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Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat to
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
via
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s. A basic thermophotovoltaic system consists of a hot object emitting
thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...
and a
photovoltaic Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially us ...
cell similar to a
solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.
but tuned to the
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
being admitted from the hot object. As TPV systems generally work at lower temperatures than solar cells, their efficiencies tend to be low. Offsetting this through the use of multi-junction cells based on non-silicon materials is common, but generally very expensive. This currently limits TPV to niche roles like
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
power and
waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility ...
collection from larger systems like
steam turbine A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam turbin ...
s.


General concept


PV

Typical photovoltaics work by creating a
p–n junction A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains ...
near the front surface of a thin
semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
material. When photons above the
bandgap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...
energy of the material hit atoms within the bulk lower layer, below the junction, an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
is photoexcited and becomes free of its atom. The junction creates an
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
that accelerates the electron forward within the cell until it passes the junction and is free to move to the thin
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
s patterned on the surface. Connecting a wire from the front to the rear allows the electrons to flow back into the bulk and complete the circuit. Photons with less energy than the bandgap do not eject electrons. Photons with energy above the bandgap will eject higher-energy electrons which tend to thermalize within the material and lose their extra energy as heat. If the cell's bandgap is raised, the electrons that are emitted will have higher energy when they reach the junction and thus result in a higher
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
, but this will reduce the number of electrons emitted as more photons will be below the bandgap energy and thus generate a lower
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
. As
electrical power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions of ...
is the product of voltage and current, there is a sweet spot where the total output is maximized. Terrestrial
solar radiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...
is typically characterized by a standard known as Air Mass 1.5, or AM1.5. This is very close to 1,000 W of energy per square meter at an
apparent temperature Apparent temperature, also known as feels like, is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The measure is most commonly applied to the perceived outdoo ...
of 5780 K. At this temperature, about half of all the energy reaching the surface is in the
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
. Based on this temperature, energy production is maximized when the bandgap is about 1.4 eV, in the
near infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
. This just happens to be very close to the bandgap in doped
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
, at 1.1 eV, which makes solar PV inexpensive to produce. This means that all of the energy in the infrared and lower, about half of AM1.5, goes to waste. There has been continuing research into cells that are made of several different layers, each with a different bandgap, and thus tuned to a different part of the solar spectrum. , cells with overall efficiencies in the range of 40% are commercially available, although they are extremely expensive and have not seen widespread use outside of specific roles like powering
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
, where cost is not a significant consideration.


TPV

The same process of photoemission can be used to produce electricity from any spectrum, although the number of semiconductor materials that will have just the right bandgap for an arbitrary hot object is limited. Instead, semiconductors that have tuneable bandgaps are needed. It is also difficult to produce solar-like thermal output; an
oxyacetylene torch Principle of burn cutting Oxy-fuel welding (commonly called oxyacetylene welding, oxy welding, or gas welding in the United States) and oxy-fuel cutting are processes that use fuel gases (or liquid fuels such as gasoline or petrol, diesel, ...
is about 3400 K (~3126 °C), and more common commercial heat sources like
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...
and
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
burn at much lower
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
s around 900 °C to about 1300 °C. This further limits the suitable materials. In the case of TPV most research has focused on
gallium antimonide Gallium antimonide (GaSb) is a semiconducting compound of gallium and antimony of the III-V family. It has a lattice constant of about 0.61 nm. It has a band gap of 0.67 eV. History The intermetallic compound GaSb was first prepared in 1926 by Vi ...
(GaSb), although
germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
(Ge) is also suitable. Another problem with lower-temperature sources is that their energy is more spread out, according to
Wien's displacement law Wien's displacement law states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. The shift of that peak is a direct consequence of the Planck r ...
. While one can make a practical solar cell with a single bandgap tuned to the peak of the spectrum and just ignore the losses in the IR region, doing the same with a lower temperature source will lose much more of the potential energy and result in very low overall efficiency. This means TPV systems almost always use multi-junction cells in order to reach reasonable double-digit efficiencies. Current research in the area aims at increasing system efficiencies while keeping the system cost low, but even then their roles tend to be niches similar to those of multi-junction solar cells.


Actual designs

TPV systems generally consist of a heat source, an emitter, and a waste heat rejection system. The TPV cells are placed between the emitter, often a block of metal or similar, and the cooling system, often a passive radiator. PV systems in general operate at lower efficiency as the temperature increases, and in TPV systems, keeping the photovoltaic cool is a significant challenge. This contrasts with a somewhat related concept, the "thermoradiative" or "negative emission" cells, in which the photodiode is on the hot side of the heat engine. Systems have also been proposed that use a thermoradiative device as an emitter in a TPV system, theoretically allowing power to be extracted from both a hot photodiode and a cold photodiode.


Applications


RTGs

Conventional
radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioacti ...
s (RTGs) used to power spacecraft use a radioactive material whose radiation is used to heat a block of material and then converted to electricity using a
thermocouple A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...
. Thermocouples are very inefficient and their replacement with TPV could offer significant improvements in efficiency and thus require a smaller and lighter RTG for any given mission. Experimental systems developed by Emcore (a multi-junction solar cell provider), Creare, Oak Ridge and
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
's
Glenn Research Center NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facilit ...
demonstrated 15 to 20% efficiency. A similar concept was developed by the
University of Houston The University of Houston (UH) is a Public university, public research university in Houston, Texas. Founded in 1927, UH is a member of the University of Houston System and the List of universities in Texas by enrollment, university in Texas ...
which reached 30% efficiency, a 3 to 4-fold improvement over existing systems.


Thermal storage

Another area of active research is using TPV as the basis of a thermal storage system. In this concept, electricity being generated in off-peak times is used to resistance heat a large block of material, typically
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
, to very high temperatures. The carbon is surrounded by TPV cells which are in turn backed by a reflector and insulation. During storage, the TPV is turned off and the photons pass through them and reflect back into the carbon. When power is needed, the TPV is connected to a load.


Waste heat collection

TPV cells have been proposed as auxiliary power conversion devices for capture of otherwise lost heat in other power generation systems, such as steam turbine systems or solar cells.


History

Henry Kolm Henry Herbert Kolm (September 9, 1924 in Vienna – July 29, 2010 in Concord, Massachusetts) was an American physicist associated with Massachusetts Institute of Technology (MIT) for many years, with extensive expertise in high-power magnets and ...
constructed an elementary TPV system at
MIT The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...
in 1956. However, Pierre Aigrain is widely cited as the inventor based on lectures he gave at MIT between 1960–1961 which, unlike Kolm's system, led to research and development. In the 1980s, efficiency reached around 30%. In 1997 a prototype TPV hybrid car was built, the "Viking 29" (TPV) powered automobile, designed and built by the Vehicle Research Institute (VRI) at
Western Washington University Western Washington University (WWU or Western) is a public university in Bellingham, Washington. The northernmost university in the contiguous United States, WWU was founded in 1893 as the state-funded New Whatcom Normal School, succeeding a pri ...
. In 2022,
MIT The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...
/
NREL The National Renewable Energy Laboratory (NREL) in the US specializes in the research and development of renewable energy, energy efficiency, energy systems integration, and sustainable transportation. NREL is a federally funded research and d ...
announced a device with 41% efficiency. The absorber employed multiple
III-V semiconductor Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way. Because of t ...
layers tuned to absorb variously, ultraviolet, visible, and infrared photons. A gold reflector recycled unabsorbed photons. The device operated at 2400 °C, at which temperature the tungsten emitter reaches maximum brightness.


Details


Efficiency

The upper limit for efficiency in TPVs (and all systems that convert heat energy to work) is the Carnot efficiency, that of an ideal
heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...
. This efficiency is given by: :\eta = 1 - \frac where Tcell is the temperature of the PV converter. Practical systems can achieve Tcell= ~300 K and Temit= ~1800 K, giving a maximum possible efficiency of ~83%. This assumes the PV converts the radiation into electrical energy without losses, such as thermalization or
Joule heating Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of an electric current through a conductor (material), conductor produces heat. Joule's first law (also just Joule's law), also known in c ...
, though in reality the photovoltaic inefficiency is quite significant. In real devices, as of 2021, the maximum demonstrated efficiency in the laboratory was 35% with an emitter temperature of 1,773 K. This is the efficiency in terms of heat input being converted to electrical power. In complete TPV systems, a necessarily lower total system efficiency may be cited including the source of heat, so for example, fuel-based TPV systems may report efficiencies in terms of fuel-energy to electrical energy, in which case 5% is considered a "world record" level of efficiency. Real-world efficiencies are reduced by such effects as heat transfer losses, electrical conversion efficiency (TPV voltage outputs are often quite low), and losses due to active cooling of the PV cell.


Emitters

Deviations from perfect absorption and perfect black body behavior lead to light losses. For selective emitters, any light emitted at wavelengths not matched to the bandgap energy of the photovoltaic may not be efficiently converted, reducing efficiency. In particular, emissions associated with
phonon In physics, a phonon is a collective excitation in a periodic, Elasticity (physics), elastic arrangement of atoms or molecules in condensed matter physics, condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phon ...
resonances are difficult to avoid for wavelengths in the deep
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
, which cannot be practically converted. An ideal emitter would emit no light at wavelengths other than at the bandgap energy, and much TPV research is devoted to developing emitters that better approximate this narrow emission spectrum.


Filters

For black body emitters or imperfect selective emitters, filters reflect non-ideal wavelengths back to the emitter. These filters are imperfect. Any light that is absorbed or scattered and not redirected to the emitter or the converter is lost, generally as heat. Conversely, practical filters often reflect a small percentage of light in desired wavelength ranges. Both are inefficiencies. The absorption of suboptimal wavelengths by the photovoltaic device also contributes inefficiency and has the added effect of heating it, which also decreases efficiency.


Converters

Even for systems where only light of optimal wavelengths is passed to the photovoltaic converter, inefficiencies associated with
non-radiative recombination In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are ...
and Ohmic losses exist. There are also losses from Fresnel reflections at the PV surface, optimal-wavelength light that passes through the cell unabsorbed, and the energy difference between higher-energy photons and the bandgap energy (though this tends to be less significant than with solar PVs). Non-radiative recombination losses tend to become less significant as the light intensity increases, while they increase with increasing temperature, so real systems must consider the intensity produced by a given design and
operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
.


Geometry

In an ideal system, the emitter is surrounded by converters so no light is lost. Realistically, geometries must accommodate the input energy (fuel injection or input light) used to heat the emitter. Additionally, costs have prohibited surrounding the filter with converters. When the emitter reemits light, anything that does not travel to the converters is lost. Mirrors can be used to redirect some of this light back to the emitter; however, the mirrors may have their own losses.


Black body radiation

For black body emitters where photon recirculation is achieved via filters,
Planck's law In physics, Planck's law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature , when there is no net flow of matter or energy between the body and its environment. At ...
states that a black body emits light with a spectrum given by: I'(\lambda,T) =\frac\frac where ''I''′ is the light flux of a specific wavelength, ''λ'', given in units of 1 m–3⋅s–1. ''h'' is the
Planck constant The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivale ...
, k is the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...
, ''c'' is the speed of light, and ''T''emit is the emitter temperature. Thus, the light flux with wavelengths in a specific range can be found by integrating over the range. The peak wavelength is determined by the temperature, ''T''emit based on
Wien's displacement law Wien's displacement law states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. The shift of that peak is a direct consequence of the Planck r ...
: :\lambda_ = \frac , where ''b'' is Wien's displacement constant. For most materials, the maximum temperature an emitter can stably operate at is about 1800 °C. This corresponds to an intensity that peaks at or an energy of ~0.75 eV. For more reasonable operating temperatures of 1200 °C, this drops to ~0.5 eV. These energies dictate the range of bandgaps that are needed for practical TPV converters (though the peak spectral power is slightly higher). Traditional PV materials such as Si (1.1 eV) and GaAs (1.4 eV) are substantially less practical for TPV systems, as the intensity of the black body spectrum is low at these energies for emitters at realistic temperatures.


Active components and materials selection


Emitters

Efficiency, temperature resistance and cost are the three major factors for choosing a TPV emitter. Efficiency is determined by energy absorbed relative to incoming radiation. High temperature operation is crucial because efficiency increases with operating temperature. As emitter temperature increases, black-body radiation shifts to shorter wavelengths, allowing for more efficient absorption by photovoltaic cells.


Polycrystalline silicon carbide

Polycrystalline
silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal sin ...
(SiC) is the most commonly used emitter for burner TPVs. SiC is thermally stable to ~1700 °C. However, SiC radiates much of its energy in the long wavelength regime, far lower in energy than even the narrowest bandgap photovoltaic. Such radiation is not converted into electrical energy. However, non-absorbing selective filters in front of the PV, or mirrors deposited on the back side of the PV can be used to reflect the long wavelengths back to the emitter, thereby recycling the unconverted energy. In addition, polycrystalline SiC is inexpensive.


Tungsten

Tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...
is the most common
refractory metal Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group diff ...
that can be used as a selective emitter. It has higher emissivity in the visible and near-IR range of 0.45 to 0.47 and a low emissivity of 0.1 to 0.2 in the IR region. The emitter is usually in the shape of a cylinder with a sealed bottom, which can be considered a cavity. The emitter is attached to the back of a thermal absorber such as SiC and maintains the same temperature. Emission occurs in the visible and near IR range, which can be readily converted by the PV to electrical energy. However, compared to other metals, tungsten oxidizes more easily.


Rare-earth oxides

Rare-earth oxides such as
ytterbium Ytterbium is a chemical element with the symbol Yb and atomic number 70. It is a metal, the fourteenth and penultimate element in the lanthanide series, which is the basis of the relative stability of its +2 oxidation state. However, like the othe ...
oxide (Yb2O3) and
erbium Erbium is a chemical element with the symbol Er and atomic number 68. A silvery-white solid metal when artificially isolated, natural erbium is always found in chemical combination with other elements. It is a lanthanide, a rare-earth element ...
oxide (Er2O3) are the most commonly used selective emitters. These oxides emit a narrow band of wavelengths in the near-infrared region, allowing the emission spectra to be tailored to better fit the absorbance characteristics of a particular PV material. The peak of the emission spectrum occurs at 1.29 eV for Yb2O3 and 0.827 eV for Er2O3. As a result, Yb2O3 can be used a selective emitter for silicon cells and Er2O3, for GaSb or InGaAs. However, the slight mismatch between the emission peaks and band gap of the absorber costs significant efficiency. Selective emission only becomes significant at 1100 °C and increases with temperature. Below 1700 °C, selective emission of rare-earth oxides is fairly low, further decreasing efficiency. Currently, 13% efficiency has been achieved with Yb2O3 and silicon PV cells. In general selective emitters have had limited success. More often filters are used with black body emitters to pass wavelengths matched to the bandgap of the PV and reflect mismatched wavelengths back to the emitter.


Photonic crystals

Photonic crystals A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomi ...
allow precise control of electromagnetic wave properties. These materials give rise to the photonic bandgap (PBG). In the spectral range of the PBG, electromagnetic waves cannot propagate. Engineering these materials allows some ability to tailor their emission and absorption properties, allowing for more effective emitter design. Selective emitters with peaks at higher energy than the black body peak (for practical TPV temperatures) allow for wider bandgap converters. These converters are traditionally cheaper to manufacture and less temperature sensitive. Researchers at
Sandia Labs Sandia National Laboratories (SNL), also known as Sandia, is one of three research and development laboratories of the United States Department of Energy's National Nuclear Security Administration (NNSA). Headquartered in Kirtland Air Force Bas ...
predicted a high-efficiency (34% of light emitted converted to electricity) based on TPV emitter demonstrated using tungsten photonic crystals. However, manufacturing of these devices is difficult and not commercially feasible.


Photovoltaic cells


Silicon

Early TPV work focused on the use of silicon. Silicon's commercial availability, low cost, scalability and ease of manufacture makes this material an appealing candidate. However, the relatively wide bandgap of Si (1.1eV) is not ideal for use with a black body emitter at lower operating temperatures. Calculations indicate that Si PVs are only feasible at temperatures much higher than 2000 K. No emitter has been demonstrated that can operate at these temperatures. These engineering difficulties led to the pursuit of lower-bandgap semiconductor PVs. Using selective radiators with Si PVs is still a possibility. Selective radiators would eliminate high and low energy photons, reducing heat generated. Ideally, selective radiators would emit no radiation beyond the band edge of the PV converter, increasing conversion efficiency significantly. No efficient TPVs have been realized using Si PVs.


Germanium

Early investigations into low bandgap semiconductors focused on
germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
(Ge). Ge has a bandgap of 0.66 eV, allowing for conversion of a much higher fraction of incoming radiation. However, poor performance was observed due to the high
effective electron mass In solid state physics, a particle's effective mass (often denoted m^*) is the mass that it ''seems'' to have when responding to forces, or the mass that it seems to have when interacting with other identical particles in a thermal distribution ...
of Ge. Compared to
III-V semiconductors Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way. Because of t ...
, Ge's high electron effective mass leads to a high density of states in the conduction band and therefore a high intrinsic carrier concentration. As a result, Ge
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...
s have fast decaying "dark" current and therefore, a low open-circuit voltage. In addition, surface passivation of germanium has proven difficult.


Gallium antimonide

The
gallium antimonide Gallium antimonide (GaSb) is a semiconducting compound of gallium and antimony of the III-V family. It has a lattice constant of about 0.61 nm. It has a band gap of 0.67 eV. History The intermetallic compound GaSb was first prepared in 1926 by Vi ...
(GaSb) PV cell, invented in 1989, is the basis of most PV cells in modern TPV systems. GaSb is a III-V semiconductor with the
zinc blende Sphalerite (sometimes spelled sphaelerite) is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in sedimentary exhalative, Mississippi-Va ...
crystal structure. The GaSb cell is a key development owing to its narrow bandgap of 0.72 eV. This allows GaSb to respond to light at longer wavelengths than silicon solar cell, enabling higher power densities in conjunction with manmade emission sources. A solar cell with 35% efficiency was demonstrated using a bilayer PV with GaAs and GaSb, setting the
solar cell efficiency Solar-cell efficiency refers to the portion of energy in the form of sunlight that can be converted via photovoltaics into electricity by the solar cell. The efficiency of the solar cells used in a photovoltaic system, in combination with lat ...
record. Manufacturing a GaSb PV cell is quite simple. Czochralski
tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...
-doped n-type GaSb wafers are commercially available. Vapor-based zinc diffusion is carried out at elevated temperatures (~450 °C) to allow for p-type doping. Front and back electrical contacts are patterned using traditional
photolithography In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protect ...
techniques and an anti-reflective coating is deposited. Efficiencies are estimated at ~20% using a 1000 °C black body spectrum. The radiative limit for efficiency of the GaSb cell in this setup is 52%.


Indium gallium arsenide antimonide

Indium gallium arsenide antimonide Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat to electricity via photons. A basic thermophotovoltaic system consists of a hot object emitting thermal radiation and a photovoltaic cell similar to a solar cell b ...
(InGaAsSb) is a compound
III-V semiconductor Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way. Because of t ...
. (InxGa1−xAsySb1−y) The addition of
GaAs Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circui ...
allows for a narrower bandgap (0.5 to 0.6 eV), and therefore better absorption of long wavelengths. Specifically, the bandgap was engineered to 0.55 eV. With this bandgap, the compound achieved a photon-weighted internal quantum efficiency of 79% with a fill factor of 65% for a black body at 1100 °C. This was for a device grown on a GaSb substrate by organometallic vapour phase epitaxy (OMVPE). Devices have been grown by
molecular beam epitaxy Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors, and it is considered one of the fundamental tools for the devel ...
(MBE) and
liquid phase epitaxy Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epi ...
(LPE). The internal quantum efficiencies (IQE) of these devices approach 90%, while devices grown by the other two techniques exceed 95%. The largest problem with InGaAsSb cells is phase separation. Compositional inconsistencies throughout the device degrade its performance. When phase separation can be avoided, the IQE and fill factor of InGaAsSb approach theoretical limits in wavelength ranges near the bandgap energy. However, the Voc/Eg ratio is far from the ideal. Current methods to manufacture InGaAsSb PVs are expensive and not commercially viable.


Indium gallium arsenide

Indium gallium arsenide Indium gallium arsenide (InGaAs) (alternatively gallium indium arsenide, GaInAs) is a ternary alloy (chemical compound) of indium arsenide (InAs) and gallium arsenide (GaAs). Indium and gallium are ( group III) elements of the periodic table whil ...
(InGaAs) is a compound III-V semiconductor. It can be applied in two ways for use in TPVs. When lattice-matched to an InP substrate, InGaAs has a bandgap of 0.74 eV, no better than GaSb. Devices of this configuration have been produced with a fill factor of 69% and an efficiency of 15%. However, to absorb higher wavelength photons, the bandgap may be engineered by changing the ratio of In to Ga. The range of bandgaps for this system is from about 0.4 to 1.4 eV. However, these different structures cause strain with the InP substrate. This can be controlled with graded layers of InGaAs with different compositions. This was done to develop of device with a quantum efficiency of 68% and a fill factor of 68%, grown by MBE. This device had a bandgap of 0.55 eV, achieved in the compound In0.68Ga0.33As. It is a well-developed material. InGaAs can be made to lattice match perfectly with Ge resulting in low defect densities. Ge as a substrate is a significant advantage over more expensive or harder-to-produce substrates.


Indium phosphide arsenide antimonide

The InPAsSb quaternary alloy has been grown by both OMVPE and LPE. When lattice-matched to InAs, it has a bandgap in the range 0.3–0.55 eV. The benefits of such a low band gap have not been studied in depth. Therefore, cells incorporating InPAsSb have not been optimized and do not yet have competitive performance. The longest spectral response from an InPAsSb cell studied was 4.3 μm with a maximum response at 3 μm. For this and other low-bandgap materials, high IQE for long wavelengths is hard to achieve due to an increase in
Auger recombination In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are ...
.


Lead tin selenide/Lead strontium selenide quantum wells

PbSnSe/PbSrSe quantum well materials, which can be grown by MBE on silicon substrates, have been proposed for low cost TPV device fabrication. These IV-VI semiconductor materials can have bandgaps between 0.3 and 0.6 eV. Their symmetric band structure and lack of valence band degeneracy result in low Auger recombination rates, typically more than an order of magnitude smaller than those of comparable bandgap III-V semiconductor materials.


Applications

TPVs promise efficient and economically viable power systems for both military and commercial applications. Compared to traditional nonrenewable energy sources, burner TPVs have little NOx emissions and are virtually silent. Solar TPVs are a source of emission-free renewable energy. TPVs can be more efficient than PV systems owing to recycling of unabsorbed photons. However, losses at each energy conversion step lower efficiency. When TPVs are used with a burner source, they provide on-demand energy. As a result,
energy storage Energy storage is the capture of energy produced at one time for use at a later time to reduce imbalances between energy demand and energy production. A device that stores energy is generally called an accumulator or battery. Energy comes in ...
may not be needed. In addition, owing to the PV's proximity to the radiative source, TPVs can generate current densities 300 times that of conventional PVs.


Energy storage


Man-portable power

Battlefield dynamics require portable power. Conventional diesel generators are too heavy for use in the field. Scalability allows TPVs to be smaller and lighter than conventional generators. Also, TPVs have few emissions and are silent. Multifuel operation is another potential benefit. Investigations in the 1970s failed due to PV limitations. However, the GaSb photocell led to a renewed effort in the 1990s with improved results. In early 2001, JX Crystals delivered a TPV based battery charger to the
US Army The United States Army (USA) is the land service branch of the United States Armed Forces. It is one of the eight U.S. uniformed services, and is designated as the Army of the United States in the U.S. Constitution.Article II, section 2, cla ...
that produced 230 W fueled by
propane Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...
. This prototype utilized an SiC emitter operating at 1250 °C and GaSb photocells and was approximately 0.5 m tall. The power source had an efficiency of 2.5%, calculated as the ratio of the power generated to the thermal energy of the fuel burned. This is too low for practical battlefield use. No portable TPV power sources have reached troop testing.


Grid storage

Converting spare electricity into heat for high-volume, long-term storage is under research at various companies, who claim that costs could be much lower than
lithium-ion batteries A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
.


Spacecraft

Space power generation systems must provide consistent and reliable power without large amounts of fuel. As a result, solar and
radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
fuels (extremely high power density and long lifetime) are ideal. TPVs have been proposed for each. In the case of solar energy, orbital spacecraft may be better locations for the large and potentially cumbersome concentrators required for practical TPVs. However, weight considerations and inefficiencies associated with the more complicated design of TPVs, protected conventional PVs continue to dominate. The output of isotopes is thermal energy. In the past thermoelectricity (direct thermal to electrical conversion with no moving parts) has been used because TPV efficiency is less than the ~10% of thermoelectric converters. Stirling engines have been deemed too unreliable, despite conversion efficiencies >20%. However, with the recent advances in small-bandgap PVs, TPVs are becoming more promising. A TPV radioisotope converter with 20% efficiency was demonstrated that uses a tungsten emitter heated to 1350 K, with tandem filters and a 0.6 eV bandgap InGaAs PV converter (cooled to room temperature). About 30% of the lost energy was due to the optical cavity and filters. The remainder was due to the efficiency of the PV converter. Low-temperature operation of the converter is critical to the efficiency of TPV. Heating PV converters increases their dark current, thereby reducing efficiency. The converter is heated by the radiation from the emitter. In terrestrial systems it is reasonable to dissipate this heat without using additional energy with a
heat sink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, th ...
. However, space is an isolated system, where heat sinks are impractical. Therefore, it is critical to develop innovative solutions to efficiently remove that heat. Both represent substantial challenges.


Commercial applications


Off-grid generators

TPVs can provide continuous power to off-grid homes. Traditional PVs do not provide power during winter months and nighttime, while TPVs can utilize alternative fuels to augment solar-only production. The greatest advantage for TPV generators is
cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elect ...
of heat and power. In cold climates, it can function as both a heater/stove and a power generator. JX Crystals developed a prototype TPV heating stove/generator that burns natural gas and uses a SiC source emitter operating at 1250 °C and GaSb photocell to output 25,000
BTU The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. The modern SI u ...
/hr (7.3kW of heat) simultaneously generating 100W (1.4% efficiency). However, costs render it impractical. Combining a heater and a generator is called
combined heat and power Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elect ...
(CHP). Many TPV CHP scenarios have been theorized, but a study found that generator using boiling coolant was most cost efficient. The proposed CHP would utilize a SiC IR emitter operating at 1425 °C and GaSb photocells cooled by boiling coolant. The TPV CHP would output 85,000 BTU/hr (25kW of heat) and generate 1.5 kW. The estimated efficiency would be 12.3% (?)(1.5kW/25kW = 0.06 = 6%) requiring investment or 0.08 €/kWh assuming a 20 year lifetime. The estimated cost of other non-TPV CHPs are 0.12 €/kWh for gas engine CHP and 0.16 €/kWh for fuel cell CHP. This furnace was not commercialized because the market was not thought to be large enough.


Recreational vehicles

TPVs have been proposed for use in recreational vehicles. Their ability to use multiple fuel sources makes them interesting as more sustainable fuels emerge. TPVs silent operation allows them to replace noisy conventional generators (i.e. during "quiet hours" in national park campgrounds). However, the emitter temperatures required for practical efficiencies make TPVs on this scale unlikely.


References


External links


6th International Conference on Thermophotovoltaic Generation of Electricity

NASA Radioisotope Power Conversion Technology NRA OverviewNew thermophotovoltaic materials could replace alternators in cars and save fuel
{{Photovoltaics Photovoltaics Thermodynamics