the building and 
15

its environment. An energy-efficient envelope 

should provide most of the building’s heating, 

ventilation, cooling (HVAC), and lighting needs.

Today, standard HVAC equipment (e.g., furnaces and air-conditioners) and electric lighting are usually required to supplement the work 

of an energy-efficient envelope. In the future, the 

envelope should supply most of the building’s 

HVAC and lighting needs, with contributions 

from renewable energy. To move us closer to that 

future, the CBI team recently analyzed these six 

buildings:

• Adam Joseph Lewis Center for Environmental 

Studies, Oberlin College, Oberlin, Ohio

• BigHorn Home Improvement Center, Silverthorne, Colorado

• Cambria Office Building, Ebensburg, Pennsylvania

• Chesapeake Bay Foundation Philip Merrill 

Environmental Center, Annapolis, Maryland

• NREL Thermal Test Facility (TTF), Golden, 

Colorado

• Zion National Park Visitor Center Complex, 

Springdale, Utah.

Each one incorporates various energy efficiency 

and renewable energy design elements, such as 

daylighting (natural lighting), passive solar strategies, radiant heating, natural ventilation, evaporative cooling, ground-source heat pumps, and 

PV systems. NREL staff helped to design three of 

the six: the BigHorn Center, the TTF, and the Zion 

Visitor Center. They monitored all of them for a 

year or more when completed and analyzed data 

to compare actual energy performance with goals 

specified in original designs.

How the Buildings Measured Up

The thermal envelopes of all six buildings exceed 

those specified in ASHRAE Standard 90.1, published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. The 

buildings consume between 25% and 70% less 

energy than a comparable building would if 

designed to be ASHRAE code-compliant.

The monitoring revealed some gaps between 

design goals and actual performance, but later 

modifications helped to close those gaps. At the 

top of the list were problems with controls, insulation placement, and following the building 

plans. Here is a brief summary of the results of 

NREL’s monitoring and analysis; see the table on 

page 17 for data from the six-building study.

Lewis Center. The Adam Joseph Lewis Center 

for Environmental Studies at Oberlin College in 

Ohio was designed to be an energy producer as 

well as an energy consumer and a teaching aid 

for students. The 13,600-ft2 (1260-m2) building, constructed in 2000, houses classrooms, 

offices, and an atrium. Features include passive 

solar design, daylighting, natural ventilation, an 

enhanced thermal envelope, and geothermal heat 

pumps for heating and cooling. A roof-integrated 

PV system provides electricity.

After construction, NREL staff monitored and 

evaluated the building’s energy performance. The 

building uses 47% less energy than a comparable code-compliant 

one, and it demonstrates that a highperformance building 

works well even in 

a climate with high 

heating and cooling 

loads.

BigHorn Center. The 

BigHorn Development Project in Silverthorne, Colorado, 

is a retail complex in 

a mountain community. The building that 

NREL helped to design 

is a 36,980-ft2 (3,436-

m2) structure housing 

a hardware store and a 

warehouse/lumberyard building. Calibrated simulations were used to estimate the performance 

of the building before construction. Monitoring 

and analysis since then show that the building 

has 53% lower energy costs than a typical codecompliant retail building of its size.

Daylighting and other design features reduced 

lighting energy requirements by 80%. Reducing electric lighting and controlling solar gains 

allowed the designers to use natural ventilation to 

meet the cooling load. 

An 8.9-kilowatt (kW) 

PV system initially 

provided 2.5% of the 

building’s electricity 

needs. PV system and 

other improvements 

increased that to 

about 8%.

Cambria Building. 

The Cambria office 

building in Ebensburg, Pennsylvania, is one of the high-performing 

buildings constructed for the Pennsylvania 

Department of Environmental Protection. Completed in 2000, it features efficient wall and 

roof insulation, high-performance windows, 

The Lewis Center for 

Environmental Studies at 

Oberlin College, Ohio

The BigHorn Home Improvement 

Center, Silverthorne, Colorado

PIX09202/Jim Yost

The Cambria Office Building in the Commonwealth of 

Pennsylvania Department of 

Environmental Protection, 

Ebensburg, Pennsylvania

PIX10140/Jim Schafer 

PIX10860/Robb Williamson
16

ventilation. And an energy management system 

tracks outdoor temperatures and humidity to 

control the HVAC system. At CBF’s request, NREL 

staff assessed the building’s performance through 

a combination of monitoring and computer simulations; results showed a 12% annual energy 

cost savings compared with costs for a standard 

design.

Thermal Test Facility. NREL built the 10,000-

ft2 Thermal Test Facility (TTF) in 1996 to house 

buildings research and to refine and test a new 

integrated design process. The TTF’s clerestory 

windows maximize the use of natural light and 

minimize summer cooling loads. A rigid exterior 

finish minimizes envelope heat transfer. A directindirect evaporative air-conditioning system 

meets cooling loads. And an energy management 

system monitors and controls internal temperatures, humidity, air pressure, duct pressure, light 

levels, and carbon dioxide levels.

Using computer simulations, NREL tested several design concepts before construction. The goal 

was to reduce building energy requirements by 

70%. After construction, simulated performance 

was compared with actual performance; results 

included energy cost savings of 51%.

Zion Visitor Center. To develop a new visitor 

center complex in Zion National Park in southwestern Utah, the National Park Service requested 

technical support from NREL. Their collaboration 

lasted throughout the design, construction, and 

evaluation phases.

The 8,800-ft2 (817-m2) main building that 

opened in May 2000 houses interpretative displays, offices, and retail space. Features include 

passive direct evaporative cooling, natural ventilation, external shading devices, and glazing that 

minimizes solar gain in summer and uses it in 

winter. Also featured are thermal mass sized for 

the direct-gain system, a Trombe wall, daylighting, PV power, and digital controls to integrate 

energy operations. The building has 67% lower 

energy costs than a similar one meeting applicable energy codes.

PIX04116/Warren Gretz

ground-source heat pumps, an under-floor air 

distribution system, energy recovery ventilators, 

and daylighting.

The design also specified an 18.2-kW PV system 

for on-site power production. These elements 

helped to achieve a LEED (Leadership in Energy 

and Environmental Design) 2.0 Gold Certification from the U.S. Green Building Council, and 

a 43% reduction in energy use in comparison to 

a standard building design. NREL staff collected 

performance data for comparison with a standard baseline computer model and then recommended further performance improvements.

Merrill Center. The Chesapeake Bay Foundation 

(CBF), dedicated to restoring and protecting the 

bay’s resources, built the 31,000-ft2 (2,880-m2) 

Philip Merrill Environmental Center in Annapolis, Maryland, as its headquarters in 2000. The 

CBF incorporates many energy-saving features, 

including a ground-source heat pump to provide 

heat in winter and cooling in summer.

The building features large, south-facing windows for passive solar heating and daylighting. 

Sensors automatically dim lights when daylight 

levels are sufficient. Bay breezes provide natural 

The Zion National Park Visitor 

Center in Utah

The Chesapeake Bay 

Foundation Philip Merrill 

Environmental Center, 

Annapolis, Maryland

PIX09243/Robb Williamson

PIX10884/Robb Williamson

NREL’s Thermal Test Facility, Golden, Colorado 
Metric Oberlin Zion Cambria CBF TTF1 BigHorn

Benchmark ASHRAE Standard 90.1 

Version

2001 19952 2001 2001 19952 2001

Savings3

Net Source Energy 

Savings4

79% 65% 42% 22% 45% 54%

Net Site Energy Savings5 79% 65% 42% 25% 42% 36%

Site Energy Savings6 47% 62% 40% 25% 42% 35%

Energy Cost Savings 35% 67% 43% 12% 51% 53%

Project Goal 

Comparison

Design Goal or Predicted 

Performance

Net Site Energy Use: 0.0 

kBtu/ft2·yr

Energy Cost 

Saving: 80%7

Energy Cost 

Saving: 66%7

LEED 1.0 Platinum RatingEnergy Cost 

Saving: 70%7

Energy Cost 

Saving: 60%7

Measured or Simulated 

Performance

Net Site Energy Use: 16.4 

kBtu/ft2·yr 

Energy Cost 

Saving: 67%

Energy Cost 

Saving: 44%

LEED 1.0 Platinum RatingEnergy Cost 

Saving: 51%

Energy Cost 

Saving: 53%

Summary of energy savings for the six buildings studied

1 TTF monitored for select periods; actual data used to calibrate 

simulations.

2 Code used was 1995 Federal Energy Code, 10 CFR 435 (DOE 1995); 

based on ASHRAE 90.1-1989 with more stringent lighting power 

densities.

3 Energy savings for Oberlin, Bighorn, TTF, and Cambria calculated with 

simulations of as-built and base-case buildings with typical weather 

data; Zion and CBF savings calculated with measured data and basecase simulations run with measured weather data.

4 Source energy is the energy required to supply energy in the form it 

is used at the site; the Cambria office building purchases 100% green 

power, so source energy could be calculated assuming 9% loss for transmission and distribution (EIA 2004).

5 Net site energy use allows credit for electricity generated on site by the 

PV system.

6 Total site energy use is the total electricity consumed.

7 Predicted energy costs, calculated before construction, may not indicate 

future performance because of volatile energy prices.

17

Selected Best Practices

The results of this study suggested several best 

practices for high-performing buildings:

• Use an integrated design process to systemengineer the building

• Use computer simulations to guide the design 

process; these help designers analyze tradeoffs and examine the energy impacts of architecture and HVAC choices

• Simulate and measure the building’s energy 

performance at design, construction, and 

occupancy stages

• Set specific, quantifiable energy performance 

goals

• Design the building envelope to meet or minimize as many HVAC and lighting loads as 

possible

• Size HVAC and lighting systems to meet loads 

not met by the envelope

• Use daylighting in all zones adjacent to 

ex­terior walls or roofs

PIX11001/Robb Williamson

• Install highly reflective surfaces in all daylit 

zones, especially ceilings

• Monitor and evaluate post-occupancy energy 

performance

• Implement standardized measurement procedures using standard metrics

• Carefully design and implement the use, control, and integration of economizers, natural 

ventilation, and energy recovery ventilators.

Practices like these can move us closer to new 

commercial buildings that save considerable 

energy and produce it, as well.

For more information, contact Paul Torcellini, Commercial Building Integration Team 

Leader, Paul_Torcellini@nrel.gov, and see 

www.nrel.gov/buildings/highperformance/.

Paul Torcellini examines 

an in-floor diffuser for the 

raised-floor air distribution 

system in the Cambria office 

building in Pennsylvania.
18

Closing the Gaps

Semi-mismatched semiconductor 

materials can boost solar device 

efficiency

I

n the world of solar cell design, conventional 

wisdom dictates that things work best when 

everything lines up properly—in other words, 

when the spacing of the atoms in one semicon-

ductor layer of a solar device closely matches 

that of the adjacent layer. However, innovative 

research often calls for unconventional methods 

if we are to “build a better mousetrap”—or in this 

case, a more efficient solar device. The National 

Renewable Energy Laboratory’s work on so-called 

“semi-mismatched” multijunction solar cells is 

one example of this.

A primary goal of NREL’s solar energy research 

and development (R&D) is to create photovoltaic 

(PV) devices—cells and modules—that have 

excellent conversion efficiencies. A device’s con-

version efficiency is a measure of how well it 

uses the energy of sunlight to produce electrical 

energy. However, three critical “gaps” still get in 

the way of high efficiencies, as illustrated in Fig. 1.

Peering into the Gaps

These three gaps correlate with different aspects 

of solar cell R&D. The first gap is the difference 

between the theoretical efficiency of the semicon-

ductor material used in a solar device and the effi-

ciency of the fabricated device, as measured under 

the best laboratory conditions. For example, given 

certain assumptions, the theoretical best effi-

ciency of a crystalline silicon cell is about 29%. 

But the top efficiency to date for a crystalline sili-

con cell measured in the lab is nearly 25%, which 

leaves us with a 4% gap.

The reasons for this gap include the efficiency 

losses inherent in the solar conversion process 

and the difficulty of fabricating a cell with the 

necessary properties. To overcome this gap, R&D 

is needed that will identify, explain, and mini-

mize losses; enable the device to collect each pho-

ton or packet of light incident upon it; allow the 

photons to create the maximum number of elec-

trical charge carriers; and make sure the charge 
19

PV System Cost/kWh

Generated in 5-Year Period ($/kWh)

0

10

20

30 3.3

2.2

1.1

0.0

typical

cell cost:

2¢-4¢

typical

cell cost:

$3-$10

PV System Cost/Power Output ($/W)

0.001 0.01 0.1 1 10

15%

20%

25% cell eff.

30%

40%

50% cell eff.

20%

10%

5-Year Payback Threshold, at $0.15/kWh

Fixed Flat-Plate 500X Point-Focus Conc.

Cell Cost ($/cm2)

semi-mismatched solar cell being developed at 

NREL. NREL’s research is being carried out to 

develop laboratory cells with measured 

efficiencies closer to theoretical efficiencies, and 

to address design factors that will lead to more 

efficient manufactured cells and modules.

NREL’s innovations could result in increasingly 

higher efficiencies as new devices are optimized. 

In turn, these devices will play an important role 

in the need for high-efficiency PV devices in space 

applications, as well as on land, for large-scale 

power production.

The multijunction or tandem cell has been the 

focus of R&D at NREL and other laboratories for 

a few decades. In single-junction cells, the energy 

of the illuminating light that is below the bandgap of the cell material is lost; thus, it cannot be 

used to generate electricity. (The bandgap—not 

Why Worry About High Efficiencies?

High efficiencies are important because they 

are indicators of cost-effective solar devices. 

The cost-effectiveness of a PV system depends 

on the cell’s cost per unit area. To determine a 

suitable cost range for a cell, researchers from Spectrolab, Inc., and NREL adapted1 an analytical methodology by Swanson2. 

The PV system’s cost per kilowatt-hour over a 5-year payback period was calculated as a function of the cell’s cost per unit 

area, taking into account various assumptions relating to efficiencies, costs, packaging, and solar resources.

As shown, electricity costs for both flat-plate and concentrator systems can fall below 15 cents per kilowatt-hour (a nearterm R&D target value) when high efficiencies are combined with low cell costs. Note two families of curves: those for flatplate systems, requiring very low cell costs, and those for concentrator systems, which can tolerate relatively high cell costs.

The cell costs of flat-plate systems need to range from 0.1 to 1 cent per square centimeter, for cost-effective, 20%-efficient 

cells. In contrast, for concentrator systems with 30%-efficient cells, a cell cost of 1 to 5 dollars per square centimeter—which 

is 500 to 1000 times greater—is cost-effective. The challenge for flat-plate technologies is to reduce today’s typical cell costs 

of 2 to 4 cents per square centimeter by an order of magnitude, while maintaining cell efficiencies of 20%. For concentrator 

systems, 30%-efficient cells are cost-effective now; the challenge is to reach efficiencies in the 40% to 50% range for even 

greater cost-effectiveness.

Sunlight is a “dilute” resource, and low-efficiency systems require very large areas of active PV material and all packaging 

and structural materials, such as metal, glass, and plastic. However, high-efficiency solar cells have at least two benefits: 

the module area needed for a given electricity output is greatly reduced, and module and balance-of-system costs are highly leveraged. At 500x concentration, relatively high cell costs per area are acceptable. Therefore, taking cell efficiencies into 

the 40% to 50% range may well be the best path to cost-effectiveness.

1. R.R. King and others, Intl. Conf. on Solar Concentrators for the Generation of Electricity or Hydrogen, 1–5 May 2005, Scottsdale, AZ. 

NREL/CD-520-38172 (2005).

2. R. Swanson, Prog. Photovolt. Res. Appl. 8, 93–111 (2000).

Figure 1. The three efficiency gaps for solar cells and 

modules

carriers last long enough to contribute to generating a current.

The second gap is the difference between the efficiencies of laboratory cells and the efficiencies of 

those produced in commercial production lines. 

This gap may be a result of scaling up the fabrication process to produce larger devices. Also, in the 

manufacturing environment, higher throughput 

is required and fabrication conditions are often 

less controlled.

The third gap is the difference between cell efficiencies and module efficiencies. This gap will 

narrow when we can do at least three things: minimize the electrical losses that occur when cells 

are wired into circuits, bring the active area of the 

module closer to the cell area, and maximize the 

optical transmission of the protective encapsulant 

material.

Working in Tandem

Here we address several issues surrounding 

the first two gaps, focusing on an inverted 
20

Inverting and Mismatching to 

Increase Efficiency

Design options for some PV device structures 

based on lattice-matched epitaxy are constrained 

by the limited number of commercially available 

crystalline substrate materials (Ge and GaAs, for 

example). However, the “design space” increases 

substantially when we consider designs involving 

lattice-mismatched layers.

Traditionally, lattice-mismatched approaches 

have been dismissed because of problems such 

as high defect density, rough surfaces, and cracking or bowing of layers. At NREL, the goal is to 

produce semi-mismatched lattice heterostructures that look and perform like lattice-matched 

ones. In the process, we want to attain the highest 

charge-carrier lifetimes, the thinnest structures, 

and the flattest wafers possible.

Mark Wanlass and his colleagues in NREL’s 

High-Efficiency Solar Cell team are pursuing a 

novel tandem cell approach. One key innovation involves growing the cell layers in inverted 

order; this approach could have several cost and 

manufacturing benefits and boost efficiency, 

as well. Another innovation is to incorporate a 

transparent, compositionally step-graded layer to 

accommodate lattice mismatching without penalizing cell efficiency.

Figure 2 illustrates the basic layer-cake structure 

of the inverted GaInP / GaAs / gallium indium 

arsenide (GaInAs) cell. The structure is considered to be inverted because it is grown upside 

down—that is, the top layer is grown on a substrate, then the next two main “active” subcell 

layers are deposited. In actual operation, the stack 

is flipped so that sunlight strikes the top subcell 

first.

The top two active subcell layers are lattice 

matched. A 1.9-eV top active layer of GaInP is 

deposited on a GaAs substrate before a 1.4-eV 

middle active layer is added. Each active subcell 

layer actually has a double-heterostructure configuration; this means the active material is sandwiched between thin layers of material with a 

somewhat higher bandgap. This configuration 

confines the charge carriers to the active subcell 

layer. Each active subcell layer also contains an 

n/p junction. The GaInP active subcell layer is 

sandwiched between negatively (n) and positively 

(p) doped aluminum indium phosphide (AlInP) 

to form the top subcell. Similarly, the GaAs active 

subcell layer is sandwiched between n- and 

p-doped GaInP to form the middle subcell.

The lowest (third) active subcell layer is made of 

GaInAs, with a bandgap of ~1.0 eV, and is sandwiched between GaInP layers to form a doubleheterostructure bottom subcell. The GaInAs lattice is mismatched 2.2% in comparison to the 

lattices of GaInP and GaAs. To accommodate 

this mismatch, a compositionally step-graded 

one of the “gaps” discussed here—is the energy 

needed to dislodge an outer electron from its 

bond.) The multijunction concept allows a PV 

device to capture and use this lost energy, resulting in a higher conversion efficiency and thus 

greater electrical output.

During the early to mid-1990s, NREL researchers 

developed and patented a two-junction device 

with high efficiency. The top cell was made of 

gallium indium phosphide (GaInP), which has a 

bandgap of ~1.9 electron-volts (eV). The bottom 

cell was made of gallium arsenide (GaAs), with 

a bandgap of ~1.4 eV. Earlier GaAs cells had efficiencies exceeding 25% under concentrated sunlight. Adding the GaInP layer to the GaAs layer 

further boosted the overall cell efficiency by capturing more of the energy in the spectrum.

This concept later evolved into three-junction 

devices, again with the goal of capturing more 

useful energy otherwise lost to the cell. One 

prominent system is made of GaInP / GaAs / germanium (Ge). The third or lowest layer, with 

a bandgap of ~0.7 eV, captures the long-wavelength, low-energy “red” light that passes unused 

through the two upper layers. This device is lattice matched, which means that the spacing of 

atoms in the Ge layer very closely mimics the 

spacing of the atoms in the GaAs layer above it.

When a crystal is grown through an epitaxial process—in which the overlying crystal has the same 

orientation as the underlying one—matching 

keeps the crystal lattice from being deformed at 

the transition point from one layer to another. 

Deformed layers—structural defects in the regularity of the crystal lattice, such as threading 

dislocations and stacking 

faults—negatively affect the 

movement of charge carriers through the material and 

thus reduce the performance 

of the device.

Researchers at NREL and elsewhere decided to develop 

a third layer having a more 

optimal bandgap; analyses indicated that material 

with a 1-eV bandgap could 

improve overall device efficiency as much as 10%–

12%. The dilute nitride of 

a GaInAsN layer has such a 

favorable bandgap and is lattice matched to GaAs. However, efficiency measurements of GaInP / GaAs / 

GaInAsN devices have not been promising.

A logical next step was to investigate a fourjunction device consisting of GaInP / GaAs / 

GaInAsN / Ge, which has a bandgap sequence 

of 1.9 / 1.4 / 1.0 / 0.7 eV. The efficiency of this 

device is about 31% at 500 suns; further optimization may boost that number.

Concentrating Technologies 

Microdish configured for 

Spectrolab multijunction cell

PIX13740/Arizona Public Service
21

layer of GaInP is included between the middle 

and bottom subcells. In detail, the transparent 

graded layer consists of nine 0.25-micrometerthick steps that differ in composition from one 

another by 0.03 fractional percent Ga. Significantly, although cross-sectional transmission 

electron microscopy shows substantial defects in 

this graded layer, it identifies only a modest dislocation density in the active subcell layer.

The overall tandem cell is a two-terminal, monolithic device. In other words, individual subcells are not grown separately and then stacked 

mechanically. Rather, all subcells are grown in 

one continuous depositional process—in this 

case, by atmospheric-pressure metal-organic 

vapor-phase epitaxy in a system built by NREL. 

Layers of heavily p- and n-doped GaAs form 

“tunnel junctions” between the subcells, thus 

allowing charge carriers to move through the 

entire device, and front and back electrical contacts gather the cell’s current.

The device’s monolithic structure makes it 

relatively easy to manufacture in comparison 

to mechanically stacked devices. But to optimize the device, we must adjust the thicknesses 

of each subcell so that the photocurrents of each 

are the same. Otherwise, in a series-connected 

device, the subcell with the lowest photocurrent 

will be the limiting factor in regard to the current 

of the overall cell.

For more information

R.R. King, R.A. Sherif, G.S. Kinsey, S. 

Kurtz, C. M. Fetzer, K.M. Edmondson, D.C. Law, H.L. Cotal, D.D. Krut, 

J.H. Ermer, and N.H. Karam. “Bandgap Engineering in High-Efficiency 

Multijunction Concentrator Cells.” 3rd 

International Conference on Solar 

Concentrators for the Generation 

of Electricity or Hydrogen, May 1-5, 

2005, Scottsdale, AZ, NREL/CP-520-

38172.

S. Kurtz, M. Wanlass, C. Kramer, M. 

Young, J. Geisz, S. Ward, A. Duda, 

T. Moriarty, J. Carapella, P. Ahrenkiel, K. Emery, K. Jones, M. Romero, 

A. Kibbler, J. Olson, D. Friedman, W. 

McMahon, and A. Ptak. “A New GaInP/

GaAs/GaInAs, Triple-Bandgap, Tandem Solar Cell for High-Efficiency Terrestrial Concentrator Systems.” 2005 

Solar Program Review Meeting, Denver, CO, November 2005, NREL/CP520-38997.

M.W. Wanlass, S.P. Ahrenkiel, R.K. 

Ahrenkiel, D.S. Albin, J.J. Carapella, 

A. Duda, J.F. Geisz, S. Kurtz, T. Moriarty, R.J. Wehrer, and B. Wernsman. 

“Lattice-Mismatched Approaches for 

High-Performance, III-V Photovoltaic 

Energy Converters.” 31st IEEE PV Specialists Conference, Lake Buena Vista, 

FL, January 3-7, 2005, NREL/CP-520-

37440.

M.W. Wanlass, S.P. Ahrenkiel, D.S. 

Albin, J.J. Carapella, A. Duda, J.F. 

Geisz, K. Jones, S. Kurtz, T. Moriarty, and M.J. Romero. “GaInP/GaAs/

GaInAs Monolithic Tandem Cells for 

High-Performance Solar Concentrators.” NREL/CP-520-38646, 2005.

Figure 2. Structure of the inverted, 

semi-mismatched tandem device

Seeing the Benefits

As PV researchers work diligently to overcome such performance gaps as the discrepancies 

in conversion efficiencies found 

in solar cells, this inverted GaInP 

/ GaAs / GaInAs cell is a significant step forward. NREL has confirmed an efficiency of 37.9% at 

10.1 suns (and a temperature of 

25°C, a low aerosol optical depth 

air mass of 1.5 direct, and an 

area of 0.2428 cm2); this is a very 

encouraging number for a device 

that has yet to be optimized.

The peak efficiency at such a 

low concentration ratio indicates that efficiencies exceeding 

40%—perhaps as high as 45%—

at several hundred suns should 

be achievable in the near future, 

once a true concentrator version 

is fabricated. Such high efficiencies should enhance the viability of terrestrial concentrator 

systems for cost-effective power 

generation.

Some key benefits of this design 

are that the device can be mounted on 

a surrogate substrate—or “handle”—of 

choice, and contact with a heat sink can 

also be achieved, which helps to manage the thermal issues of the concentrator cell. If it is composed of some strong, 

flexible material such as metal foil or 

kapton polymer, the handle can provide 

a robust device in comparison to one 

processed on relatively fragile Ge or GaAs 

substrates.

Handle-mounted, ultra-thin device fabrication is 

a natural consequence of the inverted-structure 

approach, which also has a number of advantages. These include potentially low cost, the 

ability to make use of back-surface reflectors, and 

possible reclamation or reuse of the parent crystalline substrate (e.g., to reclaim the relatively 

scarce, expensive Ga in a GaAs substrate).

Looking Ahead

Optimizing the inverted cell will shrink its 

performance gaps further. And other materials, designs, and methods will undoubtedly be 

explored in the pursuit of higher efficiencies. 

So, whether NREL’s researchers are developing 

silicon-based, thin-film, or so-called “thirdgeneration” PV technologies, one key objective 

prevails: to close the gaps and thus create even 

more efficient cells—cells that can be produced 

at lower cost while exhibiting greater reliability 

and stellar performance.
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

Operated for the U.S. Department of Energy

by Midwest Research Institute • Battelle

Contract No. DE-AC36-99-GO10337

NREL/BR-840-38668 • June 2006

Printed with renewable-source ink on paper containing at least 

50% wastepaper, including 10% post consumer waste. 

Notice

This report was prepared as an account of work sponsored by an agency of the United 

States government. Neither the United States government nor any agency thereof, nor any 

of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

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state or reflect those of the United States government or any agency thereof.