Exergoeconomic analysis and multi objective optimization of a solar based integrated energy system for hydrogen production

In the current study, an integrated renewable based energy system consisting of a solar flat plate collector is employed to generate electricity while providing cooling load and hydrogen. A parametric study is carried out in order to determine the main design parameters and their effects on the objective functions of the system. The outlet temperature of generator, inlet temperature to organic Rankine cycle turbine, solar irradiation intensity $\left(I\right)$, collector mass flow rate $\left({\dot{m}}_{col}\right)$ and flat plate collector area $\left(A_P\right)$ are considered as five decision variables. The results of parametric study show that the variation of collector mass flow rate between 3 kg/s and 8 kg/s has different effects on exergy efficiency and total cost rate of the system. In addition, the result shows that increment of inlet temperature to the ORC evaporator has a negative effect on cooling capacity of the system. It can lead to a decrease the cooling capacity from 44.29 kW to 22.6 kW, while the electricity generation and hydrogen production rate of the system increase. Therefore, a multi objective optimization is performed in order to introduce the optimal design conditions based on an evolutionary genetic algorithm. Optimization results show that exergy efficiency of the system can be enhanced from 1.72% to 3.2% and simultaneously the cost of the system can increase from 19.59 $/h to 22.28 $/h in optimal states.     

A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production

Nano-sized ${\mathrm{TiO}}_2$ photocatalytic water-splitting technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy. Presently, the solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor activation of ${\mathrm{TiO}}_2$ by visible light. In response to these deficiencies, many investigators have been conducting research with an emphasis on effective remediation methods. Some investigators studied the effects of addition of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement of photocatalysis by modification of ${\mathrm{TiO}}_2$ by means of metal loading, metal ion doping, dye sensitization, composite semiconductor, anion doping and metal ion-implantation. This paper aims to review the up-to-date development of the above-mentioned technologies applied to ${\mathrm{TiO}}_2$ photocatalytic hydrogen production. Based on the studies reported in the literature, metal ion-implantation and dye sensitization are very effective methods to extend the activating spectrum to the visible range. Therefore, they play an important role in the development of efficient photocatalytic hydrogen production.     

Hydrogen production by coal plasma gasification for fuel cell technology

Coal gasification in steam and air atmosphere under arc plasma conditions has been investigated with Podmoskovnyi brown coal, Kuuchekinski bituminous coal and Canadian petrocoke. It was found that for those coals the gasification degree to synthesis gas were 92.3%, 95.8 and 78.6% correspondingly. The amount of produced syngas was 30–40% higher in steam than in air gasification of the coal.The reduction of the carbon monoxide content in the hydrogen-rich reformate gas for low-temperature fuel cell applications normally involves high- and low-temperature water gas shift reactors followed by selective oxidation of residual carbon monoxide. It is shown that the carbon monoxide content can be reduced in one single reactor, which is based on an iron redox cycle. During the reduction phase of the cycle, the raw gas mixture of ${\mathrm{H}}_2$ and CO reduces a ${\mathrm{Fe}}_3{\mathrm{O}}_4–{\mathrm{CeO}}_2–{\mathrm{ZrO}}_2$ sample, while during the oxidation phase steam re-oxidizes the iron and simultaneously hydrogen is being produced. The integration of the redox iron process with a coal plasma gasification technology in future allows the production of ${\mathrm{CO}}_x$-free hydrogen.     

Numerical investigation of a high pressure hydrogen jet of 82 MPa with adaptive mesh refinement: Concentration and velocity distributions

To investigate the safety properties of high-pressure hydrogen discharge or leakage, an under-expanded hydrogen jet flow with a storage pressure of 82 MPa from a small jet orifice with a diameter of 0.2 mm is studied by three-dimensional (3D) numerical calculations. The full 3D compressible Navier-Stokes equations are utilized in a domain with a size of about 3 × 3 × 6 m which is discretized by employing an adaptive mesh refinement (AMR) technology to reduce the number of grid cells. By AMR, the local mesh resolutions can narrowly cover the Taylor microscale $l_T$ and direct numerical simulations (DNS) are performed. Both the instantaneous and mean hydrogen concentration distributions in the present jet are discussed. The instantaneous concentrations of hydrogen $C_{{\text{H}}_2}$ on the axis presents significant turbulent pulsating oscillations. The centerline value of the intensity of concentration fluctuation ${\stackrel{?}{\sigma }}_{{\text{H}}_2}$ asymptotically comes to 0.23, which is in a good agreement with the existing experimental results. It substantiates the conclusion that the asymptotic centerline value of ${\stackrel{?}{\sigma }}_{{\text{H}}_2}$ is independent of jet density ratio. The probability distributions function (PDF) of instantaneous axial $C_{{\text{H}}_2}$ agree approximately with the Gaussian distribution while skewing a little to the higher range. The time averaged hydrogen concentration ${\overline{C}}_{{\text{H}}_2}$ along the radial directions can also be described as a Gaussian distribution. The axial ${\overline{C}}_{{\text{H}}_2}$ of 82 MPa hydrogen jet tends to obey the distribution discipline approximated with ${\overline{C}}_{{\text{H}}_2}=4200/(z/\theta )$ where z is the axial distance from the nozzle and $\theta$ is the effective ejection diameter, which is consistent with the experimental results. In addition, the hydrogen tip penetration $Z_{\text{tip}}$ is found to be in a linear relationship with the square root of jet flow time $\sqrt{t}$. Meanwhile, the jet's velocity half-width $L_{\text{Vh}}$ approximately gains an linear relation with z which can be expressed as $L_{\text{Vh}}=0.09z$.     

Effects of content of hydrogen on the characteristics of co-flow laminar diffusion flame of hydrogen/nitrogen mixture in various flow conditions

Effect of content of hydrogen (H₂) in fuel stream, mole fraction of H₂$\left(X_{H_2}\right)$ in fuel composition, and velocity of fuel and co-flow air $\left(V_{avg}\right)$ on the flame characteristics of a co-flow H₂/N₂ laminar diffusion flame is investigated in this paper. Co-flow burner of Toro et al. [1] is used as a model geometry in which the governing conservation transport equations for mass, momentum, energy, and species are numerically solved in a segregated manner with finite rate chemistry. GRI3 reaction mechanisms are selected along with the weight sum of grey gas radiation (WSGG) and Warnatz thermo-diffusion models. Reliability of the newly generated CFD (computational fluid dynamics) model is initially examined and validated with the experimental results of Toro et al. [1]. Then, the method of investigation is focused on a total of 12 flames with $X_{H_2}$ varying between 0.25 and 1, and $V_{avg}$ between 0.25 and 1 ms^?1. Increase of flame size, flame temperature, chemistry heat release, and NOx emission formation resulted are affected by the escalation of either $X_{H_2}$ or $V_{avg}$. Significant effect on the flame temperature and NOx emission are obtained from a higher $X_{H_2}$ in fuel whereas the flame size and heat release are the result of increasing $V_{avg}$. Along with this finding, the role of N₂ and its higher content reducing the flame temperature and NOx emission are presented.     

The effects of hydrogen addition on silica aggregate growth in atmospheric-pressure, 1-D methane/air flames with hexamethyldisiloxane admixture

The effect of hydrogen addition on silica growth in burner-stabilized methane/air flames with trace amounts of hexamethyldisiloxane are reported. Profiles of the aggregates' radius of gyration $R_g$ and monomer radius $a$ versus residence time were measured by laser light scattering. Experiments were performed at equivalence ratios of 0.8, 1.0 and 1.3, with mole fractions of 0–0.4 of hydrogen in the fuel. At equal mass flux, the addition of hydrogen was found to result in decreasing $R_g$ and $a$. However, keeping the flame temperature rather than the mass flux constant upon hydrogen addition resulted in the same measured profiles.     

Transport equations for reaction rate in laminar and turbulent premixed flames characterized by non-unity Lewis number

Transport equations for (i) the rate $W$ of product creation and (ii) its Favre-averaged value $\stackrel{?}{W}$ are derived from the first principles by assuming that $W$ depends solely on the temperature and mass fraction of a deficient reactant in a premixed turbulent flame characterized by the Lewis number $Le$ different from unity. The right hand side of the transport equation for $\stackrel{?}{W}$ involves seven unclosed terms, with some of them having opposite signs and approximately equal large magnitudes when compared to the left-hand-side terms. Accordingly, separately closing each term does not seem to be a promising approach, but a joint closure relation for the sum $\overline{T_{\text{Σ}}}$ of the seven terms is sought. For this purpose, theoretical and numerical investigations of variously stretched laminar premixed flames characterized by $Le<1$ are performed and the linear relation between $T_{\text{Σ}}$ integrated along the normal to a laminar flame and a product of (i) the consumption velocity $u_c$ and (ii) the stretch rate ${\dot{s}}_w$ evaluated in the flame reaction zone is obtained. Based on this finding and simple physical reasoning, a joint closure relation of $\overline{T_{\text{Σ}}}\propto \overline{\rho W\dot{s}}$ is hypothesized, where $\rho$ is the density and $\dot{s}$ is the stretch rate. The joint closure relation is tested against 3D DNS data obtained from three statistically 1D, planar, adiabatic, premixed turbulent flames in the case of a single-step chemistry and $Le=0.34$, 0.6, or 0.8. In all three cases, the agreement between $\overline{T_{\text{Σ}}}$ and $\overline{\rho W\dot{s}}$ extracted from the DNS is good with exception of large ($\overline{c}>0.4$) values of the mean combustion progress variable $\overline{c}$ in the case of $Le=0.34$. The developed linear relation between $\overline{T_{\text{Σ}}}$ and $\overline{\rho W\dot{s}}$ helps to understand why the leading edge of a premixed turbulent flame brush can control its speed.     

Influence of boundary conditions on the structure of laminar boundary layer with hydrogen combustion on a permeable surface

A numerical study was performed to examine how thermal and diffusion boundary conditions affect the structure of laminar diffusion flame in air flow with porous blowing and combustion of hydrogen. Boundary conditions of two types were compared, with lengthwise-constant porous-wall temperature ($T_W=\mathit{const}$ throughout the whole range of blowing ratios), and with lengthwise-constant temperature of the fuel supplied into the main flow ($T^{\prime }=\mathit{const}$). With a liquid fuel having constant evaporating-surface temperature, we deal with boundary conditions of the first type. With a gas fuel, as a rule, we encounter the regime with $T^{\prime }=\mathit{const}$. It is shown that, in spite of the significant difference in velocity and temperature profiles, in both cases the surface friction coefficients have close values. Also, it is found that the wall heat flux exhibits a maximum if considered as a function of fuel supply intensity. Nonetheless, the function of relative heat transfer monotonically decreases with blowing intensity, much like it does in non-reactive flow.     

Alloy design of V-based hydrogen permeable membrane under given temperature and pressure condition

For alloy design of hydrogen permeable membrane, it is important to control pressure–composition–isotherm (PCT curve) in an appropriate manner in order to obtain high hydrogen permeability with strong resistance to hydrogen embrittlement. Based on this concept, V-based alloy membranes are designed under some given pressure conditions at a temperature in view of the partial molar enthalpy change, $\Delta {\overline{H}}_{0.2}$, and entropy change, $\Delta {\overline{S}}_{0.2}$, of hydrogen for hydrogen dissolution. It is demonstrated that the PCT curve can be controlled very precisely in view of $\Delta {\overline{H}}_{0.2}$ and $\Delta {\overline{S}}_{0.2}$. Also, the required membrane area to obtain 300 Nm³ h^?1 of hydrogen flow is estimated. It is found that, in view of the membrane area, it is favorable to apply at least 400 kPa of hydrogen pressure at feed side.     

Hydrogen production from woody biomass by steam gasification using a CO2 sorbent

In ${\mathrm{H}}_2$ production from woody biomass by steam gasification using CaO as a ${\mathrm{CO}}_2$ sorbent, the effect of reaction parameters such as the molar ratio of CaO to carbon in the woody biomass $([\mathrm{Ca}]/[\mathrm{C}])$, reaction pressure, and reaction temperature was investigated on ${\mathrm{H}}_2$ yield and conversion to gas. In the absence of CaO, the product gas contained ${\mathrm{CO}}_2$. On the other hand, in the presence of CaO ($[\mathrm{Ca}]/[\mathrm{C}]=1,2$, and 4), no ${\mathrm{CO}}_2$ was detected in the product gas. At a $[\mathrm{Ca}]/[\mathrm{C}]$ of 2, the maximum yield of ${\mathrm{H}}_2$ was obtained. The ${\mathrm{H}}_2$ yield and conversion to gas were largely dependent on the reaction pressure, and exhibited the maximum value at $0.6\mathrm{MPa}$. It is noteworthy that ${\mathrm{H}}_2$ was obtained from woody biomass at a much lower pressure compared to other carbonaceous materials such as coal $(>12\mathrm{MPa})$ and heavy oil $(>4.2\mathrm{MPa})$ in steam gasification using a ${\mathrm{CO}}_2$ sorbent. ${\mathrm{H}}_2$ yield increased with increasing reaction temperature. Woody biomass is the one of the most appropriate carbonaceous materials in ${\mathrm{H}}_2$ production by steam gasification using CaO as a ${\mathrm{CO}}_2$ sorbent, taking the reaction pressure into account.     

Effect of vent size on vented hydrogen-air explosion

In this paper, the effect of vent size on vented hydrogen-air explosion in the room was studied by numerical simulation. Analysis of the explosion temperature, overpressure, dynamic pressure and wind velocity under different vent sizes indicate that these explosion parameters have different change rules inside and outside the room. Inside the room, the vent size has little effect on the explosion temperature, dynamic pressure and wind velocity, but it has a significant impact on the explosion overpressure. As the scaled vent size $K_v$ ($A_v/V^{2/3}$) increases from 0.1 to 0.3, the difference between the maximum internal peak overpressure is 87.8%. Outside the room, as the vent size increases, the high-temperature range (above 800 K) first decreases and then increases, while the explosion dynamic pressure and hurricane zone caused by explosion wind gradually decrease. The maximum high-temperature range (32.5 m for $K_v$ = 0.1) and hurricane zone (41.1 m for $K_v$ = 0.1) can reach 7.0 times and 8.9 times the length of the room, respectively. The explosion dynamic pressure can reach the same order of magnitude as the explosion overpressure under the same vent size. Therefore, these damage effects outside the room cannot be ignored. During the change of vent sizes, for $K_v$ ≤ 0.3, the explosion parameters change drastically and the disaster effect is significant. For example, external explosion that affect the discharge of internal explosion overpressure occur; explosion that occurs in masonry structures can destroy the structural integrity of the brick walls. Therefore, $K_v$ = 0.3 can be used as a reference for hydrogen-air venting safety design.