Thermophotovoltaic Research Articles

Comprehensive analysis of an optimized near-field tandem thermophotovoltaic converter

It is well known that performance of a thermophotovoltaic (TPV) device can be enhanced if the vacuum gap between the thermal emitter and the TPV cell becomes nanoscale due to the photon tunneling of evanescent waves. Having multiple bandgaps, multi-junction TPV cells have received attention as an alternative way to improve its performance by selectively absorbing the spectral radiation in each subcell. In this work, we comprehensively analyze the optimized near-field tandem TPV system consisting of the thin-ITO-covered tungsten emitter (at 1500 K) and GaInAsSb/InAs monolithic interconnected tandem TPV cell (at 300 K). We develop a simulation model by coupling the near-field radiation solved by fluctuational electrodynamics and the diffusion-recombination-based charge transport equations. The optimal configuration of the near-field tandem TPV system obtained by the genetic algorithm achieves the electrical power output of 8.41 W/cm$^2$ and the conversion efficiency of 35.6\% at the vacuum gap of 100 nm. We show that two resonance modes (i.e., surface plasmon polaritons supported by the ITO-vacuum interface and the confined waveguide mode in the tandem TPV cell) greatly contribute to the enhanced performance of the optimized system. We also show that the near-field tandem TPV system is superior to the single-cell-based near-field TPV system in both power output and conversion efficiency through loss analysis. Interestingly, the optimization performed with the objective function of the conversion efficiency leads to the current matching condition for the tandem TPV system regardless of the vacuum gap distances.

Sustaining efficiency at elevated power densities in InGaAs airbridge thermophotovoltaic cells

Recent work has demonstrated record-high thermophotovoltaic efficiency using thin-film InGaAs cells, but the power density of devices remains low. Elevated power densities are relevant to many thermophotovoltaic (TPV) applications, ranging from mobile generators to stationary energy storage of renewable electricity, and require effective management of heat and charge carriers. Here we investigate the use of single-junction InGaAs airbridge cells (ABCs) under such conditions. Experimental characterization of an InGaAs ABC with varying emitter and cell temperature is used to develop a predictive device model where carrier lifetimes and series resistances are the sole fitting parameters. The utility of this model is demonstrated through its use in identifying near-term opportunities for improving performance at elevated power densities, and for designing a thermal management strategy that maximizes overall power output. After accounting for the power necessary to cool the cells, this model shows that an InGaAs ABC with high material quality can reach a peak efficiency of ∼41% at 0.5 W/cm2, corresponding to an emitter temperature of 1070 °C, and sustain efficiencies above 36% up to 1.5 W/cm2.

Application of the detailed balance model to thermoradiative cells based on a p-type two-dimensional indium selenide semiconductor

Thermoradiative (TR) cells are energy conversion devices that convert low-temperature waste heat to electricity. TR cells work on the same principles as photovoltaics, but they produce a reverse bias voltage due to higher cell temperature than the environment temperature. Depending on the energy gap of the material, temperature difference would generate electrical energy by electron-hole pair recombination. In this work, we propose a two-dimensional (2D) InSe for applications in the TR cells. The electronic properties of 2D InSe are obtained by using first-principles calculations. Then, the calculated energy gap is used to estimate output power density and efficiency according to the Shockley-Queisser framework through a detailed balance model adapted with the TR cells. Using a heat source at = 1000 K and the ambient temperature = 300 K, an ideal TR cell of 2D InSe at the maximum power point can achieve output power density and efficiency up to 0.061 W/m2 and 4.41%, respectively, with an energy gap of 1.43 eV. However, sub-bandgap and non-radiative losses will degenerate the cell's performance significantly.

Latent heat thermophotovoltaic batteries

<h2>Summary</h2> Latent heat thermophotovoltaic (LHTPV) batteries store electricity as latent heat at very high temperatures (>1,000°C) and convert this heat back to electricity on demand using thermophotovoltaics (TPVs). In this study, we discuss the techno-economics of LHTPV systems, focusing on parameters such as the round-trip efficiency, energy-to-power ratio, cost per energy and power capacities, and levelized cost of storage. The very low cost of the heat storage media (<4 €/kWh) results in optimal designs with high energy-to-power ratios, fitting long-duration storage (LDS) applications. Shorter-duration storage applications are also possible by increasing the overall round-trip conversion efficiency through cogeneration, that is, combined heat and power (CHP) generation. Results indicate that LHTPV systems can provide lower levelized cost of storage than Li-ion batteries in both LDS and CHP applications. Preliminary experimental results are provided to illustrate the real operation of a LHTPV system.

Fabrication of highly lattice mismatched AlInSb diodes on GaAs substrates for thermophotovoltaic cells

We have fabricated and investigated the optical properties of highly lattice mismatched AlInSb thermophotovoltaic cells grown on GaAs substrates. Defects, such as dislocations and surface damage, were minimized by controlling the strain of the film structure and by using a silicon nitride passivation layer. An output power density of 9.7 mW cm−2 was achieved under 1000 °C blackbody radiation with an incident power density of 1.63 W cm−2, which gave a power conversion efficiency of 0.59 %. Optimal efficiency of above 5% was estimated by a simulation using recombination parameters obtained from electroluminescence analysis. Additionally, a six-fold increase in spectral efficiency was confirmed by using a 3.3 μm monochromatic light source, which indicated that a thermophotovoltaic cell with a high efficiency of more than 30% would be possible in combination with wavelength-selective emitter.

Design and Optimization of One-Dimensional TiO2/GO Photonic Crystal Structures for Enhanced Thermophotovoltaics

In this paper, we theoretically explore the spectroscopic features of various one-dimensional photonic crystal (1D-PC)-based spectrally selective filters. The 1D-PC structure is composed of alternating layers of titanium dioxide (TiO2) and graphene oxide (GO). Employing the transfer matrix method (TMM), the impacts of the incidence angle, the number, and thicknesses of TiO2/GO layers in various 1D-PC stacks on the spectroscopic features of the filters are explored in detail. The proposed 1D-PC structures are designed for practical use for thermophotovoltaic (TPV) applications to act as filters that selectively transmit light below 1.78 μm to a GaSb photovoltaic cell, while light with longer wavelengths is reflected back to the source. The optimal design presented here consists of two Bragg quarter-wave 1D-PC filters with different central frequencies stacked to form a single structure. We demonstrate that our optimized 1D-PC filter exhibits a large omnidirectional stop band as well as a broad pass band and weak absorption losses. These features meet the fundamental exigencies to realize high-efficiency TPV devices. Additionally, we show that when integrated in a TPV system, our optimized filter leads to a spectral efficiency of 64%, a device efficiency of 39%, and a power density of 8.2 W/cm2, at a source temperature of 1800 K.

Analytical and numerical evaluation of flammability limit of magnesium-fueled thermophotovoltaic combustor considering inter-regional radiation, heat loss and O2-N2,Ar,He oxidizers

Flame stability and fuel energy density are among the main parameters in the design of a thermophotovoltaic (TPV) combustion system. In this research, the flammability limit of the TPV combustor is investigated considering magnesium as the energy carrier. By proposing an asymptotic flame structure and considering the inter-regional radiation and heat loss, the premixed combustion of magnesium in O2/N2,Ar,He media is modeled. The governing equations and boundary conditions are derived and analytically-numerically solved to determine the lean flammability limit (LFL) as well as the flame features. Subsequently, by modifying flame structure and with the aid of analytical method, the closed-form expressions for flame velocity and temperature are obtained. The results indicate that in N2-O2 medium, the increase in the ambient temperature from 300 to 800 K declines the equivalence ratio (ER) of the LFL from 0.16 to 0.03; at 900 K, the flame get stable and has no extinction. Such a rise in the temperature of He- and Ar-containing media decrements the ERs of instability onset by 6.8 and 5.6 times, respectively. For oxygen mass fractions below 0.25, the LFL of Ar-O2 is higher than He-O2. The trend, however, exhibits a reverse pattern at oxygen mass fractions greater than 0.35.

Ellipsoidal Optical Cavities for Enhanced Thermophotovoltaics

Herein, we present an optical cavity in the form of a prolate ellipsoid that can greatly enhance the performance of solar thermophotovoltaic (STPV) systems. The geometrical parameters of the cavity can be designed to control the degree of photon recycling, the temperature of the emitter within the STPV system, gap distance and effective view factor between the PV cell and the emitter, and to minimize the emission losses. Numerical analysis shows the ellipsoidal optical cavity can be designed to achieve an effective view factor of 88.7% between the emitter and PV cell within an STPV system. Results show an efficiency of 5.62% in an STPV system with a GaSb PV cell and a black-body emitter under solar radiation at a concentration factor of 350X. Further, assuming the surface of the ellipsoidal optical cavity is capable of reflecting 99% of the radiation incident onto its surface, efficiencies of 15.54% can be attained when the solar concentration factor is 1400X. These results are attained for STPV systems without using selective absorbers, emitters or filters. The ellipsoidal optical cavity can be integrated into the design of advanced thermophotovoltaics (TPVs) systems and bring them closer to the high theoretical efficiencies TPV systems are capable of.

Efficiency of Radiative Heat Loss Controlled Solar Thermophotovoltaic (STPV) Cells with Back Reflecting Mirror Surfaces and Double Junction PV Cells

Efficiency of Radiative Heat Loss Controlled Solar Thermophotovoltaic (STPV) Cells with Back Reflecting Mirror Surfaces and Double Junction PV Cells

Fabrication methods for high reflectance dielectric-metal point contact rear mirror for optoelectronic devices

The patterned dielectric back contact (PDBC) structure can be used to form a point-contact architecture that features a dielectric spacer with spatially distributed, reduced-area metal point contacts between the semiconductor back not recognized contact layer and the metal back contact. In this structure, the dielectric-metal region provides higher reflectance and is electrically insulating. Reduced-area metal point contacts provide electrical conduction for the back contact but typically have lower reflectance. The fabrication methods discussed in this article were developed for thermophotovoltaic cells, but they apply to any III-V optoelectronic device requiring the use of a conductive and highly reflective back contact. Patterned dielectric back contacts may be used for enhanced sub-bandgap reflectance, for enhanced photon recycling near the bandgap energy, or both depending on the optoelectronic application. The following fabrication methods are discussed in the article•PDBC fabrication procedures for spin-on dielectrics and commonly evaporated dielectrics to form the spacer layer.•Methods to selectively etch a parasitically absorbing back contact layer using metal point contacts as an etch mask.•Methods incorporating a dielectric etch through different process techniques such as reactive ion and wet etching.

GeSn (0.524 eV) single-junction thermophotovoltaic cells based on the device transport model

Based on the transport equation of the semiconductor device model for 0.524 eV GeSn alloy and the experimental parameters of the material, the thermal–electricity conversion performance governed by a GeSn diode has been systematically studied in its normal and inverted structures. For the normal p+/n (n+/p) structure, it is demonstrated here that an optimal base doping N d(a) = 3 (7) × 1018 cm−3 is observed, and the superior p+/n structure can achieve a higher performance. To reduce material consumption, an economical active layer can comprise a 100 nm–300 nm emitter and a 3 μm–6 μm base to attain comparable performance to that for the optimal configuration. Our results offer many useful guidelines for the fabrication of economical GeSn thermophotovoltaic devices.

Evaluation of the emitter structure and temperature effect on a thermophotovoltaic system with an optimal cavity

Thermophotovoltaic (TPV) systems convert thermal energy to electrical power. In this paper, a TPV system with an optimal cavity which has three identical photovoltaic arrays and contains InGaAsSb cells considered as reference system. TracePro used to obtain the irradiance density on the surface of PV cells in TPV systems and then electrical parameters calculated using MATLAB. Short-circuit current (Isc) and open-circuit voltage (Voc) for each array are 4.39 A and 49.19 V, respectively. The whole system generates 433.2 W power with an efficiency of 10.91%. Some cases with various conditions based on the reference model are considered to evaluate the emitter structure and temperature’s effect on the TPV system. In the reference system, by changing the cylindrical emitter to a three-sided emitter, the system efficiency increased to 14.33%. Besides, the three arrays containing the InGaAs cells are used as the second layer in the reference system. In this regard, the measured characteristics for the second layer are 1.15 A for Isc and 46.58 V for Voc, respectively. The second layer generated 35.96 W power and demonstrated an increased efficiency of 2.71%, meaning 13.63% efficiency for the whole system. Moreover, system performance at different temperatures for the emitter is investigated. For the lowest emitter temperature (i.e. 1000 K) the Isc = 1.46 A, Voc = 41.14 V, 108.18 W total output power and the 6.96% system efficiency are obtained. While for the highest temperature (i.e. 1900 K) the electrical characteristics for each array are Isc = 10.56 A and Voc = 54.35 V with a total generated power of 911.91 W and the efficiency is 12.36%.

Thermodynamics of Light Management in Near-Field Thermophotovoltaics

Thermophotovoltaic (TPV) systems hold promise for recycling heat into electricity, but a key challenge is to lower the operating temperature to the range of industrial waste heat, at high efficiency. The authors offer a thermodynamic analysis of the open-circuit voltage of TPV systems operating in the far and near fields, and compare to solar PVs. An analytic model shows that although nonradiative losses increase in the near field, luminescence efficiency improves significantly in this range. This work suggests that an ultrathin near-field TPV system can exhibit ultrahigh open circuit voltage and high efficiency, for emitter temperatures much lower than its far-field counterpart.

Statistical study of thermoradiative and photovoltaic cells based on a two-level model

We use a two-level energy model to understand the conversion process that takes place in thermoradiative cells and to compare it with the conversion process that happens in photovoltaic cells. In this way, we show that in both kinds of converters the conversion process can be studied as the succession of a change in the populations of the levels that occur at constant chemical potential and a change in the value of the chemical potential of the two levels that happens while keeping their populations constant. As an application of the model, we will discuss why in thermoradiative cells the open-circuit voltage is negative while it is positive in photovoltaic cells. We also show that the expression for the open-circuit voltage is the same in both kinds of cells but that due to the values of the temperatures it is negative in thermoradiative cells and positive in photovoltaic ones.

Performance optimization and parametric selection strategies of a sodium thermal electrochemical converter-thermophotovoltaic cell integrated system

Explorations of efficient thermoelectric devices to abandon fossil fuels are widely concerned. Sodium thermal-electrochemical converter can efficiently convert heat into electricity, but much heat dissipated from the converter restricts its performance. A novel integrated system composed of a sodium thermal-electrochemical converter and a thermophotovoltaic cell is first proposed to realize hierarchical utilization of energy. Expressions for the power output densities and efficiencies of each subsystem and integrated system are derived. Performance characteristics of the integrated system are theoretically analyzed. Influences of the current density of the converter, the voltage output and bandgap energy of the cell, and the electrode area ratio of the two subsystems on the integrated systemic performance are comprehensively evaluated. The maximum power output density and efficiency of the integrated system attain, respectively, 18.54 Wcm −2 and 48.93%, which increase, respectively, 350.0% and 35.16% compared with those of the two-stage sodium thermal-electrochemical converter, and 188.3% and 41.42% compared with those of the sodium thermal-electrochemical converter-thermoelectric generator integrated system. The optimum selection criteria of main parameters are given, providing valuable references for the actual manufacture and operation. The results show that the materials with bandgap energy around 0.290 eV are preferred for the fabrication of the proposed integrated devices. • A thermophotovoltaic cell is coupled to a sodium thermal electrochemical converter. • The waste heat from the converter is efficiently utilized by the cell. • The maximum efficiency 48.93% and power output density 18.54 Wcm −2 are obtained. • Optimal selection criteria of main parameters are given. • Advantages of the present system over other relevant coupled systems are expounded.

Improving the performance of solar thermophotovoltaic (STPV) cells with spectral selected absorbers and small apertured radiation shields

An integrated analysis considering radiative energy transport among each component and carrier generation in the photovoltaic (PV) material is established for solar thermophotovoltaic (STPV) devices. STPV has been estimated with previous thermodynamics analysis to provide up to ∼85% solar energy conversion efficiency (ECE) because of its capability to convert broadband solar radiation to narrowband thermal radiation that can be fully used with PV materials. However, in previous demonstrations, the ECE of STPVs is less than 10%, even with ∼200 solar radiation concentrations. Based on our analysis, the maximum ECE of STPV without any radiation heat loss control is nearly zero when the solar concentration ratio is 1 and is ∼18.78% when the solar concentration ratio is 1000. Maximum ECE of STPV with spectral selected absorber optimized for 1000 K level temperature, which has been proposed in recent STPV studies, can be ∼9.56% when the solar concentration is 1 and can be ∼31.72% when the solar concentration ratio is 1000. Maximum ECE of STPV with our proposed multi-layered radiation shield to prevent radiative heat loss from the STPV to the ambient can provide ECE up to ∼9.53% and ∼57.02%, when the solar concentration ratio is 1 and 1000, respectively. When we combine both the spectral selected absorber and our designed radiation shield, the ECE value of STPV can be up to ∼20.65% and ∼59.23% when the solar concentration ratio is 1 and 1000, respectively. It is also observed that a ∼1 µm level bandwidth of photonic crystal (PhC), a device that can control the emission bandwidth, is required for STPV to achieve high ECE under each solar concentration ratio with difference radiative heat loss control.

Hybrid Thermoelectric–Photovoltaic Generators under Negative Illumination Conditions

This paper analyses the working principles of hybrid thermoelectric photovoltaic generators under negative illumination (also referred to as thermoradiative configuration). These kinds of systems combine a thermoradiative photovoltaic cell (TR-PV cell) and a thermoelectric generator (TEG), placed in thermal contact with each other. In this configuration, the TR-PV part cools while irradiating toward the cold sky. For this reason, in addition to the generation of electrical output, the cell can set a difference of temperature (ΔT) across the TEG legs. A theoretical model describing the behavior of these kinds of hybrid devices is reported as a function of the emitter energy gap and temperature, the sky temperature, and the ΔT across the TEG. In analogy with the positive illumination case, the key parameter is found to be the cell temperature sensitivity, which sets the convenience of the hybrid approach. The results show that while the hybrid power density is in general smaller than the sole TR-PV case, a wide window of positive efficiency gains exists. This is possible because the outgoing power density varies with the cell temperature, in contrast with the positive illumination case where the incoming power density is fixed by the temperature of the Sun. This work sets the first theoretical attempt to understand the convenience of TR-hybrid thermoelectric–photovoltaic generators (TR-HTEPVG), quantitatively assessing the suitability of these novel kinds of devices.

A hybrid system integrating solid oxide fuel cell and thermo-radiative-photovoltaic cells for energy cascade utilization

A novel hybrid system coupling solid oxide fuel cell (SOFC) with thermoradiative (TR) and photovoltaic (PV) cells is proposed, evaluated, and optimized for energy cascade utilization. Theories of electrochemistry, Planck radiative heat transfer, and first law of thermodynamics are applied to assess and optimize the performance of the hybrid system. Firstly, energy balance analysis is conducted to obtain suitable area ratio between the subsystems and the SOFC. A homo-structure InAs–InAs is chosen as an example of the TP-PV cells. The peak power density of 0.669 W cm −2 and the maximum efficiency of 0.770 and the relevant work conditions are achieved through parametric optimal analysis. It is also found that decreasing the leakage resistance of the SOFC can enhance electricity production and efficiency of the hybrid system. Secondly, a GaSb-InSb TP-PV cells are adopted to couple with the SOFC for performance enhancement. Finally, the positive effects of back surface reflector and the negative effects of irreversible heat transfers on the hybrid system are discussed. The obtained results are helpful for designing and optimizing the SOFC-TR-PV hybrid systems. • An SOFC-TR-PV hybrid system is proposed and studied to enhance power and efficiency. • An optimum structure of TP-PV cells is designed as GaSb-InSb. • The effects of optical filter and Newton's heat transfers on the system are revealed. • The maximum power and efficiency and the conditions are obtained under various cases. • The present work are of great importance to design and optimize the hybrid systems.

Performance analysis of indium antimonide thermophotovoltaic system with varied material and geometrical properties

Thermophotovoltaic (TPV) system generates electricity using the selective emission of radiation on a photovoltaic (PV) cell by heating an emitter to a very high temperature. In this work, Indium Antimonide (InSb) is used as it has a low bandgap of 0.23 eV and the performance of the InSb TPV system is compared to an Indium Gallium Arsenide (InGaAs) TPV system. TPV system of different geometries like cylindrical, flat planar, and polygonal shapes are simulated to understand the influence of TPV structure in the efficiency and reliable operating conditions of the TPV system. To enhance the efficiency of the TPV cell, a photonic crystal (PhC) filter is also inserted between the cell and the emitters that consists of a blackbody, Erbium Oxide (Er2O3), and Tungsten to allow only selective radiation from the emitter to the PV cell. The proposed TPV system has a simple structure, it is thermally stable, easy-to-manufacture, and it contains no moving parts. In summary, the performance of an InSb TPV system of different geometries is analysed by matching it with a filter and emitters made of different materials. The performance of the InSb TPV system is then benchmarked against an InGaAs TPV system.

Effect of Front Electrode on the Structure and Performances of Ga x In1-x As y Sb1-y Thermophotovoltaic Cell

We have theoretically investigated the effect of front electrode on the structure and performances of Ga x In $_{1-{x}}$ As y Sb $_{1-{y}}$ thermophotovoltaic (TPV) cells. By sampling a comb-like electrode, it is clearly demonstrated here that the optimum grid separation ${d}_{G}$ depending on the cell bandgap yields a quadratic function for a given spectrum illumination, while the desired coefficients are strongly temperature-dependent and can be well modeled by an exponential decay expression. Comparing with previous report with constant grid separation, the denser front electrode promises an inferior cell efficiency, but the similar phenomenological method can be still applicable to predict the structure and performance of Ga x In $_{1-{x}}$ As y Sb $_{1-{y}}$ TPV cells for a given operation environment.