Brief Introduction to Thermophotovoltaics
A thermophotovoltaic (TPV) system converts heat to electricity using light as an intermediary and consists of (at least) three components: a heat source, an emitter, and a photovoltaic (PV) cell with a low bandgap. The heat source brings the emitter to high temperature (), causing the emitter to emit thermal radiation, which is absorbed and converted to electricity by the PV cell. Some advantages of this energy conversion scheme include the static and quiet conversion process, the physically separated paths of heat conduction and electricity generation, and the lack of fundamental temperature gradients across materials.1 In addition, several heat sources can be used, of which there are three major kinds: radioisotope decay, chemical fuel, and sunlight that is concentrated and absorbed (Fig. 1). The radiated power density from the TPV emitter is fundamentally limited only by Planck’s law for blackbody emission.2 However, high-performance TPV systems are particularly challenging to realize in part because of the need to coordinate multiple subsystems and the difficulties in designing a good emitter.
The scope of our review is the TPV emitter, which we consider the critical component toward high system performance, and specifically on its practical implementation in TPV systems. The emitter is any material that is heated up to a high temperature. A particularly useful emitter is a selective emitter, which preferentially emits in a specific wavelength region. There is no one way to make an emitter; there are many types of emitters that can each involve a different geometric configuration and a separate set of materials, some of which we touch upon in Sec. 2.
In our review, we propose five metrics to evaluate the practicality of TPV emitters, and in particular, we examine emitters that have been used in system demonstrations of TPV prototypes. Most work on TPV emitters has focused on achieving good optical performance, but there has been little consideration of the challenges associated with implementing the emitter in and operating a TPV system.
Our review is organized as follows: in Sec. 2, we classify TPV emitters from the literature into three categories. In Sec. 3, we present and discuss the five metrics for practical TPV emitters as well as their submetrics. In Sec. 4, we provide an at-a-glance evaluation, based on the five metrics, of the five types of emitters used in prototype system demonstrations. The evaluations are summarized in two tables, with one more detailed table in the Appendix (Sec. 6).
Classification of TPV Emitters
Before evaluating the practicality of TPV emitters, we first classify emitters in the literature into the following categories: (i) used in system demonstrations of TPV prototypes, (ii) fabricated and optical performance measured, and (iii) optical performance simulated. These categories are shown in Fig. 2. In this figure, we only include emitters with emission in the range of 1 to , which corresponds approximately to the peak emission wavelengths of typical emitters at temperatures of 1000 to 1500 K. It is not within the scope of this review to discuss the mechanisms behind different emitters, nor have we provided a complete list of all emitters that have been proposed and fabricated. Detailed discussion of emitter types, mechanisms, and more examples of emitters can be found in reviews elsewhere, such as that by Pfiester and Vandervelde.3
Our focus for this review is on the emitters in the first category that have been used for system demonstrations: in Sec. 4, we evaluate the practical aspects of these emitters, following the discussion of our metrics.
Metrics for Practical TPV Systems
Although the primary purpose to develop an emitter is for its optical performance, the emitter with the best optical performance is not necessarily the best emitter for practical implementation.
For this reason, we present five practical metrics, as shown in Fig. 3: (1) optical performance, (2) ability to scale to large areas, (3) long-term high-temperature stability, (4) ease of integration within the TPV system, and (5) cost.
In the following subsections, we discuss each metric, including some approaches that researchers have taken to address key challenges.
An emitter with good optical performance has, at all angles, preferential emission of in-band photons and suppression of out-of-band photons. Optical performance refers to the emission of photons as a function of both angle and photon energy, in particular, in two regimes for the latter, in-band photons that have energy higher than the PV cell bandgap and out-of-band photons that have energy lower. Spectral control refers to the methods that enable preferential in-band emission. Some designs of spectral control are designed for broadband emission while others for narrow-band emission [the ideal cases which are shown in Fig. 4(a)]. In the latter case, the emitted photons have energies slightly above the PV cell bandgap. Typically, broadband emitters yield higher output electrical power density while narrow-band emitters can increase the TPV conversion efficiency.91
The purpose of angular control, which is often an implicit aspect of spectral control, is to ensure spectral control over all angles (polar and azimuthal, and ), because an emitter radiates photons over a wide range of angles [see Fig. 4(b)]. This is especially important because most thermal radiation is off-normal as according to Lambert’s law.
For TPV, the wavelength regions of interest are around 1 to , approximately the regions of peak emission. For an emitter heated to realistic temperatures of 1000 to 1500 K, the peak emission wavelengths are 1.9 to , as according to Wien’s displacement law. As such, one of the main requirements of TPV is to have low-bandgap PV cells, with typical bandgaps in the range of about 0.50 to 0.74 eV or 1.7 to .
Although a PV cell can generate electricity only from in-band photons, a real emitter emits both in-band and out-of-band photons at any given angle. This leads to the following problems: (a) if out-of-band photons reach the PV cell, they overheat the PV cell and reduce the PV cell efficiency and (b) when out-of-band photons are emitted and not recovered, this leads to both reduced heat-to-radiation efficiency and emitter temperature.
While we have initially defined good optical performance as that achieved by designing selective emitters, there are actually two main approaches of spectral control. The first is to enhance in-band and suppress out-of-band emission via selective emitters. The second is to reflect out-of-band photons back to the emitter, or photon recycling, via cold side filters or reflectors (CSFR) in front of (front side filter) or behind the PV cell (back surface reflector). (These are shown in Fig. 1.) It is also possible to combine both approaches, for example, to have a selective emitter and a filter or reflector, or even all three in theory.
Although an emitter with a CSFR performs better than a blackbody or graybody (relatively higher temperature and mitigated PV cell efficiency reduction), it suffers from view factor and absorption losses. In view factor loss, which is inherent to systems with diffuse emitters, photons are lost in the finite gap between the emitter and the filter/reflector [Fig. 4(c)], and in absorption loss, photons are absorbed at any interface (at the filter, reflector, or PV cell). Although it is possible to reduce the view factor loss by reducing the emitter area relative to the PV cell area, keeping an emitter arbitrarily small decreases its absolute radiated power.
On the other hand, selective emitters suppress out-of-band emission relative to in-band emission. This reduces view factor and absorption losses for out-of-band photons, although both losses, especially view factor losses, remain significant for in-band photons.
For the approach of selective emitters, the objective is to find or engineer a TPV emitter that emits mostly in-band photons and little to no out-of-band photons. While this is not within the scope of this review, there are many design questions regarding specific emission characteristics:
• Is it better to prioritize high in-band emission, even if the out-of-band emission is moderately high, or to prioritize low out-of-band emission, even if the in-band emission is only moderately high?
• Is it better to prioritize broadband or narrow-band emission? Narrow-band emitters are intended to prevent thermalization82 in PV cells, in which the excess energy (difference between photon and bandgap energies) is absorbed and lost. However, this comes at the cost of a reduction in the radiated in-band power density. One proposed way to mitigate thermalization, then, is to use PV cells of multiple bandgaps.91
• For engineered emitters, which is better: (a) select a material with naturally high emission, and suppress it in out-of-band wavelength regions or (b) select a material with naturally low emission, and enhance it in the in-band wavelength regions? Generally, suppression of naturally high emission works only for a limited wavelength range;27,92 ideally, the emission should be suppressed for wavelengths up to about , which accounts for of the energy emitted by a blackbody at 1000 K.
However, an emitter is not better than others simply because it has reached high temperatures, because temperatures beyond 1500 K are hard to achieve, the amount of input power required to heat an emitter may vary widely, and it is unclear if a given emitter can sustain high optical performance at high temperature for prolonged periods. The issue of stability at high temperature is discussed as another metric later on.
One submetric is the in-band radiated power density because the ultimate system goal is to have high-output electrical power, which is enabled by maximizing the in-band power density that is emitted and can be converted.
The radiated in-band power density, , can be calculated from the hemispherical emittance , the cutoff or bandgap wavelength , and the blackbody spectrum :93
The hemispherical emittance is the emittance across all angles, where the emittance is a measure of how close the emission is to that of a blackbody. Ideally, the in-band emittance is close to 1, while the out-of-band emittance is close to 0. In addition, the hemispherical emittance is temperature dependent, as the optical properties of a material change with temperature:
However, many papers often report only the emittance at a single angle at room temperature, since it is difficult to measure the emittance across all angles as well as at high temperature.
The common metric spectral selectivity or spectral efficiency , which is the fraction of the radiated energy that is convertible by the PV cell, can be calculated using the hemispherical emittance:94,95
It is important to point out that spectral selectivity describes in-band emission relative to the total or out-of-band emission and is distinct from the absolute values of in-band and out-of-band emittance. In other words, it is possible to have a highly selective emitter with low absolute in-band emittance or an emitter that has high in-band emittance but low selectivity (such as a graybody emitter).
Finally, we are also interested in the efficiency of the radiated in-band power to the input power, , which is calculated from the radiated in-band power density , the area of the emitter , and the input power :
One qualitative metric, which we do not use in our evaluation, is that the design of the emitter itself should be robust to fabrication imperfections across a large area, such as the lack of uniformity in critical feature dimensions. For example, a very thin film with 30-nm thickness is less robust to effects of surface roughness compared to a much thicker film.
Scalability to Large Areas
Because the fundamental limit for emitters is on the power radiated per unit area, one way to increase the absolute radiated power is by increasing the emitter area (its macroscopic exterior dimensions).
In terms of practical implementation, it is important to consider the following: (1) the substrates must be available in large sizes and (2) the fabrication methods should accommodate large-area samples relatively easily. For example, for 1, the single-crystalline substrates of tungsten and tantalum are typically available in small diameters 1 to 1.5 cm (area to ),33,57,58 while naturally selective emitters made of rare earth metals can be on the order of tens of .2324.–25 An example for 2 is that electron beam lithography, which is typically used for features , is both costly and time-consuming. The overall complexity of the fabrication process, including the number of steps and the complexity of each individual step, can impact the scalability as well as the cost. However, many of the fabrication techniques used in papers may be those best suited for proof-of-concept demonstrations, rather than mass production.
Long-Term High-Temperature Stability
The TPV emitter must sustain its optical performance at high temperatures for extended periods of time, either continuously or over multiple thermal cycles.
However, at high temperatures, the kinetic energy of atoms increases and atoms diffuse more easily, leading to a number of potential thermodynamic effects:1
• A phase change may occur (e.g., the emitter might melt),101 accompanied also by changes in morphology and optical properties.51 This can happen also for crystalline phases.45 However, it is important to keep in mind that the melting point of a material at nanometer scale is lower than for bulk.101
• The sizes of grains can grow in polycrystalline materials.51,58,9697.98.99.–100101.102.–103 However, this can actually stabilize the material, so some substrates such as polycrystalline tantalum are annealed prior to use.58 It is also possible to use large-grain or single-crystal substrates.51,57,102,104,105
• Chemical degradation may occur, such as the formation of tungsten oxides96,97,99,100 and tantalum carbide.102,105 This can necessitate that the emitter operate in inert atmosphere or vacuum,97,99,100 which requires special packaging and complicates the TPV system integration. Chemical degradation of 2-D and 3-D tungsten and 2-D tantalum photonic crystals can be mitigated by capping the surface with a 20 to 40 nm protective coating of hafnium dioxide ().1,67,68,102103.–104 One comparison of and in 3-D photonic crystals67 has found to be more thermally robust than , but is less expensive and has been used to protect a metasurface emitter.73
• Thermal expansion could lead to the cracking of a material.67,68,95,103 Also, emitters with interfaces between different materials are at risk of delamination because different materials have different thermal expansion coefficients.
Some strategies for improving the high-temperature stability include selecting materials that are known to have good high temperature properties, alloying to promote a solute drag effect,97,99,100 and modifying the geometry of a structure to change diffusion rates.100,106
There do not appear to be any published long-term () studies; in some cases, it appears the emitter is only heated to measure its high-temperature optical properties. One long study is 168 h (7 days) at 1000°C (1273 K) for a 2-D structure made with tungsten and carbon nanotubes.61 We have included some studies of emitter stability at high temperatures in Table 1.
A few (this list is not exhaustive) studies of high-temperature stability of TPV emitters.
|Emitter||Used in system demo.?||Length||Temperature (K)||Environment||Result|
|-coated W 3-D inverse opal photonic crystal67||—||12 h||1273||Forming gas, 5% in Ar||Little to no degradation|
|Same as above||—||12 h||1673||Forming gas, 5% in Ar||Collapse of structure|
|-coated W 3-D inverse opal photonic crystal67||—||12 h||1273||Forming gas, 5% in Ar||Little to no degradation|
|Same as above||—||12 h||1673||Forming gas, 5% in Ar||Some grain growth, increased surface roughness|
|W-coated 3-D inverse opal photonic crystal68||—||12 h||1273||—||Little to no degradation|
|Same as above||—||—||—||Collapse of structure|
|W inverse 3-D inverse opal photonic crystal68||—||12 h||1273||—||Little to no degradation|
|-coated W inverse 3-D inverse opal photonic crystal68||—||1 h||1673||—||Little to no degradation|
|2-D structure made with W, carbon nanotubes61||—||168 h, 7 days||1273||with He protection||Little to no degradation|
|Single crystal Er doped yttrium aluminum garnet21||Yes||300 h||In sun||In sun||Emitter cracked and darkened, but may be due to water leak|
|2-D photonic crystal made of Ta3%W alloy coating1||Yes31,32||300 h||1273||Vacuum ()||Little to no degradation|
|Same as above104||Yes31,32||1 h||1473||Vacuum ()||Little to no degradation|
|foam ceramic25||Yes||200 cycles||—||—||Little to no degradation|
|2-D photonic crystal made of polycrystalline Ta coating102||Yes29,30||144 h, 6 days||1173||Ar,||Little to no degradation|
|Same as above||Yes29,30||1 h||1273||Ar,||Little to no degradation|
|A multilayer stack made of W and 107||Yes34||1473||Vacuum,||Degradation|
|Same as above||Yes34||Two rapid thermal cycles||1250 max||—||Little to no degradation|
The longest reported studies we know of are 300 h each for an erbium-doped yttrium aluminum garnet (Er-YAG) crystal used in an solar TPV system21 and a 2-D photonic crystal made of tantalum-tungsten alloy and capped with 20 to 40 nm 1 used in a radioisotope TPV prototype.31 The former cracked and darkened after 300 h in the sun, although the authors attribute it potentially to a water leak. The 2-D photonic crystal showed little to no degradation in optical performance after annealing for 300 h at 1000°C (1273 K)1 and also 1 h at 1200°C (1473 K).104
The only other emitters we know of that were used in both system demonstrations and some high-temperature stability experiments include a foam ceramic,25 a 2-D photonic crystal made of polycrystalline tantalum and coated with 20 to 40 nm ,102 and a multilayer stack made of tungsten and .34 The foam ceramic was robust under 200 thermal cycles, the 2-D tantalum photonic crystal showed no visible degradation after 144 h at 900°C (1173 K) and 1 h at 1000°C (1273 K),102 and the multilayer stack showed little to no degradation after at least 1 h at 1423 K in vacuum and two rapid thermal cycles up to 1250 K, but showed degradation after 1 h at 1473 K.
Ease of Integration within the TPV System
The design choices for the emitter can present several challenges for its integration within the TPV system, in particular, when (1) putting the emitter and heat source physically together for thermal contact and (2) packaging the system for operation in vacuum or inert gas environment.
In some cases, the emitter and heat source are made out of the same material, such as silicon carbide12 or platinum,12 or the emitter is directly fabricated onto the heat source, for example, through the deposition of emitter materials, such as silicon and silicon dioxide6,27 or tantalum59 onto a microcombustor, or the fabrication of a combined absorber/emitter for solar TPV.16,33,34
In other cases, it may be required to cut the emitter to the correct size and to machine and weld it onto the heat source, such as a microcombustor. It is possible to use foil, sputtered coating, or a solid-state substrate. In the last case especially, the mechanical properties of the emitter material become significant. As an example, three different substrates have been explored in the development of 2-D photonic crystal emitters. These include single crystalline tungsten, polycrystalline tantalum, and tantalum-tungsten alloy.57,58,104 Tungsten is brittle and difficult to machine and weld,58 while polycrystalline tantalum is more compliant and easier to weld and machine but is soft so needs to be thicker than tungsten to achieve the same mechanical stability. Tantalum-tungsten alloy combines the better thermomechanical properties of tungsten with tantalum’s ability to be more easily machined and welded.1,104
In addition, high temperature stability concerns also apply: operating the heat source and emitter and high temperatures can lead to cracking and delamination of the emitter or heat source.
Also, the emitter often must be in vacuum or inert gas environment in order to prevent chemical degradation processes, such as oxidation and heat losses, due to convective heat transfer processes.
In our comparison of system demonstrations of prototype TPV, we have noted where the system was operated under vacuum or inert atmosphere, and whether the emitter was fabricated onto or together with the heat source.
The overall cost of emitter production depends on the cost of the raw materials, fabrication, as well as system integration. In particular, many emitters make use of relatively scarce materials, such as hafnium and rare earth metals. The cost of fabrication may increase with increased number of processing steps or complexity of fabrication processes.
For our evaluation, because it is generally difficult to project the cost, considering the emitter area can be scaled and improvements in fabrication technology may reduce costs, we have not done any assessment.
At-a-Glance Evaluation of Emitters Used in Prototype System Demonstrations
In Table 2, we summarize reported features of the emitters that have been implemented in prototype system demonstrations. In this table, we show the cutoff wavelength (which corresponds to the bandgap of the PV cell used in the system demonstration), the average in-band emittance , the average out-of-band emittance , the emittance measurement angle , the emitter temperature , whether we have found high temperature studies, whether the system is operated in vacuum or an inert gas environment, and whether the emitter is fabricated onto or with the heat source.
At-a-glance summary of prototype TPV system demonstrations.
|Emitter||Cutoff (μm)||εin||εout||θ||Temitter (K)||Area (cm2)||High temperature studies?||Vacuum/inert?||Emitter fab. w/heat src?|
|SiC, with filter7||1.8||—||—||—||—||101.4||—||—||—|
|SiC, with filter8||1.7||0.5||0.5||45 deg||1053||—||—||—||—|
|Commercial SiC burner9,10||1.8||—||—||—||1323||—||—||—||Yes|
|SiC, with filter11||2.07||—||—||—||1312||29.16||—||—||—|
|Combined emitter-combustor: SiC12||1.8||—||—||—||1120||—||—||—||Yes|
|ARC on W on SiC13||1.8||0.86||0.27||—||1548||469||—||—||—|
|ARC-coated Pt foil14||1.8||—||—||—||1287||>38||—||Yes||—|
|-coated W foil15||1.88||—||—||—||1300–1500||2.83||—||Yes||—|
|Absorber/emitter: -coated, textured W16||1.8||0.611||0.052||—||1456||6.25||—||Yes||Yes|
|Combined emitter-combustor: Pt12||1.8||0.099||0.044||—||1450||—||—||—||Yes|
|Burner made of unspecified “super alloy”19||1.73||—||—||—||1458||1282||—||—||Yes|
|Absorber/Emitter: W (Ta?) foil18||1.82||0.69||0.42||—||1720||1.67||—||—||—|
|Same as above, larger area18||1.82||0.74||0.52||—||1400||5.09||—||—||—|
|Commercial gas mantle20,108||1.1||—||—||—||1570–1970||—||—||—||—|
|Absorber/emitter: SiC plate, Emitter: Er-YAG21||1.1, 1.65||—||—||—||1133–1473||—||Yes21||—||—|
|composite, with pinhole22||1.8||0.41||0.25||—||1254||1||—||—||—|
|mantle, with filter23||1.1||—||—||—||—||231.7||—||—||—|
|-coated or SiC foam ceramic25,109||1.1||—||—||—||—||Yes25||—||—|
|1-D PhC of Si, on Si microreactor6||2.27||—||—||—||1073||1||—||Yes||Yes|
|1-D PhC of Si, on Si27||2.25||0.86||0.32||—||1285||1||—||Yes||Yes|
|1-D PhC of Si, , on Si with filter28||2.25||—||—||—||—||4||—||Yes||Yes|
|-coated, 2-D PhC made in polycrystalline Ta29||2.3||0.52||0.29||All||1270||—||Yes102||Yes||Yes|
|-coated, 2-D PhC made in polycrystalline Ta30||2.0||0.58||0.18||All||1327||4.41||Yes30||Yes||No|
|-coated, 2-D PhC made in Ta3%W alloy31,32||2.25||0.75||0.26||—||1233||1||Yes1,104||Yes||No|
|Combined emitter-absorber: W, yttria-stabilized zirconia multilayer stack33||1.85||0.84||0.21||—||1640||1.77||—||Yes||Yes|
|Combined emitter-absorber: Mo, multilayer stack34||1.85||0.71||0.15||All||1640||0.636||Yes107||Yes||Yes|
Section 6 contains tables with more details, including estimations of the in-band radiated power density (power per area) and the absolute in-band radiated power to input power efficiency, . Because spectral selectivity is a commonly reported metric, and also in part because many papers do not report the long-wavelength emittance, we have not included this in our evaluation.
Broadly, there are five types of emitters that have been implemented in prototype system demonstrations (the emitters in Table 2 are organized by these types, in this order):
1. Bulk emitters
a. Graybody emitters, such as silicon and silicon carbide,22.214.171.124.10.11.–12 are typically inexpensive, easy to fabricate in large areas, and often can be fabricated onto or with the heat source. However, they have both high in-band emission and out-of-band emission, which is why they are often coupled with cold side filters.
b. Metals with1314.15.–16 or without antireflection coating (ARC)12,1718.–19 can be easy and inexpensive to fabricate in large areas. The emission depends on the metal optical properties; the role of the ARC layer is typically to enhance emittance in a narrow band around the bandgap.
2. Naturally, selective emitters2021.22.23.24.25.–26,108,109 have been made primarily from rare earth metals, especially erbium and ytterbium. They are easy to fabricate in large areas and with high-temperature stability, especially by doping high-temperature ceramics. However, the emission wavelength range of naturally selective emitters is not tunable and narrow-band, which can lead to low in-band emitted power density.
3. 1-D photonic crystals,6,27,28 also known as dielectric mirrors, consist of alternating layers of materials with a high contrast in indices of refraction. Interference effects in this structure lead to a fairly broad reflection bandwidth, which can be used to suppress high natural emittance of a material for a wavelength region. They are easy and inexpensive to fabricate at large areas, but have multiple interfaces and are not typically made from high-temperature materials though can be directly fabricated onto a heat source. They may have high out-of-band emission outside of the region of suppression.
4. 2-D photonic crystals2930.31.–32 for TPV typically are a 2-D array of features on top of or in a substrate, such as cylindrical posts or air cavities, with feature sizes on the order of the wavelength of interest. For photonic crystals with air cavities, each individual cavity acts as a waveguide to enhance emission of wavelengths below a cutoff (half a wavelength corresponds roughly to the cavity diameter). Whether an emitter can be fabricated inexpensively with large area and can be integrated with the heat source depends largely on the substrate, for which a high-temperature material is often used.
5. The multilayer stacks33,34 (which differ from 1-D photonic crystals in that there is no periodicity in the thicknesses of the layers) featured in this review are combined absorber/emitters for solar TPV that consist of alternating metal and dielectric layers of varying thicknesses. The layer thicknesses, which are sometimes subwavelength, are optimized to enable both broadband absorption and emission. Although the optical performance is good and the fabrication costs likely low, the ability to fabricate large areas depends on the available sizes of the metal. Although high temperature materials are used, there are many interfaces and the long-term high-temperature stability is unclear.
In our review, we have presented five metrics for evaluating the practicality of TPV emitters and have used this approach to evaluate the five broad categories of emitters that have been used in TPV system prototypes. We have categorized emitters in the literature based on whether they have been used in prototype system demonstrations, fabricated, or simulated, and have evaluated the emitters in the first category according to our metrics. The emitters in this category include bulk emitters, naturally selective emitters, 1-D photonic crystals, 2-D photonic crystals, and multilayer stacks. The metrics for a practical TPV emitter include optical performance, scalability to large areas, long-term high temperature stability, ease of integration within a TPV system, and cost.
However, none of the emitters or types of emitters identified have yet satisfied all five criteria for practical TPV emitters. An emitter with good optical performance shows high, broadband, preferential in-band emittance. Some emitters with promising optical performance include the 1-D photonic crystal, 2-D photonic crystal, and multilayer stacks. For the latter two types, there have been some studies on the high-temperature stability and system integration. The multilayer stack is fabricated directly with the absorber (heat source), but its high-temperature stability has not been shown beyond 1 h. On the other hand, a 2-D photonic crystal made in refractory metals that can be fabricated on the order of has some slightly high out-of-band emittance but has been successfully integrated with microcombustor heat sources and has shown promising high-temperature stability of a few hundred hours at 900°C to 1000°C.
Moving forward, studies on the practical aspects of each TPV emitter, such as the five metrics we have presented in this review, are critical for the maturation of TPV technology. We have yet to find an emitter that satisfies the following performance metrics: spectral selectivity exceeding 90% to 95%, high in-band emittance of 0.9 to 0.95, and high-temperature stability on the order of thousands of hours and hundreds of cycles (a typical Li ion battery lasts 2 to 3 years and 110). Both of these as well as large-area fabrication can be addressed independently of the overall TPV system setup. One promising class of emitter is metamaterials, which show high optical performance; however, studies of high-temperature stability are at the moment limited. The ability of TPV systems to be commercialized hinges on the practical attributes of selective emitters, for which our five-metric approach can serve as a framework.
Appendix: Evaluation of Emitters Used in Published System Demonstrations of Prototype TPV
1. Reported the numerical values for power provided in the paper:
2. Calculation is based on radiation spectrum (power density as a function of photon energy):
3. Calculation is based on the measured or simulated spectrum of emittance or absorptance, emitter temperature , and PV cell bandgap or cutoff:
a. Hemispherical emittance spectrum at ,
b. Hemispherical emittance spectrum at a temperature that is unknown or not close to ,
c. Emittance at a single or unknown angle, at ,
d. Absorptance at a single or unknown angle, at a temperature that is not close to ,
e. Emittance at a single or unknown angle, at a temperature that is unknown or not close to .
4. Calculation is based on assumption of graybody emitter behavior with unless otherwise stated in the paper, ignoring any effects of a cold side filter if included.
5. No calculation:
Details on prototype TPV system demonstrations, part I. Bulk emitters: Si, SiC, metals with and without ARC.
|Emitter||Cutoff μm/(eV)||Temitter (K)||Mrad,in (W cm−2)||Prad,in (W)||Pinput (W)||ηri-input||Method||Area||High temp. studies||Vacuum/inert?||Emitter fab. w/heat src?|
|SiC, with filter7||1.8||—||—||—||—||—||5b||1.5 in., 3.5 in.||—||—||—|
|Double-chambered SiC cylinder, with filter8||1.7||1053||—||1.62; 0.246 W transmit. through filter||497||0.0326||1a||—||—||—||Yes|
|Commercial SiC radiant burner9,10||1.8||1323||2.28||—||—||—||4||—||—||—||Yes|
|SiC, with filter11||2.07 (0.6)||1312||3.37||98.3||—||—||4||—||—||—|
|Combined emitter-combustor: SiC12||1.8||1120||0.593||—||—||—||4||—||—||—||Yes|
|ARC on W on SiC13||1.8||1548||5.2||2439||—||—||1a||, cylindrical||—||—||—|
|ARC-coated Pt foil14||1.8||1287||—||69||497||0.0994||1a||—||Designed for inert gas||—|
|Same as above, different test condition||1.8||1287||—||142||952||0.149||1a||—||Designed for inert gas||—|
|-coated W foil15||1.88||1300–1500||—||—||—||—||5a||D 12 mm, L 25 mm||—||Ar flow||—|
|Absorber/emitter: -coated, laser-textured W16||1.8 (0.67)||1456||2.74||17.1||68.8||0.249||3e||Absorber: , Emitter:||—||—|
|W-coated SiSiC cylinder17||1.8||1463||—||—||1800||—||5a||55 mm;||—||Yes|
|Combined emitter-combustor: Pt12||1.8||1450||0.362||—||—||—||3e||—||—||—||Yes|
|Burner made of unspecified “super alloy”19||1.73 (0.72)||1458||—||5.86||11.77 kW||0.250||1b||8.0 cm, 25.5 cm; effective emitter area:||—||—||Yes|
|Absorber/Emitter: W (Ta?) foil cylinder18||1.82||1720||8.90||0.60||64.8; ()||0.0093||3d||Concentrator: , Emitter: , 15 mm||—||—||—|
|Same as above, but longer cylinder18||1.82||1400||2.31||0.47||64.8; ()||0.0072||3d||Concentrator: , Emitter: 12 mm, 45 mm||—||—||—|
Details on prototype TPV system demonstrations, part II. Naturally selective emitters based on rare earth materials, 1-D photonic crystals, 2-D photonic crystals.
|Emitter||Cutoff μm/(eV)||Temitter (K)||Mrad,in (W cm−2)||Prad,in (W)||Pinput (W)||ηri-input||Method||Area||High temperature studies||Vacuum/inert?||Emitter fab. w/heat src?|
|Gas mantle in a commercial campinglantern20,108||1.1 (1.1)||1570–1970||—||—||—||—||5a||—||—||—||—|
|Absorber/emitter: SiC plate, Emitter: single crystal Er doped yttrium aluminum garnet21||1.1,1.65||1133–1473||—||—||—||5a||—||Post 300 h in sun, emitter cracked and darkened, but may be due to water leak21||—||—|
|composite, w/polished Mo pinhole outsideemitter22||1.8||1254||0.634||0.63||100 to 450||0.0014 to 0.0063||3e||—||—||—|
|mantle, w/filter23||1.1||—||—||—||11,723||—||5b||1.5 in., 8 in.||—||—||—|
|mantle24,25||1.1 (1.1)||1735||1.32||71.1||2000||0.0356||2b||Actual , Effective||—||—||—|
|-coated or SiC foam ceramic25,109||1.1 (1.1)||1.27||—||—||—||2b||—||200 thermalcycles25||—||—|
|1-D photonic crystal (PhC), 5 alternating layers of Si, directly deposited on Si microreactor6||2.27 (0.457)||0.584||0.584||13.7||0.0426||2a||—||Yes||Yes|
|Absorber: vertically aligned multiwall carbon nanotubes, emitter: 1-D PhC of Si, on Si27||2.25 (0.55)||1285||3.44||3.44||0.215||3c||Absorber , emitter:||—||Yes|
|As above, absorber: carbon nanotubes, emitter: 1-D PhC; w/cold side rugate filter28||2.25 (0.55)||—||—||—||—||—||5b||Emitter: , 2 ratios : 12 and 7, filter:||—||Yes|
|Absorber & emitter each: 2-D photonic crystal made of polycrystalline Taw/ coating29||2.3 (0.54)||1270||1.37||—||(130 suns), if||0.106||3a||—||1 h at 1000°C, 144 h at 900°C102||—|
|2-D photonic crystal made of polycrystalline Taw/ coating30||2.0||1327||4.08||18||100||0.18||1a and 3b||at 900°C, no degradation in structure or opticalperf.30||Yes||No|
|2-D photonic crystal made of Ta3%W alloy w/ coating31,32||2.25 (0.55)||1233||2.01||2.01||53||0.0379||1b and 3-d||300 h at 1000°C1, 1 h at 1200°C104||No|
Details on prototype TPV system demonstrations, part III. Multilayer stacks. D=diameter, L=length.
|Emitter||Cutoff μm/(eV)||Temitter (K)||Mrad,in (W cm−2)||Prad,in (W)||Pinput (W)||ηri-input||Method||Area||High temperature studies||Vacuum/inert?||Emitter fab. w/heat src?|
|Combined emitter absorber: multilayer stack w/subwavelength layers of W, yttria-stabilized zirconia33||1.85 (0.67)||1640||9.79||17.3||79||0.219||3e||Absorber : 15 mm, emitter : 15 mm||—||Yes|
|Combined emitter absorber: multilayer stack w/subwavelength layers of Mo, 34||1.85 (0.67)||1640||6.62||4.21||0.260||16.2||3b||Absorber D: 6 mm, Emitter D: 9 mm||No degradation post 1 h at 1423 K, , 2 rapid thermal cycles 500–1250 K, but degradation post 1 h at 1473 K107||Yes|
This work was supported by the U.S. Army Research Office through the Institute for Soldier Nanotechnologies under Contract No. W911NF-13-D-0001, the Micro Autonomous Systems and Technology Collaborative Technology Alliance under Contract No. 892730, the Solid-State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No. DE-SC0001299/DE-FG02-09ER46577. R.S. would like to acknowledge Samantha Dale Strasser of the MIT EECS Communication Lab for help with the manuscript.