Thermoelectric materials

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Thermoelectric materials show the thermoelectric effect in a strong or convenient form.

 

The thermoelectric effect refers to phenomena by which either a temperature difference creates an electric potential or an electric potential creates a temperature difference. These phenomena are known more specifically as the Seebeck effect (converting temperature to current), Peltier effect (converting current to temperature), and Thomson effect (conductor heating/cooling). While all materials have a nonzero thermoelectric effect, in most materials it is too small to be useful. However, low-cost materials that have a sufficiently strong thermoelectric effect (and other required properties) could be used in applications including power generation and refrigeration. A commonly used thermoelectric material in such applications is bismuth telluride (Bi
₂Te
₃).

Thermoelectric materials are used in thermoelectric systems for cooling or heating in niche applications, and are being studied as a way to regenerate electricity from waste heat.

Materials selection criteria

The usefulness of a material in thermoelectric systems is determined by the two factors device efficiency and power factor. These are determined by the material's electrical conductivity, thermal conductivity, Seebeck coefficient and behavior under changing temperatures.

Device efficiency

The efficiency of a thermoelectric device for electricity generation is given by ${\displaystyle \eta }$, defined as

${\displaystyle \eta ={{\text{energy provided to the load}} \over {\text{heat energy absorbed at hot junction}}}.}$

The ability of a given material to efficiently produce thermoelectric power is related to its dimensionless figure of merit given by

${\displaystyle ZT={\sigma S^{2}T \over \kappa }}$,

which depends on the Seebeck coefficient S, thermal conductivity κ, electrical conductivity σ, and temperature T. 

In an actual thermoelectric device, two materials are used. The maximum efficiency ${\displaystyle \eta _{\mathrm {max} }}$ is then given by

${\displaystyle \eta _{\mathrm {max} }={T_{H}-T_{C} \over T_{H}}{{\sqrt {1+Z{\bar {T}}}}-1 \over {\sqrt {1+Z{\bar {T}}}}+{T_{C} \over T_{H}}},}$

where ${\displaystyle T_{H}}$ is the temperature at the hot junction and ${\displaystyle T_{C}}$ is the temperature at the surface being cooled. ${\displaystyle Z{\bar {T}}}$ is the modified dimensionless figure of merit, which takes into consideration the thermoelectric capacity of both thermoelectric materials being used in the device and, after geometrical optimization regarding the legs sections, is defined as

${\displaystyle Z{\bar {T}}={(S_{p}-S_{n})^{2}{\bar {T}} \over [(\rho _{n}\kappa _{n})^{1/2}+(\rho _{p}\kappa _{p})^{1/2}]^{2}}}$

where ${\displaystyle \rho }$ is the electrical resistivity, ${\displaystyle {\bar {T}}}$ is the average temperature between the hot and cold surfaces and the subscripts n and p denote properties related to the n- and p-type semiconducting thermoelectric materials, respectively. Since thermoelectric devices are heat engines, their efficiency is limited by the Carnot efficiency, hence the ${\displaystyle T_{H}}$ and ${\displaystyle T_{C}}$ terms in ${\displaystyle \eta _{\mathrm {max} }}$. Regardless, the coefficient of performance of current commercial thermoelectric refrigerators ranges from 0.3 to 0.6, one-sixth the value of traditional vapor-compression refrigerators.

Power factor

To determine the usefulness of a material in a thermoelectric generator or a thermoelectric cooler the power factor is calculated by its Seebeck coefficient and its electrical conductivity under a given temperature difference:

${\displaystyle \mathrm {Power~factor} =\sigma S^{2}.}$

where S is the Seebeck coefficient, and σ is the electrical conductivity. 

Materials with a high power factor are able to 'generate' more energy (move more heat or extract more energy from that temperature difference) in a space-constrained application, but are not necessarily more efficient in generating this energy.

Aspects of materials choice

For good efficiency, materials with high electrical conductivity, low thermal conductivity and high Seebeck coefficient are needed.

State density: metals vs semiconductors

The band structure of semiconductors offers better thermoelectric effects than the band structure of metals. 

The Fermi energy is below the conduction band causing the state density to be asymmetric around the Fermi energy. Therefore, the average electron energy of the conduction band is higher than the Fermi energy, making the system conducive for charge motion into a lower energy state. By contrast, the Fermi energy lies in the conduction band in metals. This makes the state density symmetric about the Fermi energy so that the average conduction electron energy is close to the Fermi energy, reducing the forces pushing for charge transport. Therefore, semiconductors are ideal thermoelectric materials. Due to the small Seebeck coefficient metals have a very limited performance and the main materials of interest are Semiconductors.

Conductivity

In the efficiency equations above, thermal conductivity and electrical conductivity compete.

The thermal conductivity κ has mainly two components:

κ = κ _electron + κ _phonon

According to the Wiedemann–Franz law, the higher the electrical conductivity, the higher κ _electron becomes. Thus in metals the ratio of thermal to electrical conductivity is about fixed, as the electron part dominates. In semiconductors, the phonon part is important and can not be neglected. It reduces the efficiency. For good efficiency a low ratio of κ _phonon / κ _electron is desired. 

Therefore, it is necessary to minimize κ _phonon and keep the electrical conductivity high. Thus semiconductors should be highly doped. 

G. A. Slack proposed that in order to optimize the figure of merit, phonons, which are responsible for thermal conductivity must experience the material as a glass (experiencing a high degree of phonon scattering—lowering thermal conductivity) while electrons must experience it as a crystal (experiencing very little scattering—maintaining electrical conductivity). The figure of merit can be improved through the independent adjustment of these properties.

Quality Factor (detailed theory on semiconductors)

The maximum ${\displaystyle Z{\bar {T}}}$ of a material is given by the material's Quality Factor: 

${\displaystyle B={\frac {2k_{B}^{2}\hbar }{3\pi }}{\frac {N_{v}C_{l}}{m_{l}^{*}\Xi ^{2}\kappa _{L}}}T}$

where ${\displaystyle k_{B}}$ is the Boltzmann constant, ${\displaystyle \hbar }$ is the reduced Planck constant, ${\displaystyle N_{v}}$ is the number of degenerated valleys for the band, ${\displaystyle C_{l}}$ is the average longitudinal elastic moduli, ${\displaystyle m_{l}^{*}}$ is the inertial effective mass, ${\displaystyle \Xi }$ is the deformation potential coefficient, ${\displaystyle \kappa _{L}}$ is the lattice thermal conduction, and ${\displaystyle T}$ is temperature. The figure of merit, ${\displaystyle Z{\bar {T}}}$, depends on doping concentration and temperature of the material of interest. The material Quality Factor: ${\displaystyle B}$ is useful because it allows for an intrinsic comparisons of possible efficiency between different materials. This relation shows that improving the electronic component ${\displaystyle {\frac {N_{v}}{m_{l}^{*}\Xi ^{2}}}}$, which primarily affects the Seebeck coefficient, will increase the quality factor of a material. A large density of states can be created due to a large number of conducting bands (${\displaystyle N_{v}}$) or by flat bands giving a high band effective mass (${\displaystyle m_{b}^{*}}$). For isotropic materials ${\displaystyle m_{b}^{*}=m_{l}^{*}}$. Therefore, it is desirable for thermoelectric materials to have high valley degeneracy in a very sharp band structure. Other complex features of the electronic structure are important. These can be partially quantified using an electronic fitness function.

Materials of interest

Strategies to improve thermoelectrics include both advanced bulk materials and the use of low-dimensional systems. Such approaches to reduce lattice thermal conductivity fall under three general material types: (1) Alloys: create point defects, vacancies, or rattling structures (heavy-ion species with large vibrational amplitudes contained within partially filled structural sites) to scatter phonons within the unit cell crystal; (2) Complex crystals: separate the phonon glass from the electron crystal using approaches similar to those for superconductors (the region responsible for electron transport should be an electron crystal of a high-mobility semiconductor, while the phonon glass should ideally house disordered structures and dopants without disrupting the electron crystal, analogous to the charge reservoir in high-T_c superconductors); (3) Multiphase nanocomposites: scatter phonons at the interfaces of nanostructured materials, be they mixed composites or thin film superlattices. 

Materials under consideration for thermoelectric device applications include:

Bismuth chalcogenides and their nanostructures

Materials such as Bi
₂Te
₃ and Bi
₂Se
₃ comprise some of the best performing room temperature thermoelectrics with a temperature-independent figure-of-merit, ZT, between 0.8 and 1.0. Nanostructuring these materials to produce a layered superlattice structure of alternating Bi
₂Te
₃ and Sb
₂Te
₃ layers produces a device within which there is good electrical conductivity but perpendicular to which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type). Note that this high value of ZT has not been independently confirmed due to the complicated demands on the growth of such superlattices and device fabrication; however the material ZT values are consistent with the performance of hot-spot coolers made out of these materials and validated at Intel Labs. 

Bismuth telluride and its solid solutions are good thermoelectric materials at room temperature and therefore suitable for refrigeration applications around 300 K. The Czochralski method has been used to grow single crystalline bismuth telluride compounds. These compounds are usually obtained with directional solidification from melt or powder metallurgy processes. Materials produced with these methods have lower efficiency than single crystalline ones due to the random orientation of crystal grains, but their mechanical properties are superior and the sensitivity to structural defects and impurities is lower due to high optimal carrier concentration.

The required carrier concentration is obtained by choosing a nonstoichiometric composition, which is achieved by introducing excess bismuth or tellurium atoms to primary melt or by dopant impurities. Some possible dopants are halogens and group IV and V atoms. Due to the small bandgap (0.16 eV) Bi₂Te₃ is partially degenerate and the corresponding Fermi-level should be close to the conduction band minimum at room temperature. The size of the band-gap means that Bi₂Te₃ has high intrinsic carrier concentration. Therefore, minority carrier conduction cannot be neglected for small stoichiometric deviations. Use of telluride compounds is limited by the toxicity and rarity of tellurium.

Lead telluride

Heremans et al. (2008) demonstrated that thallium-doped lead telluride alloy (PbTe) achieves a ZT of 1.5 at 773 K. Later, Snyder et al. (2011) reported ZT~1.4 at 750 K in sodium-doped PbTe, and ZT~1.8 at 850 K in sodium-doped PbTe_1−xSe_x alloy. Snyder's group determined that both thallium and sodium alter the electronic structure of the crystal increasing electronic conductivity. They also claim that selenium increases electric conductivity and reduces thermal conductivity. 

In 2012 another team used lead telluride to convert 15 to 20 percent of waste heat to electricity, reaching a ZT of 2.2, which they claimed was the highest yet reported.

Inorganic clathrates

Inorganic clathrates have the general formula A_xB_yC_46-y (type I) and A_xB_yC_136-y (type II), where B and C are group III and IV elements, respectively, which form the framework where “guest” A atoms (alkali or alkaline earth metal) are encapsulated in two different polyhedra facing each other. The differences between types I and II come from the number and size of voids present in their unit cells. Transport properties depend on the framework's properties, but tuning is possible by changing the “guest” atoms.

The most direct approach to synthesize and optimize the thermoelectric properties of semiconducting type I clathrates is substitutional doping, where some framework atoms are replaced with dopant atoms. In addition, powder metallurgical and crystal growth techniques have been used in clathrate synthesis. The structural and chemical properties of clathrates enable the optimization of their transport properties as a function of stoichiometry. The structure of type II materials allows a partial filling of the polyhedra, enabling better tuning of the electrical properties and therefore better control of the doping level. Partially filled variants can be synthesized as semiconducting or even insulating.

Blake et al. have predicted ZT~0.5 at room temperature and ZT~1.7 at 800 K for optimized compositions. Kuznetsov et al. measured electrical resistance and Seebeck coefficient for three different type I clathrates above room temperature and by estimating high temperature thermal conductivity from the published low temperature data they obtained ZT~0.7 at 700 K for Ba₈Ga₁₆Ge₃₀ and ZT~0.87 at 870 K for Ba₈Ga₁₆Si₃₀.

Magnesium group IV compounds

Mg₂B^IV (B^IV=Si, Ge, Sn) compounds and their solid solutions are good thermoelectric materials and their ZT values are comparable with those of established materials. The appropriate production methods are based on direct co-melting, but mechanical alloying has also been used. During synthesis, magnesium losses due to evaporation and segregation of components (especially for Mg₂Sn) need to be taken into account. Directed crystallization methods can produce single crystalline material. Solid solutions and doped compounds have to be annealed in order to produce homogeneous samples - with the same properties throughout. At 800 K, Mg₂Si_0.55−xSn_0.4Ge_0.05Bi_x has been reported to have a figure of merit about 1.4, the highest ever reported for these compounds.

Silicides

Higher silicides display ZT levels with current materials. They are mechanically and chemically strong and therefore can often be used in harsh environments without protection. Possible fabrication methods include Czochralski and floating zone for single crystals and hot pressing and sintering for polycrystalline.

Skutterudite thermoelectrics

Recently, skutterudite materials have sparked the interest of researchers in search of new thermoelectrics. These structures are of the form (Co,Ni,Fe)(P,Sb,As)
₃ and are cubic with space group Im3. Unfilled, these materials contain voids into which low-coordination ions (usually rare-earth elements) can be inserted in order to alter thermal conductivity by producing sources for lattice phonon scattering and decrease thermal conductivity due to the lattice without reducing electrical conductivity. Such qualities make these materials exhibit PGEC behavior. However, recently Khan et al. (2017) showed that it is possible to reduce the thermal conductivity without filling these voids and enhance the figure of merit by 100%, with special architecture containing nano and micro pores.

Skutterudites have the chemical formula LM₄X₁₂, where L is a rare-earth metal, M a transition metal and X a metalloid, a group V element or pnictogen whose properties lie between those of a metal and nonmetal such as phosphorus, antimony, or arsenic. These materials could be potential in multistage thermoelectric devices as it has been shown that they have ZT>1.0, but their properties are not well known.

Oxide thermoelectrics

Homologous oxide compounds (such as those of the form (SrTiO
₃)_n(SrO)
_m—the Ruddlesden-Popper phase) have layered superlattice structures that make them promising candidates for use in high-temperature thermoelectric devices. These materials exhibit low thermal conductivity perpendicular to the layers while maintaining good electronic conductivity within the layers. ZT values are relatively low (~0.34 at 1,000K), but their enhanced thermal stability, as compared to conventional high-ZT bismuth compounds, makes them superior for use in high-temperature applications.

Interest in oxides as thermoelectric materials was reawakened in 1997 when Na_xCoO₂ was found to exhibit good thermoelectric behavior. In addition to their thermal stability, other advantages of oxides are their non-toxicity and high oxidation resistance. Simultaneously controlling both the electric and phonon systems may require nanostructured materials. Some layered oxide materials are thought to have ZT~2.7 at 900 K. If the layers in a given material have the same stoichiometry, they will be stacked so that the same atoms will not be positioned on top of each other, impeding phonon conductivity perpendicular to the layers. Recently, oxide thermoelectrics have gained a lot of attention so that the range of promising phases increased drastically. Novel members of this family include ZnO, MnO₂, and NbO₂, to name but a few.

Half Heusler alloys

Half Heusler alloys have a great potential for high- temperature power generation applications. Half-Heusler (HH) are alloys with a Formula ABX . Examples of Half-Heusler include NbFeSb, NbCoSn and VFeSb. HH possesses cubic MgAgAs type structure, forming three interpenetrating face-centered-cubic (FCC). The ability to substitute any of these three sublattices opens the door for wide variety of HH compounds to be synthesized. A and B sites substitutions are employed to reduce the thermal conductivity, while the X site substitution is used to enhance the carrier concentration and thus the electrical conductivity.

Previously, ZT could not peak more than 0.5 for p-type and 0.8 for n-type HH compound. However, in the past few years, researchers were able to achieve ZT≈1 for both n-type and p-type. Nano-sized grains is one of the approaches used to lower the thermal conductivity via grain boundaries- assisted phonon scattering. Other approach was to utilize the principles of nanocomposites, by which certain combination of metals were favored on others due to the atomic size difference. For instance, Hf and Ti is more effective than Hf and Zr, when reduction of thermal conductivity is of concern, since the atomic size difference between the former is larger than that of the latter.

Electrically conducting organic materials

Some electrically conducting organic materials may have a higher figure of merit than existing inorganic materials. Seebeck coefficient can be even millivolts per Kelvin but electrical conductivity is usually low, resulting in small ZT values. Quasi-one-dimensional organic crystals are formed from linear chains or stacks of molecules that are packed into a 3D crystal. Under certain conditions some Q1D organic crystals may have ZT~20 at room temperature for both p- and n-type materials. This has been credited to an unspecified interference between two main electron-phonon interactions leading to the formation of narrow strip of states in the conduction band with a significantly reduced scattering rate as the mechanism compensate each other, yielding high ZT.

Silicon-germanium

Silicon-germanium alloys are currently the best thermoelectric materials around 1000 ℃ and are therefore used in some radioisotope thermoelectric generators (RTG) (notably the MHW-RTG and GPHS-RTG) and some other high temperature applications, such as waste heat recovery. Usability of silicon-germanium alloys is limited by their price and mid-range ZT (~0.7).

Sodium cobaltate

Experiments on crystals of sodium cobaltate, using X-ray and neutron scattering experiments carried out at the European Synchrotron Radiation Facility (ESRF) and the Institut Laue-Langevin (ILL) in Grenoble were able to suppress thermal conductivity by a factor of six compared to vacancy-free sodium cobaltate. The experiments agreed with corresponding density functional calculations. The technique involved large anharmonic displacements of Na
_0.8CoO
₂ contained within the crystals.

Amorphous materials

In 2002, Nolas and Goldsmid have come up with a suggestion that systems with the phonon mean free path larger than the charge carrier mean free path can exhibit an enhanced thermoelectric efficiency. This can be realized in amorphous thermoelectrics and soon they became a focus of many studies. This ground-breaking idea was accomplished in Cu-Ge-Te, NbO₂, In-Ga-Zn-O, Zr-Ni-Sn, Si-Au, and Ti-Pb-V-O amorphous systems. It should be mentioned that modelling of transport properties is challenging enough without breaking the long-range order so that design of amorphous thermoelectrics is at its infancy. Naturally, amorphous thermoelectrics give rise to extensive phonon scattering, which is still a challenge for crystalline thermoelectrics. A bright future is expected for these materials.

Functionally graded materials

Functionally graded materials make it possible to improve the conversion efficiency of existing thermoelectrics. These materials have a non-uniform carrier concentration distribution and in some cases also solid solution composition. In power generation applications the temperature difference can be several hundred degrees and therefore devices made from homogeneous materials have some part that operates at the temperature where ZT is substantially lower than its maximum value. This problem can be solved by using materials whose transport properties vary along their length thus enabling substantial improvements to the operating efficiency over large temperature differences. This is possible with functionally graded materials as they have a variable carrier concentration along the length of the material, which is optimized for operations over specific temperature range.

Nanomaterials and superlattices

In addition to nanostructured Bi
₂Te
₃/Sb
₂Te
₃ superlattice thin films, other nanostructured materials, including nanowires, nanotubes and quantum dots show potential in improving thermoelectric properties.

PbTe/PbSeTe quantum dot superlattice

Another example of a superlattice involves a PbTe/PbSeTe quantum dot superlattices provides an enhanced ZT (approximately 1.5 at room temperature) that was higher than the bulk ZT value for either PbTe or PbSeTe (approximately 0.5).

Nanocrystal stability and thermal conductivity

Not all nanocrystalline materials are stable, because the crystal size can grow at high temperatures, ruining the materials' desired characteristics. 

Nanocrystalline materials have many interfaces between crystals, which Physics of SASER scatter phonons so the thermal conductivity is reduced. Phonons are confined to the grain, if their mean free path is larger than the material grain size. 

Measured lattice thermal conductivity in nanowires is known to depend on roughness, the method of synthesis and properties of the source material.

Nanocrystalline transition metal silicides

Nanocrystalline transition metal silicides are a promising material group for thermoelectric applications, because they fulfill several criteria that are demanded from the commercial applications point of view. In some nanocrystalline transition metal silicides the power factor is higher than in the corresponding polycrystalline material but the lack of reliable data on thermal conductivity prevents the evaluation of their thermoelectric efficiency.

Nanostructured skutterudites

Skutterudites, a cobalt arsenide mineral with variable amounts of nickel and iron, can be produced artificially, and are candidates for better thermoelectric materials. 

One advantage of nanostructured skutterudites over normal skutterudites is their reduced thermal conductivity, caused by grain boundary scattering. ZT values of ~0.65 and > 0.4 have been achieved with CoSb₃ based samples; the former values were 2.0 for Ni and 0.75 for Te-doped material at 680 K and latter for Au-composite at T > 700 K.

Even greater performance improvements can be achieved by using composites and by controlling the grain size, the compaction conditions of polycrystalline samples and the carrier concentration.

Graphene

Graphene is known for its high electrical conductivity and Seebeck coefficient at room temperature. However, from thermoelectric perspective, its thermal conductivity is notably high, which in turn limits its ZT. Several approaches were suggested to reduce the thermal conductivity of graphene without altering much its electrical conductivity. These include, but not limited to, the following:

1- Doping with carbon isotopes to form isotopic heterojunction such as that of 12C and 13C. Basically, those isotopes possess different phonon frequency mismatch, which ultimately lead to the scattering of the heat carriers (the phonons). Fortunately, this approach has been shown to not affecting neither the power factor nor the electrical conductivity.

2- Wrinkles and cracks in the graphene structure were shown to contribute to the reduction in the thermal conductivity. Reported values of thermal conductivity of suspended graphene of size 3.8 µm show a wide spread from 1500 to 5000 W/mK. A recent study attributed that to the microstructural defects present in graphene, such as wrinkles and cracks, which can drop the thermal conductivity by 27%. These defects help scatter phonons. 

3- Introduction of defects with techniques such as oxygen plasma treatment: a more systemic way of introducing defects in graphene structure is done through O₂ plasma treatment. Ultimately, the graphene sample will contain prescribed-holes spaced and numbered according to the plasma intensity. People were able to improve ZT of graphene from 1 to a value of 2.6 when the defect density is raised from 0.04 to 2.5 (this number is an index of defect density and usually understood when compared to the corresponding value of the un-treated graphene, 0.04 in our case). Nevertheless, this technique would lower the electrical conductivity as well, which can be kept unchanged if the plasma processing parameters are optimized.

4- Functionalization of graphene by oxygen: the thermal behavior of graphene oxide has not been investigated extensively as compared to its counterpart; graphene. However, it was shown theoretically by Density Functional Theory (DFT) model that adding oxygen into the lattice of graphene reduces a lot its thermal conductivity due to phonon scattering effect. Scattering of phonons result from both acoustic mismatch and reduced symmetry in graphene structure after doping with oxygen. The reduction of thermal conductivity can easily exceed 50% with this approach.

Superlattices and roughness

Superlattices - nano structured thermocouples, are considered a good candidate for better thermoelectric device manufacturing, with materials that can be used in manufacturing this structure.

Their production is expensive for general-use due to fabrication processes based on expensive thin-film growth methods. However, since the amount of thin-film materials required for device fabrication with superlattices, is so much less than thin-film materials in bulk thermoelectric materials (almost by a factor of 1/10,000) the long-term cost advantage is indeed favorable. 

This is particularly true given the limited availability of tellurium causing competing solar applications for thermoelectric coupling systems to rise.

Superlattice structures also allow the independent manipulation of transport parameters by adjusting the structure itself, enabling research for better understanding of the thermoelectric phenomena in nanoscale, and studying the phonon-blocking electron-transmitting structures - explaining the changes in electric field and conductivity due to the material's nano-structure.

Many strategies exist to decrease the superlattice thermal conductivity that are based on engineering of phonon transport. The thermal conductivity along the film plane and wire axis can be reduced by creating diffuse interface scattering and by reducing the interface separation distance, both which are caused by interface roughness.

Interface roughness can naturally occur or may be artificially induced. 

In nature, roughness is caused by the mixing of atoms of foreign elements. Artificial roughness can be created using various structure types, such as quantum dot interfaces and thin-films on step-covered substrates.

Problems in superlattices

Reduced electrical conductivity: reduced phonon-scattering interface structures often also exhibit a decrease in electrical conductivity. 

The thermal conductivity in the cross-plane direction of the lattice is usually very low, but depending on the type of superlattice, the thermoelectric coefficient may increase because of changes to the band structure.

Low thermal conductivity in superlattices is usually due to strong interface scattering of phonons. Minibands are caused by the lack of quantum confinement within a well. The mini-band structure depends on the superlattice period so that with a very short period (~1 nm) the band structure approaches the alloy limit and with a long period (≥ ~60 nm) minibands become so close to each other that they can be approximated with a continuum.

Superlatice structure countermeasures: counter measures can be taken which practically eliminate the problem of decreased electrical conductivity in a reduced phonon-scattering interface. These measures include the proper choice of superlattice structure, taking advantage of mini-band conduction across superlattices, and avoiding quantum-confinement. It has been shown that because electrons and phonons have different wavelengths, it is possible to engineer the structure in such a way that phonons are scattered more diffusely at the interface than electrons.

Phonon confinement countermeasures: another approach to overcome the decrease in electrical conductivity in reduced phonon-scattering structures is to increase phonon reflectivity and therefore decrease the thermal conductivity perpendicular to the interfaces. This can be achieved by increasing the mismatch between the materials in adjacent layers, including density, group velocity, specific heat, and the phonon-spectrum. 

Interface roughness causes diffuse phonon scattering, which either increases or decreases the phonon reflectivity at the interfaces. A mismatch between bulk dispersion relations confines phonons, and the confinement becomes more favorable as the difference in dispersion increases. 

The amount of confinement is currently unknown as only some models and experimental data exist. As with a previous method, the effects on the electrical conductivity have to be considered.

Attempts to Localize long wavelength phonons by aperiodic superlattices or composite superlattices with different periodicities have been made. In addition, defects, especially dislocations, can be used to reduce thermal conductivity in low dimensional systems.

Parasitic heat: parasitic heat conduction in the barrier layers could cause significant performance loss. It has been proposed but not tested that this can be overcome by choosing a certain correct distance between the quantum wells.

The Seebeck coefficient can change its sign in superlattice nanowires due to the existence of minigaps as Fermi energy varies. This indicates that superlattices can be tailored to exhibit n or p-type behavior by using the same dopants as those that are used for corresponding bulk materials by carefully controlling Fermi energy or the dopant concentration. With nanowire arrays, it is possible to exploit semimetal-semiconductor transition due to the quantum confinement and use materials that normally would not be good thermoelectric materials in bulk form. Such elements are for example bismuth. The Seebeck effect could also be used to determine the carrier concentration and Fermi energy in nanowires.

In quantum dot thermoelectrics, unconventional or nonband transport behavior (e.g. tunneling or hopping) is necessary to utilize their special electronic band structure in the transport direction. It is possible to achieve ZT>2 at elevated temperatures with quantum dot superlattices, but they are almost always unsuitable for mass production. 

However, in superlattices, where quantum-effects are not involved, with film thickness of only a few micrometers (µm) to about 15 µm, Bi₂Te₃/Sb₂Te₃ superlattice material has been made into high-performance microcoolers and other devices. The performance of hot-spot coolers are consistent with the reported ZT~2.4 of superlattice materials at 300 K.

Nanocomposites are promising material class for bulk thermoelectric devices, but several challenges have to be overcome to make them suitable for practical applications. It is not well understood why the improved thermoelectric properties appear only in certain materials with specific fabrication processes.

SrTe nanocrystals can be embedded in a bulk PbTe matrix so that rocksalt lattices of both materials are completely aligned (endotaxy) with optimal molar concentration for SrTe only 2%. This can cause strong phonon scattering but would not affect charge transport. In such case, ZT~1.7 can be achieved at 815 K for p-type material.

Tin selenide

In 2014, researchers at Northwestern University discovered that tin selenide (SnSe) has a ZT of 2.6 along the b axis of the unit cell. This is the highest value reported to date. This high ZT figure of merit has been attributed to an extremely low thermal conductivity found in the SnSe lattice. Specifically, SnSe demonstrated a lattice thermal conductivity of 0.23 W·m⁻¹·K⁻¹, which is much lower than previously reported values of 0.5 W·m⁻¹·K⁻¹ and greater. This SnSe material also exhibited a ZT of 2.3±0.3 along the c-axis and 0.8±0.2 along the a-axis. These excellent figures of merit were obtained by researchers working at elevated temperatures, specifically 923 K (650 °C). As shown by the figures below, SnSe performance metrics were found to significantly improve at higher temperatures; this is due to a structural change that is discussed below. Power factor, conductivity, and thermal conductivity all reach their optimal values at or above 750 K, and appear to plateau at higher temperatures. However, these reports have become controversial as reported in Nature because other groups have not been able to reproduce the reported bulk thermal conductivity data.

SnSe Performance Metrics

 

Although it exists at room temperature in an orthorhombic structure with space group Pnma, SnSe has been shown to undergo a transition to a structure with higher symmetry, space group Cmcm, at higher temperatures. This structure consists of Sn-Se planes that are stacked upwards in the a-direction, which accounts for the poor performance out-of-plane (along a-axis). Upon transitioning to the Cmcm structure, SnSe maintains its low thermal conductivity but exhibits higher carrier mobilities, leading to its excellent ZT value.

One particular impediment to further development of SnSe is that it has a relatively low carrier concentration: approximately 10¹⁷ cm⁻³. Further compounding this issue is the fact that SnSe has been reported to have low doping efficiency.

However, such single crystalline materials suffer from inability to make useful devices due to their brittleness as well as narrow range of temperatures, where ZT is reported to be high. Further, polycrystalline materials made out of these compounds by several investigators have not confirmed the high ZT of these materials.

Production methods

Production methods for these materials can be divided into powder and crystal growth based techniques. Powder based techniques offer excellent ability to control and maintain desired carrier distribution. In crystal growth techniques dopants are often mixed with melt, but diffusion from gaseous phase can also be used. In the zone melting techniques disks of different materials are stacked on top of others and then materials are mixed with each other when a traveling heater causes melting. In powder techniques, either different powders are mixed with a varying ratio before melting or they are in different layers as a stack before pressing and melting. 

There are applications, such as cooling of electronic circuits, where thin films are required. Therefore, thermoelectric materials can also be synthesized using physical vapor deposition techniques. Another reason to utilize these methods is to design these phases and provide guidance for bulk applications.

Significant improvement on 3D printing skills makes it possible for thermoelectric materials prepared from 3D printing inks, which are usually synthesized by dispersing inorganic powders to organic solvent or making a suspension. Thermoelectric products are made from special materials that absorb heat and create electricity. The requirement of having complex geometries that fit in tightly constrained spaces, makes 3D printing the ideal manufacturing technique. Also, printable materials that demonstrate good mechanical flexibility could be utilized for wearable thermoelectrics, which convert body energy to electricity.

Applications

Refrigeration

Thermoelectric materials can be used as refrigerators, called "thermoelectric coolers", or "Peltier coolers" after the Peltier effect that controls their operation. As a refrigeration technology, Peltier cooling is far less common than vapor-compression refrigeration. The main advantages of a Peltier cooler (compared to a vapor-compression refrigerator) are its lack of moving parts or refrigerant, and its small size and flexible shape (form factor). 

The main disadvantage of Peltier coolers is low efficiency. It is estimated that materials with ZT>3 (about 20–30% Carnot efficiency) would be required to replace traditional coolers in most applications. Today, Peltier coolers are only used in niche applications, especially small scale, where efficiency is not important.

Power generation

Thermoelectric efficiency depends on the figure of merit, ZT. There is no theoretical upper limit to ZT, and as ZT approaches infinity, the thermoelectric efficiency approaches the Carnot limit. However, no known thermoelectrics have a ZT greater than 3. As of 2010, thermoelectric generators serve application niches where efficiency and cost are less important than reliability, light weight, and small size.

Internal combustion engines capture 20–25% of the energy released during fuel combustion. Increasing the conversion rate can increase mileage and provide more electricity for on-board controls and creature comforts (stability controls, telematics, navigation systems, electronic braking, etc.) It may be possible to shift energy draw from the engine (in certain cases) to the electrical load in the car, e.g. electrical power steering or electrical coolant pump operation.

Cogeneration power plants use the heat produced during electricity generation for alternative purposes. Thermoelectrics may find applications in such systems or in solar thermal energy generation.

Concentrator photovoltaics

From Wikipedia, the free encyclopedia

This Amonix system in Las Vegas, USA consists of thousands of small Fresnel lenses, each focusing sunlight to ~500X higher intensity onto a tiny, high-efficiency multi-junction solar cell. A Tesla Roadster is parked beneath for scale.

 

Concentrator photovoltaics (CPV) modules on dual axis solar trackers in Golmud, China

Concentrator photovoltaics (CPV) (also known as Concentration Photovoltaics) is a photovoltaic technology that generates electricity from sunlight. Contrary to conventional photovoltaic systems, it uses lenses and curved mirrors to focus sunlight onto small, but highly efficient, multi-junction (MJ) solar cells. In addition, CPV systems often use solar trackers and sometimes a cooling system to further increase their efficiency. Ongoing research and development is rapidly improving their competitiveness in the utility-scale segment and in areas of high insolation. This sort of solar technology can be thus used in smaller areas.

Systems using high-concentration photovoltaics (HCPV) especially have the potential to become competitive in the near future. They possess the highest efficiency of all existing PV technologies, and a smaller photovoltaic array also reduces the balance of system costs. Currently, CPV is not used in the PV rooftop segment and is far less common than conventional PV systems. For regions with a high annual direct normal irradiance of 2000 kilowatt-hour (kWh) per square meter or more, the levelized cost of electricity is in the range of $0.08–$0.15 per kWh and installation cost for a 10-megawatt CPV power plant was identified to lie between €1.40–€2.20 (~$1.50-$2.30) per watt-peak (W_p).

In 2016, cumulative CPV installations reached 350 megawatts (MW), less than 0.2% of the global installed capacity of 230,000 MW. Commercial HCPV systems reached instantaneous ("spot") efficiencies of up to 42% under standard test conditions (with concentration levels above 400) and the International Energy Agency sees potential to increase the efficiency of this technology to 50% by the mid-2020s. As of December 2014, the best lab cell efficiency for concentrator MJ-cells reached 46% (four or more junctions). Under outdoor, operating conditions, CPV module efficiencies have exceeded 33% ("one third of a sun"). System-level AC efficiencies are in the range of 25-28%. CPV installations are located in China, the United States, South Africa, Italy and Spain.

HCPV directly competes with concentrated solar power (CSP) as both technologies are suited best for areas with high direct normal irradiance, which are also known as the Sun Belt region in the United States and the Golden Banana in Southern Europe. CPV and CSP are often confused with one another, despite being intrinsically different technologies from the start: CPV uses the photovoltaic effect to directly generate electricity from sunlight, while CSP – often called concentrated solar thermal – uses the heat from the sun's radiation in order to make steam to drive a turbine, that then produces electricity using a generator. Currently, CSP is more common than CPV.

History

Research into concentrator photovoltaics has taken place since the mid 1970s, initially spurred on by the energy shock from a mideast oil embargo. Sandia National Laboratories in Albuquerque, New Mexico was the site for most of the early work, with the first modern-like photovoltaic concentrating system produced there late in the decade. Their first system was a linear-trough concentrator system that used a point focus acrylic Fresnel lens focusing on water-cooled silicon cells and two axis tracking. Cell cooling with a passive heat sink and use of silicone-on-glass Fresnel lenses was demonstrated in 1979 by the Ramón Areces Project at the Institute of Solar Energy of the Technical University of Madrid. The 350 kW SOLERAS project in Saudi Arabia—the largest until many years later—was constructed by Sandia/Martin Marietta in 1981.

Research and development continued through the 1980s and 1990s without significant industry interest. Improvements in cell efficiency were soon recognized as essential to making the technology economical. However the improvements to Si-based cell technologies used by both concentrators and flat PV failed to favor the system-level economics of CPV. The introduction of III-V Multi-junction solar cells starting in the early 2000s has since provided a clear differentiator. MJ cell efficiencies have improved from 34% (3-junctions) to 46% (4-junctions) at research-scale production levels. A substantial number of multi-MW CPV projects have also been commissioned worldwide since 2010.

Challenges

Modern CPV systems operate most efficiently in highly concentrated sunlight (i.e. concentration levels equivalent to hundreds of suns), as long as the solar cell is kept cool through the use of heat sinks. Diffuse light, which occurs in cloudy and overcast conditions, cannot be highly concentrated using conventional optical components only (i.e. macroscopic lenses and mirrors). Filtered light, which occurs in hazy or polluted conditions, has spectral variations which produce mismatches between the electrical currents generated within the series-connected junctions of spectrally "tuned" multi-junction (MJ) photovoltaic cells. These CPV features lead to rapid decreases in power output when atmospheric conditions are less than ideal.

To produce equal or greater energy per rated watt than conventional PV systems, CPV systems must be located in areas that receive plentiful direct sunlight. This is typically specified as average DNI greater than 5.5-6 kWh/m²/day or 2000kWh/m²/yr. Otherwise, evaluations of annualized DNI vs. GNI/GHI irradiance data have concluded that conventional PV should still perform better over time than presently available CPV technology in most regions of the world. 

CPV Strengths	CPV Weaknesses
High efficiencies under direct normal irradiance	HCPV cannot utilize diffuse radiation. LCPV can only utilize a fraction of diffuse radiation.
Low cost per watt of manufacturing capital	Power output of MJ solar cells is more sensitive to shifts in radiation spectra caused by changing atmospheric conditions.
Low temperature coefficients	Tracking with sufficient accuracy and reliability is required.
No cooling water required for passively cooled systems	May require frequent cleaning to mitigate soiling losses, depending on the site
Additional use of waste heat possible for systems with active cooling possible (e.g.large mirror systems)	Limited market – can only be used in regions with high DNI, cannot be easily installed on rooftops
Modular – kW to GW scale	Strong cost decrease of competing technologies for electricity production
Increased and stable energy production throughout the day due to (two-axis) tracking	Bankability and perception issues
Low energy payback time	New generation technologies, without a history of production (thus increased risk)
Potential double use of land e.g. for agriculture, low environmental impact	Optical losses
High potential for cost reduction	Lack of technology standardization
Opportunities for local manufacturing	–
Smaller cell sizes could prevent large fluctuations in module price due to variations in semiconductor prices	–
Greater potential for efficiency increase in the future compared to single-junction flat plate systems could lead to greater improvements in land area use, BOS costs, and BOP costs	–

Ongoing research and development

International CPV-x Conference - Historical Participation Statistics. Data Source - CPV-x Proceedings

 

CPV research and development has been pursued in over 20 countries for more than a decade. The annual CPV-x conference series has served as a primary networking and exchange forum between university, government lab, and industry participants. Government agencies have also continued to encourage a number of specific technology thrusts. 

ARPA-E announced a first round of R&D funding in late 2015 for the MOSAIC Program (Microscale Optimized Solar-cell Arrays with Integrated Concentration) to further combat the location and expense challenges of existing CPV technology. As stated in the program description: "MOSAIC projects are grouped into three categories: complete systems that cost effectively integrate micro-CPV for regions such as sunny areas of the U.S. southwest that have high Direct Normal Incident (DNI) solar radiation; complete systems that apply to regions, such as areas of the U.S. Northeast and Midwest, that have low DNI solar radiation or high diffuse solar radiation; and concepts that seek partial solutions to technology challenges."

In Europe the CPVMATCH Program (Concentrating PhotoVoltaic Modules using Advanced Technologies and Cells for Highest efficiencies) aims "to bring practical performance of HCPV modules closer to theoretical limits". Efficiency goals achievable by 2019 are identified as 48% for cells and 40% for modules at greater than 800x concentration.

The Australian Renewable Energy Agency (ARENA) extended its support in 2017 for further commercialization of the HCPV technology developed by Raygen. Their 250kW dense array receivers are the most powerful CPV receivers thus far created, with demonstrated PV efficiency of 40.4% and include usable heat co-generation.

Optical design

The design of macroscopic sunlight concentrators for CPV introduces a very specific optical design problem, with features that makes it different from any other optical design. It has to be efficient, suitable for mass production, capable of high concentration, insensitive to manufacturing and mounting inaccuracies, and capable of providing uniform illumination of the cell. All these reasons make nonimaging optics the most suitable for CPV. 

For very low concentrations, the wide acceptance angles of nonimaging optics avoid the need for active solar tracking. For medium and high concentrations, a wide acceptance angle can be seen as a measure of how tolerant the optic is to imperfections in the whole system. It is vital to start with a wide acceptance angle since it must be able to accommodate tracking errors, movements of the system due to wind, imperfectly manufactured optics, imperfectly assembled components, finite stiffness of the supporting structure or its deformation due to aging, among other factors. All of these reduce the initial acceptance angle and, after they are all factored in, the system must still be able to capture the finite angular aperture of sunlight.

Efficiency

Reported records of solar cell efficiency since 1975. As of December 2014, best lab cell efficiency reached 46% (for multi-junction concentrator, 4+ junctions).

 

All CPV systems have a concentrating optic and a solar cell. Generally, active solar tracking is necessary. Low-concentration systems often have a simple booster reflector, which can increase solar electric output by over 30% from that of non-concentrator PV systems. Experimental results from such LCPV systems in Canada resulted in energy gains over 40% for prismatic glass and 45% for traditional crystalline silicon PV modules.

Semiconductor properties allow solar cells to operate more efficiently in concentrated light, as long as the cell Junction temperature is kept cool by suitable heat sinks. Efficiency of multi-junction photovoltaic cells developed in research is upward of 44% today, with the potential to approach 50% in the coming years. The theoretical limiting efficiency under concentration approaches 65% for 5 junctions, which is a likely practical maximum.

Types

CPV systems are categorized according to the amount of their solar concentration, measured in "suns" (the square of the magnification).

Low concentration PV (LCPV)

An example of a Low Concentration PV Cell's surface, showing the glass lensing

 

Low concentration PV are systems with a solar concentration of 2–100 suns. For economic reasons, conventional or modified silicon solar cells are typically used, and, at these concentrations, the heat flux is low enough that the cells do not need to be actively cooled. There is now modeling and experimental evidence that standard solar modules do not need any modification, tracking or cooling if the concentration level is low and yet still have increased output of 35% or more.

Medium concentration PV

From concentrations of 100 to 300 suns, the CPV systems require two-axis solar tracking and cooling (whether passive or active), which makes them more complex. 

A 10×10 mm HCPV solar cell

High concentration photovoltaics (HCPV)

High concentration photovoltaics (HCPV) systems employ concentrating optics consisting of dish reflectors or fresnel lenses that concentrate sunlight to intensities of 1,000 suns or more. The solar cells require high-capacity heat sinks to prevent thermal destruction and to manage temperature related electrical performance and life expectancy losses. To further exacerbate the concentrated cooling design, the heat sink must be passive, otherwise the power required for active cooling will reduce the overall conversion efficiency and economy. Multi-junction solar cells are currently favored over single junction cells, as they are more efficient and have a lower temperature coefficient (less loss in efficiency with an increase in temperature). The efficiency of both cell types rises with increased concentration; multi-junction efficiency rises faster. Multi-junction solar cells, originally designed for non-concentrating PV on space-based satellites, have been re-designed due to the high-current density encountered with CPV (typically 8 A/cm² at 500 suns). Though the cost of multi-junction solar cells is roughly 100 times that of conventional silicon cells of the same area, the small cell area employed makes the relative costs of cells in each system comparable and the system economics favor the multi-junction cells. Multi-junction cell efficiency has now reached 44% in production cells. 

The 44% value given above is for a specific set of conditions known as "standard test conditions". These include a specific spectrum, an incident optical power of 850 W/m², and a cell temperature of 25 °C. In a concentrating system, the cell will typically operate under conditions of variable spectrum, lower optical power, and higher temperature. The optics needed to concentrate the light have limited efficiency themselves, in the range of 75–90%. Taking these factors into account, a solar module incorporating a 44% multi-junction cell might deliver a DC efficiency around 36%. Under similar conditions, a crystalline silicon module would deliver an efficiency of less than 18%.

When high concentration is needed (500–1000 times), as occurs in the case of high efficiency multi-junction solar cells, it is likely that it will be crucial for commercial success at the system level to achieve such concentration with a sufficient acceptance angle. This allows tolerance in mass production of all components, relaxes the module assembling and system installation, and decreasing the cost of structural elements. Since the main goal of CPV is to make solar energy inexpensive, there can be used only a few surfaces. Decreasing the number of elements and achieving high acceptance angle, can be relaxed optical and mechanical requirements, such as accuracy of the optical surfaces profiles, the module assembling, the installation, the supporting structure, etc. To this end, improvements in sunshape modelling at the system design stage may lead to higher system efficiencies.

Reliability requirements

The maximum operating temperatures (T_{max cell}) of CPV systems are limited to less than approximately 100–125 °C on account of the intrinsic reliability limitation of their multi-junction PV cells. This contrasts to CSP and other CHP systems which may be designed to function at temperatures in excess of several hundred degrees. More specifically, the cells are fabricated from a layering of thin-film III-V semiconductor materials having intrinsic lifetimes during operation that rapidly decrease with an Arrhenius-type temperature dependence. The system receiver must therefore provide for highly efficient and uniform cell cooling, where an ideal receiver would provide T_{max coolant} ~ T_{max cell}. In addition to material and design limitations in receiver heat-transfer performance, numerous extrinsic factors, such as the frequent system thermal cycling, further reduce the practical T_{max coolant} compatible with long system life to below about 80 °C. 

The higher capital costs, lesser standardization, and added engineering & operational complexities (in comparison to zero and low-concentration PV technologies) make long-life performance a critical demonstration goal for the first generations of CPV technologies. Performance certification standards (e.g. IEC 62108, UL 8703, IEC 62789, IEC 62670) include stress testing conditions that may be useful to uncover some predominantly infant and early life (less than 1–2 year) failure modes at the system, module, and sub-component levels. However, such standardized tests – as typically performed on only a small sampling of units – are generally incapable to evaluate comprehensive long-term lifetimes (10 to 25 or more years) for each unique system design and application under its broader range of actual operating conditions. Reliability of these complex systems is therefore assessed in the field, and is improved through aggressive product development cycles which are guided by the results of accelerated component/system aging, performance monitoring diagnostics, and failure analysis. Significant growth in the deployment of CPV can be anticipated once these concerns are better addressed to build confidence in system bankability.

Installations

Concentrator photovoltaics technology has established its presence in the solar industry over the last several years. The first CPV power plant that exceeded 1 MW-level was commissioned in Spain in 2006. By the end of 2015, the number of CPV power plants around the world accounted for a total installed capacity of 350 MW. Field data collected over six years is also starting to benchmark the prospects for long-term system reliability.

The emerging CPV segment has comprised ~0.1% of the fast-growing utility market for PV installations over the past decade. Unfortunately, by the end of 2015, the near term outlook for CPV industry growth has faded with closure of all of the largest CPV manufacturing facilities: including those of Suncore, Soitec, Amonix, and Solfocus. Nevertheless, the growth outlook for the overall PV industry continues to appear strong.

Cumulative CPV Installations in MW by country by November 2014

 

Yearly Installed CPV Capacity in MW from 2002 to 2015.

 

Yearly Installed PV Capacity in GW from 2002 to 2015.

List of large CPV systems

The largest CPV power plant currently in operation is of 80 MW_p capacity located in Golmud, China, hosted by Suncore Photovoltaics.

Power station	Capacity (MWp)	Location	Vendor/Builder
Golmud 2	79.83	in Golmud/Qinghai province/China	Suncore
Golmud 1	57.96	in Golmud/Qinghai province/China	Suncore
Touwsrivier	44.19	in Touwsrivier/Western Cape/South Africa	Soitec
Alamosa Solar Project	35.28	in Alamosa, Colorado/San Luis Valley/United States	Amonix

Concentrated photovoltaics and thermal

Concentrator photovoltaics and thermal (CPVT), also sometimes called combined heat and power solar (CHAPS) or hybrid thermal CPV, is a cogeneration or micro cogeneration technology used in the field of concentrator photovoltaics that produces usable heat and electricity within the same system. CPVT at high concentrations of over 100 suns (HCPVT) utilizes similar components as HCPV, including dual-axis tracking and multi-junction photovoltaic cells. A fluid actively cools the integrated thermal–photovoltaic receiver, and simultaneously transports the collected heat. 

Typically, one or more receivers and a heat exchanger operate within a closed thermal loop. To maintain efficient overall operation and avoid damage from thermal runaway, the demand for heat from the secondary side of the exchanger must be consistently high. Under such optimal operating conditions, collection efficiencies exceeding 70% (up to ~35% electric, ~40% thermal for HCPVT) are anticipated. Net operating efficiencies may be substantially lower depending on how well a system is engineered to match the demands of the particular thermal application.

The maximum temperature of CPVT systems is typically too low alone to power a boiler for additional steam-based cogeneration of electricity. Such systems may be economical to power lower temperature applications having a constant high heat demand. The heat may be employed in district heating, water heating and air conditioning, desalination or process heat. For applications having lower or intermittent heat demand, a system may be augmented with a switchable heat dump to the external environment in order to maintain reliable electrical output and safeguard cell life, despite the resulting reduction in net operating efficiency. 

HCPVT active cooling enables the use of much higher power thermal–photovoltaic receiver units, generating typically 1–100 kilowatts electric, as compared to HCPV systems that mostly rely upon passive cooling of single ~20W cells. Such high-power receivers utilize dense arrays of cells mounted on a high-efficiency heat sink. Minimizing the number of individual receiver units is a simplification that should ultimately yield improvement in the overall balance of system costs, manufacturability, maintainability/upgradeability, and reliability.

This 240 x 80 mm 1,000 suns CPV heat sink design thermal animation, was created using high resolution CFD analysis, and shows temperature contoured heat sink surface and flow trajectories as predicted.

Demonstration projects

The economics of a mature CPVT industry is anticipated to be competitive, despite the large recent cost reductions and gradual efficiency improvements for conventional silicon PV (which can be installed alongside conventional CSP to provide for similar electrical+thermal generation capabilities). CPVT may currently be economical for niche markets having all of the following application characteristics:

• high solar direct normal incidence (DNI)
• tight space constraints for placement of a solar collector array
• high and constant demand for low-temperature (less than 80 °C) heat
• high cost of grid electricity
• access to backup sources of power or cost-efficient storage (electrical and thermal)

Utilization of a power purchase agreement (PPA), government assistance programs, and innovative financing schemes are also helping potential manufacturers and users to mitigate the risks of early CPVT technology adoption.

CPVT equipment offerings ranging from low (LCPVT) to high (HCPVT) concentration are now being deployed by several startup ventures. As such, longer-term viability of the technical and/or business approach being pursued by any individual system provider is typically speculative. Notably, the minimum viable products of startups can vary widely in their attention to reliability engineering. Nevertheless, the following incomplete compilation is offered to assist with the identification of some early industry trends.

LCPVT systems at ~14x concentration using reflective trough concentrators, and receiver pipes clad with silicon cells having dense interconnects, have been assembled by Cogenra with a claimed 75% efficiency (~15-20% electric, 60% thermal). Several such systems are in operation for more than 5 years as of 2015, and similar systems are being produced by Absolicon  and Idhelio at 10x and 50x concentration, respectively.

HCPVT offerings at over 700x concentration have more recently emerged, and may be classified into three power tiers. Third tier systems are distributed generators consisting of large arrays of ~20W single-cell receiver/collector units, similar to those previously pioneered by Amonix and SolFocus for HCPV. Second tier systems utilize localized dense-arrays of cells that produce 1-100 kW of electrical power output per receiver/generator unit. First tier systems exceed 100 kW of electrical output and are most aggressive in targeting the utility market.

Several HCPVT system providers are listed in the following table. Nearly all are early demonstration systems which have been in service for under 5 years as of 2015. Collected thermal power is typically 1.5x-2x the rated electrical power. 

Provider	Country	Concentrator Type	Unit Size in  kWe
			Generator	Receiver
		- Tier 1 -
Raygen	Australia	Large Heliostat Array	250	250
		- Tier 2 -
Airlight Energy/dsolar	Switzerland	Large Dish	12	12
Rehnu	United States	Large Dish	6.4	0.8
Solartron	Canada	Large Dish	20	20
Southwest Solar	United States	Large Dish	20	20
Sun Oyster	Germany	Large Trough + Lens	4.7	2.35
Zenith Solar/Suncore	Israel/China/USA	Large Dish	4.5	2.25
		- Tier 3 -
BSQ Solar	Spain	Small Lens Array	13,44	0.02
Silex Power	Malta	Small Dish Array	16	0.04
Solergy	Italy/USA	Small Lens Array	20	0.02