This paper provides an overview of the developments in thermophotovoltaic (TPV) generation of electricity that have occurred relatively recently-from about 1994 to October 1998. The components considered are the semiconductor converter; the radiator; and the means of recirculating unusable, long-wavelength photons. A short account of the functions and performance of each of these components is given. Also discussed are operational systems and progress in modeling TPV systems

Coutts, T.J.

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An Overview ofThermophotovoltaic Generationof ElectricitySeptember 1999      •      NREL/CP-520-26904Timothy J. CouttsPresented at the 11th International PhotovoltaicsScience and Engineering Conference (PVSEC-11)Sapporo, JapanSeptember 20-24, 1999National Renewable Energy Laboratory1617 Cole BoulevardGolden, Colorado 80401-3393NREL is a U.S. Department of Energy LaboratoryOperated by Midwest Research Institute • Battelle • BechtelContract No. DE-AC36-98-GO10337NOTICEThis report was prepared as an account of work sponsored by an agency of the United Statesgovernment. Neither the United States government nor any agency thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights.  Reference herein to any specific commercialproduct, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarilyconstitute or imply its endorsement, recommendation, or favoring by the United States government or anyagency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflectthose of the United States government or any agency thereof.Available to DOE and DOE contractors from:Office of Scientific and Technical Information (OSTI)P.O. Box 62Oak Ridge, TN  37831Prices available by calling 423-576-8401Available to the public from:National Technical Information Service (NTIS)U.S. Department of Commerce5285 Port Royal RoadSpringfield, VA  22161703-605-6000 or 800-553-6847orDOE Information Bridgehttp://www.doe.gov/bridge/home.htmlPrinted on paper containing at least 50% wastepaper, including 20% postconsumer waste1An Overview of Thermophotovoltaic Generation of ElectricityTimothy J. CouttsNational Renewable Energy LaboratoryGolden CO  80401, USAABSTRACTThis paper provides an overview of thedevelopments in thermophotovoltaic (TPV)generation of electricity that have occurredrelatively recently—from about 1994 to October1998.  The components considered are thesemiconductor converter; the radiator; and themeans of recirculating unusable, long-wavelengthphotons.  A short account of the functions andperformance of each of these components isgiven.  Also discussed are operational systemsand progress in modeling TPV systems.1.  IntroductionThe origins of TPV date back to the early1960s, but it is only in relatively recent years thatit has shown real promise.  Incident and outputpower densities are much larger than flat-plate PVmodules because of geometric considerations.The output power densities from the cells mayeventually be as large as 5 watts cm-2, i.e., 50times that of a flat-plate PV system, assuming thesemiconductor converters work as efficiently assolar cells.  Indeed, researchers have alreadydemonstrated cell outputs of about 1 watt cm-2.The sun radiates mainly in the visible range ofwavelengths, whereas a TPV radiator emitsmainly in the near-infrared range.  This demandssemiconductor bandgaps of 0.4-0.7 eV forefficient conversion.  TPV development has beenable to exploit the extraordinary progress made inthe fields of III-V semiconductors and high-efficiency solar cells.Potential attractions of TPV include: highpower density, fuel versatility, portability, silentoperation, operation that is independent of thesun, and low maintenance.  Possible applicationsinclude: stand-alone domestic gas-furnaces, powersystems for navigation of sailing boats, silentpower supplies on recreational vehicles, co-generation of electricity and heat, large-scalerecovery of high-temperature waste heat fromindustrial processes such as glass-manufacture,and many others.  Military organizations havealso taken a keen interest in TPV conversionbecause of possible strategic advantages that maybe realized.  Despite its potential advantages andapplications, TPV is not yet widely accepted and,so far as the author is aware, only JX Crystals,Inc., is actually manufacturing complete systems.The most recently archived publications onTPV are to be found in the proceedings of the four2NREL conferences on the subject.  Interestedreaders are referred to those publications fordetails.2.  The generic TPV systemA generic TPV system consists of a: (i) sourceof heat (e.g., a flame, radiative isotope, the sun),(ii) radiator, (iii) semiconductor converter, (iv)means of recirculating the sub-bandgap photons toconserve system energy, and (v) powerconditioning system.  The various options foritems (ii), (iii), and (iv), as well as theirperformance are discussed in this paper.  It mustbe appreciated that each of these processes has anefficiency of less than unity, thus implying thatthe overall system efficiency must necessarily berelatively modest.  In the short-term, a systemefficiency of 20% would be a very ambitious goal.Nearly all work undertaken to date has concernedthe optimization of individual components, withrelatively little work being devoted to the systemas a whole.  This rather unbalanced approach hasbeen addressed, to some extent, in recent yearsbut more work on complete systems—andespecially the demonstration of systems—must beperformed to demonstrate the full potential ofTPV.3.  The semiconductor converterEarly work on TPV converters focused mainlyon silicon and germanium converters(references).  However, the quality of theseelemental semiconductors was poor.  It is theimpressive progress in III-V devices that has ledto the high performance of modern TPV devicesand the consequent resurgence of interest in thisfield.  Potential reductions in cost may beachieved with development of thin-filmconverters.Most, but not all, recent device work hasconcerned the semiconductors GaInAs andGaInAsSb grown by organo-metallic vapor-phaseepitaxy (OMVPE) or by molecular-beam epitaxy(MBE).  GaInAs is lattice-matched to an InPsubstrate when the atomic proportions of Ga:Inare in the ratio 47:53.  However, this compositioncorresponds to a bandgap of 0.73 eV, which is toolarge for optimum efficiency.  Hence, much efforthas been devoted to fabricating InGaAs deviceswith bandgaps of as little as 0.5 eV.  Thismaterial, of course, is lattice-mismatched to theInP substrate, and success in growing it relativelyfree of damaging defects (caused by strainrelaxation) is of major importance to thetechnology.  In addition, considerable effort hasbeen devoted to the characterization of the alloys.The quaternary alloy GaInAsSb can be grownlattice-matched to either GaSb or InAs substrates,although the former is mainly used.  The optimumbandgap can be selected (in the range 0.38-0.7eV) without compromising lattice-matching, andgreat progress on GaInAsSb devices has beenmade.  At present, at least up to October 1998,there seems to be little to choose between thesetwo rival alloy systems.  Despite this progress intwo appropriate multi-component alloys, the only3commercially manufactured TPV system is theMidnight Sun , marketed by JX Crystals, Inc.This uses the binary semiconductor GaSb, with abandgap of about 0.7 eV.  Although this is non-ideal, GaSb can be grown in large volume and caneasily be processed into a device that performswell.  The devices are fabricated from wafers ofp-GaSb, with the junctions being formed bydiffusion.  Arrays of GaSb cells are connected inseries and parallel combinations, and systemoutputs of as high as 600 watts have beenrecorded.4.  The radiatorThe radiator may be either broad- or narrow-band.  If the former is used, then the spectralemittance must be as close to unity as possible,and the material most commonly used has beensilicon carbide.  This has an emittance of about0.9, and it will withstand temperatures up to about1900 K.  Optimization of the bandgap depends, asexpected, on the radiator temperature, and there isinevitably a large proportion of the incidentphotons that are of too low an energy to beabsorbed.  For the system to have an acceptableefficiency, these must be re-absorbed to theradiator.  This aspect will be discussed in the nextsection.  Narrow-band radiators considered to dateinclude ytterbia, erbia, and holmia.  These radiateat energies of about 1.1, 0.7, and 0.5 eV,respectively.  The selective radiation propertyarises from the electronic structure; valenceelectrons are screened from mutual interaction,thereby preventing energy bands from forming.Great progress has been made with selectiveradiators in recent years, and earlier objections ofless-than-ideal performance have largely beenovercome.  Again, at this stage, there seems to belittle to choose between the two approaches.5.  Photon recirculationPhoton recirculation involves the reflection ofthe unusable (and possibly deleterious) long-wavelength photons back to the radiator to be re-absorbed.  The ideal characteristic of thiscomponent is near-zero reflectance up to thebandgap of the semiconductor and unity beyond.This may be achieved in at least four differentways, which will be discussed now.Dielectric filtersA multilayer stack of dielectric films ofalternating high and low refractive indexes can bedesigned to have a specific optical characteristicby appropriate choice of the number andthicknesses of the layers.  However, thisapproach, although demonstrated to worksatisfactorily in R&D, may not be feasible forlarge-volume manufacture.Plasma filtersA plasma filter is based on a transparentconducting oxide.  By choosing the carrierconcentration of these materials appropriately (≅1020 cm-3), the required optical characteristicoccurs at the required wavelength (≅  2.5 µm).Operation is based on resonance of the electronsin the film.  The performance of the filter is4adequate but not ideal because, at resonance, thefree electrons absorb energy.  This prevents thereflectance of the filter from being abrupt, and itreduces the long-wave reflectance to significantlyless than 100%.  Nevertheless, plasma filters havebeen used successfully in conjunction withdielectric filters in operational systems.Resonant antennae arraysAn array of features on a dielectric substrate,with dimensions close to the wavelength of theincident radiation (≅  2.5 µm), forms an activeoptical filter.  For slots or crosses in a metallicfilm, a band-pass filter is formed.  For crosses ona dielectric substrate, a band-reject filter isformed.  For a TPV converter, the inductive filteris required.  High reflectances at both long andshort wavelengths have been achieved.  Thisminimizes losses due to both above- and below-bandgap photons.  Thermalization of hot carriersand absorption by free electrons are prevented.Back-surface reflectorsThe entire device may be grown on a semi-insulating substrate, in which case, sub-bandgapphotons pass through the substrate withoutsignificant absorption.  This has been achievedwith InP substrates and mismatched InGaAsactive device layers.  With the back surface of thesubstrate metallized with a specular metal layer,the unabsorbed photons are reflected back to theradiator.  This has also been demonstratedsatisfactorily.6.  SystemsModeling of TPV systems, using broad-bandradiators suggests that efficiencies of 15% may beachievable in the short-term, but a longer-termeffort would be required to reach 20%.  There aremany issues to consider in the construction of asystem, including: non-uniformity of photon flux,photon loss mechanisms, converter temperature,and mismatched performances of individual cells.These factors are difficult to model.  Given thatexisting efficiencies are still significantly less thanthe above figures, there seems little reason toquestion modeling data.  The relatively lowpredicted and measured efficiencies are forelectrical conversion only.  If the waste heat fromthe burner, e.g., the Midnight Sun® manufacturedby JX Crystals, Inc., or from an industrial process,is used, then the efficiency may be claimed to bemuch higher.7.  SummaryThe individual components of a TPVsystem have performed well, but there have notbeen enough demonstrations of complete systems.At present, there is little to choose between thetwo main semiconducting alloys and between theselective and broad-band radiators.  Back-surfacereflectors, however, seem to be the mostpromising filters, at present.  More work oncollective optimization of the main componentsalso needs to be done.

An overview of thermophotovoltaic generation of electricity

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