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Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat differentials to electricity via photons. 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 carriers, 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. ¹ Much research and development in TPVs therefore concerns methods for controlling the emitter's properties.

Many attribute the idea of this system to the French scientist Pierre Aigrain (1956).

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" ² was the World’s first thermophotovoltaic (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 thermocouples to provide a 3 to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators.

Background

Schematic of TPV systems.
Left: Here a blackbody emitter is heated(either by solar concentration or combustion) causing it emit radiation. The radiation passes through a filter that allows only optimal wavelengths for conversion and reflects others back to the emitter where they can absorbed again generating additional heat and photons. That light passed through the filter is absorbed by the PV converter to generate electricity. Any unabsorbed light passes to the reflector where it is sent back to the PV to be converter or to the emitter to be reabsorbed. In practical systems reflector geometries must be used to surround the material to prevent emitted light from escaping to the surroundings.
Right: A similar system, where the blackbody emitter and narrow band filter are replaced with a selective emitter.

Top: Normally, in PVs, light emitted with energy below the band gap energy cannot be absorbed and is transmitted. Light with energy greater than the bandgap energy is absorbed, but that excess energy is lost to heat.
Bottom: In the case of narrow band emission, the emission spectra can be tailored for optimal absorption.

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, ΔG = E_photon − E_g , 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 T_cell is the temperature of the PV converter. For the best reasonable values in a practical system, T_cell~300K and T_emit~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 temperature).

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 T_emit 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, T_emit 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.

Active Components and Materials Selection

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 PV³, or mirrors deposited on the back side of the PV⁴ 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 region⁵. 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 crystals⁶. However, manufacturing of these devices is difficult and not currently commercially feasible.

Photovoltaic Cells

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 1989⁷, 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 GaSb⁷, setting the world record for solar cell 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 spectrum⁸. 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°C⁹. 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%¹⁰. 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 V_oc/E_g ratio is far from the ideal¹⁰. 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%¹¹. 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 epitaxy⁹. This device also had a band gap of 0.55eV achieved by a compound of In_0.68Ga_0.33As. 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 3μm¹⁰. 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.

Government Applications

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 tall¹². 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 TEs¹³. 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)¹⁴. 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.¹⁴

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 systesm, 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.¹³

Commercial Applications

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 production

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 estimate¹⁵, 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. However the emitter temperatures required for practical efficiencies make TPVs on this scale extremely unlikely¹⁶.

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

References