A novel three-step GeO2/GeO thermochemical water splitting cycle for solar hydrogen production

This investigation reports the thermodynamic exploration of a novel three-step GeO₂/GeO water splitting (WS) cycle. The thermodynamic computations were performed by using the data obtained from HSC Chemistry thermodynamic software. Numerous process parameters allied with the GeO₂/GeO WS cycle were estimated by drifting the thermal reduction ($T_H$) and water splitting temperature ($T_L$). The entire analysis was divided into two section: a) equilibrium analysis and b) efficiency analysis. The equilibrium analysis was useful to determine the $T_H$ and $T_L$ required for the initiation of the thermal reduction (TR) of GeO₂ and re-oxidation of GeO via WS reaction. Furthermore, the influence of $P_{O_2}$ on the $T_H$ required for the comprehensive dissociation of GeO₂ into GeO and O₂ was also studied. The efficiency analysis was conducted by drifting the $T_H$ and $T_L$ in the range of 2080 to 1280 K and 500–1000 K, respectively. Obtained results indicate that the minimum ${\dot{Q}}_{solar-cycle}=624.3\text{kW}$ and maximum ${\eta }_{solar-to-fuel}=45.7\text{\%}$ in case of the GeO₂/GeO WS cycle can be attained when the TR of GeO₂ was carried out at 1280 K and the WS reaction was performed at 1000 K. This ${\eta }_{solar-to-fuel}=45.7\text{\%}$ was observed to be higher than the SnO₂/SnO WS cycle (39.3%) and lower than the ZnO/Zn WS cycle (49.3%). The ${\dot{Q}}_{solar-cycle}$ can be further decreased to 463.9 kW and the ${\eta }_{solar-to-fuel}$ can be upsurged up to 61.5% by applying 50% heat recuperation.     

The instability of laminar methane/hydrogen/air flames: Correlation between small and large-scale explosions

Darrieus–Landau (D-L) instability can cause significant acceleration in freely expanding spherical flames, which can lead to accidental large-scale gas explosions. To evaluate the potential of using high-pressure lab-scale experiments to predict the onset of cellular instabilities in large-scale atmospheric explosions, experimental measurements of the cellular instabilities for hydrogen and methane mixtures are conducted, in laboratory spherical explosions at elevated pressures. These measurements are compared with those from several large-scale atmospheric experiments. Comprehensive correlations of the pressure effect on a critical Karlovitz number, $K_{cl}$, together with those of strain rate Markstein number, $M{a}_{sr}$, are developed for hydrogen/air mixtures. The regime of stability reduces for all mixtures, as $M{a}_{sr}$ becomes negative. Values derived from large-scale experiments closely follow the same correlation of $K_{cl}$ with $M{a}_{sr}$. As a result, the extent of the regime where the laminar explosion flames become unstable can be predicted as a function of $M{a}_{sr}$ and pressure.     

Effect of hydrogen addition on the detonation performances of methane/oxygen at different equivalence ratios

Detonation performances of methane/hydrogen/oxygen (CH₄/H₂/O₂) mixtures were investigated experimentally in a 3000 mm long tube with an inner diameter of 30 mm at different initial pressures $p_0$ (ranging from 10 kPa to 50.5 kPa). Mixtures with different proportions of H₂ in the total fuel $\alpha$ (0%, 14.29% and 25%) and different equivalence ratios $\Phi$ (0.8, 1.0 and 1.2) were tested. Signals of flame front and pressure were obtained by ion probes and high frequency pressure transducers, respectively. Results showed that with the increase of $p_0$, $\alpha$and $\Phi$, the average velocity of steady detonation $V_{ave}$ increased. For mixtures with the given $\alpha$, when $\Phi$ increased by 0.2, $V_{ave}$ increased by 100 m/s. In the present study, velocity deficits were found to be within 5%, and when $p_0$ was higher than 20 kPa, the velocity deficits were within 2%. The average peak pressure of steady detonation $p_{ave}$ was close to the von Neumann pressure $p_{vN}$. Both the increase of $p_0$ and $\Phi$ led to the increase of the $p_{ave}$. But the addition of H₂ led to the decrease of $p_{ave}$, and $p_{ave}$ decreased with the increased of $\alpha$.     

Hybrid solar-driven hydrogen generation by sorption enhanced–chemical looping and hydrocarbon reforming coupled with carbon capture and Rankine cycle

Hydrogen ($H_2$) production from fossil fuels using Hydrocarbon Reforming Methods (HRM) accounts for nearly 95% of Global $H_2$ production. Unlike hybrid Chemical Looping Steam Reforming (CL-SR) systems, the Integrated Solar-Driven Sorption Enhanced–Chemical Looping of Hydrocarbon Reforming (SE-CL-HR) utilises solar thermal energy from the Concentrating Solar Power (CSP) system to drive the endothermic decomposition of feedstocks. Furthermore, the simulated hybrid systems utilise recovered heat to generate electricity, reuse of by-product ${CO}_2$ for more syngas production and finaly, ${CO}_2$ capture by reaction of $CaO$ to form ${CaCO}_3$. This work focused on simulating hybrid CSP systems and SE-CL-HR plants with Heat Transfer Fluid (HTF) output temperatures between 750 and 1050 °C. In this study, System Advisor Model (SAM) and MATLAB software are used to develop the CSP system. While the CSP result saved in the MATLAB workspace gets exported to Simulink to feed SE-CL-SMR, SE-CL-POX and SE-CL-ATR Aspen plus models. The integrated system was fed with ${CH}_4$ as the working fluid of the solar furnace. Stoichiometric and Gibbs free-energy minimisation were employed to investigate the effect of operating parameters. The output of the integrated system shows ≥9.5% exergy efficiency in comparison to conventional HRM. In addition, ${CO}_2$ capture by $CaO$ and high-pH water ($Ca$, $Mg$, $N{a}^{+}$, $O_2$, $O{H}^{-}$ and $C{l}^{-}$) to produce ${CaCO}_3$, ${MgCO}_3$ and other valuable products was also investigated in a process simulation. The research results revealed that for 8.1 $tons/hr$ of ${CH}_4$ and 277.1 $tons/hr$ $H_{2}O$ (steam) flowrates, 62 $tons/hr$ of $H_2$ can be generated and 338.5 $tons/hr$ of ${CO}_2$ emission can be reused and captured by the adoption of these new innovative technologies.     

Study on the correlation between ion current integral signal and combustion pressure

Based on the self-designed and developed gas mixture combustion and flame ion current measurement and control system, this paper studies correlation research between the ion current integral signal and combustion pressure under different initial pressures, equivalent ratios and hydrogen and carbon dioxide volume fractions. The experiments obtained the change rules under the different initial parameters of the ion current waveform and combustion pressure waveform along with the combustion time, the change rules of the characteristic parameters of ion current integral signal ($I_{\theta 80}$, $t_{80}$) and combustion pressure ($P_{max}$, $t_{Pmax}$) are analysed, revealing a high correlation between the $t_{80}$ and the $t_{Pmax}$; a method for timely and effective determining the pressure peak moment using ion current integral signals is presented. The conclusions of this study provide new technological approaches for effectively and accurately acquiring the combustion pressure information of gas mixture in cylinders using ion current integral signals, thereby realising accurate combustion control.     

Effect of burner diameter and diluents on the extinction strain rate of syngas-air non-premixed Tsuji-type flames

The present study focuses on the experimental determination of the global extinction strain rate ($a_g$) for different syngas-air combinations using the Tsuji type configuration. To study the effect of porous burner diameter (D), $a_g$ values were obtained for four values of D at atmospheric pressure. The experimentally obtained $a_g$ for a given fuel-oxidizer combination decreases with an increase in burner diameter (D). This trend is consistent with the limited data available in the literature for hydrocarbon fuels. Other geometric and flow-field effects namely, (1) plug flow, (2) flow-field blocking by the burner, and (3) heat loss by the flame to sidewalls that can affect $a_g$ were also experimentally quantified. The results from this study show that the plug flow boundary condition is always satisfied for oxidizer inlet distance > 2 times the largest porous burner diameter. Burner diameter less than 1/4 times side wall length (as is the case for all burners used in this study) does not significantly modify the flow. Hence, these two flow-field modifications do not affect $a_g$. However, heat loss from the flame to the ambient through the side walls can cause a 4–9 % decrease in $a_g$. Experiments showed that, CO/H₂ mixtures diluted with N₂ yield 1.6–2.25 times higher $a_g$ in comparison to CO/H₂ mixtures diluted with CO₂. Increasing H₂ from 1 to 5 % leads to 2.5–3.8 times increase in $a_g$, compared to 5 to 10 % increase in H₂ which leads to only 1.3–1.7 times increase in $a_g$ for 70 % of N₂ (v/v) in fuel mixture. Global extinction strain rate ($a_g$) increases by 1.5–2.4 times with 10 % increase in CO for fuel mixtures consisting of H₂ (1 and 5 % by v/v), CO₂ (50, 60 and 70 % by v/v) and N₂ (50, 60, 70 and 80 % by v/v). The change in overall reactivity (${\omega }_o$) due to different diluents is used to quantitatively explain the variation of $a_g$ for different fuel compositions. These effects are also qualitatively explained using OH radical concentration change with H₂ % in the fuel mixtures.     

A comparative study of γAl2O3–C2H6O2 and γAl2O3–H2O near a vertical curved surface having porous medium

The present analysis reveals the impact of porous medium and mixed convection nano-fluid flow near an exponentially curved surface. For this purpose alumina is treated as nano-particles along with two base fluids such as $H_{2}O$ and $C_2{H}_6{O}_2$. Under different assumptions the mathematical model of energy and momentum equations are developed by using curvilinear coordinates. The PDEs are transformed into non-linear differential equations by using similarity transformation variables. The table of skin friction and Nusselt number are drawn by fluctuating the values of different parameters for both fluids. Moreover, the numerical solutions are generated in MATLAB by using Bvp4c. Influence of effective and without effective Pr numbers for an incompressible $\gamma A{l}_2{O}_3-C_2{H}_6{O}_2$ and $\gamma A{l}_2{O}_3-H_{2}O$ are investigated. We observe that the velocity profile increases for increasing values of curvature, mixed convection and volume fraction of nanoparticles for two different cases effective and with-out effective Prandtl numbers. Moreover, in case of effective $Pr$ number, the temperature profile decreases but for with-out effective $Pr$ number temperature profile increases. Furthermore, the graphs are drawn to check the behavior various parameters for effective and with-out effective Pr numbers.     

Parametric study of cascade latent heat thermal energy storage (CLHTES) system in Concentrated Solar Power (CSP) plants

Recently, Concentrated Solar Power (CSP) is attracting numerous research attentions, and thermal energy storage (TES) system filled with energy storage media is a critical component in all CSP plants. To realize a high energy storage efficiency ($\xi$) and exergy efficiency ($\eta$), a comprehensive study to the cascade latent heat thermal energy storage (CLHTES) system is necessary from the perspective of heat transfer. In this study, a dimensionless parametric study was presented using an enthalpy-based 1D transient model for energy storage/extraction in CLHTES system. A dimensionless parameter space (${\tau }_r,H_{CR},St{f}^{\text{*}}$) was constructed by considering $\xi$ and $\eta$ as the objective functions to explore the effects from dimensionless material properties (such as latent heat, specific heat at solid and liquid phases) and dimensionless operational parameters (such as charging/discharging time period, TES tank height and diameter). It is recommended that when $H_{CR}$<0.5 and ${\Pi }_c/{\Pi }_d$<1.0, the system performance is very sensitive to $H_{CR}$ and ${\Pi }_c/{\Pi }_d$, furthermore for the same TES tank volume (H/D = D/H = 1.0), the sensitivity by varying its diameter alone is double than that from changing its height. The novelty of this study is to provide the design criteria for the CLHTES system, so that it can easily be designed and its efficiencies can be competitive to sensible heat thermal energy storage (SHTES) system. The results from this parametric study and sensitivity analysis are expected to benefit the solar thermal research and industry community to design the CLHTES system.