Thermophotovoltaic
Encyclopedia
Thermophotovoltaic energy conversion is a direct conversion process from heat differentials to electricity
via photon
s. A basic thermophotovoltaic system consists of a thermal
emitter
and a photovoltaic diode
cell.
The temperature
of the thermal emitter varies between different systems from about 900 °C to about 1300 °C, although in principle TPV devices can extract energy from any emitter with temperature elevated above that of the photovoltaic device (forming an optical heat engine
). The emitter can be a piece of solid material or a specially engineered structure. A conventional solar cell
is effectively a TPV device in which the Sun
functions as the emitter. Thermal emission
is the spontaneous emission of photons due to thermal motion of charges in the material. For normal TPV temperatures, this radiation is mostly at near infrared and infrared
frequencies. The photovoltaic diodes can absorb some of these radiated photons and convert them into free charge carrier
s, that is electricity.
Thermophotovoltaic systems have few, if any, moving parts
and are therefore very quiet and require low maintenance. These properties make thermophotovoltaic systems suitable for remote-site and portable electricity-generating applications. Their efficiency
-cost
properties, however, are often rather poor compared to other electricity-generating technologies. Current research in the area aims at increasing the system efficiencies while keeping the system cost low.
In the design of a TPV system, it is usually desired to match the thermal emission's optical properties (wavelength
, polarization, direction) with the most efficient conversion characteristics of the photovoltaic cell, since unconverted thermal emission is a major source of inefficiency. Most groups focus on Gallium antimonide (GaSb) cells. Germanium (Ge) is also suitable. Much research and development in TPVs therefore concerns methods for controlling the emitter's properties.
Though Henry Kolm had constructed an elementary TPV system at MIT in 1956, Pierre Aigrain is widely cited as the inventor of TPV based on the content of some lectures he gave at MIT between 1960-1961 which, unlike Kolm's system, led to R&D work. A review of the historical development of TPV is presented in Nelson (2003).
TPV cells have often been proposed as auxiliary power conversion devices for regeneration of lost heat in other power generation systems, such as steam turbine systems or solar cells.
A protoype TPV hybrid car was even built. The "Viking 29" (TPV) powered automobile, designed and built by the Vehicle Research Institute (VRI) at Western Washington University
.
TPV research is a very active area. Among others, the University of Houston
TPV Radioisotope Power Conversion Technology development effort is aiming at combining thermophotovoltaic cell concurrently with thermocouple
s to provide a 3 to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators.
The emitter can be heated by sunlight or combustion. In this sense, TPVs provide a great deal of versatility in potential fuels. In the case of solar TPVs, extremely large concentrators are needed to provide reasonable temperatures for efficient operation.
Vast improvements can be made on this basic concept by taking advantage of filters or selective emitters to create emissions in a narrow wavelength range that is optimized for the specific photovoltaic (PV) converter used in the system. In this way TPVs can overcome a fundamental challenge for traditional PVs, making efficient use of the entire solar spectrum. For blackbody emitters, photons with energy less than the bandgap of the converter cannot be absorbed to generate electron/hole pairs and are either reflected and lost or passes through the cell. Photons with energy above the bandgap can be absorbed, but the excess energy,
, is again lost, generating undesirable heating in the cell. In the case of TPVs, similar issues can exist, but the use of either selective emitters (emissivity over only a narrow wavelength range), or optical filters that only pass a narrow range of wavelengths and reflect all others, can be used to generate emission spectra that can be optimally converted by the PV converter. In this way, these photons are not lost or used inefficiently, in principle, drastically increasing the overall system efficiency. In the case of reflective filters, the emitter must be able to absorb over this range to make effective use those photons not converted.
In order to achieve the maximum efficiency, all photons should be converted. A process often termed photon recycling can be used to approach this. Here reflectors are placed behind the converter and anywhere else in the system that photons might not be efficiently directed to the collector. These photons are directed back to the concentrator where they can be converted, or back to the emitter, where they can be reabsorbed to generate heat and additional photons. An idealized TPV system would use photon recycling and selective emission to utilize all photons and allow them to be optimally converted.
where Tcell is the temperature of the PV converter. For the best reasonable values in a practical system, Tcell~300K and Temit~1800, giving a maximum efficiency of ~83%. This limit sets the upper limit for the system efficiency. At 83% efficiency, all heat energy is converted to radiation by the emitter which is then converted by the PV into electrical energy without losses, such as thermalization or ohmic losses. At the maximum efficiency, we also assume that there is no entropy change, which is only possible if the emitter and cell are at the same temperature. Still, as an upper limit, it is useful. Due to the complexity of TPV systems and the many sources of inefficiency, more accurate models for efficiency become quite complicated, but a discussion of the various sources of inefficiency that cause real systems to fall far short of this limit is worthwhile.
).
where I' is the flux of light of a specific wavelength, λ, given in units of 1/m3/s. Here, h is Planck’s constant, k is Boltzmann’s constant, c is the speed of light, and Temit is the temperature of the emitter. Thus, the flux of light with wavelengths in a specific range can be found by integrating over the range. The peak wavelength is determined by the temperature, Temit based on Wien’s displacement law:
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 which is peaked at λ~1600 nm or an energy of ~0.75 eV. For more reasonable operation temperatures of 1200°C, this drops to ~0.5 eV. These energies dictate the range of band gaps 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 blackbody spectrum is extremely low at these energies for emitters at realistic temperatures.
However, 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 above and below the band edge of the PV converter, increasing conversion efficiency significantly. However, selective emitters today are far from ideal. Consequently, no efficient TPVs have been realized using a Si PVs.
.
The manufacturing process for the GaSb PV cell is quite simple. Czochralski Te-doped n-type GaSb wafers are readily commercially available. Vapor based Zn diffusion is then carried out at elevated temperatures ~450°C to allow for p-type doping. Lastly, front and back electrical contacts are patterned using traditional photolithography techniques and an anti-reflective coating is deposited. Current efficiencies are estimated to be ~20% using a 1000°C blackbody spectrum. The radiative limit for efficiency of the GaSb cell in this setup is 52%, so vast improvements can still be made.
Early investigations into TPVs in the 1970s proved to be impossible due to PV limitations. However, with the realization of the GaSb photocell, a renewed effort in the 1990s produced greater results. In early 2001, JX Crystals delivered a TPV based battery charger to the Army that produced an output of 230 W by burning propane. This prototype utilized 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 by the ratio of the power generated to the thermal energy of the fuel burned. This is too low for practical use on the battlefield. In order to increase efficiency, narrow band emitters would need to be realized and the temperature of the burner would need to be raised. In order to accommodate this, further thermal management steps, such as water cooling or coolant boiling, must be implemented.
Although many successful proof-of-concept prototypes have been demonstrated, no TPV portable power sources have been developed for troop testing or battlefield implementation. Further advances in the ruggedness of components and thermal management must be made in order to renew interest in TPV man-portable power sources for use on the battlefield.
Probably more interesting is the prospect of using TPVs for conversion of radioisotope energy. The output of isotopes is already thermal energy, so in this sense TPVs are optimal. In the past thermoelectric (TEs) (also direct thermal to electrical conversion with no moving parts) have been used over TPVs because of the extremely low demonstrated efficiencies when compared to the ~10% of TEs. Stirling engines have also been considered, but are undesirable due to reliability concerns, which are unacceptable for space missions, despite improved conversion efficiencies (>20% demonstrated). However, more recently with large advances in small bandgap PVs critical for effective operation, TPVs are becoming more promising candidates. Recently, a TPV radioisotope converter with ~20% efficiency was demonstrated that used 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.
However space offers one unique challenge for TPV systems. Low temperature operation of the converter is critical to the efficiency of TPV. For PV converters raised temperature increases the dark current substantially, reducing overall efficiency. For all TPV systems, the converter will be heated by the radiation from the emitter. In terrestrial systems it is reasonable to dissipate this heat without using additional energy by heat sinking the converter. However, space is an isolated system, and such heat sinks are not practical. As a result it is critical to develop innovative solutions to efficiently remove that heat, or optimized TPV cells that can operate efficiently with higher temperature converters. Both represent substantial challenges. Despite this, TPVs offer substantial promise for use in future space travel.
The greatest advantage for TPV generators is cogeneration of heat and power. In cold climates, it can function as both a heater or stove and a power generator. JX Crystals has developed a prototype TPV heating stove and generator. It burns natural gas and uses a SiC source emitter operating at 1250°C and GaSb photocell to output 25,000 BTU/hr and generate 100 W at the same time. However, costs must be significantly reduced in order to render it commercially viable.
When a furnace is used as a heater and a generator, it is called Combined Heat and Power (CHP). Many TPV CHP scenarios have been theorized but in a recent cost estimate, a generator using boiling coolant was determined to be 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 and generate 1.5 kW. The estimated efficiency would be 12.3% and the investment would be 8 EURcents/kWh provided that the lifetime of the CHP furnace is 20 years. The estimated cost of other non-TPV CHPs are 12 EURcents/kWh for gas engine CHP and the 16 EURcents/kWh for fuel cell CHP. This proposed furnace has not been developed because there is comparatively a very small market for off-grid power generation and no funding is available to develop the boiling liquid cooled GaSb PV array.
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
via photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s. A basic thermophotovoltaic system consists of a thermal
Thermal
A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...
emitter
Emitter
-In general:*A device used to exude any signal, beacon, light, odor, liquid, fragrance, ionizing particles or any other type of signal.-In horticulture:*A device used in drip irrigation.-In electronics and instrument physics:...
and a photovoltaic diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
cell.
The temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
of the thermal emitter varies between different systems from about 900 °C to about 1300 °C, although in principle TPV devices can extract energy from any emitter with temperature elevated above that of the photovoltaic device (forming an optical heat engine
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...
). The emitter can be a piece of solid material or a specially engineered structure. A conventional solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
is effectively a TPV device in which the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
functions as the emitter. Thermal emission
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
is the spontaneous emission of photons due to thermal motion of charges in the material. For normal TPV temperatures, this radiation is mostly at near infrared and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
frequencies. The photovoltaic diodes can absorb some of these radiated photons and convert them into free charge carrier
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
s, that is electricity.
Thermophotovoltaic systems have few, if any, moving parts
Moving parts
The moving parts of a machine are those parts of it that move. Machines comprise both moving and fixed parts. The moving parts have controlled and constrained motions....
and are therefore very quiet and require low maintenance. These properties make thermophotovoltaic systems suitable for remote-site and portable electricity-generating applications. Their efficiency
Energy conversion efficiency
Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...
-cost
Cost
In production, research, retail, and accounting, a cost is the value of money that has been used up to produce something, and hence is not available for use anymore. In business, the cost may be one of acquisition, in which case the amount of money expended to acquire it is counted as cost. In this...
properties, however, are often rather poor compared to other electricity-generating technologies. Current research in the area aims at increasing the system efficiencies while keeping the system cost low.
In the design of a TPV system, it is usually desired to match the thermal emission's optical properties (wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
, polarization, direction) with the most efficient conversion characteristics of the photovoltaic cell, since unconverted thermal emission is a major source of inefficiency. Most groups focus on Gallium antimonide (GaSb) cells. Germanium (Ge) is also suitable. Much research and development in TPVs therefore concerns methods for controlling the emitter's properties.
Though Henry Kolm had constructed an elementary TPV system at MIT in 1956, Pierre Aigrain is widely cited as the inventor of TPV based on the content of some lectures he gave at MIT between 1960-1961 which, unlike Kolm's system, led to R&D work. A review of the historical development of TPV is presented in Nelson (2003).
TPV cells have often been proposed as auxiliary power conversion devices for regeneration of lost heat in other power generation systems, such as steam turbine systems or solar cells.
A protoype TPV hybrid car was even built. The "Viking 29" (TPV) powered automobile, designed and built by the Vehicle Research Institute (VRI) at Western Washington University
Western Washington University
Western Washington University is one of six state-funded, four-year universities of higher education in the U.S. state of Washington. It is located in Bellingham and offers bachelor's and master's degrees.-History:...
.
TPV research is a very active area. Among others, the University of Houston
University of Houston
The University of Houston is a state research university, and is the flagship institution of the University of Houston System. Founded in 1927, it is Texas's third-largest university with nearly 40,000 students. Its campus spans 667 acres in southeast Houston, and was known as University of...
TPV Radioisotope Power Conversion Technology development effort is aiming at combining thermophotovoltaic cell concurrently with thermocouple
Thermocouple
A thermocouple is a device consisting of two different conductors that produce a voltage proportional to a temperature difference between either end of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a...
s to provide a 3 to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators.
Background
Thermophotovoltaics (TPVs) are a class of power generating systems that are used to convert thermal energy to electrical energy. They consist of, at a minimum, an emitter and a photovoltaic power converter. However, most TPV systems also include additional components such as concentrators, filters and reflectors. The basic principle of operation is similar to that of traditional photovoltaics (PV) where a p/n junction is used to absorb optical energy, generate and separate electron/hole pairs, and in doing so convert that energy into electrical power. The difference is that the optical energy is not directly generated by the sun, but instead by a material at high temperature (termed the emitter), causing it to emit light. In this way thermal energy is converted to electrical energy.The emitter can be heated by sunlight or combustion. In this sense, TPVs provide a great deal of versatility in potential fuels. In the case of solar TPVs, extremely large concentrators are needed to provide reasonable temperatures for efficient operation.
Vast improvements can be made on this basic concept by taking advantage of filters or selective emitters to create emissions in a narrow wavelength range that is optimized for the specific photovoltaic (PV) converter used in the system. In this way TPVs can overcome a fundamental challenge for traditional PVs, making efficient use of the entire solar spectrum. For blackbody emitters, photons with energy less than the bandgap of the converter cannot be absorbed to generate electron/hole pairs and are either reflected and lost or passes through the cell. Photons with energy above the bandgap can be absorbed, but the excess energy,
, is again lost, generating undesirable heating in the cell. In the case of TPVs, similar issues can exist, but the use of either selective emitters (emissivity over only a narrow wavelength range), or optical filters that only pass a narrow range of wavelengths and reflect all others, can be used to generate emission spectra that can be optimally converted by the PV converter. In this way, these photons are not lost or used inefficiently, in principle, drastically increasing the overall system efficiency. In the case of reflective filters, the emitter must be able to absorb over this range to make effective use those photons not converted.In order to achieve the maximum efficiency, all photons should be converted. A process often termed photon recycling can be used to approach this. Here reflectors are placed behind the converter and anywhere else in the system that photons might not be efficiently directed to the collector. These photons are directed back to the concentrator where they can be converted, or back to the emitter, where they can be reabsorbed to generate heat and additional photons. An idealized TPV system would use photon recycling and selective emission to utilize all photons and allow them to be optimally converted.
Efficiency
To understand the overall benefit of TPV systems, a discussion of the basic principles of efficiency in TPVs is useful. The absolute 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. This efficiency is given by:where Tcell is the temperature of the PV converter. For the best reasonable values in a practical system, Tcell~300K and Temit~1800, giving a maximum efficiency of ~83%. This limit sets the upper limit for the system efficiency. At 83% efficiency, all heat energy is converted to radiation by the emitter which is then converted by the PV into electrical energy without losses, such as thermalization or ohmic losses. At the maximum efficiency, we also assume that there is no entropy change, which is only possible if the emitter and cell are at the same temperature. Still, as an upper limit, it is useful. Due to the complexity of TPV systems and the many sources of inefficiency, more accurate models for efficiency become quite complicated, but a discussion of the various sources of inefficiency that cause real systems to fall far short of this limit is worthwhile.
Emitters
For the emitter, deviations from perfect absorbing and perfect blackbody behavior lead to light losses. For the case of selective emitters, any light emitted at wavelengths not matched to the bandgap energy of the PV may not be efficiently converted (for reasons discussed above) and leads to reduced efficiency. In particular, emissions associated with phonon resonances are difficult to avoid for wavelengths of in the deep IR, which cannot be practically converted. Ideally, an emitter will not emit in this range, and energy will only be converted at wavelengths that are easily converted.Filters
For blackbody emitters or imperfect selective emitters, filters are needed to reflect nonideal wavelengths back to the emitter. In practice, these filters are rarely perfect. Any light that is absorbed or scattered and not redirected to the emitter or the converter is lost. Additionally, practical filters often reflect a small percentage of light in desired wavelength ranges or transmit light of non ideal wavelengths. Both can lead to inefficiencies.Converters
Even for systems where only light of optimal wavelengths is passed to the converter, inefficiencies associated with non-radiative recombination and ohmic losses exist. Since these losses can depend on the intensity of light incident on the cell, real systems must consider the intensity produced by a given set of conditions (emitter material, filter, operating temperatureOperating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
).
Geometry
In an ideal system, the emitter would be surrounded by PV converters so no light is lost. However, realistically, geometries must accommodate the input energy (fuel injection or input light) used to heat the emitter. Additionally, high costs prohibit the placement of converters everywhere. 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.Blackbody Radiation
To understand some of the practical demands of real TPV components, looking at some basic numbers is useful. For the purposes of these arguments we will discuss blackbody emitters where photon recirculation is achieved via filters; however, similar concepts can be applied towards selective emission emitters. Planck's law states that a blackbody will emit light with a spectrum given by:where I' is the flux of light of a specific wavelength, λ, given in units of 1/m3/s. Here, h is Planck’s constant, k is Boltzmann’s constant, c is the speed of light, and Temit is the temperature of the emitter. Thus, the flux of light with wavelengths in a specific range can be found by integrating over the range. The peak wavelength is determined by the temperature, Temit based on Wien’s displacement law:
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 which is peaked at λ~1600 nm or an energy of ~0.75 eV. For more reasonable operation temperatures of 1200°C, this drops to ~0.5 eV. These energies dictate the range of band gaps 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 blackbody spectrum is extremely low at these energies for emitters at realistic temperatures.
Emitters
Efficiency, temperature resistance, and cost are the three major factors when choosing the radiator for TPVs. Efficiency is determined by energy absorbed relative to total incoming radiation. Ability to operate at high temperatures is a crucial factor because efficiency increases with operating temperature. As emitter temperature increases, the blackbody radiation shifts to shorter wavelengths, allowing for more efficient absorption by photovoltaic cells. Lastly, cost is a major limitation in commercialization of TPVs.Polycrystalline Silicon Carbide
Polycrystalline Silicon Carbide (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 band gap photovoltaic. This radiation, in turn, 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 extremely cheap to manufacture, making it a good choice for commercial applications.Tungsten
Refractory metals are often used as selective emitters for burner TPVs. Tungsten is the most common choice. Tungsten 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.Rare-earth Oxides
Rare-earth oxides such as ytterbium oxide (Yb2O3) and erbium oxide (Er2O3) are the most commonly used selective emitters for TPVs. These oxides emit a narrow band of wavelengths in the near infrared regions, allowing the tailoring of the emission spectra to better fit the absorbance characteristics of a particular PV cell. The peak of the emission spectrum occurs at 1.29eV for Yb2O3 and 0.827eV for Er2O3. As a result, Yb2O3 can be used a selective emitter for Si PV cells and Er2O3, for GaSb or InGaAs. However, the slight mismatch between the emission peaks and band gap of the absorber results in a significant loss of efficiency. In addition, selective emission only becomes significant at 1100°C and increases with temperature, per Planck’s Law. At reasonable operating temperatures (below 1700°C), selective emission of rare-earth oxides is fairly low, resulting in a further decrease in efficiency. Currently, only 13% efficiency has been achieved with Yb2O3 and silicon PV cells. In general selective emitters have had limited success. More often spectral control filters are used with blackbody emitters to pass wavelengths matched to the band gap of the PV and reflect mismatched wavelengths back to the emitter.Photonic Crystals
Photonic crystals are a class of novel periodic materials that allow the precise control of electromagnetic wave properties. These materials give rise to the photonic band gap (PBG). In the spectral range of the PBG, electromagnetic waves cannot propagate. The engineering of these materials allows some ability to tailor their emission and absorption properties, allowing for more effective design of selective emitters. Selective emitters with peaks at higher energy than the blackbody peak (for practical TPV temperatures) allow for wider band gap converters. These converters are traditionally cheaper to manufacture and less temperature sensitive. Recently, researchers at Sandia Labs have demonstrated a high-efficiency (34% of light emitted from PBG selective emitter was converted to electricity) TPV system using tungsten photonic crystals. However, manufacturing of these devices is difficult and not currently commercially feasible.Silicon
Early work in TPVs focused on the use of Si PVs. Silicon’s commercial availability, extremely low cost, scalability, and ease of manufacture makes this material an extremely appealing candidate. However, the relative wide band gap of Si (1.1eV) is not ideal for use with a blackbody emitter at lower operating temperatures. Calculations using Planck’s law, which describes the blackbody spectrum as a function of temperature, indicates that Si PVs would only be 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 band gap semiconductor PVs for conversion of the blackbody spectrum.However, 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 above and below the band edge of the PV converter, increasing conversion efficiency significantly. However, selective emitters today are far from ideal. Consequently, no efficient TPVs have been realized using a Si PVs.
Germanium
Early investigations into low band gap semiconductors focused on germanium (Ge). Ge has a band gap of 0.66eV, allowing for a much higher percent of incoming radiation to be converted. However, poor performance was observed due Ge’s extremely high effective electron mass. Compared to III-V semiconductors, 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 diodes have fast decaying “dark” current and therefore, a low open-circuit voltage. In addition, surface passivation of germanium has proven to be extremely difficult. For these two reasons, germanium is an unlikely candidate for use in TPVs.Gallium Antimonide
The gallium antimonide (GaSb) PV cell, invented in 1989, is the basis of most PV cells in modern TPV systems. GaSb is a III-V semiconductor with a zinc blende crystal structure. The GaSb cell is recognized a key development in the TPV community due to its narrow band gap of 0.72 eV. This allows GaSb to respond to light at longer wavelengths than the conventional silicon solar cell thus enabling higher power densities when used in conjunction with manmade emission sources. A 35% efficient solar cell was demonstrated by the inventors at Boeing in 1989 using a bilayer PV with GaAs and GaSb, setting the world record for solar cell efficiencySolar cell efficiency
The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency...
.
The manufacturing process for the GaSb PV cell is quite simple. Czochralski Te-doped n-type GaSb wafers are readily commercially available. Vapor based Zn diffusion is then carried out at elevated temperatures ~450°C to allow for p-type doping. Lastly, front and back electrical contacts are patterned using traditional photolithography techniques and an anti-reflective coating is deposited. Current efficiencies are estimated to be ~20% using a 1000°C blackbody spectrum. The radiative limit for efficiency of the GaSb cell in this setup is 52%, so vast improvements can still be made.
Indium Gallium Arsenide Antimonide
Indium Gallium Arsenide Antimonide (InGaAsSb) is a compound III-V semiconductor. The addition of GaAs allows for a narrower band gap (0.5 to 0.6 eV), and therefore better absorption of long wavelengths. Specifically, the band gap has been engineered to 0.55eV. With this band gap, the compounded achieved a photon-weighted internal quantum efficiency of 79% with a fill factor of 65% for a blackbody at 1100°C. This was for a device grown on a GaSb substrate by OMVPE. Devices have also been grown by MBE and LPE. The internal quantum efficiencies (IQE) of these devices have all been impressive. The IQE of the LPE-grown devices are approaching 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 and are extremely detrimental to its performance. When phase separation can be avoided, the IQE and fill factor of InGaAsSb are approaching theoretical limits in wavelength ranges near the bandgap energy, however, the Voc/Eg ratio is far from the ideal. Improving this ratio through photon recycling and tandem cell structures would be the next area in which the performance of this material could be significantly improved. In addition, current methods to manufacture InGaAsSb PVs are expensive and not commercially viable.Indium Gallium Arsenide
Indium Gallium Arsenide (InGaAs) is also 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 band gap of 0.74eV, which is not an improvement on traditional GaSb. Devices of this configuration have been produced with a fill factor of 69% and an efficiency of 15%. However, in order to absorb higher wavelength photons, the band gap may be engineered by changing the ratio of In to Ga. The range of band gaps for this system is from about 0.4eV to 1.4eV. 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 molecular beam epitaxy. This device also had a band gap of 0.55eV achieved by a compound of In0.68Ga0.33As. InGaAs devices have the advantage of being a well-developed material. InGaAs can also be made to lattice match perfectly with Ge resulting in very low defect densities. Being able to use 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 engineered to be lattice-matched to InAs, it has a band gap range from 0.3-0.55eV. The benefits of a TPV system with such a low band gap have not been studied significantly. Therefore, cells incorporating InPAsSb have not been optimized and do not yet have very competitive properties and performance. The longest spectral response from an InPAsSb cell studied was out to 4.3μm with a maximum response at 3μm. While this is a promising material in the very low band gap range, it has yet to be developed. For this and other extremely low band gap materials, high IQE for long wavelengths is hard to achieve due to an increase in Auger recombination.Applications of Thermophotovoltaics
TPVs have significant promise for 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, on the other hand, are a source of entirely renewable energy with no emissions. Compared to photovoltaics, TPVs have the potential to be more efficient due to recycling of unabsorbed photons. However, the structure of TPVs is more complex, and losses at each energy conversion step can result in a lower efficiency than that of photovoltaics. Further developments must be made to the absorber/emitter and PV cell in order to realize its full potential as a renewable energy source. Unlike PVs, however, when TPVs are used with a burner source, they provide on-demand energy. As a result, no form of energy storage is needed. In addition TPVs, due to the PV’s proximity to the radiative source, can generate current densities 300 times that of conventional PVs.Man-Portable Power
With the increased usage of electronics on the battlefield, there is a need to provide portable power sources to soldiers. Conventional diesel generators are far too heavy for personal use in the field. Scalability allows TPVs to be smaller and lighter than conventional generators. In addition TPVs have very little emission and are silent, making it feasible for tactical field application. Multifuel operation is another potential future benefit.Early investigations into TPVs in the 1970s proved to be impossible due to PV limitations. However, with the realization of the GaSb photocell, a renewed effort in the 1990s produced greater results. In early 2001, JX Crystals delivered a TPV based battery charger to the Army that produced an output of 230 W by burning propane. This prototype utilized 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 by the ratio of the power generated to the thermal energy of the fuel burned. This is too low for practical use on the battlefield. In order to increase efficiency, narrow band emitters would need to be realized and the temperature of the burner would need to be raised. In order to accommodate this, further thermal management steps, such as water cooling or coolant boiling, must be implemented.
Although many successful proof-of-concept prototypes have been demonstrated, no TPV portable power sources have been developed for troop testing or battlefield implementation. Further advances in the ruggedness of components and thermal management must be made in order to renew interest in TPV man-portable power sources for use on the battlefield.
Spacecraft
For space travel power generation systems are needed that provide consistent and reliable power without requiring storage of large amounts of fuel. As a result, solar and radioisotope fuels (extremely high power density and long lifetime) are ideal sources of energy. TPVs have been proposed as sources for conversion 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, because weight is an important consideration and due to inefficiencies associated with the somewhat more complicated design of TPVs, conventional PVs will almost surely be more effective for these applications. However, if the efficiency of individual components can be improved to the point that TPVs can offer substantially higher conversion efficiencies than PVs due to photon recycling then they might become useful for solar conversion in space.Probably more interesting is the prospect of using TPVs for conversion of radioisotope energy. The output of isotopes is already thermal energy, so in this sense TPVs are optimal. In the past thermoelectric (TEs) (also direct thermal to electrical conversion with no moving parts) have been used over TPVs because of the extremely low demonstrated efficiencies when compared to the ~10% of TEs. Stirling engines have also been considered, but are undesirable due to reliability concerns, which are unacceptable for space missions, despite improved conversion efficiencies (>20% demonstrated). However, more recently with large advances in small bandgap PVs critical for effective operation, TPVs are becoming more promising candidates. Recently, a TPV radioisotope converter with ~20% efficiency was demonstrated that used 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.
However space offers one unique challenge for TPV systems. Low temperature operation of the converter is critical to the efficiency of TPV. For PV converters raised temperature increases the dark current substantially, reducing overall efficiency. For all TPV systems, the converter will be heated by the radiation from the emitter. In terrestrial systems it is reasonable to dissipate this heat without using additional energy by heat sinking the converter. However, space is an isolated system, and such heat sinks are not practical. As a result it is critical to develop innovative solutions to efficiently remove that heat, or optimized TPV cells that can operate efficiently with higher temperature converters. Both represent substantial challenges. Despite this, TPVs offer substantial promise for use in future space travel.
Off-grid Generators
Many homes in North America as well as developing countries are located in remote regions not connected to the power grid. Where available, power line extensions can be extremely expensive and impractical. TPVs can provide a continuous supply of power in off-grid homes. Traditional PVs on the other hand, would not provide sufficient power during the winter months and nighttime when solar energy is at a minimum, while TPVs can utilize alternative fuels to augment solar only productionThe greatest advantage for TPV generators is cogeneration of heat and power. In cold climates, it can function as both a heater or stove and a power generator. JX Crystals has developed a prototype TPV heating stove and generator. It burns natural gas and uses a SiC source emitter operating at 1250°C and GaSb photocell to output 25,000 BTU/hr and generate 100 W at the same time. However, costs must be significantly reduced in order to render it commercially viable.
When a furnace is used as a heater and a generator, it is called Combined Heat and Power (CHP). Many TPV CHP scenarios have been theorized but in a recent cost estimate, a generator using boiling coolant was determined to be 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 and generate 1.5 kW. The estimated efficiency would be 12.3% and the investment would be 8 EURcents/kWh provided that the lifetime of the CHP furnace is 20 years. The estimated cost of other non-TPV CHPs are 12 EURcents/kWh for gas engine CHP and the 16 EURcents/kWh for fuel cell CHP. This proposed furnace has not been developed because there is comparatively a very small market for off-grid power generation and no funding is available to develop the boiling liquid cooled GaSb PV array.
Recreational vehicles
TPVs have been proposed for use in recreational vehicles. With the advent of hybrid and other electrically powered vehicles, power generators with electrical outputs have become more interesting. In particular the versatility of TPVs for fuel choice and the ability to use multiple fuel sources makes them interesting as a wider variety of fuels are being with better sustainability are being investigated today. Furthermore, the silent operation of TPVs would both allow the generation of electricity when the use of noisy conventional generators is not allowed, and not disturb others when the use of generators is permitted. However the emitter temperatures required for practical efficiencies make TPVs on this scale extremely unlikely.External links
- Thermophotovoltaics at Western Washington University.
- Thermophotovoltaics at Massachusetts Institute of Technology.
- Thermophotovoltaics at the Ioffe Institute.
- 6th International Conference on Thermophotovoltaic Generation of Electricity
- NASA Radioisotope Power Conversion Technology NRA Overview
- New thermophotovoltaic materials could replace alternators in cars and save fuel