Search Results (52)

Interest in germanium electronic devices is experiencing a comeback thanks to their suitability for a wide range of new applications, like CMOS transistors, quantum technology or infrared photonics. Among these applications, Ge-based thermophotovoltaic converters could become the backbone of thermo-electrical batteries. However, these devices are still far from the efficiency threshold needed for industrial deployment, with surface recombination as the main limiting factor for the material. In this work, we discuss the main passivation techniques developed for germanium photovoltaic and thermophotovoltaic devices, summarizing their main advantages and disadvantages. The analysis reveals that surface recombination velocities as low as 2.7 cm/s and 1.3 cm/s have already been reported for p-type and n-type germanium, respectively, although improving surface recombination velocities below 100 cm/s would result in marginal efficiency gains. Therefore, the main challenge for the material is not reducing this parameter further but developing robust and reliable processes for integrating the current techniques into functional devices. Full article

The micro combustor is the energy source of micro-thermophotovoltaic systems; thus, optimizing its design is one of the key parameters that lead to an increase in output energy. Therefore, to enhance the system’s overall efficiency, this numerical work introduces a new design configuration for parallel-flow (PF) and counter-flow (CF) hydrogen-fueled micro cylindrical combustors integrated into a micro planar combustor. To overcome the short residence time in micro combustor applications causing high heat dissipation, the micro cylindrical combustors house heat-recirculating channels to allow more heat to transfer to the external walls. In pursuit of this target, simulations are carried out to analyze the thermodynamic and system efficiency parameters. In addition, different initial operating conditions are varied to optimize the system, including inlet velocity and equivalence ratio. The results reveal that the PF and CF structures result in significantly higher wall temperatures and more uniform wall temperature variations than the conventional design (CD). Despite the high entropy generations, the exhaust gas temperatures of the PF and CF are 591 K and 580 K lower than the CD, respectively, and both the PF and CF result in 14% increases in radiation efficiency. Increasing the inlet velocity improves the key thermal parameters in the new designs; however, the system efficiency experiences a drastic reduction. The power output density highlights the unity equivalence ratio as optimal. The PF and CF designs yield roughly identical findings, but the CF exhibits more uniform wall temperatures in most cases due to the equal thermal energy from opposite sides. Full article

Advancements in thermophotovoltaic (TPV) technologies enable a new alternative for the electrification of nuclear power. These solid-state heat engines are more robust and likely cheaper to manufacture than the turbomachinery used in traditional microreactor concepts. The Radiantly Integrated TPV-microreactor system (RITMS) described in this work takes a novel approach to utilizing direct electric conversion of thermal power radiated from the active core. Without intermediary energy transfer, this direct coupling allows for system efficiencies well above 30%. While providing an introduction to the concept, the early RITMS work lacked an integrated computational sequence and economics-by-design approach, resulting in a failure to fully capture the physics of the system or to properly evaluate design parameter importance. The primary purpose of this paper is to describe and demonstrate a computational sequence that fully couples the conductive-radiative heat transfer with a neutronic solution and to provide design-specific cost estimation. This new computational framework is deployed in re-examining the multi-physics behavior of the RITMS design and to perform consistent trade-off studies. A favorable RITMS design was selected based on performance and fuel cycle costs, which was deemed feasible when considering cost uncertainty. Able to operate on 7% enriched fuel, this RITMS case was selected to balance fuel utilization with total power output. Full article

To improve the heat transfer mechanisms from the thermal energy to the walls, the current work presents a new structure for a micro combustor fueled by hydrogen featuring a diamond-shaped bifurcated inner-tube configuration. For this purpose, a series of three-dimensional (3D) numerical analyses are conducted to investigate the effects of the length of the diamond-shaped structure, width of inner flame channels, inlet equivalence ratio, and hydrogen volume flow rate on the key performance and thermodynamic parameters. In comparison to the conventional design, the outcomes reveal that the proposed configuration exhibits remarkable improvements in energy conversion efficiency, as it reduces the mean exhaust gas temperature by 585.98 K and boosts the exergy and radiation efficiencies by 7.78% and 14.08%, respectively. The parametric study of the design parameters indicates that elongating the diamond-shaped structure and widening the inner flame channels enhance the thermal dynamics and consequently improve the rates of heat absorption by the walls. The increase in the hydrogen volume flow rates feeds the system with additional energy and, therefore, advances the average wall temperature and its uniformity across the external surface. Nevertheless, it also reduces system efficiency due to the limited capacity of the micro combustor to utilize a large energy input along with the high magnitude of entropy production resulting particularly from the mechanism of chemical entropy generation. Operating under a stoichiometric condition balances hydrogen and oxygen in the premixed charge, achieving optimal thermal performance for the micro combustor. Full article

To enhance the performance of combustors in micro thermophotovoltaic systems, this study employs numerical simulations to investigate a planar microscale combustor featuring a counter-flow flame configuration. The analysis begins with an evaluation of the effects of (1) equivalence ratio Φ and (2) inlet flow rate V_i on key thermal and combustion parameters, including the average temperature of the combustor main wall (${\overline{T}}_w$), wall temperature non-uniformity (${\overline{R}}_{Tw}$) and radiation efficiency (${\eta }_r$). The findings indicate that increasing Φ causes these parameters to initially increase and subsequently decrease. Similarly, increasing the inlet flow rate leads to a monotonic decline in ${\eta }_r$, while the ${\overline{T}}_w$ and ${\overline{R}}_{Tw}$ exhibit a rise-then-fall trend. A comparative study between the proposed combustor and a conventional planar combustor reveals that, under identical inlet flow rate and equivalence ratio conditions, the use of the counterflow flame configuration can increase the ${\overline{T}}_w$ while reducing the ${\overline{R}}_{Tw}$. The Nusselt number analysis shows that the counter-flow flame configuration micro-combustor achieves a larger area with positive Nusselt numbers and higher average Nusselt numbers, which highlights improved heat transfer from the fluid to the solid. Furthermore, the comparison of blow-off limits shows that the combustor with counter-flow flame configuration exhibits superior flame stability and a broader flammability range. Overall, this study provides a preliminary investigation into the use of counter-flow flame configurations in microscale combustors. Full article

The current study proposes a novel design configuration of a hydrogen-fueled micro cylindrical combustor. The newly developed design consists of a single-channel inlet and a double-channel outlet with a heat-recirculating structure aimed at enhancing the heat transfer mechanism from the combustion to the walls. Investigations are conducted using three-dimensional numerical simulation means, and emphasis is placed on assessing the effects of the novel design structure on key thermal parameters and nitrogen oxide (NO_x) emissions. The numerical modeling approach is first validated against the experimental and numerical data available in the literature. A parametric study is then conducted by means of varying the length and width of the heat-recirculating channel, inlet velocity, and inlet equivalence ratio. The findings revealed that the novel design configuration significantly improves thermal performance and curtails NO_x emissions in comparison with those of the conventional structure. For example, the proposed design leads the radiation efficiency to increase by roughly 10%. The increase in the width of the preheating channel yields further optimization by boosting the heat transfer process from the flame to the walls. Elevating the inlet velocity exhibits a pronounced increase in the mean wall temperature and a more uniform distribution of the wall temperature. However, the exhaust gas temperature increases with increasing inlet velocity, leading to a reduction in the exergy and radiation efficiencies. The equivalence ratio of unity optimizes key thermal parameters, as the lean and rich conditions suffer from low hydrogen and oxygen contents, respectively. Full article

Ultrathin cells are gaining popularity due to their lower weight, reduced cost, and enhanced flexibility. However, compared to bulk cells, light absorption in ultrathin cells is generally much lower. This study presents a numerical simulation of a metamaterial light management structure made of ultrathin lead sulfide colloidal quantum dots (PbS CQDs) sandwiched between a top ITO grating and a tungsten backing to develop an efficient hybrid solar/thermophotovoltaic cell (HSTPVC). The optical properties were computed using both the finite integration technique (FIT) and the finite element method (FEM). The absorptance enhancement was attributed to the excitations of magnetic polaritons (MP), surface plasmon polaritons (SPP), and lossy mode resonance (LMR). The HSTPVC with the metamaterial optical light management structure was assessed for short-circuit current density, open-circuit voltage, and conversion efficiency. The results show a conversion efficiency of 18.02% under AM 1.5 solar illumination and a maximum thermophotovoltaic conversion efficiency of 12.96% at TB = 1600 K. The HSTPVC can operate in a hybrid solar/thermal conversion state when the ITO grating is included by combining the advantages of QDs and metamaterials. This work highlights the potential for developing a new generation of hybrid STPV cells through theoretical modeling and numerical simulations. Full article

The combustion characteristics of micro-combustors significantly impact the performance of micro-thermophotovoltaic (MTPV) systems. This study aims to investigate the effects of sinusoidal-shaped walls and catalyst segmentation on flame stability and combustion performance in a micro-combustor by using numerical methods. The numerical simulation with detailed gas-phase and surface reaction mechanisms is reliable, as the results of numerical simulation align with experimental data. The results show that the interplay between flame stability and sinusoidal-shaped walls is crucial, particularly because of the cavities formed by the sinusoidal-shaped walls of the micro-combustor. The gas-phase ignition position of the sinusoidal-shaped wall combustor moves upstream by 0.050 m compared to the planar-wall combustor, but the flame is stretched. The catalyst segments coated on the crest can shorten the flame length and increase the average temperature by a maximum 62 K, but delay the gas-phase ignition. Conversely, catalyst segments coated on the trough can advance ignition, but this results in flame elongation and a decrease in the average temperature. The rational combination of catalyst segmentation and sinusoidal-shaped walls facilitates moving the ignition position upstream by a maximum of 0.065 m while substantially reducing the length of the combustor required for complete fuel conversion by more than 60%. These attributes are highly beneficial for improving efficiency and minimizing the length of the micro-combustor for MTPV application. Full article

Efficient solar thermal conversion is crucial for renewable clean energy technologies such as solar thermal power generation, solar thermophotovoltaic and seawater desalination. To maximize solar energy conversion efficiency, a solar selective absorber with tailored absorption properties designed for solar applications is indispensable. In this study, we propose a broadband selective absorber based on amorphous carbon (a-C) metamaterials that achieves high absorption in the ultraviolet (UV), visible (Vis) and near-infrared (NIR) spectral ranges. Additionally, through metal doping, the optical properties of carbon matrix materials can be modulated. We introduce Ti@a-C thin film into the nanostructure to enhance light absorption across most of the solar spectrum, particularly in the NIR wavelength band, which is essential for improving energy utilization. The impressive solar absorptivity and photothermal conversion efficiency reach 97.8% and 95.6%, respectively. Notably, these superior performances are well-maintained even at large incident angles with different polarized states. These findings open new avenues for the application of a-C matrix materials, especially in fields related to solar energy harvesting. Full article

Utilizing integrated solar systems and renewable energy sources has the potential to not only decrease the CO₂ emissions of buildings but also provide access to more affordable energy alternatives compared to fossil fuels, especially considering the recent rise in prices. Nevertheless, many designers and project decision makers are hesitant to embrace solar technologies due to the uncertainty surrounding the cost–benefit balance. This paper presents a case study of the design process, highlighting the energy-saving and cost–benefit aspects of a solar façade featuring solar thermophotovoltaics (STPVs) and wavelength-selective thermophotovoltaics (TPVs) in a high-rise residential building situated in the semi-arid climate of Tehran, Iran. The simulation methodology consists of EnergyPlus Engines in Rhino, along with the Ladybug and Honeybee plugins throughout the solar façade design process. The solar façade incorporating STPV yielded energy savings of 25 kWh per square meter, marking a 34% reduction compared to a standard façade. In contrast, the TPV demonstrated energy savings of 35 kWh per square meter, indicating a 48% decrease in energy consumption compared to a regular façade. This research indicates that, particularly in semi-arid climatic conditions, TPV systems exhibit a superior performance when integrated into the façades of high-rise structures; yet, due to the low electricity prices in the region, neither STPVs nor TPVs are financially viable. The study contributes to raising awareness, fostering technological innovations, influencing policy discussions, and promoting the adoption of sustainable practices in the field of energy and architecture. Full article

A theoretical model is developed to determine the Shockley–Queisser efficiency limit of solar thermophotovoltaic (STPV) cells with single- or double-junction photovoltaic (PV) cells and a simple radiation shield considering the divergence nature of concentrated solar radiation. A combination of adaptive parametric sweep and graphic-based methods is developed to solve the highly nonlinear correlations of energy and carrier transports in the theoretical model to find the optimized operating conditions of STPVs with high stability. The theoretical model predicts that the Shockley–Queisser efficiency limit of STPV under 1000× solar concentration and a simple radiation shield is ~50.1% with InGaAsSb PV cells, ~49.1% with GaSb PV cells, and ~53.2% with InGaAsSb/GaSb double-junction PV cells. The operating temperatures are ~1719.5 K, ~1794.1 K, and 1640.0 K, respectively. An observation from the modeling is that the energy loss due to the thermalization of hot carriers in the STPV with spectrally selected emitters is ~40% less than that in single-junction solar cells. Also determined from the modeling is that ~20% of the collected solar energy is still lost through thermal radiation, even with a simple radiation shield to block the radiative heat loss to the surroundings. Following this understanding, a further improvement in the Shockley–Queisser efficiency of STPVs can be achieved by adopting advanced designs of radiation shields that can separate the absorber of the STPVs far away from the aperture of the radiation shield without using a large-area absorber. Full article

The specific work presents an optical and thermal investigation of a hybrid thermo-photovoltaic solar collector with an asymmetrical compound parabolic mirror. Such collectors offer an innovative and sustainable approach to address both the thermal and electrical demands of residents on islands using renewable sources of energy and thus reducing the dependency on fossil fuels. The main goal of this investigation involves an analysis of the prementioned type of solar collector, incorporating an innovative and cost-effective numerical modelling technique aiming to enhance comprehension of its energy and exergy performance. The optical performance of the collector was calculated first with ray tracing for the month of June, and the ideal slope was determined for the same month. After the optical analysis, the energy and exergy performance were both estimated by implementing a novel numerical method in both COMSOL and SolidWorks. Based on the optical analysis, it was determined that the most favorable inclination angle for achieving optimum optical efficiency on the mean day of June is 10°. The thermal analysis, focusing on thermal efficiency, showed a maximum deviation of 5.3% between the two solutions, which indicates the reliability of the method. The collector achieved a maximum thermal efficiency of 58.55% and a maximum exergy efficiency of 16.94%. Full article

Since the use of chemical fuels is permanently damaging the environment, the need for new energy sources is urgent for mankind. Given that solar energy is a clean and sustainable energy source, this study investigates and proposes a six-layer composite ultra-wideband high-efficiency solar absorber with an annular microstructure. It achieves this by using a combination of the properties of metamaterials and the quantum confinement effects of semiconductor materials. The substrate is W–Ti–Al₂O₃, and the microstructure is an annular InAs-square InAs film–Ti film combination. We used Lumerical Solutions’ FDTD solution program to simulate the absorber and calculate the model’s absorption, field distribution, and thermal radiation efficiency (when it is used as a thermal emitter), and further explored the physical mechanism of the model’s ultra-broadband absorption. Our model has an average absorption of 95.80% in the 283–3615 nm band, 95.66% in the 280–4000 nm band, and a weighted average absorption efficiency of 95.78% under AM1.5 illumination. Meanwhile, the reflectance of the model in the 5586–20,000 nm band is all higher than 80%, with an average reflectance of 94.52%, which has a good thermal infrared suppression performance. It is 95.42% under thermal radiation at 1000 K. It has outstanding performance when employed as a thermal emitter as well. Additionally, simulation results show that the absorber has good polarization and incidence angle insensitivity. The model may be applied to photodetection, thermophotovoltaics, bio-detection, imaging, thermal ion emission, and solar water evaporation for water purification. Full article

A variety of nanostructures capable of generating strong local electromagnetic fields (hot spots) in interaction with radiation have been under intensive investigation towards plasmonic applications in surface-enhanced Raman scattering (SERS), biosensing, broadband absorbers, thermophotovoltaics, photocatalysis, etc. In many cases, these nanostructures are formed on a surface or embedded into a near-surface layer of the dielectric substrate, making some part of the field dissipate into the bulk and not contribute to the desired plasmonic functionality. To reduce such losses, the interface between the metallic nanostructures and the dielectric environment should be engineered. In the current work, Au nanodiscs are fabricated on Si posts of very small diameter (pin-shaped structures), enabling them to decouple the strong optical near fields localized at the nanodiscs from the bulk Si substrate. The Si post diameter is optimized by adjusting the gas flow rates in reactive-ion etching, resulting in a minimum post diameter of 20 nm at the nanodisc interface. The effect of this diameter on the localized surface plasmon resonance of the nanodisc is investigated with linear optical spectroscopic measurements, where a significant spectral blue shift of the resonance band is noticed compared to similar discs formed on bulk Si surface. The experimental results are compared with modelling where a 3-fold increase in the electric field enhancement is demonstrated. The fabricated pin-shaped nanostructures are tested in SERS measurements showing a significant increase in the enhancement factor in the order of 10⁶. Thus, this work suggests a way of engineering 3D morphology to tune the substrate influence on the plasmonic properties of nanostructures and to develop efficient nanofabrication technologies. Full article

Terahertz absorbers have been extensively investigated by researchers due to their applications in thermophotovoltaic energy conversion and sensors, but a key factor limiting their development is the lack of vital and versatile materials. Ferromagnetic shape memory alloys (FSMAs) offer a novel remedy for tunable metamaterials due to their brilliant recovery of deformation, remote control, and transient response. In this study, we propose a tunable absorber based on magnetic field tuning, consisting of Ni–Mn–Sn ferro-magnetic shape memory alloy films in fractal geometry and optically excited Si plates. Numerical analysis shows that the proposed absorber has an absorbance bandwidth of 1.129 THz above 90% between 1.950 THz and 3.079 THz. The absorber geometry can be regulated by an external magnetic field, allowing dynamic switching between broadband and narrowband absorption modes, the latter showing an ultra-narrow bandwidth and a high-quality factor Q of ~25.8. The proposed terahertz absorber has several advantages over current state-of-the-art bifunctional absorbers, including its ultra-thin structure of 10.39 μm and an additional switching function. The absorption can be continuously tuned from 90% to 5% when the light-excited silicon plate is transferred from the insulator state to the metal state. This study presents a promising alternative strategy for developing actively regulated and versatile terahertz-integrated devices. Full article