National Renewable Energy Laboratory 2005 Research Review June 2006 ISSUE 4 a publication about NREL R&D highlights CONTENTS feature 6 On the Road to Future Fuels & Vehicles America’s dependence on imported petroleum can be reduced by using clean, domestic trans- portation 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 fuel- cell vehicles in the long term. departments articles 1 Perspective 14 High- The President has told the nation that we are Performing “…addicted to oil…[and] the best way to break Commercial this addiction is through technology.” NREL’s R&D in sustainable, domestic energy technolo- Buildings gies is leading the way to a long-term solution NREL’s R&D shows that it is possible to design to this problem. and construct attractive residential and commer- cial buildings that are much more energy efficient 2 NREL in Focus than current standard practice. Take a look at six such “high-performing” buildings. NREL has a new deputy director/chief operat- ing officer, has partnered with others to form a fuel-cell research center, has expanded its com- 18 Closing the Gaps puting capabilities beyond the 1-teraflop High-efficiency solar milestone, and is about to open a major cells are one of many new facility. The Science and Technology areas of solar research Facility will greatly facilitate the transfer of at NREL. Read about renewable energy technology to industry. the basic rationale of multijunction cells and Awards & Honors some innovative design 4 twists that are boosting During 2005, NREL received two R&D 100 their efficiencies. 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. TheResearch 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.

i Perspective

Dr. Stanley R. Bull, Associate Director, Science & Technology

Securing Energy for the future directions in integrate renewable areas the President energy technologies Future Through Science and particularly noted in into these buildings. Technology his Address: biofuels

PIX006943/Warren Gretz PIX006943/Warren The President’s and vehicles, energy- Science and technology. They are at Advanced Energy efficient buildings, the heart of everything we do at the Initiative will invest National Renewable Energy Laboratory, and solar energy. in renewable energy technologies that as we pursue innovative, robust, and Biofuels and Vehicles—The alternative “change how we power our homes sustainable ways to produce energy— fuels industry is helping to reduce our and offices.” In this way, we can con- and as we seek to understand and illu- dependence on imports by introducing tinue to assist the U.S. building indus- minate the physics, chemistry, biology, more domestic ethanol and biodiesel try in constructing energy-smart, and engineering behind alternative options into the transportation fuels “high-performing” buildings, like the energy technologies. market. This issue describes NREL’s con- ones described in this issue. Although our nation is learning to use tributions, which include expanding Solar Energy—As part of NREL’s energy more wisely—for example, by feedstock sources, developing meth- research and development in photo- producing and buying more energy- ods for breaking down feedstock for fur- voltaics, our scientists work closely with efficient vehicles and appliances—the ther processing, and making conversion other expert groups to investigate and demand is still great. Americans con- methods for fuels and other products improve the way in which solar cells sumed 99.7 quadrillion Btu (quads) of more efficient and cost effective. directly convert sunlight to electricity. energy in 2004; the Energy Information The President proposes to “fund addi- The President’s Solar America Initiative Administration projects that this will tional research in cutting-edge meth- will support the development of solar increase to 127 quads by 2025. ods of producing ethanol, not just from cells that convert the sunlight much more efficiently than they do now. In his 2006 State of the Union address, corn, but from wood chips and stalks, President George W. Bush noted the or switchgrass. Our goal is to make this Work in high-efficiency solar cells is one seriousness of the situation and pointed new kind of ethanol practical and com- of many relevant areas of solar research the way to a remedy. The President said, petitive within six years.” Through the conducted at NREL. In the article in this “Keeping America competitive requires Advanced Energy Initiative, we will issue, we discuss the basic rationale of affordable energy. And here we have a continue working toward this impor- multijunction cells and some innova- serious problem: America is addicted to tant goal in feedstock development and tive twists on their design that can boost oil, which is often imported from un- conversion methods. efficiencies to higher levels. stable parts of the world. The best way Buildings—Our buildings consume From NREL’s inception in 1977 to the to break this addiction is through nearly 40% of the energy Americans use present, the science and technology of technology.” each year. NREL’s buildings research alternative energy has been the core of With our emphasis on research and and development demonstrate that it is our mission. And this will continue in development in market-relevant, possible to design and construct attrac- the decades to come. The advances we domestic energy technologies, NREL is tive residential and commercial build- make in all areas of renewable energy working toward a long-term solution to ings that are much more energy efficient and energy efficiency will play a vital this problem. In this issue of the NREL than the current standard practice. role in assuring the security, strength, Research Review, we present three stories Our work also develops good ways to and competitiveness of the nation. that highlight recent advances and

1 provided $2 million in start-up fund- NREL In Focus ing, and the partners are contributing $1 million. The CFCC aims to provide state leadership in the development and application of fuel cell technol- ogies, increase the public’s under- standing of the technologies through education and outreach activities, and foster economic development through partnerships with local businesses. Bill Glover Is New NREL vision. He will also serve as a vice pres- Deputy Director and COO ident for Midwest Research Institute, The CFCC will also coordinate the which has operated and managed development of fuel cell R&D pro- On February NREL for DOE since NREL opened as grams, facilitate access to experts in 1, 2006,NREL the Solar Energy Research Institute in NREL and CSM research labs, assist in welcomed 1977. forming strategic alliances among local William (Bill) developers and university research cen- Glover as the New Colorado Research ters, and serve as an independent test- Laboratory’s ing agency. In addition, the center will new Deputy Partnership Formed to promote the development of fuel cell Director and Develop Fuel Cells PIX14712/Mike Linenberger PIX14712/Mike curriculums in colleges, universities, Chief Oper- Power generation in the 21st Century and trade schools in order to build a ating Officer. faces some tough challenges: it has technically viable labor force for this “I’m glad to to score very high in energy efficiency Deputy Director and Chief young industry. Operating Officer Bill Glover be here,” he and, at the same time, very low in envi- said. “I’m very The tremendous growth excited about the Lab’s mission and I expected in the fuel cell am looking forward to working with industry in the coming everyone at NREL.” decade will mean new high-tech jobs in R&D Bill has had more than 35 years of and in manufacturing that executive-level experience. His exten-

PIX14679/Ray David PIX14679/Ray will eventually extend well sive experience with the Department beyond Colorado and the of Energy (DOE) has included senior industry itself. management responsibilities in stan- dards identification and management, At the end of two years, self-assessment and independent over- the CFCC should be sight, quality assurance, performance self-sustaining through measurement and evaluation, and research and develop- readiness reviews. He was on a head- ment contracts, consulting quarters level group overseeing the The new joint-venture Colorado Fuel Cell Center is housed agreements, and other rollout of DOE’s Standards Program, at the Colorado School of Mines a few miles away from the sources of funding. The and he led a facilities maintenance pro- NREL campus. center will then empha- gram assessment team at Los Alamos size the development National Laboratory. ronmental impacts. Right now, the fuel of the Colorado fuel cell industry as cell is one of the few technologies that it responds to national solicitations. Before joining NREL, Bill was an inde- appears to do both these things at once. For more information, contact CFCC pendent consultant, President/General Executive Director Robert J. Remick, Manager of TENERA Federal Services So, to help bring advanced fuel cell [email protected]. LLC, and Director of Performance technologies to the marketplace, Assurance with EG&G, Rocky Flats. He NREL has formed a partnership with NREL Computing Power served with distinction for 26 years in the Gas Technology Institute, the the United States Navy, including man- Colorado School of Mines (CSM), and Surpasses Teraflop aging a nuclear submarine squadron Versa Power Systems. Their joint ven- NREL’s high-performance computing with more than 3,500 personnel. ture, known as the Colorado Fuel Cell capability recently surpassed the peak- Center (CFCC), is focusing on fuel performance mark of 1 teraflop—or Bill has a B.A. in chemistry from the cell research, development, testing, one trillion floating point operations University of Rochester and the equiv- and commercialization in a state that per second. “We’ve gone from 96 giga- alent of an M.S. in nuclear engineering is home to several related high-tech flops of power to 1.2 teraflops in less (from the Navy Senior Officer Nuclear industries. than four years,” said Steve Hammond, Power Training). director of NREL’s Computational Sci- The new center is housed in the Gen- As Chief Operating Officer, Bill will ences Center (CSC), highlighting this eral Research Laboratory at the Colo- oversee Laboratory operations. As Dep- significant milestone in building high- rado School of Mines in Golden. The uty Director, he will support the devel- performance computing expertise and Colorado Governor’s Office of Energy opment and implementation of the capabilities at the Laboratory. Management and Conservation has Laboratory’s operational strategic  PIX14532/Jack Dempsey scientific data. scientific stereo 3D analysis andof visualization complex tools and for techniques include cutting-edge NREL’s Computational Sciences capabilities study by direct experiment. sive, dangerous, or even impossible to phenomena that would be too expen ers study complex systems and natural practice. The NREL CSC helps research scientific knowledge and engineering with theory and experiment to advance nomena—is an indispensable partner ulate real-world processes and phe of using computers to model and sim Computational science—the process process costs. lead to higher ethanol yields and lower these chemical reactions, which can tool for understanding the kinetics of puting capability has been an essential mass pretreatment. The increased com loss of fermentable sugars during bio modeling, they have investigated the using CSC’s quantum mechanical Center have likewise benefited. By Scientists in the National Bioenergy best performance. ments, leading to a design with the numerous different tab sizes and place optimize the geometry by considering types. Simulations help time-consuming proto out building expensive, heat exchanger with patent-pending tab-fin models of their novel tional fluid dynamics ter have run computa Thermal Systems Cen ers in the Buildings & For example, research aids scientific pursuits. ing capability greatly performance comput This increase in high------centerpiece of the building will be the PDIL, an 11,400 an PDIL, the be will building the of centerpiece The industry. to R&D from technology of transfer the accelerate and barriers manufacturing and technical reduce to designed especially was S&TF The door. next Facility Research Energy Solar the to S&TF the connects bridge elevated An fans. exhaust as such equipment, mechanical contains level third The (PDIL). Laboratory Integration and Development Process open alarge, and labs additional houses level second The level. ground facility’s the on are space office and Laboratories consumers. to ultimately, manufacturing—and commercial to bench laboratory the from move to technologies new takes it time the shorten and researchers among collaboration increase to designed is building The Initiatives. Hydrogen and America Solar the in highlighted technologies energy renewable important most the of several are which cells, fuel and energy, hydrogen nanostructures, films, thin cells, solar in development expect occupancy to begin in July 2006. July in begin to occupancy expect managers project made, progress excellent the of Because selected. contractors and completed were designs after 2004 fall in began S&TF the of Construction building. (LEED) Design Environmental and Energy in Leadership Council Building Green aU.S. as rating a“Gold” achieve to designed been has facility the Overall, system. distribution air raised-floor a as well as featured, also is cooling evaporative climate, dry Colorado’s with line In used. be to systems cooling and heating smaller allows air. This supply cool or preheat to air exhaust use technologies recovery energy addition, In leader. project senior NREL an Geet, Van Otto says spaces,” office all in daylighting have 100% to was goal “Our labs. the of some even and areas office open the illuminates clerestories and windows numerous through light natural S&TF, the In daytime. the cooler—in much that facility the keeping lights—and artificial of use the by minimizing use energy reduce to helps abuilding, in light natural of use abundant the is which example, for Daylighting, building. laboratory conventional amore would than energy less 40% as much as consume to expected is It use. energy efficient and design sustainable of amodel also is S&TF the facilities, laboratory NREL other Like says. Sheldon Pete manager project technical future,” the in and today both needs research our meet will that space laboratory flexible and functional designing on was emphasis our project, this into “Going access. computer and board awhite features area tion interac Each innovations. other and breakthroughs R&D to lead often that sions discus informal of kind the encourage spaces” “interaction building’s the And needed. as ones, open more larger, form to combined be to labs smaller allowing design, the guided model laboratory Aflexible techniques. deposition novel and analysis, surface materials, advanced for laboratories includes also S&TF The together.” working skills diverse with people of bonus added the and data, integrated tools, integrated us give will PDIL “The says, project, integration process the leading is who Nelson, Brent modules. PV as such devices turing manufac in application early for processes up scale they as partners industry for available be will bays research Six characterization. and processing, tion, deposi thin-film for tools of class anew accommodate to designed specifically S&TF, is a71,000-ft is S&TF, or Facility, Technology and Science The Colorado. Golden, in campus Mountain Table South the on facility research newest its out roll to ready nearly is NREL Open to Set Technology and Science Facility Energy-Efficient 2 (6,596-m 2 ), multilevel facility dedicated to research and and research to dedicated facility multilevel ), -ft 2 laboratory space space laboratory - - - - 

PIX14651/Mike Linenberger Technology Transfer Awards & Honors Awards The Federal Laboratory Consortium for Technology Transfer (FLC) recog- nized NREL with two “Notable Tech- nology Development” awards in 2005 R&D 100 Awards for technology transfer activities. The NREL won two R&D100 awards from purpose of the FLC awards is to rec- Research & Development (R&D) Maga- ognize achievements in moving tech- zine in 2005. These awards recognize nologies from the laboratory into the what the magazine editors consider to marketplace. be the most significant products intro- The first award recognized NREL’s duced into the marketplace over the Thermal Comfort Project, which has past year. NREL won the awards for its resulted in several tools to analyze the work in advancing residential energy efficiency of climate control in vehi- analysis software and improving the cles. The tools include the Advanced testing of silicon used in solar cell Automotive Manikin (ADAM), production. which mimics human physiological Advances in Residential Energy responses to hot and cold to predict Analysis The first R&D100 award rec- how comfortable a vehicle occupant ognizes the software application might feel. The project is managed called TREAT—Targeted Residential by NREL’s Center for Transportation Energy Analysis Tools—created by Technologies and Systems. NREL and the New York State Energy Research and Development Author- The FLC also recognized this center’s ity (NYSERDA) and its partners. Advanced Vehicles and Fuels Project. This software is a comprehensive The project created a partnership with energy analysis tool that mod- vehicle manufacturers, fuel provid- els building energy consump- ers, and others to incorporate innova- tion and identifies the most tive technologies into the industry to cost-effective energy-efficiency reduce fuel use and emissions. upgrades for both single and The FLC also awarded Lawrence multifamily buildings. TREAT “Marty” Murphy, manager of NREL’s is based on SUNREL, a building Enterprise Development Programs, energy simulation tool that NREL a Distinguished Service Award. The developed in the mid-1980s. Several award recognizes his leadership dur- innovations make TREAT particularly ing the last outstanding; for example, the software nine years predicts occupant behavior, reports in build- health and safety issues, and links users ing and pro- to outside data resources. moting the Advances in Silicon Testing The sec- program. In ond R&D100 award highlights NREL’s addition, he

novel method of detecting impurities Gretz PIX14684/Warren has worked and defects in silicon boules, the mate- tirelessly on rial from which most commercial solar the Industry cells are made. Designed by subcon- Growth Ven- tractor Ron Sinton of Sinton Consult- ture Forums, ing, the instrument is called the Sinton which Quasi-Steady-State Minority-Carrier Marty Murphy encourage the Lifetime Analyzer. It provides valu- creation and able information to manufactur- growth of companies that promote PIX13344/Sinton Consulting, Inc. ers, identifying substandard silicon clean, efficient, and renewable energy. before it is fabricated into cells, The FLC is a nationwide network of and thereby increases the num- federal laboratories that strives to link ber of high-quality cells produced, developed technologies and expertise while boosting manufacturing with the private sector. yields and reducing manufacturing costs. The evaluation system helps the photovoltaics industry be com- petitive by allowing manufacturers to produce consistently better sili- con at the lowest possible price. 4 Academy of Engineering Ghirardi Among Nation’s NREL’s device development Elects Kazmerski Brightest work, within Lawrence The Hispanic Engineers National the National Kazmerski Achievement Awards Corporation Center for was elected (HENAAC) named NREL’s Maria Photovoltaics. a member of Ghirardi as one of the nation’s best PIX14683/Warren Gretz PIX14683/Warren This involves the National and brightest engineers and scien- developing Academy of tists. Ghirardi is a senior scientist at and opti- Engineer- NREL and a Research Associate Profes- mizing both ing in 2005. sor at the Colorado School of Mines. single- and Member- Her research at NREL involves photo- polycrystal- ship in the biological hydrogen production and Timothy Coutts line solar Academy is includes developing a system for pro- cells. The bestowed ducing large quantities of active algal annual award honors photovoltaics on engi- hydrogenase enzyme, a major break- pioneer William R. Cherry and Lawrence Kazmerski neers who through in research. She has authored is bestowed each year at the IEEE have made outstanding contribu- 45 publications, and her research Photovoltaics Specialists Conference. tions to engineering research, prac- was the subject of a Discovery Chan- tice, or education, and to the pioneers nel program that aired in 2003. NREL Lawson Appointed to of new and developing fields of tech- Air Quality Commission nology. Kazmerski, who is the direc- Colorado Governor Bill Owens tor of NREL’s National Center for appointed Douglas Lawson to serve Photovoltaics, has worked at NREL on the state’s Air Quality Control since its inception. He has published Commission. Lawson, principal scien- more than 290 journal papers in the tist in NREL’s Center for Transportation areas of solar cells, thin films, semi- Technologies and Systems, manages conductor materials and devices, sur-

PIX13524/Warren Gretz PIX13524/Warren the Environmental Science and Health face and interface analysis, scanning Impacts program for the Department probe microscopy, nanoscale tech- of Energy. He has completed the first nology, high-temperature supercon- year of his three-year appointment to ductivity, and semiconductor defects. the commission, which consists of Kazmerski is also a Fellow of the Insti- nine members representing different tute of Electrical and Electronics Engi- disciplines and interests. neers, the American Physical Society, and the American Vacuum Society. Lawson was appointed because of his Under Secretary of Energy David Gar- Maria Ghirardi strong technical background. “I bring man, invoking Kazmerski’s oft-used Director Dan Arvizu adds, “Not only the air pollution science background nickname, said, “Kaz is known around is her research on alternative fuels to the commission,” Lawson said. The the world for his leadership in solar helping set the stage for our nation’s commission energy research. NREL, DOE, and the energy future, she is a role model for is a rule- nation are fortunate to have such a students who want to pursue careers making body talented and tireless researcher who in engineering, science, technology, that reviews has devoted his life to this and math.” rules and important work.” regulations Coutts Wins IEEE Award proposed by Timothy J. Coutts received the 2005 the state and William R. Cherry award from the the U.S. Envi- PIX14716/Mike Linenberger PIX14716/Mike Institute of Electrical and Electronics ronmental Engineers (IEEE) for his long-term Protec- contributions to the science tion Agency. and technology of Lawson has photovoltaic energy published Douglas Lawson conversion, including more than the dissemination of 70 journal articles on various aspects substantial publications and of air pollution and served on two presentations. Coutts leads National Research Council committees on air quality. .

 On the Road to Future Fuels and Vehicles NREL’s research helps move the nation toward energy independence

he total price that Americans pay for gaso- line and diesel fuel isn’t the one shown on T the pump. Because we import about 60% of our crude oil, we are vulnerable to price fluc- tuations, 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 coun- try—were registered in 2004, according to Depart- ment of Transportation statistics. It takes a lot of fuel to keep all those wheels moving. The Depart- ment of Energy reports that the average American household uses more than 1,000 gallons of gas- oline 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 bil- lion daily miles. Addressing these issues, in February 2006, Pres- ident 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

 hydrogen-powered vehicles, as well as alternative The Alternative Fuels Data transportation fuels, particularly ethanol. Center. NREL is helping to expand the market for With the support of the Department of Energy’s alternative fuels and vehi- Office of Energy Efficiency and Renewable Energy, cles through the Alterna- and in partnership with both public and private tive Fuels Data Center, or organizations, NREL’s scientists and engineers AFDC. Supported by the are assisting in this effort by conducting R&D in Department of Energy’s innovative fuel and vehicle technologies. These Office of Energy Efficiency technologies will help to reduce our nation’s and Renewable Energy and dependence on imports, enhance our energy housed at NREL for more security, and improve the quality of our air. than a decade, the AFDC is a Here, we review where we are now, in terms of searchable electronic library R&D and market readiness, as well as where we of technical data, publica- plan to be in 5 to 10 years and where we want tions, and information on to be in 20 years. This work has been, and will advanced and alternative be, key to developing tomorrow’s fuel and vehi- fuels and vehicles (see www. cle technologies. These technologies include eere.energy.gov/afdc). It also advanced hybrid electric vehicles, plug-in hybrids, contains information about biodiesel and other biofuels, ethanol produced financial incentives, regula- and Bensinger, PIX13531/Charles of New Mexico Energy Partners Renewable from cellulosic biomass, advanced batteries, tions, fuel performance, emissions, funding and Options for reducing our use of imported oil are hydrogen fuels, fuel cells, and more. training resources, industry contacts, and updates available to consumers Where We Are Now on advanced vehicles. now. Diesel vehicles can use In addition, the AFDC Vehicle Buyer’s Guide 20% biodiesel. Flexible- Today, ethanol, biodiesel, flexible-fuel vehi- fuel vehicles can use 85% helps both fleet managers and consumers evalu- cles, and hybrid-electric vehicles are all available ethanol. And 10% ethanol ate various fuel and vehicle options. An Alterna- in some form, but in limited quantities, in the is widely used for fuel- tive Fuel Station Locator provides maps showing efficient hybrid-electric or United States. Since the Energy Policy Act of 2005 U.S. fueling stations. And additional databases any regular vehicle. NREL establishes a renewable-fuels standard requir- provide useful technical data—for example, on is working to improve the ing total U.S. transportation fuel sales in 2012 to vehicle emissions and performance. competitiveness of these include 7.5 billion gallons of renewable fuel, we current alternatives and to should see steady increases in the use of biofuels Hybrid-electric vehicles. Though they are still develop new technologies and the vehicles that run on them. new kids on the road, hybrid-electric vehicles— for the future. HEVs—are becoming increasingly popular; about Today, more than 30% of U.S. gasoline is E10— 1.3% of light vehicles sales in 2005 were HEVs. To gasoline blended with 10% ethanol. About 90% improve fuel efficiency and performance, HEVs of the U.S.-produced ethanol in this blend is combine a smaller, but more efficient, gasoline made from the starch and simple sugars in corn. engine with an electric motor. Estimates indi- The availability of E10 depends on local prices cate that if everyone drove an HEV, and air-quality regulations, however. Fuel Cell Vehicles we could reduce our petroleum Flexible-fuel vehicles. In some parts of the coun- consumption right now by at try, particularly the Midwest, consumers can pur- least 20% and perhaps as chase E85—an ethanol blend containing 15% much as 40%. gasoline. But E85 can be used only in “FFVs”— flexible-fuel vehicles that run on either gasoline Hydrogen or ethanol blends. At present, U.S. automakers manufacture flexible-fuel versions of several Increased popular models, such as the Ford F150 truck Transportation Use of and the Chevrolet Suburban sport utility vehi- Fuels Based on Plug-in Electric Vehicles cle. About 4½ million to 5 million FFVs are estimated to be on the road today. Renewables Cellulosic However, many FFV owners are unaware that Ethanol they have one. A recent survey of FFV owners by an ethanol producer found that 70% didn’t know Hybrid-Electric & they owned a vehicle that can run on E85. Those Flexible-Fuel Vehicles who do know might wonder where they can pur- chase this fuel; less than 600 service stations Long-Term offered E85 in January 2006. A tax credit in the Energy Policy Act of 2005 provides for developing Grain an alternative fuel infrastructure, and this could Ethanol help increase that number. And at least one auto- Mid-Term maker, GM, has stepped up public information efforts regarding FFVs and E85.

Near-Term  U.S. Dependence on Foreign Oil Where We Should Be in 30 5 to 10 Years 1978 Adoption of CAFE Mid-term options such as cellulose-based ethanol Standards 25 and “plug-in” hybrid-electric vehicles are tech- 1985 Freeze of nologies in the latter stages of development, but CAFE Standards they are still too expensive to compete in the mar- 20 ketplace. Therefore, NREL’s researchers and engi- neers are working to improve these technologies so they can be competitive and widely available. 15 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 10 plant-based materials and waste products—

Millions of Barrels Per Day Per Barrels of Millions that could be used. Two other components of plants, cellulose and hemicellulose, are also made 5 of sugars, but those sugars are linked in long polymer chains that are not easy to convert to ethanol. Advanced biomass conversion technol- 0 1973 1977 1981 1985 1989 1993 1997 2001 2005 2009 2013 2017 2021 2025 2029 ogies break down the polymer chains into their Actual Projected component sugars and then ferment them into Source: Historic data from Energy Information Administration (EIA), Monthly alcohol to produce cellulosic ethanol. Energy Review 2006, Table 3.1A; projections from EIA Annual Energy Outlook 2006, Table 11. This technology thus turns ordinary, low-value plant materials—such as corn stalks, sawdust, Working with major U.S. automakers, NREL wood chips, wastepaper, and fast-growing trees researchers have been contributing to the devel- and grasses—into ethanol and other valuable opment of HEVs for more than a decade. Today, fuels and chemicals. Cellulosic ethanol could do they are still investigating ways to improve the much to reduce our dependence on imported performance of HEV components and working oil and curb U.S. greenhouse gas emissions. The with industry to put those improvements into technology works—but it’s still too expensive. So, practice. national laboratories and private-sector groups One key to making HEVs more practical is to alike are developing more cost-effective produc- incorporate low-cost, integrated power electron- tion methods. ics. NREL, other national laboratories, DOE, and For many years, NREL has been developing tech- industry are working on advanced components nologies that produce ethanol and other valuable that condition the electrical signal between the fuels and chemicals from cellulosic biomass. Our power generation unit (a battery or fuel cell) and researchers have conducted much of the basic the electric motor. research underpinning a process in which a dilute Heavy-duty vehicles are prime subjects for R&D, acid is used to break down hemicellulose. In a because they consume much more fuel per step known as enzymatic hydrolysis, enzymes vehicle than passenger cars do. Working with break cellulose down into its component sug- industry, NREL is developing advanced hybrid ars. While we focused on the process, our research components and systems that could increase the partners made great strides in reducing the cost of fuel efficiency of heavy trucks as much as 100%. the enzymes. Four years ago, these enzymes were too expensive to be used in a cellulosic ethanol Biodiesel. For diesel-powered vehicles, one process. Today, enzymatic hydrolysis is the lowest renewable fuel option is B20—a blend consisting cost option for hydrolyzing cellulose to glucose. of 20% fatty acid methyl ester (known as biodiesel) and 80% petroleum diesel fuel. NREL researchers are also leading the develop- Biodiesel can be made from any animal or vege- ment of better pretreatment methods, which use table fat or oil and used in just about any diesel dilute acid at elevated temperatures and pressures vehicle. It can reduce environmental emissions to break down hemicellulose into component dramatically, depending on the blend. U.S. sugars. And they engineered one of only three biodiesel is largely produced from soybean oil organisms to date that can co-ferment cellulosic and recycled restaurant cooking oil. and hemicellulosic sugars. Staff at NREL have been working to reduce the In NREL’s one-ton-per-day feedstock pilot plant, technical barriers that stand in the way of pro- researchers continue to partner with companies ducing biodiesel and using it more widely. They in the vanguard of this emerging industry to vali- also test biodiesel products in engines supplied date new biomass-to-ethanol technologies. They by various industry partners. Their work will help still need to be simpler and less expensive; for to make biodiesel more cost-competitive, reli- example, the capital equipment and sophisticated able, and plentiful. processing steps required are costly. So NREL is

 Cost of Ethanol from Cellulosic Biomass (Costs in 2002 Dollars) 6.00 NREL continues to investigate technologies that allow us to produce ethanol economically from corn stover— 5.00 the husks, leaves, and stalks that today are left behind in the field after the corn is harvested. This feedstock is both high in 4.00 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 3.00 $2.26/gal Enzymes made from corn kernels. Though NREL has made great progress in reducing the cost of cellulosic ethanol technology, 2.00 some key hurdles remain to be overcome. DOE 2012 Conversion Target NREL’s analysts calculate that a price $1.07/gal DOE 2020 Yield 65 gal/ton Target of $2.26 per gallon is needed to allow $0.59/gal 1.00 industry to build a profitable plant today. Minimum Ethanol Selling Price ($/gal) Yield 75 gal/ton And a price of $1.07 per gallon is needed Feed $53/ton for large-scale market penetration— Feedstock Yield 94 gal/ton Feed $45/ton Feed $30/ton a goal we hope to reach by 2012. 0.00 2001 2005 2010 2012 2015 2020 Actual Future Goals Source: U.S. Department of Energy, Biomass Program data

working to change this. When all process design convert the resulting synthesis gas to diesel targets are met and ethanol can be produced for a substitutes and other fuels. NREL has also little more than $1.00 per gallon, cellulosic etha- partnered with industry to explore whether nol will be competitive with ethanol from starch a bio-oil produced through the pyrolysis and probably even with gasoline. of biomass could be a useful intermediate for producing clean biofuels. Several small NREL researchers and their partners have received biomass gasification and pyrolysis plants are several technology awards for novel biomass con- already up and running. version methods. And they are working to enable In the biorefineries of the future, selected sug- industry to produce fuels, chemicals, and other ars and intermediates would be made into products in biorefineries that would manufacture high-value products for various markets. The a variety of products from biomass—much as remaining sugars would be fermented to eth- today’s oil refineries and petrochemical plants do anol fuel. The lignin in plants might also be from petroleum. gasified or pyrolyzed and converted to fuels Other mid-term biofuels. NREL researchers and chemicals. have helped to develop processes for gasifying or liquefying biomass by heating it with little or NREL scientists and their industrial part- no oxygen. Using the Fischer-Tropsch conver- ners are also evaluating an option known as sion process and others, they can catalytically “green diesel.” Biodiesel contains molecules

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

Critics of corn-based ethanol often say that it takes more energy to ethanol totally replaces gasoline energy, and relatively little petro- produce the ethanol than we get from the resulting fuel. But official leum product is used to produce it. studies demonstrate just the opposite. The efficiency of both corn It is true that modern corn farming and ethanol production are both farming and ethanol production has dramatically increased over the energy-intensive, and that the “net energy” gain of some early eth- years. The latest U.S. Department of Agriculture (USDA) study calcu- anol plants was relatively modest. But allegations about a negative lates a net energy balance of 1.67 for ethanol production; that is, for net energy balance seem to be based either on using old data or fail- every unit of energy that went into growing the corn and turning it ing to take into account the considerable value of the animal feed into ethanol in 2001, we got back about two-thirds more energy in co-products of ethanol production. the form of automotive fuel and animal feed. Animal feed co-products are a critical part of any effectiveness anal- Even if that net energy balance were less than one, ethanol produc- ysis because they are a major part of the economics of ethanol pro- tion and use still displaces oil imports with domestic nonpetroleum duction. Corn, the largest U.S. crop, is used primarily for high-starch energy, which is a major plus in terms of reducing our dependence animal feed. Ethanol production uses up most of the starch from the on imported fuel. On the basis of liquid fuels alone, the USDA calcu- kernels, but leaves the protein and fiber, so ethanol co-products are lated a net balance of 6.34. It is important to note that the energy in very high in protein and therefore are high-value animal feeds.  Can Biofuels Displace Gasoline? 160 159 billion gal/yr Although the amount of ethanol produced from corn grain is increasing rapidly, Ethanol Production 150 it will probably displace only a small portion of the very large U.S. market for 8.0 gasoline. However, cellulosic ethanol— 140 and the power, chemicals, and other fuels 7.0 THISthat SALE can be produced from cellulosic 6.0 130 biomass—could displace petroleum usage GALLONSsubstantially. The Energy Information Administration’s 2006 Annual Energy 5.0 120 UNLEADEDOutlook projects annual gasoline demand will be almost 20 quads, or about 160 4.0 billion gallons, in 2015. And the USDA Renewable 110 Fuels projects that corn grain production for 3.0 Standard

2014–2015 will be more than 12,000 Billions of Gallons

100 million bushels; of that, about 14% will be 2.0 used for ethanol production (Agriculture Baseline Projections to 2015, February 1.0 90 2005). DOE and the USDA estimate that Ethanol Production cellulosic ethanol and other biomass 0 energy supplies have potential to displace 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 80 30% to 40% of current petroleum usage; Year see Biomass as 70 Feedstock for a 105 Bioenergy and 104 billion gal/yr

Bioproducts Industry: THIS SALE Biodiesel Production 60 90 The Technical GALLONS 80.0 Feasibility of a Billion- UNLEADED

50 Ton Annual Supply, 75 70.0 April 2005.

60.0 Billions of Gallons (Gasoline Equivalent) Per Year* 40 60

50.0

30 45 40.0 34.7 billion gal/yr

20 Standard 10% 30 30.0

Ethanol Blend Millions of Gallons THIS SALE Entire 2015 Corn Crop Billions of Gallons Ethanol Per Year* GALLONS 20.0 10 15 2012 Renewable UNLEADED Fuel Standard 4.9 billion gal/yr 10.0 Projected 2015 Production 0 0 Projected Gasoline Potential Grain Ethanol Potential Cellulosic Ethanol 0 Demand in 2015 Production or Other Biomass Energy 1999 2000 2001 2002 2003 2004 2005 *The right- and left-hand scales are different because ethanol has only two-thirds as much Year energy per gallon as gasoline. For example, 90 billion gallons of ethanol is equivalent--in energy terms--to 60 billion gallons of gasoline.

produced by trans-esterifying triglycerides with Today’s HEVs do not depend on an external methanol. Green diesel consists of paraffin mole- means of recharging their batteries. NREL is work- cules produced by hydrogenating triglycerides by ing to move HEV technology a step further by means of a conventional petroleum refining pro- developing hybrids that can be plugged in to store cess. Green diesel has a very high cetane number, electricity for later use. Adding extra batteries to so it ignites fairly quickly after injection, and a an HEV and a means to plug them in, we would low pour point—the lowest temperature at which drive most of a typical day on domestic electricity a fuel will pour. Thus, it is a high-quality diesel and still have fuel in the tank for longer trips. fuel and is totally compatible with petroleum Today’s batteries are heavy and expensive, how- diesel. ever. So, NREL is researching advanced drive Plug-in hybrids. In addition to producing trans- trains and other vehicle components—such as portation fuels from biomass, we can also put batteries—that can be used in conventional, electricity to work to reduce our petroleum usage. hybrid-electric, and plug-in hybrid vehicles. Electric vehicles are clean and quiet, and there are Advances in batteries and other energy-storage already some vehicle recharging stations around technologies are essential to the success of plug- the country. This vehicle technology could be in hybrids. NREL is also exploring ways to make fully renewable if the recharging stations pro- plug-ins reversible, so that excess power stored in vided electricity generated by wind, solar, and the vehicle would go back into the utility grid, to other renewable energy technologies. the owner’s credit.

10 Where We Need to Be in 20 Years Where Will the Biomass Come From? Taking all this another step further, NREL fore- We are not likely to be able to replace all our petroleum-based fuels sees the emergence of “renewable communities” and chemicals with biomass-derived products. However, it is a good that would feature plug-in hybrids, zero-energy idea to set some goals for the contribution biomass can make to meet future petroleum demand. homes, and various renewable energy technolo- gies. They might not be a reality now, but they are The Biomass R&D Technical Advisory Committee to the Secretar- ies of Energy and Agriculture has stated that biomass could supply a part of our vision for the future. 5% of current U.S. power needs, 20% of transportation fuel require- We support the Department of Energy’s “30 by ments, and 25% of chemical needs by 2030 (Vision for Bioenergy and 30” goal, which means that ethanol will make up Biobased Products in the United States, October 2002). This would dis- place 30% of our petroleum use and require about a billion tons of at least 30% of our nation’s transportation fuels biomass per year to meet current U.S. consumption levels. by 2030. Research in other long-term technol- A subsequent joint DOE-USDA study headed by Oak Ridge Nation- ogy options, such as renewable-based hydrogen al Laboratory that included contributions from NREL and others was fuel and fuel cells, is also important, especially in conducted to determine whether that goal is feasible. The study terms of the basic science underlying those tech- found that forestlands and agricultural lands—the two largest sources nologies. Hydrogen fuel cells for transportation of biomass—could produce the following annual tonnages by 2030: and hydrogen vehicles are so promising that Cal- Forestlands ifornia and some states on the Atlantic Coast are Fuelwood harvest from forests 52 setting up prototype hydrogen fueling stations to Residues from wood processing & pulp & paper mills 145 test new technologies as they develop. Urban wood residues 47 Residues from logging and site-clearing operations 64 Hydrogen production. Producing hydrogen by Forest thinnings to reduce fire hazards 60 steam-reforming natural gas, today’s most eco- Forestlands subtotal 368 nomical method, would increase our reliance on an increasingly scarce fossil fuel. So, NREL is Agricultural Lands pursuing a renewable option: gasifying biomass Annual crop residues 428 and reforming the resulting syngas to hydrogen Perennial dedicated energy crops 377 through a water-gas shift reaction. Crop grains used for biofuels 87 Manures and miscellaneous 106 NREL researchers are also exploring the use Agricultural lands subtotal 998 of cost-effective solar, wind, and other renew- Total production in 2030 (in million dry tons of biomass) 1,366 able technologies to electrolyze water to pro- duce hydrogen. And they are pursuing both The study concludes that enough biomass would be available in 2030 photoelectrochemical and photobiological tech- to meet about 40% of our current petroleum use. Business-as- usual projections for petroleum use predict a substantial increase. nologies that could produce hydrogen directly. But much more efficient vehicles and the use of electricity rather than The photoelectrochemical approach integrates liquid fuels (for example, for plug-in electric hybrids) could reduce the need for petroleum dramatically. Key assumptions in the study elements of a photovoltaic cell with elements of included these: crop yields will increase by 50% by 2030; cropland an electrolyzer, so that the absorption of light will switch to no-till management; 55 million acres will be convert- energy triggers the splitting of a water mole- ed to dedicated energy crops; crop and residue harvesting technology cule in an aqueous electrolyte. Because aque- will improve; and environmentally sensitive lands and forestlands ous photovoltaic cells and electrolysis share not accessible by road would be excluded. (See Biomass as Feedstock anode, cathode, and electrolyte components, this for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a approach is potentially much more efficient than Billion-Ton Annual Supply, April 2005). the separate steps of electricity generation and electrolysis. Required Growth of Advanced Renewable Fuels to Supply The photobiological approach takes advantage 30% of Current U.S. Gasoline Demand by 2030 of the fact that certain microorganisms—such 60 as green algae—naturally split water to produce hydrogen as a way to dissipate the energy they 50 do not need in certain circumstances. Research- ers have been creating new genetic forms of the microorganisms that can sustain hydrogen pro- 40 duction in the presence of oxygen. One system 44.8 uses a metabolic switch (sulfur deprivation) to 30 cycle algal cells between a photosynthetic growth Cellulosic Ethanol 20 and “Green” Diesel

phase and a hydrogen production phase. Billion Gallons/Year Fuel-cell research. Fuel cells could revolution- 3.7 ize the way we power our nation, providing 10 Grain Ethanol and 12.8 clean, more efficient alternatives to burning fos- 9.4 Conventional Biodiesel sil fuels. However, many challenges must be over- 0 2005 2010 2015 2020 2025 2030 come before fuel cells will be competitive in the Year marketplace. Therefore, NREL’s work focuses on

11 improving the performance and cost effec- Using NREL-created models such as ADVISOR tiveness of fuel cell systems, subsystems, and (Advanced Vehicle Simulator), which is now components. Specific research areas include commercially available, we can simulate system analysis and component research. potential vehicle technologies and combina- tions of technologies in days or weeks, rather Fuel cells are rather complex, so innovative

PIX14682/Dean Armstrong than the months or years it takes to build and analysis tools are needed to design them. NREL’s Bill Kramer shows a power test prototypes. Using these computer tools, NREL is using a variety of tools to identify inverter and a new cooling system that we can advise automakers, equipment pro- critical design issues for fuel cell vehicles and NREL developed and is testing in its viders, other government programs, and our systems. We work with industry to share and new Electrical Systems Laboratory. own researchers on the most promising ave- apply robust design tools and techniques to nues to pursue to reduce the environmental address such issues as durability, cost, and New NREL Facilities Provide impacts of vehicles and improve their effi- efficiency. Key Tools for Studying ciency and performance. Surfaces and Heat Flow Simulation tools. Much of NREL’s research NREL has also developed a unique ther- Two new NREL laboratories provide in advanced vehicles makes use of computer mal comfort manikin known as ADAM (the researchers with important resources simulations of vehicle technologies to model, they can use to improve fuel and Advanced Automotive Manikin), which is evaluate, and optimize them. A basic prob- vehicle technologies. The state-of-the- designed to help the automotive industry lem underlying our dependence on imported art electron microscopes in the recently design smaller, more efficient climate-control opened Biomass Surface Characteriza- oil is that today’s cars and trucks are not very systems in vehicles. tion Laboratory (BSCL) are powerful efficient. Even a hybrid-electric vehicle uses tools for understanding the core mech- anisms underlying biomass conversion less than one-fifth of the energy—fossil or The ultimate goal of all this research and processes. The new Electrical Systems renewable—that goes into it. Because several development is to enable U.S. industry to Laboratory (ESL) provides the essen- different configurations for hybrid-electric produce advanced, low-emission, economi- tial capabilities needed to improve the and fuel-cell vehicles are possible, and cally competitive fuels and vehicles that will efficiency and thermal management of numerous alternative technologies could be meet our future transportation and environ- power electronics for hybrid-electric, fuel-cell, and other advanced vehicles. used in key vehicle components, computer mental needs. Pursuing all options, we are on Surface analysis is key to under- modeling is critically important. the way to greater energy independence. standing both biological and chem- ical reactions. These reactions are basi- cally a function of the way a catalyst interacts with a substrate (for example, acid and water acting on hemicellulose For More Information or enzymatic proteins acting on cellu- A. Aden, M. Ruth, K. Ibsen, J. Jechura, K. Neeves, J. Sheehan, B. Wallace, L. Montague, A. Slayton, J. Lukas. lose). It takes sophisticated equip- Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis ment to determine the actual shape and Enzymatic Hydrolysis for Corn Stover. June 2002. National Renewable Energy Laboratory. NREL/TP-510- of a chemical or protein and how it 32438 (www.nrel.gov/docs/fy02osti/32438.pdf). interacts with biomass. The new BSCL Biomass Technical Advisory Committee. Vision for Bioenergy and Biobased Products in the United States, provides NREL biomass researchers October 2002, and Roadmap for Biomass Technologies in the United States, December 2002 (www.biomass. with some of the best equipment avail- govtools.us/pdfs/BioVision_03_Web.pdf; www.biomass.govtools.us/pdfs/FinalBiomassRoadmap.pdf). able, including scanning-electron, atomic-force, transmission-electron, R.D. Perlack, L.L. Wright, A.F. Turhollow, R.L. Graham, B.J. Stokes, D.C. Erbach. Biomass as Feedstock for a near-field-scanning-optical, and Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply. April 2005. Oak confocal microscopes. Ridge National Laboratory for the U.S. Department of Energy and U.S. Department of Agriculture (www. For vehicle efficiency, it is important eere.energy.gov/biomass/pdfs/final_billionton_vision_report2.pdf). to understand heat flow and per- T. Penney, J. Elling. “The Race to Connect Cars, Communities and Renewables.” Geotimes. August 2005. formance under anticipated uses. American Geological Institute. The ESL’s capabilities start with a Renewable Fuels Association. From Niche to Nation: Ethanol Industry Outlook 2006 (www.ethanolrfa.org/ 125-kilowatt bidirectional power sup- industry/outlook/). ply. This allows researchers to simulate the way electricity flows from a hybrid H. Shapouri, J. Duffield, A. McAloon, M. Wang.The Energy Balance of Corn Ethanol: An Update, 2002, Agricul- vehicle’s battery to its motor and back, tural Economic Report No. 814, and The 2001 Net Energy Balance of Corn-Ethanol, presented at the Corn Uti- both to assess performance under lization and Technology Conference, June 7-9, 2004, Indianapolis, IN. U.S. Department of Agriculture, Office various drive cycles and to represent of the Chief Economist, Office of Energy Policy and New Uses (www.usda.gov/oce/reports/energy/aer-814. actual heat-flow situations. A high- pdf; www.usda.gov/oce/reports/energy/net_energy_balance.pdf). heat-flux test loop allows heat flow J. Sheehan, A. Aden, K. Paustian, K. Killian, J. Brenner, M. Walsh, R. Nelson. “Energy and Environmental to be assessed within various power Aspects of Using Corn Stover for Fuel Ethanol.” 2004. Journal of Industrial Ecology. Volume 7, Number 3-4 electronics components. Research- (http://devafdc.nrel.gov/pdfs/8427.pdf). ers are already using it to develop and test innovative cooling systems for Web sites: the power inverters needed to con- • National Renewable Energy Laboratory Biomass Research, www.nrel.gov/biomass/ vert the DC current of a battery to the • National Renewable Energy Laboratory Advanced Vehicles and Fuels Research, www.nrel.gov/ AC required by the motor. Ultraca- vehiclesandfuels/ pacitors are also being studied; they have less total power storage capabil- • U.S. Department of Energy, Energy Efficiency and Renewable Energy Alternative Fuels Data Center, ity than batteries, but can deliver much www.eere.energy.gov/afdc/ more power for a short time and could • U.S. Department of Energy, Energy Efficiency and Renewable Energy Biomass Program, www.eere. boost the efficiency of certain types of energy.gov/biomass/index.html, www.eere.energy.gov/biomass/publications.html vehicles. • U.S. Department of Energy, Energy Efficiency and Renewable Energy FreedomCAR and Vehicle Technol- ogies Program, www.eere.energy.gov/vehiclesandfuels/

12 Sophisticated Modeling Capabili-

ties for Advanced Vehicles R&D 100% 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 tech- 2.9% nologies, such as plug-in hybrids, fuel cell vehicles, and Accessories 63.4% Losses Engine advanced storage technologies. Losses

NREL’s expert analysts have considerable expe- 3.5% Driveline rience in evaluating advanced vehicle technolo- Losses gies. Using computer simulation and analysis tools, 17.2% Remaining13.0% they evaluate new technologies and their impacts on fuel Standby/Idle energy for Losses propulsion after subtracting all economy, vehicle performance, exhaust emissions, costs, and energy losses. market potential. For example, they use high-level, exploratory, and detailed mod- eling 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 characteristics can change quickly. for a particular type of vehicle. For example, Exploratory models, such as ADVISOR, allow analysts to investi- sometimes two or more design gate energy management strategies, interactions among compo- objectives—such as power and nents of various types and sizes, and the performance and energy fuel economy—compete with each implications of different powertrain designs. These are impor- other; optimizing involves evaluating and comparing options to tant in developing future plug-in and heavy-duty hybrid vehicles. achieve the best overall effect. And analysts can tap into the con- More detailed models, such as the Powertrain System Analysis siderable computational capability of NREL’s distributed comput- Toolkit, provide in-depth analyses of components, events, and ing system, making the entire process more efficient. transients. And robust design integrates the latest computer- NREL has also developed a tool to evaluate technical targets and aided engineering and design tools with advanced design tech- R&D goals, estimate market penetration, and project the impacts niques to solve technical barriers and accelerate development. of advanced vehicles on U.S. oil usage. For more information, and The Laboratory’s optimization tools and distributed computing a list of vehicle simulation and analysis tools currently used at capabilities complement this modeling work. Optimization tools NREL, please see www.nrel.gov/vehiclesandfuels/vsa/ help analysts find the best combinations of components and simulation_tools.html.

1400 180 Hybrid-electric vehicle (HEV) 1200 sales are still small, but high- efficiency vehicles are proving 160 Projected Sales of 1000 to be very popular. So, sales Sales of Flexible-Fuel Vehicles Flexible-Fuel Vehicles are expected to increase substantially. They also set the 800 140 stage for “plug-in” HEVs, which could shift transportation 600 energy use from largely 120 Projected Sales of imported oil to domestically Hybrid-Electric Vehicles 400 generated electricity. There Thousands of Vehicles are already about 5 million 100 200 flexible-fuel vehicles (FFVs) on the road that can use E85 fuel, a high-percentage blend of 80 0 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 2022 2024 ethanol. The number of service Year stations selling E85 needs to grow proportionately to take 60 advantage of the capabilities Thousands of Vehicles of this growing FFV fleet. 40 Sales of Hybrid-Electric Vehicles

20

0 2000 2001 2002 2003 2004 2005 Year 13 High-Performing Commercial Buildings Designed for maximum energy efficiency, they are easy on the eyes as well as the budget

he buildings we live and work in consume about 39% of all the primary energy the T 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

14 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., fur- naces and air-conditioners) and electric light- ing 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 PIX10860/Robb Williamson from renewable energy. To move us closer to that The Lewis Center for future, the CBI team recently analyzed these six Environmental Studies at Oberlin College, Ohio buildings: pumps for heating and cooling. A roof-integrated • Adam Joseph Lewis Center for Environmental PV system provides electricity. Studies, Oberlin College, Oberlin, Ohio After construction, NREL staff monitored and • BigHorn Home Improvement Center, Silver- evaluated the building’s energy performance. The thorne, Colorado building uses 47% less energy than a compara- • Cambria Office Building, Ebensburg, Pennsyl- ble code-compliant vania one, and it demon- strates that a high- • Chesapeake Bay Foundation Philip Merrill performance building Environmental Center, Annapolis, Maryland works well even in • NREL Thermal Test Facility (TTF), Golden, a climate with high Colorado heating and cooling • Zion National Park Visitor Center Complex, loads. Springdale, Utah. BigHorn Center. The Each one incorporates various energy efficiency BigHorn Develop- and renewable energy design elements, such as ment Project in Sil- daylighting (natural lighting), passive solar strat- verthorne, Colorado, egies, radiant heating, natural ventilation, evap- is a retail complex in orative cooling, ground-source heat pumps, and a mountain commu- PV systems. NREL staff helped to design three of nity. The building that the six: the BigHorn Center, the TTF, and the Zion NREL helped to design is a 36,980-ft2 (3,436- Visitor Center. They monitored all of them for a PIX09202/Jim Yost year or more when completed and analyzed data m2) structure housing The BigHorn Home Improvement to compare actual energy performance with goals a hardware store and a Center, Silverthorne, Colorado specified in original designs. warehouse/lumberyard building. Calibrated sim- ulations were used to estimate the performance How the Buildings Measured Up of the building before construction. Monitoring The thermal envelopes of all six buildings exceed and analysis since then show that the building those specified in ASHRAE Standard 90.1, pub- has 53% lower energy costs than a typical code- lished by the American Society of Heating, Refrig- compliant retail building of its size. erating, and Air-Conditioning Engineers. The Daylighting and other design features reduced buildings consume between 25% and 70% less lighting energy requirements by 80%. Reduc- energy than a comparable building would if ing electric lighting and controlling solar gains designed to be ASHRAE code-compliant. allowed the designers to use natural ventilation to The monitoring revealed some gaps between meet the cooling load. design goals and actual performance, but later An 8.9-kilowatt (kW) modifications helped to close those gaps. At the PV system initially top of the list were problems with controls, insu- provided 2.5% of the lation placement, and following the building building’s electricity plans. Here is a brief summary of the results of needs. PV system and NREL’s monitoring and analysis; see the table on other improvements page 17 for data from the six-building study. increased that to Lewis Center. The Adam Joseph Lewis Center about 8%. for Environmental Studies at Oberlin College in Cambria Building. PIX10140/Jim Schafer Ohio was designed to be an energy producer as The Cambria office The Cambria Office Build- well as an energy consumer and a teaching aid building in Ebens- ing in the Commonwealth of for students. The 13,600-ft2 (1260-m2) build- burg, Pennsylvania, is one of the high-performing Pennsylvania Department of ing, constructed in 2000, houses classrooms, buildings constructed for the Pennsylvania Environmental Protection, offices, and an atrium. Features include passive Department of Environmental Protection. Com- Ebensburg, Pennsylvania solar design, daylighting, natural ventilation, an pleted in 2000, it features efficient wall and enhanced thermal envelope, and geothermal heat roof insulation, high-performance windows,

15 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 sim- ulations; 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 direct- PIX10884/Robb Williamson The Chesapeake Bay PIX04116/Warren Gretz Foundation Philip Merrill Environmental Center, ground-source heat pumps, an under-floor air Annapolis, Maryland 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 Certifica- tion 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 stan- dard baseline computer model and then recom- mended further performance improvements. NREL’s Thermal Test Facility, Golden, Colorado Merrill Center. The Chesapeake Bay Foundation (CBF), dedicated to restoring and protecting the bay’s resources, built the 31,000-ft2 (2,880-m2) indirect evaporative air-conditioning system Philip Merrill Environmental Center in Annap- meets cooling loads. And an energy management olis, Maryland, as its headquarters in 2000. The system monitors and controls internal tempera- CBF incorporates many energy-saving features, tures, humidity, air pressure, duct pressure, light including a ground-source heat pump to provide levels, and carbon dioxide levels. heat in winter and cooling in summer. Using computer simulations, NREL tested sev- The building features large, south-facing win- eral design concepts before construction. The goal dows for passive solar heating and daylighting. was to reduce building energy requirements by Sensors automatically dim lights when daylight 70%. After construction, simulated performance levels are sufficient. Bay breezes provide natural was compared with actual performance; results The Zion National Park Visitor included energy cost savings of 51%. Center in Utah PIX09243/Robb Williamson Zion Visitor Center. To develop a new visitor center complex in Zion National Park in south- western 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 dis- plays, offices, and retail space. Features include passive direct evaporative cooling, natural venti- lation, 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, daylight- ing, PV power, and digital controls to integrate energy operations. The building has 67% lower energy costs than a similar one meeting applica- ble energy codes.

16 Summary of energy savings for the six buildings studied

Metric Oberlin Zion Cambria CBF TTF1 BigHorn

Benchmark ASHRAE Standard 90.1 2001 19952 2001 2001 19952 2001 Version

Net Source Energy 79% 65% 42% 22% 45% 54% Savings4

Net Site Energy Savings5 79% 65% 42% 25% 42% 36% Savings3 Site Energy Savings6 47% 62% 40% 25% 42% 35%

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

PIX04116/Warren Gretz Design Goal or Predicted Net Site En- Energy Cost Energy Cost LEED 1.0 Plat- Energy Cost Energy Cost Performance ergy Use: 0.0 Saving: 80%7 Saving: 66%7 inum Rating Saving: 70%7 Saving: 60%7 kBtu/ft2·yr Project Goal Comparison Measured or Simulated Net Site Ener- Energy Cost Energy Cost LEED 1.0 Plat- Energy Cost Energy Cost Performance gy Use: 16.4 Saving: 67% Saving: 44% inum Rating Saving: 51% Saving: 53% kBtu/ft2·yr

1 TTF monitored for select periods; actual data used to calibrate 4 Source energy is the energy required to supply energy in the form it simulations. is used at the site; the Cambria office building purchases 100% green 2 Code used was 1995 Federal Energy Code, 10 CFR 435 (DOE 1995); power, so source energy could be calculated assuming 9% loss for trans- based on ASHRAE 90.1-1989 with more stringent lighting power mission and distribution (EIA 2004). densities. 5 Net site energy use allows credit for electricity generated on site by the 3 Energy savings for Oberlin, Bighorn, TTF, and Cambria calculated with PV system. simulations of as-built and base-case buildings with typical weather 6 Total site energy use is the total electricity consumed. data; Zion and CBF savings calculated with measured data and base- 7 Predicted energy costs, calculated before construction, may not indicate case simulations run with measured weather data. future performance because of volatile energy prices.

Paul Torcellini examines an in-floor diffuser for the raised-floor air distribution • Install highly reflective surfaces in all daylit system in the Cambria office Selected Best Practices building in Pennsylvania. zones, especially ceilings The results of this study suggested several best practices for high-performing buildings: • Monitor and evaluate post-occupancy energy performance • Use an integrated design process to system- engineer the building • Implement standardized measurement proce- dures using standard metrics • Use computer simulations to guide the design process; these help designers analyze trade- • Carefully design and implement the use, con- offs and examine the energy impacts of archi- trol, and integration of economizers, natural tecture and HVAC choices ventilation, and energy recovery ventilators. • Simulate and measure the building’s energy Practices like these can move us closer to new performance at design, construction, and commercial buildings that save considerable occupancy stages energy and produce it, as well. • Set specific, quantifiable energy performance goals • Design the building envelope to meet or min- imize as many HVAC and lighting loads as possible • Size HVAC and lighting systems to meet loads not met by the envelope For more information, contact Paul Torcel- lini, Commercial Building Integration Team • Use daylighting in all zones adjacent to Leader, [email protected], and see exterior walls or roofs www.nrel.gov/buildings/highperformance/. PIX11001/Robb Williamson

17 Closing the Gaps Semi-mismatched semiconductor materials can boost solar device efficiency

n the world of solar cell design, conventional wisdom dictates that things work best when Ieverything 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

18 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 Figure 1. The three efficiency gaps for solar cells and modules applications, as well as on land, for large-scale power production. carriers last long enough to contribute to generat- The multijunction or tandem cell has been the ing a current. focus of R&D at NREL and other laboratories for The second gap is the difference between the effi- a few decades. In single-junction cells, the energy ciencies of laboratory cells and the efficiencies of of the illuminating light that is below the band- those produced in commercial production lines. gap of the cell material is lost; thus, it cannot be This gap may be a result of scaling up the fabrica- used to generate electricity. (The bandgap—not tion process to produce larger devices. Also, in the manufacturing environment, higher throughput 30 3.3 is required and fabrication conditions are often less controlled. The third gap is the difference between cell effi- ciencies and module efficiencies. This gap will narrow when we can do at least three things: min- imize the electrical losses that occur when cells 20 2.2 are wired into circuits, bring the active area of the 10% module closer to the cell area, and maximize the 20% optical transmission of the protective encapsulant 5-Year Payback Threshold, at $0.15/kWh material. 15% 30% 20% Working in Tandem 10 40% 1.1 PV System Cost/kWh System PV 25% cell eff. Here we address several issues surrounding 50% cell eff. the first two gaps, focusing on an inverted typical typical

Generated in 5-Year Period ($/kWh) Period 5-Year in Generated cell cost: cell cost: 2¢-4¢ $3-$10 PV System Cost/Power Output ($/W)

Fixed Flat-Plate 500X Point-Focus Conc. Why Worry About High Efficiencies? 0 0.0 High efficiencies are important because they 0.001 0.01 0.1 1 10 Cell Cost ($/cm2) 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 near- term R&D target value) when high efficiencies are combined with low cell costs. Note two families of curves: those for flat- plate 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 high- ly 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).

19 one of the “gaps” discussed here—is the energy Inverting and Mismatching to needed to dislodge an outer electron from its bond.) The multijunction concept allows a PV Increase Efficiency device to capture and use this lost energy, result- Design options for some PV device structures ing in a higher conversion efficiency and thus based on lattice-matched epitaxy are constrained greater electrical output. by the limited number of commercially available crystalline substrate materials (Ge and GaAs, for During the early to mid-1990s, NREL researchers example). However, the “design space” increases developed and patented a two-junction device substantially when we consider designs involving with high efficiency. The top cell was made of lattice-mismatched layers. gallium indium phosphide (GaInP), which has a bandgap of ~1.9 electron-volts (eV). The bottom Traditionally, lattice-mismatched approaches cell was made of gallium arsenide (GaAs), with have been dismissed because of problems such a bandgap of ~1.4 eV. Earlier GaAs cells had effi- as high defect density, rough surfaces, and crack- ciencies exceeding 25% under concentrated sun- ing or bowing of layers. At NREL, the goal is to light. Adding the GaInP layer to the GaAs layer produce semi-mismatched lattice heterostruc- further boosted the overall cell efficiency by cap- tures that look and perform like lattice-matched turing more of the energy in the spectrum. ones. In the process, we want to attain the highest charge-carrier lifetimes, the thinnest structures, This concept later evolved into three-junction and the flattest wafers possible. devices, again with the goal of capturing more useful energy otherwise lost to the cell. One Mark Wanlass and his colleagues in NREL’s prominent system is made of GaInP / GaAs / ger- High-Efficiency Solar Cell team are pursuing a manium (Ge). The third or lowest layer, with novel tandem cell approach. One key innova- a bandgap of ~0.7 eV, captures the long-wave- tion involves growing the cell layers in inverted length, low-energy “red” light that passes unused order; this approach could have several cost and through the two upper layers. This device is lat- manufacturing benefits and boost efficiency, tice matched, which means that the spacing of as well. Another innovation is to incorporate a atoms in the Ge layer very closely mimics the transparent, compositionally step-graded layer to spacing of the atoms in the GaAs layer above it. accommodate lattice mismatching without penal- izing cell efficiency. When a crystal is grown through an epitaxial pro- cess—in which the overlying crystal has the same Figure 2 illustrates the basic layer-cake structure orientation as the underlying one—matching of the inverted GaInP / GaAs / gallium indium keeps the crystal lattice from being deformed at arsenide (GaInAs) cell. The structure is consid- the transition point from one layer to another. ered to be inverted because it is grown upside Deformed layers—structural defects in the reg- down—that is, the top layer is grown on a sub- ularity of the crystal lattice, such as threading strate, then the next two main “active” subcell PIX13740/Arizona Public Service dislocations and stacking layers are deposited. In actual operation, the stack faults—negatively affect the is flipped so that sunlight strikes the top subcell movement of charge carri- first. ers through the material and The top two active subcell layers are lattice thus reduce the performance matched. A 1.9-eV top active layer of GaInP is of the device. deposited on a GaAs substrate before a 1.4-eV Researchers at NREL and else- middle active layer is added. Each active subcell where decided to develop layer actually has a double-heterostructure con- a third layer having a more figuration; this means the active material is sand- optimal bandgap; analy- wiched between thin layers of material with a ses indicated that material somewhat higher bandgap. This configuration with a 1-eV bandgap could confines the charge carriers to the active subcell improve overall device effi- layer. Each active subcell layer also contains an ciency as much as 10%– n/p junction. The GaInP active subcell layer is 12%. The dilute nitride of sandwiched between negatively (n) and positively a GaInAsN layer has such a (p) doped aluminum indium phosphide (AlInP) favorable bandgap and is lat- to form the top subcell. Similarly, the GaAs active Concentrating Technologies tice matched to GaAs. How- subcell layer is sandwiched between n- and Microdish configured for ever, efficiency measurements of GaInP / GaAs / p-doped GaInP to form the middle subcell. Spectrolab multijunction cell GaInAsN devices have not been promising. The lowest (third) active subcell layer is made of A logical next step was to investigate a four- GaInAs, with a bandgap of ~1.0 eV, and is sand- junction device consisting of GaInP / GaAs / wiched between GaInP layers to form a double- GaInAsN / Ge, which has a bandgap sequence heterostructure bottom subcell. The GaInAs lat- of 1.9 / 1.4 / 1.0 / 0.7 eV. The efficiency of this tice is mismatched 2.2% in comparison to the device is about 31% at 500 suns; further optimi- lattices of GaInP and GaAs. To accommodate zation may boost that number. this mismatch, a compositionally step-graded

20 Seeing the Benefits For more information As PV researchers work dili- R.R. King, R.A. Sherif, G.S. Kinsey, S. Kurtz, C. M. Fetzer, K.M. Edmond- gently to overcome such perfor- son, D.C. Law, H.L. Cotal, D.D. Krut, mance gaps as the discrepancies J.H. Ermer, and N.H. Karam. “Band- in conversion efficiencies found gap Engineering in High-Efficiency in solar cells, this inverted GaInP Multijunction Concentrator Cells.” 3rd International Conference on Solar / GaAs / GaInAs cell is a signifi- Concentrators for the Generation cant step forward. NREL has con- of Electricity or Hydrogen, May 1-5, firmed an efficiency of 37.9% at 2005, Scottsdale, AZ, NREL/CP-520- 10.1 suns (and a temperature of 38172. 25°C, a low aerosol optical depth S. Kurtz, M. Wanlass, C. Kramer, M. air mass of 1.5 direct, and an Young, J. Geisz, S. Ward, A. Duda, 2 T. Moriarty, J. Carapella, P. Ahren- area of 0.2428 cm ); this is a very kiel, K. Emery, K. Jones, M. Romero, encouraging number for a device A. Kibbler, J. Olson, D. Friedman, W. that has yet to be optimized. McMahon, and A. Ptak. “A New GaInP/ GaAs/GaInAs, Triple-Bandgap, Tan- The peak efficiency at such a dem Solar Cell for High-Efficiency Ter- low concentration ratio indi- restrial Concentrator Systems.” 2005 cates that efficiencies exceeding Solar Program Review Meeting, Den- ver, CO, November 2005, NREL/CP- 40%—perhaps as high as 45%— 520-38997. at several hundred suns should M.W. Wanlass, S.P. Ahrenkiel, R.K. be achievable in the near future, Ahrenkiel, D.S. Albin, J.J. Carapella, once a true concentrator version A. Duda, J.F. Geisz, S. Kurtz, T. Mori- is fabricated. Such high efficien- arty, R.J. Wehrer, and B. Wernsman. cies should enhance the viabil- “Lattice-Mismatched Approaches for High-Performance, III-V Photovoltaic ity of terrestrial concentrator Energy Converters.” 31st IEEE PV Spe- systems for cost-effective power cialists Conference, Lake Buena Vista, Figure 2. Structure of the inverted, generation. FL, January 3-7, 2005, NREL/CP-520- semi-mismatched tandem device 37440. Some key benefits of this design M.W. Wanlass, S.P. Ahrenkiel, D.S. are that the device can be mounted on Albin, J.J. Carapella, A. Duda, J.F. a surrogate substrate—or “handle”—of Geisz, K. Jones, S. Kurtz, T. Mori- layer of GaInP is included between the middle choice, and contact with a heat sink can arty, and M.J. Romero. “GaInP/GaAs/ and bottom subcells. In detail, the transparent also be achieved, which helps to man- GaInAs Monolithic Tandem Cells for High-Performance Solar Concentra- graded layer consists of nine 0.25-micrometer- age the thermal issues of the concentra- tors.” NREL/CP-520-38646, 2005. thick steps that differ in composition from one tor cell. If it is composed of some strong, another by 0.03 fractional percent Ga. Signif- flexible material such as metal foil or icantly, although cross-sectional transmission kapton polymer, the handle can provide electron microscopy shows substantial defects in a robust device in comparison to one this graded layer, it identifies only a modest dis- processed on relatively fragile Ge or GaAs location density in the active subcell layer. substrates. The overall tandem cell is a two-terminal, mono- Handle-mounted, ultra-thin device fabrication is lithic device. In other words, individual sub- a natural consequence of the inverted-structure cells are not grown separately and then stacked approach, which also has a number of advan- mechanically. Rather, all subcells are grown in tages. These include potentially low cost, the one continuous depositional process—in this ability to make use of back-surface reflectors, and case, by atmospheric-pressure metal-organic possible reclamation or reuse of the parent crys- vapor-phase epitaxy in a system built by NREL. talline substrate (e.g., to reclaim the relatively Layers of heavily p- and n-doped GaAs form scarce, expensive Ga in a GaAs substrate). “tunnel junctions” between the subcells, thus allowing charge carriers to move through the Looking Ahead entire device, and front and back electrical con- Optimizing the inverted cell will shrink its tacts gather the cell’s current. performance gaps further. And other materi- als, designs, and methods will undoubtedly be The device’s monolithic structure makes it explored in the pursuit of higher efficiencies. relatively easy to manufacture in comparison So, whether NREL’s researchers are developing to mechanically stacked devices. But to opti- silicon-based, thin-film, or so-called “third- mize the device, we must adjust the thicknesses generation” PV technologies, one key objective of each subcell so that the photocurrents of each prevails: to close the gaps and thus create even are the same. Otherwise, in a series-connected more efficient cells—cells that can be produced device, the subcell with the lowest photocurrent at lower cost while exhibiting greater reliability will be the limiting factor in regard to the current and stellar performance. of the overall cell.

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