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In this work, we investigated the impact of temperature on two-phase transport in low temperature (LT)-polymer electrolyte membrane (PEM) electrolyzer anode flow channels via in operando neutron imaging and observed a decrease in mass transport overpotential with increasing temperature. We observed an increase in anode oxygen gas content with increasing temperature, which was counter-intu.itive to the trends in mass transport overpotential. We attributed this counterintuitive decrease in mass transport overpotential to the enhanced reactant distribution in the flow channels as a result of the temperature increase, determined via a one-dimensional analytical model. We further determined that gas accumulation and fluid property changes are competing, temperature-dependent contributors to mass transport overpotential; however, liquid water viscosity changes led to the dominate enhancement of reactant water distributions in the anode. We present this temperature-dependent mass transport overpotential as a great opportunity for further increasing the voltage efficiency of PEM electrolyzers.

Abstract The technology landscape around distributed generation continues to evolve in response to increasing demand for high-efficiency, low-emission, low-cost power generation. While emerging distributed power technologies, such as solid oxide fuel cells (SOFCs), continue to advance, they still face challenges due to their high capital costs, and shorter lifetimes that typically arise from electrochemical stack performance degradation at high operating temperatures (>750 °C). Recent advancements in protonic ceramic fuel cells (PCFCs) offer the potential to mitigate drawbacks of their higher temperature SOFC counterparts by enabling lower operating temperatures (550 °C–600 °C) with acceptable power densities. The present work leverages the recent progress in protonic ceramic cell and stack technology development to generate viable system configurations and evaluate the energetic performance potential of PCFC-based systems for stationary power generation. Process system engineering of two water-neutral system concepts, which provide 25 kW of electric power and process hot water, are presented and evaluated through sensitivity studies. Stack design parameters are altered and used to gauge the effect on system performance characteristics, including fuel cell stack and balance-of-plant sizing requirements, and electric and cogeneration efficiencies. The study finds that the potentially high per-pass fuel utilization capability of PCFC stacks could enable unprecedented electric efficiencies approaching 70% without hybridization with other prime movers.

Abstract Comparative pyrolysis behaviors of typical hardwood ( Fagus sylvatica ) and softwood ( Cunninghamia lanceolata ) were investigated based on thermogravimetric analysis over a wide heating rate range from 5 K/min to 60 K/min. The Flynn-Wall-Ozawa model-free method was applied to estimate the various activation energy values at different conversion rates, and the Coats-Redfern model-fitting method was used to predict the possible reaction mechanism. Two pyrolysis regions were established by the trend of activation energy, divided by the threshold of conversion rate (0.4 for hardwood and 0.2 for softwood) but with the same distinguished temperature at about 580 K. For the region under the conversion rate threshold, the activation energy of hardwood increased gradually while softwood decreased. Furthermore, the activation energy remained the same for both hardwood and softwood in the region over the conversion rate threshold. However, softwood behaved greater activation energy than hardwood during the whole pyrolysis process. The pyrolysis differences of hardwood and softwood could be attributed to the chemical component, molecular structure, component proportion and various extractives. The same reaction mechanism of hardwood and softwood was verified by applying the Coats-Redfern approach. By checking activation energies obtained according to different models with those obtained through the Flynn-Wall-Ozawa method, the best model was based on diffusion mechanism when the conversion rate was less than its threshold, otherwise based on reaction order (2nd to 3rd).

Abstract Interest in solar chimney power plant (SCPP) has seen resurgence due to the continuously increasing awareness on environmental concerns, particularly greenhouse gas emissions from fossil fuels, since the 21st century. Although remarkable advances in the understanding of SCPP have been achieved through extensive theoretical, experimental, and numerical studies with different focuses on various aspects of the SCPP technology, no industrial scale SCPP has been built. In response to these new scientific advances and challenges for commercialization, seven questions, including parameter influences, turbine design, flow and heat transfer characteristics, similarity analysis, and hybrid systems, are presented in this work. In addition, answers and current understanding are included to provide succinct links to latest knowledge and identify areas that require further research.

Abstract Increasing electricity production by solar and wind energy is projected to impact the stability of electricity grids and consequently may limit the growth of renewable electricity generation. This issue can be ameliorated in part by increasing the flexibility of baseload power plants. A thermodynamic analysis of thermal energy storage (TES) coupled with a nuclear-powered Rankine cycle as one approach of increasing baseload flexibility is presented. During periods of excess capacity, the high-pressure steam supply is used to charge the TES. When electricity generation above the baseload capacity is required, the TES is discharged to generate steam for expansion in the low-pressure turbine. Pressure, temperature, and enthalpy state points within the cycle are presented over a range of charge and discharge rates. The capacity factor over a charge/discharge cycle is up to 9.8% higher than that of the same plant operated with steam bypass. This benefit increases with increasing charge and discharge power. With TES, the thermal-to-electrical efficiency is stable over a wide range of discharge rates. The results support future development of TES systems for baseload thermal power plants in a power grid in which renewable energy is prioritized.

Abstract For the safe and efficient operation of concentrator photovoltaic cells and electronic chips, low and uniform temperature should be attained. Therefore, the prime focus of this study is to design the optimal headers and to evaluate the performance of a monolithic double-layer microchannel heat sink (MDL-MCHS) operating under forced convective boiling conditions. The designed and fabricated heat sink was proved to attain a uniform temperature distribution over the entire surface of the MCHS heated wall, in a narrow temperature range around the coolant boiling point. The designs of the MCHS inlet and outlet headers were computationally optimized to avoid flow maldistribution over 10 parallel channels in each layer. Subsequently, an MDL-MCHS with an optimized header was fabricated using a metal 3D printer, and its thermal characteristics were experimentally evaluated in counterflow and parallel-flow operations under single-phase liquid flow and forced convective boiling conditions. The supplied heat flux was varied from 1.0 to 9.2 kW/m2. Ethanol and acetone with a boiling point of 78.4 °C or 56 °C were identically fed into each layer in a flowrate ( V ) range of 15–400 ml/h. At 9.2 kW/m2 (11.5 suns), the counterflow operation of forced convective boiling attained temperature uniformity below 1.6 °C and 1.8 °C in the V range of 25–100 ml/h for ethanol and 50–300 ml/h for acetone, respectively. The resultant wall temperature was nearly identical with the boiling point of operated coolant.

Abstract The coal gasification process is one of the main exergy destruction contributors in polygeneration systems and has considerable energy saving potential. In the present study, for improving the performance of the polygeneration system, the coal-steam gasification method was employed to integrate a novel methanol-electricity polygeneration system. The results indicated that the energy efficiency of the novel system was 63.3% with a chemical-to-power output ratio of 8.4, while the energy efficiency of the traditional system is 51.3% at the optimal unreacted syngas recycling ratio. Exergy analysis results revealed that the system exergy destruction in the coal–steam gasification process is 7.5% smaller than that in the GE gasification process, and eliminating the air separation unit can reduce the exergy destruction of the system by 4.3%. Additionally, the energy saving contributions of gasification process improvement and system integration were quantitatively evaluated. When the chemical-to-power output ratio increased from 1.9 to 11.9, the energy saving contributions of the system integration and gasification process improvement ranged from 9.8% to 15.1% and 11.9% to 12.9%, respectively.