[en] Highlights: • A thermodynamic model with empirical basis is developed to guarantee high fidelity. • T-Q diagram analysis evaluates feasibility of 14 potential SOFC-CHP systems. • Direct-combustion (DC) and gas-branching methods provide optimal thermal integration. • DC Branching needs the lowest parasitic power of 1.04 kW less than 5% of gross power. • DC Branching shows the highest electrical (55.5%) and exergetic (53.5%) efficiency. -- Abstract: A comparative study on various solid oxide fuel cell (SOFC)-combined heat and power system layout designs is conducted to suggest its optimized structure. Thermodynamic models of a SOFC stack and balance of plant components are developed by using empirical correlations dependent on their design variables. 14 potential system layout designs are categorized by thermal integration methods through rigorous literature review and evaluated for their thermodynamic feasibility and performance by T-Q diagram analysis. Results show that preemptive air heating prior to combustion of stack exhaust gases or fuel heating is not thermodynamically feasible due to substantial heat transfer during air heating. Independent heat recuperation of the anode exhaust gas from a SOFC stack also shows thermodynamic non-viability given its low heat capacity rate. 4 effective layouts are selected and further investigated by proceeding detailed thermodynamic analysis. The system layout employing direct combustion just after the SOFC stack and branching of the hot gas stream (combustion gas) results in the pressure drop of 23.5 kPa and parasitic power of 1.04 kW (less than 5% of gross power). These are 2–4 times smaller than those of other layouts. Accordingly, the proposed layout provides the highest electrical efficiency of 55.5% and exergetic efficiency of 53.5%.

[en] Highlights: • Optimal operation for a solid oxide co-electrolysis cell system is investigated. • The effect of key operating parameters and system performance map is explained. • High system efficiency and reactant conversion is obtained at high current density. • High current-to-reactant ratio allows high efficiency and high reactant conversion. • Air ratio should rather be chosen to support the thermal stability of the stack. This study performs thermodynamic optimization of solid oxide co-electrolysis cell system’s key operating parameters (current density, current-to-reactant ratio, and air ratio) by using high-fidelity and empirical-based system component models. For each operating parameter, a sensitivity analysis is conducted to elucidate optimal operating points for high system performance evaluated by indices such as system efficiency, reactant conversion, and H₂:CO ratio. At a high current density of 1.2 A/cm², the highest system efficiency of 58.21%, the highest reactant conversion of 63.84%, and the lowest H₂:CO ratio of 1.258 are obtained. Likewise, a high current-to-reactant ratio of 0.9 allows obtaining the high system efficiency of 58.26%, the high reactant conversion of 83.79%, and the lowest H₂:CO ratio of 1.230. Contrary to the two parameters, at the lowest air ratio of 3, the maximum system efficiency of 63.38% and the maximum H₂:CO ratio of 1.516 are obtained, whereas the highest reactant conversion of 62.78% is obtained at the highest air ratio. Based on the single parameter analysis, a performance map for each system performance index is derived as a function of current density and current-to-reactant ratio under the fixed air ratio given that the air ratio is more related to the thermal inertia of the stack. To gain high system efficiency and high reactant conversion, a high current-to-reactant ratio of 0.75 ∼ 0.90 is necessary. The high current density of 1.2A/cm² is also recommended for obtaining high performance, a small temperature gradient inside the SOEC stack, and small temperature variation with the change of the current-to-reactant ratio. However, if the system aims to obtain a low H₂:CO ratio, low current-to-reactant ratio and low current density are suggested at the expense of system efficiency and reactant conversion. The maximum system efficiency is obtained at 1.2A/cm² and the current-to-reactant ratio of 0.75, and the maximum reactant conversion of 88.91% is obtained at 1.2A/cm² and the current-to-reactant ratio of 0.90. On the other hand, the highest H₂:CO ratio is obtained at 0.2 A/cm² and the current-to-reactant ratio of 0.15. The extensive results for the solid oxide co-electrolysis cell system obtained in this study enable figuring out the coupling effects of key operating parameters and capturing optimal operating conditions of a solid oxide co-electrolysis cell system without performing costly experiments under various operating conditions.

[en] Highlights: • An analytical expression-based one-dimensional solid oxide fuel cell model is used. • Using the anode exhaust gas for fuel pre-reforming enhances electrical efficiency. • An operating window is derived from the feasible range of system parameters. • Heat to power ratio and electrical efficiency are inversely proportional. • The optimal operating conditions for summer and winter are suggested. Operational guidelines for safe and optimal load following operation of a residential solid oxide fuel cell-combined heat and power system are suggested. System layout design optimization via feasibility test and performance analysis is preceded. Then, the effect of system parameters on system performance is analyzed in a parametric study to draw an operating window and to derive optimal operating conditions. Owing to complete analytical expressions equipped in an in-house one-dimensional solid oxide fuel cell stack model, reliable estimation of the stack performance under various operating conditions is accomplished. The operating window is drawn from the feasibility criteria, which consider system thermal integration and durability. It is found that the operating range of low current density, low fuel utilization, and low air utilization values leads to unfeasible thermal integration. Moreover, the operating range of high current density, low fuel utilization, and high air utilization leads to defect in system durability. Finally, the optimal operating conditions are suggested to supply heat and power for extreme seasonal energy demands. The optimal operating condition for summer is suggested by the air utilization of 17.5%, the current density of 0.4 A/cm², and the fuel utilization of 80%, providing the net electrical power of 18.5 kW, the electrical efficiency of 44.1%, and the heat to power ratio of 0.95. The optimal operating condition for winter is suggested by the air utilization of 17.5%, the current density of 0.3 A/cm², and the fuel utilization of 60%, providing the net electrical power of 15.5 kW, the electrical efficiency of 37.0%, and the heat to power ratio of 1.23.

[en] Highlights: • Advanced infiltration technique for solid oxide regenerative fuel cells is introduced. • Highly active and thermally stable nanocatalysts are produced via in operando synthesis. • Geometric properties and crystallization behavior are precisely regulated at high temperatures. • Nano-tailoring of catalysts remarkably improves the performance in both fuel cell and electrolysis modes. • Nanocatalysts do not degrade during long-term operation under harsh environments. Solid oxide regenerative fuel cells (SORFCs), which perform the dual functions of power generation and energy storage at high temperatures, could offer one of the most efficient and environmentally friendly options for future energy management systems. Although the functionality of SORFC electrodes could be significantly improved by reducing the feature size to the nanoscale, the practical use of nanomaterials has been limited in this area due to losses in stability and controllability with increasing temperature. Here, we demonstrate an advanced infiltration technique that allows nanoscale control of highly active and stable catalysts at elevated temperatures. Homogeneous precipitation in chemical solution, which is induced by urea decomposition, promotes crystallization behavior and regulates precursor redistribution, thus allowing the precise tailoring of the phase purity and geometric properties. Controlling the key characteristics of Sm_0.5Sr_0.5CoO₃ (SSC) nanocatalysts yields an electrode that is very close to the ideal electrode structure identified by our modeling study herein. Consequently, outstanding performance and durability are demonstrated in both fuel cell and electrolysis modes. This work highlights a simple, cost-effective and reproducible way to implement thermally stable nanocomponents in SORFCs, and furthermore, it expands opportunities to effectively exploit nanotechnology in a wide range of high-temperature energy devices.

[en] Thermopower waves, which occur during combustion within hybrid structures formed from nanomaterials and chemical fuels, result in a self-propagating thermal reaction and concomitantly generate electrical energy from the acceleration of charge carriers along the nanostructures. The hybrid structures for thermopower waves are composed of two primary components: the core thermoelectric material and the combustible fuel. So far, most studies have focused on investigating various nanomaterials for improving energy generation. Herein, we report that the composition of the chemical fuel used has a significant effect on the power generated by thermopower waves. Hybrid nanostructures consisting of mixtures of picric acid and picramide with sodium azide were synthesized and used to generate thermopower waves. A maximum voltage of ∼2 V and an average peak specific power as high as 15 kW kg"−"1 were obtained using the picric acid/sodium azide/multiwalled carbon nanotubes (MWCNTs) array composite. The average reaction velocity and the output voltage in the case of the picric acid/sodium azide were 25 cm s"−"1 and 157 mV, while they were 2 cm s"−"1 and 3 mV, in the case of the picramide/sodium azide. These marked differences are attributable to the chemical and structural differences of the mixtures. Mixing picric acid and sodium azide in deionized water resulted in the formation of 2,4,6-trinitro sodium phenoxide and hydrogen azide (H-N_3), owing to the exchange of H"+ and Na"+ ions, as well as the formation of fiber-like structures, because of benzene π stacking. The negative enthalpy of formation of the new compounds and the fiber-like structures accelerate the reaction and increase the output voltage. Elucidating the effects of the composition of the chemical fuel used in the hybrid nanostructures will allow for the control of the combustion process and help optimize the energy generated from thermopower waves, furthering the development of thermopower waves as an energy source. (paper)

[en] Highlights: • Anode concentration polarization plays a key role in determining dynamic response. • Microstructure of an anode support has a negligible impact on transient behavior. • Raising the fuel utilization from 40% to 80% increases the relaxation time by 240%. • Local fuel depletion towards fuel outlet induces current undershoot and relaxation. • Controlling the local fuel concentration is critical for improving dynamic response. -- Abstract: The effect of anode microstructure (i.e., porosity, tortuosity, and thickness) and fuel utilization (i.e., the flow rate ratio of fuel consumed to fuel supply) on the potentiodynamic response of a solid oxide fuel cell is elucidated by resolving thermo-electrochemical parameters temporally. To investigate physical and electrochemical processes occurring at electrodes upon electrical load change, a high-fidelity physicochemical model is used in this study. Locally distributed thermo-fluidic flow field and thermodynamic variables are resolved spatially and temporally by performing dynamic, three-dimensional numerical modeling. Results show that relaxation time is required for current density to asymptotically recover from its excessive response to potential steps to its original magnitude upon potentiodynamic conditions. This is predominantly attributed to anodic concentration polarization, indicating that the overall dynamic characteristic is primarily governed by diffusive transport phenomena in the anode. A parametric study for the anode microstructure and fuel utilization, which may influence the species transport in the anode, is conducted to find out methodologies to control the relaxation time. The parametric study shows that the microstructure has a trivial effect on the species diffusion velocity and the transient behavior upon electrical load change. On the other hand, the relaxation time is substantially influenced by fuel utilization such that it increases by 240% (from 0.4 s to 1.36 s) when raising the fuel utilization from 40% to 80%. Its sensitivity coefficient is nearly 2.0 which is substantially larger than −0.03 to 0.4 of anode microstructure. This implies that the relaxation time under electrical load change can be primarily controlled by selecting the optimal operating conditions, in particular in the fuel side.

[en] Growing concerns over greenhouse gas emissions have driven extensive research into new power generation cycles that enable carbon dioxide capture and sequestration. In this regard, oxy-fuel combustion is a promising new technology in which fuels are burned in an environment of oxygen and recycled combustion gases. In this paper, an oxy-fuel combustion power cycle that utilizes a pressurized coal combustor is analyzed. We show that this approach recovers more thermal energy from the flue gases because the elevated flue gas pressure raises the dew point and the available latent enthalpy in the flue gases. The high-pressure water-condensing flue gas thermal energy recovery system reduces steam bleeding which is typically used in conventional steam cycles and enables the cycle to achieve higher efficiency. The pressurized combustion process provides the purification and compression unit with a concentrated carbon dioxide stream. For the purpose of our analysis, a flue gas purification and compression process including de-SO_x, de-NO_x, and low temperature flash unit is examined. We compare a case in which the combustor operates at 1.1 bars with a base case in which the combustor operates at 10 bars. Results show nearly 3% point increase in the net efficiency for the latter case.

[en] Highlights: • High-performance lanthanum nickelate-based cathode is fabricated. • The sinterability of cathode is enhance by compositional modification. • Harmful chemical reactions are suppressed by lowering processing temperatures. • The cell with lanthanum nickelate cathode outperforms state-of-the-art cells. -- Abstract: The Ruddlesden-Popper phase lanthanum nickelate, La₂NiO_4+δ (LNO), offers excellent material properties as a cathode for solid oxide fuel cells (SOFCs). However, taking full advantage of its intrinsic properties is difficult in realistic cells because of its high chemical reactivity with the electrolyte at elevated temperatures. Herein, we demonstrate high-performance SOFCs with an LNO-based cathode fabricated by a low-temperature processing route that suppresses harmful chemical reactions. The sintering capability of the composite cathode composed of LNO and gadolinia-doped ceria (GDC) was enhanced by mixing Fe-based sintering additive with GDC, which formed reliable interfacial bonding with the electrolyte at a temperature ~200 °C below the typical processing temperature. Because no interdiffusion between cathode and electrolyte occurs at such low temperatures, the cell is successfully fabricated without diffusion blocking layer, which simplifies the cell structure and manufacturing process. The cell with the LNO-based cathode outperformed state-of-the-art cells, particularly at lower operating temperatures. These results highlight that the processing parameters strongly affect the electrochemical performance of this LNO-based cathode and must be carefully engineered to fully exploit its superior intrinsic properties.

[en] One of the major problems arising with Solid-Oxide Fuel Cell (SOFC) electrolyte is conventional sintering which requires a very high temperature (>1300 °C) to fully density the electrolyte material. In the present study, the sintering temperature of SOFC electrolyte is drastically decreased down to 600 °C. Combinational effects of particle size reduction, liquid-phase sintering mechanism and microwave sintering resulted in achieving full density in such a record-low sintering temperature. Gadolinium doped Ceria (GDC) nano-particles are synthesized by co-precipitation method, Lithium (Li), as an additional dopant, is used as liquid-phase sintering aid. Microwave sintering of this electrolyte material resulted in decreasing the sintering temperature to 600 °C. Micrographs obtained from Scanning/Transmission Electron Microscopy (SEM/TEM) clearly pointed a drastic growth in grain-size of Li-GDC sample (∼150 nm) than compared to GDC sample (<30 nm) showing the significance of Li addition. The sintered Li-GDC samples displayed an ionic conductivity of ∼1.00 × 10"−"2 S cm"−"1 at 600 °C in air and from the conductivity plots the activation energy is found to be 0.53 eV. - Highlights: • Sintering temperature is drastically decreased to 600 °C. • Key factors: Particle size reduction, liquid-phase and microwave sintering. • Nano-Li-GDC sample has ionic conductivity of ∼1.00 × 10"−"2 S cm"−"1 at 600 °C in air.

[en] Highlights: • The hybrid system maximizes latent heat recovery and fuel conversion to the power. • Injecting the boiler flue gas into the gasifier improves oxy-coal gasification. • Reusing the boiler flue gas as a diluent in a gas turbine enhances its utilization. • Using additional heat exchangers provides the flexibility of thermal integration. • Extensive heat recovery and hybridization increase the net efficiency of the system. This study proposes the 2nd generation hybrid pressurized oxy-coal combustion power cycle that utilizes both fluidized-bed combustion coupled with the Rankine cycle and gasification connected to the Brayton cycle. The fluidized-bed boiler and gasifier are thermally connected by a flue gas stream flowing from the former to the latter. This improves thermal integration carbon conversion in the gasifier. The Rankine cycle is thermally connected to the Brayton cycle through the reheating process of intermediate pressure steam. This hybrid cycle enables the system to recover thermal energy from the pressurized flue gas with effective thermal integration in a way that it balances heat flow rates between hot and cold streams and increases the degree of freedom for heat flow control. The proposed system also reuses cold boiler exhaust gas as a diluent of the Brayton cycle to control its gas turbine inlet temperature, which raises the gas turbine flow rate and hence the gross power. The results indicate more than 9%-point and 1%-point increases in the gross and net efficiency, respectively, as compared to a single Rankine cycle system. The proposed hybrid system exhibits the net efficiency of 34.39% (HHV) higher than 33.04% (HHV) of the single oxy-PFBC cycle.