Advances in thermophotovoltaics: materials, devices and systems

Resume : Waste heat is an enormous and largely untapped source of power that has the potential, once appropriately harnessed, to radically improve energy utilization on a global scale. MTPV Power Corporation is presently deploying thermophotovoltaic systems into the world?s most energy-intensive industries, with waste heat recovery applications in oil & gas, chemical processing, and glass, steel, and cement manufacturing. The company?s micron-gap technology is leveraged in its commercial systems to maintain nanometer-scale distances between emitters and photovoltaic receivers over very large areas in excess of 60 square centimeters. A robust chip module design has been proven to deliver more than a million chip lifetime operating hours, and multi-module systems have delivered tens of megawatt-hours of produced energy with greater than 99% uptime. Unique deployment infrastructure and balance-of-system architectures have been implemented to address the rigors of energy capture in challenging settings such as oil & gas flares, glass foundry flues, and steel mills.

Resume : Both solar cells and batteries generate quiet DC electric power. However, while solar cells are light weight, they only operate when the sun is shining. A fuel fired thermophotovoltaic (TPV) generator can operate day and night. The development of a light weight fuel fired TPV generator will be described here. In TPV, infrared (IR) sensitive GaSb photovoltaic cells convert energy from a combustion heated glowing ceramic IR emitter into electricity. We present here the design and operation of a first stand alone TPV generator complete with a photovoltaic converter array and a burner / emitter recuperator assembly and support components. This first unit has a durable NiO/MgO ceramic IR emitter operating at 1150 C and a 108 cell photovoltaic converter array tested at up to 50 W. The TPV unit also has a novel omega recuperator to preheat the combustion air increasing the system efficiency. With all the assemblies operating together, the complete TPV unit generates 24 W with the emitter operating at 1150 C and the array operating at 60 C. Modeling presents a path to improve the recuperator to allow the emitter to operate at 1300 C thereby increasing the IR emitted power and therefore the GaSb array and TPV generator should produce 50 W. A portable light weight TPV power supply has applications both for soldiers as well as for unmanned aerial vehicles (UAVs).

Resume : Three core enablers are responsible for the resurged interest in fuel-fired thermophotovoltaics (TPV) for portable power sources: Advances in manufacturing, specifically, high resolution patterning and 3D printing of refractory materials; PV maturation, due to military and medical imaging; and advanced modeling tools for reactor and heat exchanger design. Given the complexity of the problem, however, the research focus tends toward the maturation of components, whether solely, on photovoltaics, spectral control, or heat sources. Even groups with a systems' focus tend to have a component preference. This binary distribution leaves a number of possibilities and few mechanisms to identify an optimum system path. To help navigate the space, we present a design tool ? and supporting experimental work ? that quantifies how individual components and their couplings, in a fuel-fired TPV system, determine overall system performance, from fuel to photovoltaic including balance of plant. The model is composed of two sub-models: the Emitter-Cavity-TPV model, which incorporates empirical findings to, determine TPV conversion efficiency, effective emitter flux, cell thermal management and a boundary condition required to fully define the Reduced Order Reactor model, which relates fuel flow requirements to emitter surface temperatures, thereby completing the energy chain of custody. In addition, by including properties of heat exchangers and estimates of parasitic power consumption, the limits of what can be practically achieved in efficiency, size, weight and power become clear.

Resume : This paper presents the design, optimization, and fabrication of a high-efficiency planar STPV system comprising of spectrally selective absorber and emitter surfaces, and GaSb PV cells. The selective absorber consists of a micro-textured tungsten (W) surface that provides light absorptance of more than 90% at visible and near-infrared wavelengths. The selective emitter is a multilayer metal-dielectric structure of W and Si3N4 with its spectral properties tuned to match with the quantum efficiency of the GaSb cells. A comprehensive thermodynamic model was formulated for a detailed analysis and optimization of the transport of power at multiple stages of the STPV system. The system was tested at various operating temperatures using a high-power continuous wave laser as a simulated source of concentrated solar irradiation. A heat shield was installed on the absorber side to suppress the undesired radiation loss from the absorber end. An electrical output power density of 1.71 W/cm2 with a maximum conversion efficiency of 8.4% was measured at 1676 K for an equivalent incident solar concentration factor of ~2100. This efficiency is higher than those of previously reported experimental STPV systems. Optical and thermal losses occurred at multiple stages of the energy conversion process are quantified. Combining the simulation and experimental results, guidelines to further improve the performance of STPV systems are also provided.

Resume : While thermophotovoltaics have potential for high efficiency, they are ultimately limited by the fraction of useful photons absorbed by the photovoltaic cell. However, the blackbody emission spectrum below 1500 K generally does not have enough useful photons without enhancement. Two competing methods -- selective thermal emission and external photon recycling -- can improve the efficiency, but the former has the advantage of also applying to selective solar absorbers. In this work, we report a spectrally-selective thin-film silicon emitter. Its high-temperature emittance shows strong spectral selectivity at 868 K, and thermal stability is proven by measuring its infrared reflection spectrum before and after 24 hours of thermal cycling. Furthermore, it potentially for scalable manufacturing with a base in thin-film crystalline silicon coated by thin films of earth-abundant materials. Finally, it exhibits exceptional mechanical flexibility, for compatibility with a wide range of thermophotovoltaic cells. In summary, these thin-film silicon selective thermal emitters provide a combination of spectral selectivity, thermal stability, manufacturing scalability, and mechanical flexibility that may benefit the future adoption and use of thermophotovoltaics.

Resume : Abstract: In order to tailor thermophotovoltaic emitters to match specific photovoltaic receivers we design and investigate spectrally selective high temperature stable emitters. We demonstrate selective band edge emitters based on W-HfO2 refractive multilayer metamaterials and based on monolayers of spherical particles from yttria stabilized zirconia (YSZ) on HfO2 coated W-substrates. Both emitter types are stable up to 1400°C. Since the emitted power scales with the fourth power of temperature and for better match with low band gap photovoltaic cells, very high temperatures well above 1000 °C become very important. Degradation mechanisms and conditions for sustainable selectivity and high thermal stability are discussed. The stability of nanoscaled structured materials at very high temperatures is a scientific topic of fundamental importance in various fields of physical and materials sciences. References: 1. Dyachenko, P.N et al., Nature Communications, vol. 7, no. 11809, 1?8 (2016) 2. Lang, S et al., Scientific Reports, vol. 7, no. 1, p. 13916?13916 (2017) 3. Leib, E.W et al., Journal of Materials Chemistry C, vol. 4, no. 1, pp. 62?74 (2016) 4. Biehs, S.-A. et al. ,Physical Review Letters, vol. 115, no. 17, p. 174301?174301 (2015) 5. Dyachenko, P.N. et al., Optics Express, vol. 23, no. 19, pp. A1236 (2015) 6. Chirumamilla, M. et al., Scientific Reports, vol. 9, Article number: 7241 (2019)

Resume : Low-temperature thermophotovoltaics (TPV) demand very low band gap (< 0.5 eV) semiconductors to maximize output power. As the band gap narrows, so does the number of available materials. Current technology is mostly based on epitaxially-grown alloys of III-V elements, such as InGaAs(Sb), which present some limitations. First, it is not always possible to obtain the desired band gap, due to technological constraints. Second, their fabrication methods are expensive, an issue that becomes more and more important as lower temperature systems are aimed, because the output TPV power density diminishes rapidly with temperature. Colloidal quantum dots (CQD) are an interesting alternative to epitaxial materials for low-temperature TPV. The band gap of these nanocrystals can be tuned precisely during their synthesis by changing their size. Thus, in principle, any desired optimum bad gap for low-temperature TPV could be achieved by choosing the right combination of material and nanocrystal size. In addition, CQDs are fabricated by low-cost, wet chemical methods. These characteristics allow envisaging efficient, low-cost TPV cells. We give an overview of the potential of CQDs as photovoltaic absorbers for low-temperature TPV devices and review the state-of-the-art.

Resume : A solution for short term storage that provides energy on demand is required to achieve to long sought-for dream of a 100% Renewable Electricity System. Thermal energy storage at high temperatures (1800-2500ºC) in conjunction with thermophotovoltaic (TPV) cells to convert heat into electricity can potentially achieve high efficiency and rapid response times. However, in order to provide a cost-effective solution (as compared to batteries or pumped storage) the TPV converter needs to reach high efficiencies and be manufacturable in high volumes at moderate costs. In this scenario, Germanium based converters provide a unique advantage. Being the base of the current multijunction solar cell technology ?used in space PV and in terrestrial CPV systems? there is a mature existing infrastructure for substrate fabrication, structure epitaxial growth and device manufacturing which could be leveraged for TPV. In this paper we will revisit the potential and limitations of TPV cell designs based on germanium and review the challenges in TPV cell growth using MOVPE including 1) p/n junction formation; 2) emitter passivation; 3) Ge autodoping ; and 4) tandem configurations

Resume : A grid-level energy storage system is envisioned where excess electricity is stored as high temperature heat in the 1700-2200?C range and extracted when needed using GaAs thermophotovoltaic (TPV) cells. The crucial cell factors affecting the TPV system efficiency are the sub-bandgap reflectance (Rsub) and the series resistance (Rs). To enhance Rsub in the required 0.9-10 micron range, a point-contacted GaAs TPV cell is fabricated with a low refractive index, low-loss dielectric spacer layer inserted between the semiconductor back contact layer and the metal back contact. This architecture boosts the recycling of sub-bandgap photons back to the thermal emitter by minimizing the metal absorption losses. Rsub increases with spacing and smaller diameters of the point contacts, at the expense of increased Rs. For a thermal emitter at 2200 °C, the TPV cells operate at a current density of ~10 A/cm2 where high Rs result in significant power loss, leading to a trade-off between Rsub and Rs. We have fabricated GaAs TPV cells with 10 ?m diameter point contacts spaced 50 µm apart, with a ~250 nm SiO2 spacer layer. Standard photovoltaic measurements indicate a Voc of 1.07 V and Jsc of 20 mA/cm2 (without ARC) at 1000 W/m2, and Rs of 10 mohm.cm2 for 0.1 cm2 cells. The cell efficiency peaked at 5 A/cm2 and has less than 3% relative efficiency loss at 11 A/cm2. Rsub as measured by FTIR was ~92%. Optimization of the SiO2 dielectric is expected to lead to Rsub ?96% and a TPV system efficiency ? 45%.

Resume : The study of a special thermophotovoltaic material, namely, a thin nanostructured film consisting of close-packed metal nanoscale particles (with diameter about 2-15 nm) with spatial ordering of nanoparticles in size deposited on the surface of a broadband dielectric material is presented. Due to the dimensional dependence of Fermi energy, the presence of spatially inhomogeneous distribution of metal nanoparticles by size leads to the spatial redistribution of the charge in such a system thus the potential difference in the same direction must be found. The appearance in this system of an electron excited by an external photon (even if low-energy with a wavelength greater than a micrometer) leads to the flow of the electron in the direction of the potential gradient caused by the spatial ordering of nanoclusters in size. Since nanoclusters are metal, this provides the ability to detect photons of different wavelengths and, therefore, provides a wide spectrum of radiation absorption of the proposed system. The presence of contact between nanoclusters means the preservation of electronic conductivity in such a system due to electron tunneling between nanoclusters and percolation effects. The obtained preliminary results on the formation and study of the properties of nanoparticle films have shown that the study of such systems can potentially lead to the change of energy efficiency and energy saving of modern thermal power sources to a completely new level.

Resume : Thermodynamics has been successful in finding the limits of different power conversion schemes and has advanced our understanding of physics and chemistry; thus, it is accepted as a universal truth subjecting all macroscopic objects. The photovoltaic effect, namely the radiative energy exchange of semiconductors, is, however, governed by the law of detailed-balance. To this date, it is not fully understood how thermodynamics pertains to the photovoltaic effect since the addition of detailed-balance forms an inconsistent set of constraints. In this work, we propose a unification of the first and second thermodynamic laws with the detailed-balance by identifying a third independent variable, in the form of sub-bandgap emissivity, in addition to the existing potential and temperature. This addition allows us to reformulate the three laws governing the photovoltaic effect in a consistent solvable manner, thus advancing our fundamental understanding of light-matter interactions. More importantly, this approach should point to the limiting factors of advanced photovoltaic concepts such as thermophotovoltaics, thermoradiative, and thermophotonics solar power conversion, radiative-cooling, and concentrated multi-junction solar cells for space missions, concepts that are instrumental for our ability to progress and mitigate climate change.

Resume : Germanium is regarded as an excellent substrate for the development of low-cost thermophotovoltaic devices. However, the poor properties of the germanium oxide (water-soluble and thermodynamically unstable) which is rapidly formed in air, significantly jeopardizes surface passivation. In this work, a study of the influence of an in-situ H2 plasma cleaning before PECVD deposition of an amorphous silicon carbide stack is carried out ultimately demonstrating a very high minority carrier lifetime (> 1000 µs) when germanium oxide is effectively removed from the interface. Besides of the pre-cleaning step, the parameters for the deposition of amorphous silicon carbide has been also optimized, showing a vast influence of the process temperature in the studied range (150-250 °C), with more than 5-fold lifetime enhancement. Finally, the already passivated samples were subjected to a rapid thermal annealing at temperatures between 250 °C and 500 °C, which originates an additional improvement of the minority carrier lifetime, implying a thermally activated reaction for the surface passivation mechanism with an estimated activation energy of about 1.8 eV. Capacitance-voltage measurements will be conducted in order to determine the underlying mechanism of such reaction. This study will propitiate a greater understanding on the key elements of germanium surface passivation, eventually enabling the development of low cost and highly efficient thermophotovoltaic devices.

Resume : In this work, modeling, epitaxy, fabrication and characterization of different interdigitated back contact (IBC) InGaAs thermophotovoltaic (TPV) cells are presented. PC1D modeling reveals the importance of an intrinsic base and an extremely thin emitter, both of which must be circumvented attending to practical fabrication risks. Assuming ideal growth conditions, the simulations also show how an emitter directly grown on top of the semiconductor substrate simplifies and reduces the layer structure at a minimum efficiency cost. The IBC TPV cell design is optimized using a quasi-3D distributed model fed with semi-empirical electronic parameters and solved by SPICE, leading to a fingerless (non-interdigitated) configuration that solely relies on the substrate for the lateral conduction. Different structures are grown by molecular beam epitaxy, from which 1 cm2 back contact TPV cells are fabricated according to this design. Characterization of the first prototypes shows promising results, validating the simulations. The fingerless back contact cells enable minimized shadowing and array-packing losses. Besides, the cleared front surface facilitates their integration in near-field and hybrid thermionic-photovoltaic arrangements, where micro-spacers are needed on the front side of the cell to get close enough to the incandescent source. The trade-off between substrate transparency and lateral series resistance is analyzed as a function of the substrate doping and thickness.

Resume : A Thermophotovoltaic system (TPV) converts radiant thermal energy directly into electricity using a photovoltaic cell. One of the key components for the effective functioning of the TPV is the thermal stability of the selective thermal emitters used. A selective thermal emitter is designed precisely to emit radiation that matches the band gap energy of the photovoltaic cell used. In our work, we fabricate a 1D selective emitters comprising of alternative layers of metal and dielectric materials. The intriguing choice of materials for this application is refractory materials because of their high melting points. The materials used in our selective emitters are W/HfO2, prepared by magnetron sputtering. In-situ XRD annealing experiments are carried out at temperatures above 800 °C and in a vacuum below 1e-5 mbar vacuum to validated the temperature stability. We report structural and morphological changes individually in both the layers and establish a failure mechanism at high temperatures that deteriorates the performance of the 1D selective emitter.

Resume : Heat-assisted light emission makes it possible to reach an energy converting efficiency above unity in light-emitting diodes (LEDs).Theoretical prediction and experiment verification have been reached within bulk system. However, the literature studies usually employ the optical density of the bulk system and it limits the application away from nano-emitters. As it is well-known, nanostructure modifies the optical density of states significantly.Such modification has the potential to increase the entropy of emission and lead to higher energy conversion efficiency. In this work, a self-consistent method is presented for the first time.The entropy of light emission with heat contribution for a nano-sized round sphere and the planar film can be calculated by this method. We conceptually prove the entropy is a powerful tool to understand the light emission at the nanoscale. By considering the entropy of light emission, we find a nanosphere can have much higher energy converting efficiency than bulk. In addition to the efficiency, we find that the light emission intensity per unit volume material goes up approximately inversely proportional to the sphere radius when the radius reduces from bulk to nanosize, which originally comes from the low photon recycling rate for a nanostructure. Similar results of efficiency and cooling power hold true for the thin film with a small thickness. Such findings indicate the nanostructure is a good candidate for high-efficiency LEDs and coolers.

Resume : In indoor condition, the module has been tested at control parameters at STC(ambient temperature 25ºC, wind speed 1ms-1, radiation 1000Wm-2 at AM1.5 spectrum). Whereas, the temperature of the module in outdoor condition is about 20-30 ºC more than the ambient temperature. For the temperature reduction nowadays the radiative cooling concept has been taken into consideration. The implementation of solar radiative cooler is also varying according to its location in the module. Either it is applied on the front glass of the solar cell or applied as ARC. Both the method has its own benefits and restriction. The radiative cooler applied on the top of the solar front glass can be developed as a Bragg reflector which reduces the parasitic absorption. It does not allow to improve the temperature of the module as well as it emits radiation in the atmospheric window to reduce the module temperature further. This method has a limitation with EVA heat transfer coefficient value which is very low due to that the heat transfer through EVA is very low. Another method applied on the solar cell as ARC only which improve the absorption in 300-1100nm as well as improve the cooling effect by the radiative cooling method. However, both methods have able to reduce the temperature in the range of 5-6ºC.