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This paper describes and presents the direct numerical simulation of a volumetric solar receiver under concentrated irradiation. The simulations systematically analyze the receiver performance and thermal loss mechanisms for operating conditions when outlet temperatures are in the 1000–1500 °C range. The implemented numerical method fully considers interactions between radiation, convection, and conduction using a discrete ordinates approach as a radiation model. The power over air mass (POM) and air mass flux were modified for six receiver channels with different cell sizes. The receiver efficiency decreased when the air mass flux increased beyond a certain criterion for constant POM. For a 3.8 mm cell, the receiver efficiency decreased to

To perform thermochemical cycles using non-volatile metal oxides to split water and produce hydrogen, a directly irradiated fluidized bed reactor is designed and fabricated for beam-down configuration. As the main aim of this investigation is to analyze the heat transfer and particulate flow of the reactor, chemically inert particles are used. A transient 3D heat and mass transfer model is formulated by the combined approach of discrete element method and computational fluid dynamics. The radiative transfer is solved using the discrete ordinate radiation model. Experimental validation is accomplished by the measured temperatures, obtained with the fluidized bed reactor prototype tested under 30 kWth high-flux solar simulator. The model is applied to analyze the granular flow characteristics and efficiency of the reactor for various superficial gas velocities and bed masses. The results indicate that higher gas flow rate increases the velocity and convection loss of the bed and decreases the bed temperature and efficiency of the reactor.

Two-point correlation and integral scale of curved channel turbulent flow were examined for onset and growth of large-scale vortex. Away from the two walls of the straight channel, the two-point correlation of streamwise velocity has large negative values around local minimum at y/δ = 1.6 and those of radial and spanwise components take small negative values at the same area. The large-scale motion in the straight channel is thus indicated to have conspicuous streamwise component and weak secondary motion. In the curved channel, two-point correlation of radial velocity decreased around the minimum away from the wall (y/δ = 1.6) and large negative values extended to the whole channel width. It is thus indicated that the large-scale vortex originated from the large-scale structures with conspicuous streamwise component in the outer-wall side, grew and expanded to the full channel width of the curved part. The integral scale integrating two-point correlation conditionally on the negative values greatly decreased to denote the onset and growth of the large-scale motion quantitatively. The local minima of two-point correlation for radial velocity were used as indicator of the large-scale vortex and near-wall small-scale vortex. The magnitude of it at the channel center area was as low as one thirds of those of near-wall area in the straight channel, and however increased and surpassed those of near-wall area in the curved channel. Therefore, large-scale vortices were weaker than the small-scale motion in the straight part of the channel, and they grew and surpassed the small-scale motion in strength in the curved part of the channel.

Gasification of carbonaceous materials using concentrated solar thermal radiation has been considered as one of the promising renewable pathways to produce syngas. In this study, thermal performance of a recently developed solar thermochemical reactor is presented. To analyze the gas–solid flow and heat transfer characteristics of the reactor, a transient three dimensional numerical model has been developed using discrete element method and computational fluid dynamics. Particle collision dynamics has been solved by the spring-dashpot model based on the soft-sphere method. To perform model verification, experiments have been performed using 30kWth fluidized bed reactor prototype under high flux solar simulator. The particulate and thermal characteristics of spout, annulus and fountain of the fluidized bed are analyzed for different irradiation power, loaded powder and gas flow rates. The results indicate that large and small size particles govern the bottom and fountain part of the bed respectively due to gravitational force, and the peak temperature is moved from fountain core to the fountain periphery of the bed when increasing the gas flow rate.

The surface electronic properties of the light absorber and band alignment at the p/n heterointerface are key issues for high-performance heterojunction solar cells. We investigated the band alignment of the heterointerface between cadmium sulfide (CdS) and Ge-incorporated Cu2ZnSnSe4 (CZTGSe), with Ge/(Ge + Sn) ratios ( x) between 0 and 0.4, by X-ray photoelectron, ultraviolet, and inversed photoemission spectroscopies (XPS, UPS, and IPES, respectively). In particular, we used interface-induced band bending in order to determine the conduction band offset (CBO) and valence-band offset (VBO), which were calculated from the core-level shifts of each element in both the CdS overlayer and the CZTGSe bottom layer. Moreover, the surface electronic properties of CZTGSe were also investigated by laser-irradiated XPS. The CBO at the CdS/CZTGSe heterointerface decreased linearly, from +0.36 to +0.20 eV, as x was increased from 0 to 0.4; in contrast, the VBO at the CdS/CZTGSe heterointerface was independent of Ge content. Both UPS and IPES revealed that the Fermi level at the CZTGSe surface is located near the center of the band gap. The hole concentration at the CZTGSe surface was on the order of 1011 cm-3, which is much smaller than that of the bulk (∼1016 cm-3). We discuss the differences in hole deficiencies near the surface and in the bulk on the basis of laser-irradiated XPS and conclude that hole deficiencies are due to defects distributed near the surface with densities that are lower than in the bulk, and the Fermi level is not pinned at the CZTGSe surface.

A combined numerical and experimental investigation of hydrodynamics and heat transfer of internally circulating fluidized bed reactor for solar gasification of coal coke to produce syngas is reported. As the main objective of this study is to study the thermal performance of the reactor, chemically inert quartz particles have been used. A numerical model has been developed by the combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The radiation transfer equation has been solved by the discrete ordinate (DO) model. The experimental data have been used for model validation. The thermal performance of the reactor and the spout-annulus flow have been predicted by the model and other hydrodynamic parameters such as internal solid volume fraction, velocity and temperature of the particles are analyzed as a function of superficial gas velocity (gas flow rate) at different initial conditions.

A stacked layer of ultrathin hydrogenated silicon oxide (SiOx:H) and hydrogenated amorphous silicon (a-Si:H) has been developed to passivate the crystalline silicon (c-Si) surface (see graphical abstract). Silicon oxide has the advantage of excellent optical and passivation properties. The SiOx:H layer is deposited on the c-Si surface by atomic layer deposition (ALD), with its thickness precisely controlled below 2 nm. The a-Si:H layer is deposited on the SiOx:H layer by plasma-enhanced chemical vapor deposition (PECVD) with a specific doping property, i.e. intrinsic, n- or p-type. The samples are then annealed in the range of 100 °C to 950 °C to study the fundamental passivation properties. We find that a combination of an ultrathin SiOx:H and (p) a-Si:H layers shows a favorable passivation compared to a neat (p) a-Si:H layer. The effective minority carrier lifetime, measured by quasi-steady-state photoconductance (QSSPC), is ~ 0.5 ms after low temperature annealing at 300 °C. The passivation property is discussed in terms of hydrogen concentration, bond configurations, stoichiometry x of SiOx:H, and material microstructure, characterized by Fourier transform infra-red (FTIR) and Raman spectroscopy. It is suggested that a reorganization of both the SiOx:H, and the (p) a-Si:H layers, associated with hydrogen diffusion, plays an important role in improving the passivation.

Considerable attention is being paid in recent years to develop and improve alternative technologies to replace fossil fuels due to pollution problems and limited conventional energy sources. Solar thermal energy has been considering as one of the most attractive renewable sources to provide solution to the aforementioned problems. In recent years, latent thermal energy storage systems using phase change materials (PCMs) have been developed to aid energy supply and demand of solar thermal power plants. As one of the obstacles of these systems is the low thermal conductivity of PCMs, ceramic fin has been used in this study to enhance heat transfer rate between the PCM and heat transfer fluid. To assess the heat transfer characteristics of the triplex-tube thermal energy storage system with ceramic fin, a numerical model has been developed and the effect of fin for various Stefan numbers has been studied numerically. The heat transfer and solidification rates are significantly enhanced when connecting the fins between the inner and outer tubes.

The major issues of thin-film silicon solar cells have been the light-induced metastability of hydrogenated amorphous silicon (a-Si:H) and the weak infrared light absorption of hydrogenated microcrystalline silicon (μc-Si:H). In order to overcome these challenges, we have developed a novel deposition process for a-Si:H and an improved light trapping scheme for μc-Si:H light absorbers, leading to four record independently-confirmed conversion efficiencies with single-junction and multijunction device structures. In this article, we review some technological progresses that led to the notable improvement in thin-film silicon solar cells, and address some limitations imposed by the absorber layer quality as well as the cell design.

This study analyzes the effect of cut-back inlets on the conjugate heat transfer of honeycomb-channel solar receivers. Two-way coupling simulations are reported for a plane-surface inlet, and five kinds of cut-back inlet receivers. The receivers are based on a 1.9-mm × 1.9-mm square cell with a 1.0-mm wall thickness. The cut-back inlet receivers have different amounts of material removed from the walls at the channel inlets. Numerical simulations demonstrate that receivers with a 2.4-mm cut from the channel inlet increased the exit temperature by 20.0 K or more, and decreased the pressure drop relative to the plane-surface receiver. Therefore, the results indicate that the cut-back inlet receiver’s thermal, and hydrodynamic performance is better for industrial use than is the plane-surface receiver. This performance is then explained with detailed examinations of the individual heat-transfer processes in the model. These detailed comparisons indicate that the top cuts reduce shadow effects as light irradiates the channel walls, allowing more direct irradiation to reach the wall surface, thus improving the overall performance.

Solar gasification is one of the promising techniques to convert the carbonaceous materials to clean chemical fuels, which offers the advantages of being transportable as well as storable for extended period of time. In this study, thermal performance of a recently developed 5 kWth fluidized bed reactor for solar gasification has been investigated and reported. Discrete element method (DEM) has been used for modeling the granular flow, and computational fluid dynamics (CFD) method has been used for modeling the multiphase flow. To validate the developed model, experiments were preformed and compared with modeling results. Discrete ordinate radiation model has been used to solve the radiative transfer equation. The thermal performance of the reactor and particulate flow behavior have been predicted and the effect of particle size, particle size distribution and gas flow rate are analyzed. The results indicate that the performance of the bed increases when fluidizing the annulus region particles as the high porosity increases the diffusion rate of radiation throughout the bed.

A loop thermosiphon thermal collector was developed for the waste heat recovery power generation with electric capacity of 500 W. The heat collector with heat transfer area of 0.159 m2 (500 mm width and 300 mm depth) was connected to the condenser with a shrunken heat transfer area by a loop. The thermal performance of the apparatus was declined when increasing the water filling rate to 90% as the working fluid occupied the internal volume. In the range of water filling rate between 30% and 60%, the effective thermal conductivity was around100 times of the conductivity of copper.

A new device design of microcrystalline silicon thin-film solar cell allows for approaching the geometric light-trapping limit. The solar cell is based on triangular textured surfaces in combination with optimized front and back contacts with very low optical losses. In comparison to crystalline silicon solar cells with record energy conversion efficiency the material usage of the thin-film solar cells is reduced to 1–2%, while exhibiting the potential to achieve short circuit current densities of more than 80% of their counterparts. The short circuit current density of the thin-film solar cells is approaching the geometric light-trapping limit commonly known as the Yablonovitch limit under perpendicular incidence. The design of the solar cell is described considering the electrical and optical properties of the textured solar cell.

The feasibility of using surfactant-free semiconducting silicon-tin alloy nanocrystals with quantum confinement effect for photovoltaics is demonstrated. Synthetized nanoparticles with average of 3 nm in diameter and optical bandgap of 0.81 eV at room temperature were obtained. X-ray diffraction measurements with synchrotron radiation have confirmed a Si0.88Sn0.12 alloyed composition that corresponds to a ratio of about eight Si atoms for every Sn atom. Fourier transform infrared spectroscopy revealed a reduced surface oxygen concentration compared with elemental silicon nanocrystals. Furthermore the potential of silicon-tin nanocrystals as a photovoltaic material is assessed and an enhancement of the solar cells performance is demonstrated due to the extended spectral range and increased absorption. In particular, the short circuit current density has shown improvements as the concentration of silicon-tin nanocrystals is increased.

The particles flow and heat transfer characteristics of a high temperature solar thermochemical fluidized bed reactor have been studied for solar beam-down concentrating systems. A numerical model has been developed by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) collisional model since it is an effective approach for studying the gas-solid flow. The discrete ordinate model has been used to solve the radiation heat transfer. An experimental visualization of particles circulation pattern and mixing of two-tower fluidized bed system has been presented. A good agreement has been found between the experimental measurements and numerical predictions. The effect of gas superficial velocity, bed mass and inlet gas temperature on the flow pattern and temperature characteristics of the bed have been investigated. The results showed that the maximum and average temperature of the bed, depends on the top layer position and focal point of the concentrated radiation, decreased when increasing the total mass of the bed.

We performed a direct numerical simulation (DNS) of the turbulent natural convection between two parallel plates at a Rayleigh number of Ra = 8.0 × 106, focusing on the turbulent natural co...

This paper describes a volumetric solar receiver that is vertically integrated with beam-down optics for condensed light irradiation. The heat-transfer performance of a silicon carbide honeycomb receiver was investigated using a 30-kW th solar simulator and numerical simulation. The experiments achieved an air temperature of 840 K at the receiver outlet by varying the operational parameters. Numerical simulations were performed for a vertical honeycomb block with beam-down irradiation and a horizontal honeycomb block with tower-type irradiation to elucidate the effects of buoyancy. Three blocks with different sizes were simulated for a variety of operational parameters. When the block was oriented vertically, the flow and temperature fields remained nearly symmetric in and near the receiver. In contrast, when it was oriented horizontally, the flow and temperature became asymmetric, with the hot spot moving toward the receiver's side wall and the stream in the receiver being reversed. The vertical orientation's robustness to buoyancy effects prevented any reduction in the receiver efficiency or outlet temperature and suppressed the thermal leakage.

In this study, a numerical model has been developed by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) collisional model to study the particle-fluid flow of the fluidized bed reactor for solar beam-down concentrating system. The contact forces between the particles have been calculated by the spring-dashpot model, based on the soft-sphere method. An experimental visualization of particles circulation pattern and mixing of two-tower fluidized bed system has been presented. A good agreement has been found between the experimental measurements and numerical predictions. To investigate the influence of fluid flow rate and particle size on the flow pattern of the reactor, simulations have been performed for various conditions. The results indicate that the large size particles induce three-dimensional effects as they are accumulated at the central axis region. The average bed height of the left tower increased by 23.4% when increasing the flow rate about 70%.

The flow and heat transfer in a curved channel were numerically analyzed for their spatial advancement under the influence of streamline curvature. The simulation was made for the curved channel of the radius ratio 0.92. The curved channel was provided with the fully developed velocity and thermal profile in a straight channel at the mean velocity Reynolds numbers up to 20,400. The number of grid points was more than 3.00 × 10 8 for the case of the highest Reynolds number. The local Nusselt number indicated that the heat transfer was enhanced on the outer wall of the curved channel and such enhancement was notable over the downstream area of ϕ ≧ 80°. This heat transfer enhancement came from the large-scale organized vortex which intensified the lateral convection of the secondary stream and caused high values of the turbulent heat flux. The pre-multiplied spectrum was used to analyze the spatial advancement of the large-scale organized vortex. The spectrum indicated that the large-scale vortex was produced from the initial disturbance in the straight channel and the vortex grew and expanded to nearly the channel full width. It was shown that the advancement of the vortex was almost independent from the Reynolds numbers.

We determined that the conduction band offset (CBO) and the valence band offset (VBO) at the CdS/ Cu2ZnSnSe4 (CZTSe) heterointerface are +0.56 and +0.89eV, respectively, by using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and inversed photoemission spectroscopy (IPES). A positive CBO value, so-called “spike” structure, means that the position of conduction band becomes higher than that of absorber layer. The evaluated CBO of +0.56 eV suggests that the conduction band alignment at CdS/CZTSe interface is enough to become an electron barrier. Despite such a large spike structure in the conduction band at the interface, a conversion efficiency of 8.7 % could be obtained for the CdS/CZTSe heterojunction solar cells.

Solar energy has the potential to play a central role in the future global energy system because of the scale of the solar resource, its predictability, and its ubiquitous nature. Global installed solar photovoltaic (PV) capacity exceeded 500 GW at the end of 2018, and an estimated additional 500 GW of PV capacity is projected to be installed by 2022–2023, bringing us into the era of TW-scale PV. Given the speed of change in the PV industry, both in terms of continued dramatic cost decreases and manufacturing-scale increases, the growth toward TW-scale PV has caught many observers, including many of us (1), by surprise. Two years ago, we focused on the challenges of achieving 3 to 10 TW of PV by 2030. Here, we envision a future with ∼10 TW of PV by 2030 and 30 to 70 TW by 2050, providing a majority of global energy. PV would be not just a key contributor to electricity generation but also a central contributor to all segments of the global energy system. We discuss ramifications and challenges for complementary technologies (e.g., energy storage, power to gas/liquid fuels/chemicals, grid integration, and multiple sector electrification) and summarize what is needed in research in PV performance, reliability, manufacturing, and recycling.

The surface electronic properties of the light absorber and band alignment at the p/n heterointerface are key issues for high-performance heterojunction solar cells. We investigated the band alignment of the heterointerface between cadmium sulfide (CdS) and Ge-incorporated Cu

Cyclic dehydrogenation-rehydrogenation (D-R) experiments are performed on thin oxide passivated contact (TOPCon) p-type poly-Si/SiOx stack to investigate the role of hydrogen atoms for crystalline silicon solar cell. The dehydrogenation steps are performed by thermal annealing at 450 °C for 1h under vacuum condition, and the rehydrogenation steps by hydrogen plasma treatment (HPT) at 300 °C for 1min. The ultrathin SiOx layer of 0.8±0.1 nm is realized by atomic layer deposition (ALD) technique allowing a precise control at atomic scale. The effective lifetime is enhanced to 1.4 ms by the HPT. The D-R method underlines the contribution of H atoms to the c-Si surface passivation that diffuses into the SiOx/c-Si interface. Besides, the lifetime recovery is well described by a stretched exponential function that indicates the dispersive nature of the p-type poly-Si/SiOx stack.

A transient three dimensional numerical model of the heat transfer and fluid flow of a windowed fluidized bed reactor for solar thermochemical conversions is formulated and solved using discrete element method coupled to computational fluid dynamics. Radiation transfer equation is solved by discrete ordinate radiation model and the particle collision dynamics is solved by spring-dashpot model based on soft-sphere method. The instantaneous granular flow behavior of the irradiated bed is presented along with the incident radiation and particle size distribution. The results indicate that as time progresses the average velocity of the particle increases due to high temperature and bed expansion effect.

Electronic defects in semiconductor devices play important roles in device performance and reliability. These defects are created during fabrication using plasma-based processing technology. The authors investigate the generation and annihilation of defects in amorphous silicon by measuring the photocurrent during processing with hydrogen and argon plasmas. Unfavorable species for defect generation and favorable species for defect recovery are distinguished. These results are beneficial for improving semiconductor plasma processing, which is used to make state-of-the-art devices such as FinFETs, computer memory, and solar cells.

The effects of pre-annealing the stacked metallic precursor (Zn/Cu/Sn/Mo/glass) on the structural and morphological properties of the resultant Cu 2 ZnSnSe 4 (CZTSe) thin films and their device properties were investigated. The crystalline quality and morphology of the CZTSe thin films were enhanced by the pre-annealing treatment. The pre-annealed CZTSe solar cells showed an improved conversion efficiency η of 7.02% compared with those that were not pre-annealed (η=5.65%). This enhancement in solar cell performance was mainly attributed to an improvement in shunt resistance ( R sh ) from 80 to 150 Ω cm 2 , which resulted in improvements in open-circuit voltage and fill factor.

The relationship between light trapping and microstructure of high-efficiency microcrystalline silicon thin-film solar cells on honeycomb textured substrates is investigated by experiments, optical simulations and analytical calculations. The solar cells realized on the honeycomb textured substrates exhibit short circuit current of 30 mA/cm2 and energy conversion efficiencies exceeding 11%. The microstructure of the solar cells is limited by the formation of so called “cracks” in the film, which negatively affect the short circuit current, fill factor and open circuit voltage. The formation of cracks is studied by transmission electron microscopy, optoelectrical measurements and simulations of the 3D morphology of the solar cell. Furthermore, a simple analytical model is presented to calculate the critical thickness at which cracks are formed. Both models are compared to experimental results. Guidelines are provided on how to avoid the formation of cracks in microcrystalline silicon films on the textured substrates while maximizing the light trapping properties.

This paper proposes the improvement of microfluidic mixture using the unsteady electroosmotic vortices induced by a staggered array of electrodes. The unsteady electroosmotic forces near two walls, having opposite directions near two walls, were shown to drive organized vortices to contribute for the enhanced mixing. Examination was given to the flow field involving unsteady vortices and their role for the particle mixing. Numerical simulation was made to evaluate the particle mixing process and to reveal how the particle mixing occurred under influence of disturbance by unsteady vortices. The Reynolds number was set to 0.005 and 1.0 changing the oscillation frequency and the electric field intensity. Efficiency of particle mixing was evaluated by the variance intensity of particle number fraction. The agreement between the simulation for the steady electroosmotic flow and the existing paper, and the grid convergence vindicated the simulation code presently used. The numerical results for unsteady electroosmotic flow demonstrated that the particle mixing was controlled by the oscillation frequency and the balanced time scales of the flow perturbation and convection was essential to the particle mixing. The particle flow was displayed to discuss the phenomenology related to the particle mixing. This revealed that the intermingling of particles within main and secondary vortices crucial for the mixing process whereas the outside streams exerted by the vortices were not effective on it.

Carrier trapping in hydrogenated amorphous silicon (a-Si:H) has been investigated by means of an optical pump-probe technique. The trapped carriers (electrons) at the conduction band tail are detected as an increment of the photocurrent, and their density is quantitatively determined under the assumption of carrier generation and recombination kinetics. We find that carrier trapping strongly depends on the band tail distribution in addition to the pump light intensity. Specifically, the trapped electron density increases with the Urbach energy that characterizes the valence band tail broadening. Under the condition of a pump light intensity of 10 mW/cm2 operated at 532 nm, the trapped electron density is determined to be ≈ 4 × 1017 cm− 3 for an intrinsic a-Si:H film with an Urbach energy of 45 mV. The effects of carrier trapping on the device performance are studied in single-junction a-Si:H p-i-n solar cells. The results suggest that carrier trapping causes a reduction in the fill factor of the solar cells.

This paper describes the thermal collector type loop thermosiphon for low-temperature waste heat recovery. Water is used as the working fluid for heat transport at temperatures 100 °C and higher. The loop thermosiphon comprises of a thermal receiver, a condenser, and riser and downcomer tubes. The thermal receiver is made of a cupper plate brazed by meandering heat transfer tube. This receiver collects the thermal radiation from the electric heater at the heat transfer area of 1000 cm 2 (40 cm × 25 cm), and transports the heat by vaporization of water to the condenser having heat transfer area of 62 cm 2 . In the no inclination mode, the thermosiphon is upright so that the thermal receiver and the condenser are placed vertically. In this mode, the effective thermal conductivity exceeds 60 kW/(m K) when the thermal receiver temperature was higher than 125 °C for the water filling ratio α = 30–70%. Although the effective thermal conductivity is deteriorated for the higher filling ratio α = 80% and 90%, the effects from the filling ratio are tiny for α = 30–70%. The experimental tests were also made for the negatively inclined mode where the inlet and exit ports of the receiver were directed downward and for the positively inclined modes where they were directed upward. The tests revealed that the negative inclination almost halted the heat transport. However, the tests also indicated that the positive inclination showed the performance comparable to the no inclination mode for the cases up to the inclination angle 90°.