Changingthe Faceof Energy

Renewables for the 21st Centur yA

t the dawn of the millennium, humanity isfaced with expanding economies; growingpopulations; spiraling needs for food, shelter,

and goods; and burgeoning demands that burdenEarth’s ecosystems.

Underlying these trends is the increasing demandfor energy—a demand that intensifies the world’sgrowing predicament. In the last 20 years theworld has increased its consumption of energy byover 40%, to more than 390 quads of energy peryear. More than 85% of this comes from fossilfuels. Although fossil fuels have long driven theengines of economic growth, ominousconsequences of their use are becomingapparent—dwindling supplies, acid rain, airpollution, and an alarming increase in atmosphericcarbon dioxide, which portends grave repercus-sions in the form of global warming.

But imagine a world that is clean, beautiful, andbountiful. In which each nation has sufficient,inexhaustible sources of energy. In which energyfor human enterprises is derived from renewablesources—nonpolluting and clean—and returned tothe ecosystems for regeneration.

Such a world can be more than imagination. It is adefinite possibility that daily stares us in the facein the sunshine, wind, plants, and even in thewater that surrounds us.

At the National Renewable Energy Laboratory, wedo more than simply imagine such a world. Weexplore its phenomena, synthesize the materialsthat exploit the phenomena, model the devices,

engineer the systems,and develop thetechnologies that will help replacetoday’s fossil fuel reality with thesustainable and clean world that all of usimagine. Physics is at the core of making thistransition to a renewable energy world.

The interaction of light and matter

From the time of Newton, through the formulation ofelectromagnetism, through Einstein’s conceptualiza-tion of relativity, to forging the foundations of quan-tum mechanics, light and matter and their interactionhave lain at the heart of physics.

This role of light/matter interaction in physics will con-tinue to dominate in the 21st century. In astrophysicsand cosmology, it will provide an ever-deepeningcomprehension of the universe. In solid-statephysics, it will give rise to optical discs and high-speed optical computers. And in our understanding ofrenewable energy sciences, it will spawn a revolu-tion. At the vanguard of this revolution will be scien-tists from the National Renewable Energy Laboratory.

To the casual observer, solar phenomena mayappear deceptively simple—photons interact with theatmosphere and biosphere to heat the earth, createthe winds, fill the reservoirs, and grow the plants.But exploring the phenomena to advance sustain-able technologies for generating, transmitting, andstoring energy demands all the tools, techniques,and intellectual prowess of modern physics.

In this quest, NREL scientists pursue disciplinesranging from materials science to structural dynam-ics to condensed matter physics. In the process,their endeavors weave an intricate tapestry thatdeepens our understanding of the interaction of lightand matter and that elevates the sciences and thewell being of humanity.

Nevada’ssolarresourcealone couldprovide enoughenergy to generateall of the nation’selectricity. Thiswould require only a100-mile x 100-milearea with 10%-efficientPV systems installed.

With today's wind energy technology and just a moderate use ofland, the wind resource in the Great Plains states could supply thenation with more than one-and-a-half times the electricity it currentlyuses. In this map, areas with the strongest wind resources areshown in red.

Materials Synthesis & CharacterizationA variety of materials—including photovoltaic, photoelectro-chemical, and electrochromic materials—react with sunlight toproduce electricity, to darken glass in the presence of sunlight,and to split water into oxygen and hydrogen. The challenge isto synthesize materials with optimal properties that can bestexploit the phenomena.

To meet this challenge, NREL scientists explore and developstate-of-the-art techniques for growing thin films. They deviseoriginal methods for synthesizing high-efficiency materials rang-ing from elementary silicon to more complex semiconductors—methods that enable them to precisely control important materi-al properties. And they investigate novel materials, includingnanoscale precursors and quantum dots for boosting conversionefficiencies; high-temperature superconductors for transmittingand storing energy; and carbon nanotubes for storing hydrogen.

Other NREL scientists analyze the synthesized materials todetermine their defects, structure, and material, electrical, and

optical properties. Forthese analyses, theyemploy proximal probetechniques, analyticalmicroscopy, surfaceanalysis, electro-opticalcharacterization, anddevice-performanceanalysis to measure,image, and character-ize materials from theatomic to the macro-scopic scale. They alsodevise new techniques,and design custominstruments that allowthem to move atomsaround, performnanoscale spec-troscopy, and investi-gate nonequilibriumelectron dynamics.

Device PhysicsAs world leaders in photovoltaic research, NREL scientists inves-tigate a wide spectrum of devices, especially those based on sili-con, II-VI, I-III-VI2, and III-V materials and alloys.

For crystalline silicon, scientists explore silicon growth process-es, investigate the role of impurities and defects, and modellight absorption. For amorphous silicon, they devise new devicestructures, develop deposition techniques, and model causes ofmaterial instability.

For I-III-VI2 and II-VI materials—predominantly copper indiumdiselenide, cadmium telluride, and their alloys—they have pio-neered devices with world-record efficiencies and have developedbreakthrough methods for synthesizing stable, efficient devices.

They are using III-V materials to make some of the world’s mostefficient devices. One of these—a 30.2%-efficient device thatuses a GaInP2/GaAs, two-junction structure to effectively absorbhigh- and low-energy photons—is opening the door to 40%-effi-cient, four-junction cells.

They are synthesizing III-V materials with low-energy gaps, tomake devices that convert IR photons to electricity. And they areexploring concepts that combine III-V photovoltaic structureswith photoelectrochemical concepts and liquid interfaces—anapproach that can be used to directly split water into oxygenand hydrogen with a record conversion efficiency of 12.4%.

Among the techniques NREL scien-tists use to synthesize materialsare: an NREL-customized systemthat combines MBE, MOCVD, andin situ analysis to make III-V photo-voltaic materials; and pulsed laserdeposition, which is used fordepositing a variety of thin films.

Used to analyze material surfaces, the staticSIMS distinguishes elements and moleculeswhose masses range from 1 to >10,000 amu.

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Device structure of 30.2%-efficient GaInP2/GaAs two-junction PV cell.Success with this cell is leading toward three- and four-junction cells.

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Among NREL’s efforts in amorphous sili-con are: modeling studies to explain thematerial’s “light-soaking” instability; anda customized hot-wire deposition systemfor improving the efficiency and stabilityof amorphous silicon materials.

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Solid-State SpectroscopyResearchers in solid-state spectroscopy employ state-of-the-arttechniques to explore the fundamental mechanisms that limit theperformance of photovoltaic, electrochromic, and superconductormaterials. They also investigate devices with novel architecturesand material compositions to optimize performance.

A fertile research area is the sponta-neous ordering that some III-V semi-conductors exhibit under certaingrowth conditions. By controllinggrowth parameters, material proper-ties can be adjusted for specific appli-cations. Of particular interest is theordering of GaInP2 and its use in tan-dem photovoltaic cells—the study ofwhich has involved Raman spec-troscopy and spatially resolved photo-luminescence, among otheradvanced techniques.

Researchers are also investigatingminority-carrier lifetimes and carriertransport in CdS/CdTe heterostruc-tures. These investigations involveultrafast spectroscopy for time-resolved photoluminescence, continu-ous-wavelength spectroscopy tomeasure absorption over a wide spec-tral range, and scanning confocalmicroscopy to map out the spatial vari-ation in photoluminescence intensity.

Other research topics include low-band-gap nitrides, which can be used

to lower the band gapsof ternary III-V alloys;composition modula-tion in semiconductoralloys, which offerspossibilities for spatiallyseparating free holesand electrons; andtransition metal oxides,which make excellentcathodes in lithium-ionbatteries.

Condensed Matter PhysicsInvestigating the theoretical foundations of photovoltaic materials,quantum nanostructures, and order-disorder phenomena in semi-conductor alloys constitutes just some of NREL’s research in con-densed matter physics.

For photovoltaic materials, NREL physicists pioneered theoreticalresearch into the material properties and electronic structure ofchalcopyrite semiconductors, indicating increased conversion effi-ciencies through the addition of Ga to CuInSe2. Currently, thisgroup is using first-principles electronic-structure theory to explainthe nonstoichiometry of CuInSe2 and the appearance of ordered-vacancy compounds.

The group also applies first-principles electronic-structure theoryto explain why some III-V alloys exhibit spontaneous long-rangeorder under certain growth conditions, whereas others undergo

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Psuedopotential calculations of the electronic structure of a pyramidal InAs quantumdot embedded in GaAs.

Atomic structure of single-layer, thallium-based high-Tcsuperconducting materialbeing investigated by NRELresearchers. This materialexhibits favorable attributes—including a high transition tem-perature and promising super-conducting properties.

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Spatially resolved photolu-minescence. Each frameshows the photolumines-cence spectrum taken atthe center of a 5- x 5-µmarea of a GaInP2 epilayer.Each inset shows thespatial variation in thephotoluminescence at theindicated spectral energy.

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short-range phase transitions between order and disorder underother growth conditions. Their calculations show changes inmaterial properties that depend on the order or disorder of thematerial, including changes in band gap and the anisotropy ofeffective masses of electrons and holes.

For nanostructures,NREL theorists use avariety of theoreticalmethods to calculatethe electronic, optical-transport, and structuralproperties of semicon-ductor quantum dots.

Experimentalists arealso investigatingnanostructures,because of the range ofpossibilities for innova-tive architectures andincreased efficienciesfor photovoltaic andphotoelectrochemicaldevices. Currently, theirresearch focuses on thepreparation, characteri-zation, and applicationsof III-V quantum dotsprepared through col-loidal chemistry.

Aero- and Structural DynamicsTo meet cost, lifetime, and energy demands, wind turbine com-ponents—especially rotor blades, drivetrain, and tower—mustbe well engineered. The rotor must capture large amounts ofwind energy and convert it efficiently to electricity. This is a com-plicated task because of the elaborate intercoupling betweenaerodynamic forces and the structural dynamics of components.

Wind passing over components, especially the rotor, createscomplex interactions. Turbulence or eddies contained in thewind vary in size, orientation, and strength. The interplay ofthese eddies with the rotor generates forces that may be manytimes those produced by steady winds.

NREL physicists investigate the aerodynamic and aeroelasticresponse of turbine components to steady and unsteady airflow.The airflow creates deterministic and stochastic loads and vibra-tions throughout the turbine. The resulting structural deforma-tions create fatigue in turbine materials, which must bedesigned to withstand fatigue cycles for 20 to 30 years.

The investigations translate into structural criteria for designingwind turbine components. For this task, researchers meticulouslyselect low-cost, long-life materials tailored to the correctstrength, stiffness, and fatigue life. And they take care to designcomponents that will not operate at natural resonant frequen-cies—which could quickly destroy blades and machine.

To predict performance and failure modes and to facilitate thedesign process, researchers also develop computer models.Among these are ones that simulate turbulent inflow, and result-ing rotor loads and blade fatigue. To agree with reality, the simu-lations include total system interaction under steady andunsteady wind conditions.

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Illustration ofpredictedlowest-energyconfigurationsof a film ofGaInP2(Ga = Blue,In = Red,P = Yellow)on a GaAssubstrate(As = Green),showing therelationshipamong surfacereconstruction,surface segre-gation, andsubsurfaceordering.

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National Renewable Energy Laboratory1617 Cole BoulevardGolden, Colorado 80401-3393303-275-3000

NREL is a U.S. Department of Energy National LaboratoryOperated by Midwest Research Institute • Battelle • Bechtel

Support for the renewable energy research described in thisbrochure provided by DOE’s Office of Power Technologies andOffice of Science.

NREL/BR-590-26070March 1999

Photo credits: Einstein image courtesy of the Lotte Jacobi Archives,University of New Hampshire; Earth image courtesy of NASA; solarX-ray image from the Yohkoh mission of ISAS, Japan, with theX-ray telescope prepared by the Lockheed Palo Alto ResearchLaboratory, the National Astronomical Observatory of Japan, andthe University of Tokyo with support from NASA and ISAS.

Printed with a renewable-source ink on paper containing at least50% wastepaper, including 20% postconsumer waste.

NREL Web sites:

National Renewable Energy Laboratory: http://www.nrel.gov

Center for Basic Sciences: http://www.nrel.gov/basic_sciences

National Center for Photovoltaics: http://www.nrel.gov/ncpv

National Wind Technology Center: http://www.nrel.gov/wind