Effects of photo-generated gas bubbles on the performance of tandem photoelectrochemical reactors for hydrogen production

Presence of gas bubbles in the vicinity of semiconductor electrodes interferes with their active surface areas and introduces inert voids in the electrolyte hindering its ionic conductivity. Furthermore, gas bubbles obstruct the radiation path through scattering. The aim of this work is to study the characteristic hydrogen- and oxygen-gas bubble behavior and their effects on photoelectrochemical reactor performance. Findings of gas bubble formation, electrode coverage and curtain profiles based on macroscopic bubble graphical images are reported. Effects of increased convective forces are also observed. Further, the scattering of incident light implemented through simulations based on Mie scattering theory is reported. Results show that hydrogen gas bubbles are more extensive in coverage due to formation of a froth while oxygen bubbles coalesce and rise easily. The growth of the bubble cover increases ohmic resistance reducing the current magnitude. Even at a modest current density of 10 mA/cm², the curtain thickness may rise to 2 mm or 3 mm for oxygen and hydrogen, respectively. Light scattering increases with increasing bubble size and is more pronounced for shorter wavelengths. It is also found that presence of multiple bubbles reduces light intensity by up to 2% and highest when the bubble radius is 150 μm. Increase in both photoelectrode and electrolyte resistances as well as radiation losses due to presence of bubbles hence undermine the performance of photoelectrochemical reactors.     

Efficient operation of autothermal microchannel reactors for the production of hydrogen by steam methane reforming

Efficient conversion of methane to hydrogen has emerged as a significant challenge to realizing fuel cell-based energy systems. Autothermal microchannel reactors, coupling of exothermic and endothermic reactions in parallel channels, have become one of the most promising technologies in the field of hydrogen production. Such reactors were utilized as an intensified design for conducting the endothermic steam methane reforming reaction. The energy required by the endothermic process is supplied directly through the separating plates of the reactor structure from the exothermic process occurring on the opposing side. Optimal design problems associated with transport phenomena in such an autothermal system were analyzed. Various methods for designing and operating autothermal reactors employed in steam methane reforming were discussed. Computational fluid dynamics simulations were performed to identify the underlying principles of process intensification, and to delineate several design and operational features of the intensified reforming process. The results indicated that the autothermal reactor is preferable to be thermally conductive to ensure its structural integrity and maximum operating regime. However, the thermal properties of the reactor structure are not essential due to efficient heat transfer existing between endothermic and exothermic process streams. A reactor design which minimizes the mass transfer resistance is highly required, and the channel dimension is of critical importance. Furthermore, the challenges presented by the efficient operation of the autothermal system were identified, along with demonstrating the implementation of transport management in order to improve overall reactor performance and to mitigate extreme temperature excursions.     

Hydrogen from wet air and sunlight in a tandem photoelectrochemical cell

A solid-state photoelectrochemical (SSPEC) cell is an attractive approach for solar water splitting, especially when it comes to monolithic device design. In a SSPEC cell the electrodes distance is minimized, while the use of polymer-based membranes alleviates the need for liquid electrolytes, and at the same time they can separate the anode from the cathode. In this work, we have made and tested, firstly, a SSPEC cell with a Pt/C electrocatalyst as the cathode electrode, under purely gaseous conditions. The anode was supplied with air of 80% relative humidity (RH) and the cathode with argon. Secondly, we replaced the Pt/C cathode with a photocathode consisting of 2D photocatalytic g-C₃N₄, which was placed in tandem with the photoanode (tandem-SSPEC). The tandem configuration showed a three-fold enhancement in the obtained photovoltage and a steady-state photocurrent density. The mechanism of operation is discussed in view of recent advances in surface proton conduction in absorbed water layers. The presented SSPEC cell is based on earth-abundant materials and provides a way towards systems of artificial photosynthesis, especially for areas where water sources are scarce and electrical grid infrastructure is limited or nonexistent. The only requirements to make hydrogen are humidity and sunlight.     

Experimental investigation of various copper oxide electrodeposition conditions on photoelectrochemical hydrogen production

In this study, an experimental investigation of photosensitive material copper oxide electrodeposition on various substances is performed under different experimental conditions in order to evaluate the effects on photoelectrochemical hydrogen production system. The experimental setup consists of solar simulator, electrodeposition chemicals, hydrogen sensor, pH meter, graphite and platinum electrodes, heating plate, stirrer, temperature sensors, cathode and anode plates, concentrating lens and potentiostat. The overall aim is to optimize the efficiencies by generating higher currents and eventually hydrogen as light enhances the separation of water process. The results obtained in this study are promising for photoelectrochemical hydrogen production under the solar simulator and concentrated light irradiation conditions. Furthermore, an electrolysis setup using the coated metals and graphite rod is built to investigate the amount of photocurrent production. The characterization is also conducted under light and no-light conditions, where the amount of produced current and hydrogen increased in light compared to no-light condition. At the applied voltage of ?0.6 V and ?0.4 V vs. Ag/AgCl, the photocurrent densities of 0.8 mA/cm² and 0.27 mA/cm² are obtained with a solar conversion efficiency of 0.86% and 0.24%, respectively.     

Influence of chelating agents on the growth and photoelectrochemical responses of chemical bath-synthesized AgIn5S8 film electrodes

An aqueous method for the deposition of silver-indium-sulfide ternary semiconductor film electrodes is presented. Various deposition parameters, such as reaction temperature and molar ratios of different chelating agents, were changed in order to grow uniform and adherent films on indium-tin-oxide glass substrates. With a reaction temperature higher than 65 °C, a film composed of AgIn₅S₈ was grown on the substrates in our experimental conditions. The direct and indirect energy band gaps of samples prepared in this study varied from 1.70 to 1.97 eV and 1.61 to 1.72 eV, respectively. The maximum photocurrent density of samples in this study reached 3.0 mA/cm² at an external potential of +1.0 V vs. a Pt electrode under illumination using a 300 W Xe lamp system with the light intensity set at 100 mW/cm².     

Influence of the management strategy and operating conditions on the performance of an adsorption chiller

The aim of this experimental work was to study the influence of the operation mode (i.e. cycle time and relative duration of ads-/desorption phases, R) as well as of the operating conditions on the performance of an adsorption chiller. The testing campaign demonstrated that the management optimization strongly improves the performance of such kind of machines. The Coefficient of Performance (COP) and the Specific/Volumetric Cooling Power (SCP, VCP) vary, respectively, in a range of ±133% and ±43% when the cycle time (τ_cycle) increases from 5 to 20 min at fixed boundary conditions (T_e = 15 °C, T_c = 35 °C, T_h = 90 °C) while a further increasing in performance (up to 15%) is reached, at fixed cycle time, by protracting the duration of the adsorption phase at the expense of the desorption one. The complete set of results allowed to draw a map of performance suitable for the optimization of the management mode taking into account the specific application. At T_e = 15 °C, T_c = 35 °C, T_h = 90 °C, if high SCP is required (e.g. automotive air conditioning), the optimal choice is τ_cycle = 7 min and R = 2.5 (SCP = 394 W/kg, COP = 0.60, VCP = 223 W/m³) while to assure a good efficiency (e.g. solar cooling) the proper management is τ_cycle = 20 min and R = 1 (SCP = 204 W/kg, COP = 0.69, VCP = 116 W/m³).     

变工况对带涡流发生器的PEMFC性能的影响

质子交换膜燃料电池(PEMFC)是一种高效的能量利用装置。为了提高其工作性能,在其中加装涡流发生器,并研究工况对其性能的影响,对其进行了单因素和多因素影响分析,利用响应面法建立数学关系式。结果表明：加装涡流发生器,可以明显改善PEMFC温度均匀性、提高燃料利用率、改善通道排水,对燃料电池的性能有明显提升,电压为0.4 V时电流密度提升了55.4%;在所选区间内温度为65℃,相对压力为100 kPa,湿度为60%,化学计量比为2.50时,燃料电池的功率密度最大。     

Effect of catalytic washcoat shape and properties on mass transfer characteristics of microstructured steam-methanol reformers for hydrogen production

Many attempts have been made to improve mass transfer by reducing the size of reactors. However, such reduction will fairly quickly reach practical limitations and numerous difficulties still remain. Catalytic washcoat shape and properties may be critical design factors, but the mechanisms for their effects on mass transfer characteristics are still not fully understood. To effectively eliminate problems associated with mass transport phenomena in microstructured steam-methanol reformers, the effects of washcoat shape and properties were investigated in various situations by performing computational fluid dynamics simulations. The dependence of the solution on mass transfer characteristics was reduced to a small number of dimensionless parameters. A dimensionless mass transfer analysis was carried out in terms of the Sherwood, Schmidt, and pore Reynolds numbers. The results indicated that the rate of mass transfer is predominantly controlled by washcoat properties, and porosity and effective thermal conductivity are fundamentally important. The rate of the reforming reaction is typically controlled by kinetics at a temperature of 480 K and limited by mass transfer at a temperature of 580 K. The shape of washcoats affects the overall mass transfer characteristics, depending on the structural and thermal properties of washcoats. The shape effect is limited by heat transfer. A three-fold increase in effectiveness factor can be achieved by increasing the effective thermal conductivity of the washcoat. Design recommendations were finally made to improve transport characteristics for the systems.     

Performance and parameter analysis of tandem solar cells using measurements at multiple spectral conditions

The current-voltage (I–V) characteristics of monolithic tandem solar cells have been measured with different radiation spectra. Introducing a spectral metric allows the experimental results and the match between current and efficiency value to be analysed. Simultaneous analysis of the I–V characteristics allows extraction of diode parameters of the individual sub-cells.

The measurements allow an assessment of cell performance and optimisation, both for amorphous and crystalline tandem cells.     

Counter-Rotating Type Tidal Stream Power Unit Boarded on Pillar （Perform- ances and Flow Conditions of Tandem Propellers）

The authors have invented the unique counter-rotating type tidal stream power unit composed of the tandem pro- peUers and the double rotational armature type peculiar generator without the traditional stator. The front and the rear propellers counter-drive the inner and the outer armatures of the peculiar generator, respectively. The unit has the fixftful advantages that not only the output is sufficiently higher without supplementary equipment such as a gearbox, but also the rotational moment hardly act on the pillar because the rotational torque of both propel- lers/armatures are counter-balanced in the unit. This paper discusses experimentally the performances of the power unit and the effects of the propeller rotation on the sea surface. The axial force acting on the pillar in- creases naturally with the increase of not only the stream velocity but also the drag of the tandem propellers. Be- sides, the force vertical to the stream also acts on the pillar, which is induced from the Karman vortex street and the dominant frequencies appear owing to the front and the rear propeller rotations. The propeller rotating in close to the sea surface brings the abnormal wave and the amplitude increases as the stream velocity is faster and/or the drag is stronger.     

Analytical investigations of varying cross section microstructures on charge transfer in solid oxide fuel cell electrodes

An extended surface modeling concept (electrochemical fin) is applied to charge transport within the SOFC electrode microstructure using an analytical modeling approach analogous to thermal fin analysis. This model is distinct from similar approaches applied to SOFC electrode microstructure in its application of a governing equation that allows for variable cross-section geometry. The model presented is capable of replicating experimentally observed electrode behavior inclusive of sensitivity to microstructural geometry, which stands in contrast to existing models that apply governing equations analogous to a constant cross-section thermal fin equation. Insights learned from this study include: the establishment of a suite of dimensionless parameters and performance metrics that can be applied to assess electrode microstructure, the definition of microstructure-related transport regimes relevant to electrode design, and correlations that allow performance predictions for electrodes that provide cell structural support. Of particular note, the variable cross-section modeling approach motivates the definition of a sintering quality parameter that quantifies the degree of constriction within the conducting network of the electrode, a phenomenon that exerts influence over electrode polarization. One-dimensional models are presented for electrochemical fins of several cross-sectional geometries with the ultimate goal of developing a general tool that enables the prompt performance evaluation of electrode microstructures. Such a tool would facilitate SOFC microstructural design by focusing more detailed modeling efforts on the most promising microstructures.     

Study of operating conditions and cell design on the performance of alkaline anion exchange membrane based direct methanol fuel cells

Direct methanol fuel cells using an alkaline anion exchange membrane (AAEM) were prepared, studied, and optimized. The effects of fuel composition and electrode materials were investigated. Membrane electrode assemblies fabricated with Tokuyama^® AAEM and commercial noble metal catalysts achieved peak power densities between 25 and 168 mW cm⁻² depending on the operating temperature, fuel composition, and electrode materials used. Good electrode wettability at the anode was found to be very important for achieving high power densities. The performance of the best AAEM cells was comparable to Nafion^®-based cells under similar conditions. Factors limiting the performance of AAEM MEAs were found to be different from those of Nafion^® MEAs. Improved electrode kinetics for methanol oxidation in alkaline electrolyte at Pt-Ru are apparent at low current densities. At high current densities, rapid CO₂ production converts the hydroxide anions, necessary for methanol oxidation, to bicarbonate and carbonate: consequently, the membrane and interfacial conductivity are drastically reduced. These phenomena necessitate the use of aqueous potassium hydroxide and wettable electrode materials for efficient hydroxide supply to the anode. However, aqueous hydroxide is not needed at the cathode. Compared to AAEM-based fuel cells, methanol fuel cells based on proton-conducting Nafion^® retain better performance at high current densities by providing the benefit of carbon dioxide rejection.     

The effect of variable stator on performance of a highly loaded tandem axial flow compressor stage

Increasing the aerodynamic load on compressor blades helps to obtain a higher pressure ratio in lower rotational speeds.Considering the high aerodynamic load effects and structural concerns in the design process,it is possible to obtain higher pressure ratios compared to conventional compressors.However,it must be noted that imposing higher aerodynamic loads results in higher loss coefficients and deteriorates the overall performance.To avoid the loss increase,the boundary layer quality must be studied carefully over the blade suction surface.Employment of advanced shaped airfoils (like CDAs),slotted blades or other boundary layer control methods has helped the designers to use higher aerodynamic loads on compressor blades.Tandem cascade is a passive boundary layer control method,which is based on using the flow momentum to control the boundary layer on the suction surface and also to avoid the probable separation caused by higher aerodynamic loads.In fact,the front pressure side flow momentum helps to compensate the positive pressure gradient over the aft blade's suction side.Also,in comparison to the single blade stators,tandem variable stators have more degrees of freedom,and this issue increases the possibility of finding enhanced conditions in the compressor off-design performance.In the current study,a 3D design procedure for an axial flow tandem compressor stage has been applied to design a highly loaded stage.Following,this design is numerically investigated using a CFD code and the stage characteristic map is reported.Also,the effect of various stator stagger angles on the compressor performance and especially on the compressor surge margin has been discussed.To validate the CFD method,another known compressor stage is presented and its performance is numerically investigated and the results are compared with available experimental results.     

Influence of sputter oxygen partial pressure on photoelectrochemical performance of tungsten oxide films

Polycrystalline tungsten oxide films of 1–1.2μm thickness were prepared by reactive sputtering at elevated substrate temperature (270 °C) and under different oxygen partial pressures in the range from 0.8 to 2.1 mTorr. At the lowest partial pressure the films were substoichiometric, showed increased disorder, and exhibited photocurrents of 0.6 mA/cm² at 1.8 V vs SCE in 0.33 M H₃PO₄. At partial pressures of 1.4 mTorr and greater, stoichiometric WO₃ films were produced which exhibited photocurrents of 2.4 mA/cm² at 1.8 V vs SCE. It has been determined that the photoelectrochemical performance of slightly substoichiometric films is adversely affected by changes in optical properties, while the photocurrents of severely substoichiometric films suffer additionally from poor carrier collection.     

Remarkable stability of the enhanced sharp photocurrent in methylviolet–C18 and rhodamine–C18 dye-sensitized photoelectrochemical cell with p-CuSCN

A remarkable stability in the sharp photo-current peak at 570 nm was found when p-CuSCN semiconductor photocathode was sensitized with double-dye LB films of rhodamine-C₁₈ (R-C₁₈) monolayers and methylviolet-C₁₈ (M-C₁₈) monolayers. At first, M-C₁₈ four monolayers were deposited on p-CuSCN and then, R-C₁₈ four monolayers were deposited on M-C₁₈ monolayers to make an molecular arrangement on the semiconductor (LB-(R-C₁₈(4)/M-C₁₈(4))/p-CuSCN). Such a photo-cathode exhibited a remarkable stability of steady-state photo-current in (10⁻² M) KI+(10⁻⁴ M) I₂ aqueous solution of pH=6.5, where the quantum efficiency reached was 45%, as estimated by correcting the dye absorption in the sharp photo-current peak at 570 nm. When four monolayers of M-C₁₈ and four monolayers of R-C₁₈ were separated with an arachidic acid LB films, sharp photo-current peak and its stability gradually decreased. This remarkable stability may be due to an efficient energy transfer process and an efficient charge separation caused by a strong interaction between the parallel electron-vibration planes of both M-C₁₈ and R-C₁₈ molecules.     

Influence of reaction conditions on bio-oil production from pyrolysis of construction waste wood

The pyrolysis characteristics of construction waste wood were investigated for conversion into renewable liquid fuels. The activation energy of pyrolysis derived from thermogravimetric analysis increased gradually with temperature, from 149.41 kJ/mol to 590.22 kJ/mol, as the decomposition of cellulose and hemicellulose was completed and only lignin remained to be decomposed slowly. The yield and properties of pyrolysis oil were studied using two types of reactors, a batch reactor and a fluidized-bed reactor, for a temperature range of 400–550 °C. While both reactors revealed the maximum oil yield at 500 °C, the fluidized-bed reactor consistently gave larger and less temperature-dependent oil yields than the batch reactor. This type of reactor also reduced the moisture content of the oil and improved the oil quality by minimizing the secondary condensation and dehydration. The oil from the fluidized-bed reactor resulted in a larger phenolic content than from the batch reactor, indicating more effective decomposition of lignin. The catalytic pyrolysis over HZSM-5 in the batch reactor increased the proportion of light phenolics and aromatics, which was helpful in upgrading the oil quality.     

Impact of cell design and operating conditions on the performances of SOFC fuelled with methane

An in-house-model has been developed to study the thermal and electrochemical behaviour of a planar SOFC fed directly with methane and incorporated in a boiler. The usual Ni-YSZ cermet has been considered for the anode material. It has been found that methane reforming into hydrogen occurs only at the cell inlet in a limited depth within the anode. A sensitivity analysis has allowed establishing that anode thicknesses higher than ∼400–500 μm are required to achieve both the optimal methane conversion and electrochemical performances.     

工况对两相流引射器制冷循环系统性能的影响

引射器结构简单、无运动部件,在制冷系统中代替膨胀阀可提高系统的COP。以R134a为工质,实验研究了不同工况条件下两相流引射器制冷循环系统性能,分析了蒸发温度、冷凝温度对引射比、压力提升比、制冷量和系统COP的影响。研究发现:当冷凝温度为40℃时,随着蒸发温度的提高,引射比和压力提升比均下降,制冷量和系统COP均提高;当蒸发温度为-10℃时,随着冷凝温度的增加,引射比和压力提升比均增大,制冷量和系统COP均下降。     

Physical and operating conditions effects on silica gel/water adsorption chiller performance

Adsorption refrigeration systems are commercially developed due to the need of replacing the conventional systems which utilise environmentally harmful refrigerants and consume high grade electrical power. This paper presents the key equations necessary for developing a novel empirical lumped analytical simulation model for commercial 450 kW two-bed silica gel/water adsorption chiller incorporating mass and heat recovery schemes. The adsorption chiller governing equations were solved using MATLAB® platform integrated with REFPROP® to determine the working fluids thermo-physical properties. The simulation model predicted the chiller performance within acceptable tolerance and hence it was used as an evaluation and optimisation tool. The simulation model was used for investigating the effect of changing fin spacing on chiller performance where changing fin spacing from its design value to minimum permissible value increased chiller cooling capacity by 3.0% but decreased the COP by 2.3%. Furthermore, the effect of generation temperature lift on chiller performance and the feasibility of using it as a load control tool will be discussed. Genetic Algorithm optimisation tool was used to determine the optimum cycle time corresponding to maximum cooling capacity, where using the new cycle time increased the chiller cooling capacity by 8.3%.     

Effects of variation in working fluids and operating conditions on the performance of a thermoacoustic refrigerator

This study reports on a numerical investigation of the effects of variation in working fluids and operating conditions on the performance of a thermoacoustic refrigerator. The performance of a thermoacoustic refrigerator is evaluated based on the cooling power, coefficient of performance (COP), and the entropy generation rate within the device. The effect of the variation of the working fluid is observed by changing the Prandtl number (Pr) between 0.7 and 0.28. The operating conditions investigated are drive ratio (DR), stack plate spacing (y₀), and mean pressure (p_m). The present research shows that lowering the Pr of the working fluid does not improve the performance of a thermoacoustic refrigerator for all of the selected operating conditions. COP increases 78% by reducing the Pr from 0.7 to 0.28 at y₀ = 3.33δ_k, at atmospheric pressure and a DR of 1.7%. While the COP decreases by reducing the Pr from 0.7 to 0.28 at y₀ = 1.0δ_k, at atmospheric pressure, and a DR of 1.7%. The results are compared with the available experimental data and found good agreement.