An experimental study on the practical performance of organic Rankine cycle (ORC) system using zeotropic mixture is performed by using a small scale ORC power generation experimental setup. R601a/R600a is selected as the working fluid. The effects of mixture composition, heat source temperature, and working fluid flow rate on the performance of ORC system are investigated. The experimental results indicate that the net power output first increases and then decreases as the R600a concentration increases. The optimal mixture composition with the maximum net power output is 0.6/0.4 (mass fraction) at the heat source temperature of 115°C. The net power output of R601a/R600a (0.6/0.4) is higher than that of R601a by 25%, indicating that the performance of ORC system can be clearly improved by using the zeotropic mixture. For a fixed working fluid flow rate, both net power output and thermal efficiency first decrease slowly and then drop sharply with the decrease of the heat source temperature. The appropriate superheat degree of R601a/R600a is in the range of 15 to 20°C when the heat source temperature has a small variation. In addition, the optimal working fluid volume flow rates yielding the maximum net power output are obtained for different compositions of R601a/R600a. The experimental results in the study can be of great significance for the design and operation of ORC power system using zeotropic mixture. Copyright © 2016 John Wiley & Sons, Ltd.

This paper examines the instability and energy dissipation of a high-head Francis turbine during shutdown using improved delayed detached eddy simulation, which involves linearly reducing the guide vane opening from 9.84° (best efficiency point) to 0.8° [Spin-No-Load (SNL)]. The results demonstrate a linear increase in water head, with hydraulic efficiency dropping from 93% to 30%. Pressure fluctuations in the vaneless space are mainly due to blade passing frequency and low-frequency components at SNL. High-amplitude pressure fluctuations occur below 37.4% opening in the draft tube, with the dominant frequency under 0.32 times the blade passing frequency. Three vortex structures are identified within the draft tube, a columnar vortex rope in the first stage, multiple helical vortex ropes in the second stage, and discrete vortex structures in the third stage. The most significant axial and radial velocity fluctuations are evident during the second and third stages. Turbulent kinetic energy generation and work done by Reynolds stress are the main sources of power loss. Energy dissipation primarily occurs at the outlet of the runner blades, while it corresponds to the positions of the vortex structures in the draft tube, suggesting that vortex flow structures are the primary mechanism of energy dissipation in the turbine. This study introduces a numerical shutdown model using the dynamic mesh technique, clarifies the relationship between guide vane opening and performance parameters, and identifies the three-stage vortex evolution and energy dissipation mechanisms, offering novel insights into transient instabilities in high-head Francis turbines.

This work used the established mathematic models of an intermediate-temperature solid oxide fuel cell (IT-SOFC) and gas turbine (GT) hybrid system fueled with wood chip gas to investigate the load performance and safe characteristic under off-design conditions. Three different operating modes (mode A: regulating the fuel proportionally, and the air is passively regulated. Mode B: regulating the fuel only. Mode C: simultaneously regulating the fuel and air) were chosen, and the component safety factors (such as fuel cell maximum temperature, compressor surge margin, carbon deposition in reformer) were considered. Results show that when the operation modes A and C are executed, the hybrid system output power can be safely changed from 41% to 104%, and 45% to 103%, respectively. When mode B is executed, the load adjustment range of hybrid system is from 20% to 134%, which is wider than that of two above operation modes. However, the safety characteristic in this case is very complicated. The system will suffer from two potential malfunctions caused by too lower temperature entering turbine and CH4/CO cracking in reforming reactor when it operates in low load conditions. When the system operates in the high load conditions exceeding 130% of relative power, the potential thermal cracking of fuel cell will be occurred.

Faced with environment pollution and climate change, it is essential to develop advanced low-carbon energy system based on renewable sources. Micro-energy networks with a high proportion of renewable energy are one of the most important methods to achieve the complementary use of renewable energy and fossil energy with high efficiency of energy utilization. Research on urban-level energy networks would be an important step for applying and integrating renewable sources. Faced with high penetration rate of renewable energy, the robust ability and safety operation are challenging from both sides of energy generation and the electrical grid. Meanwhile, the increasing number of electrical and fuel cell powered vehicles put additional difficulties for optimization and scheduling. Therefore, research on urban energy systems with a high proportion of renewable energy is extremely valuable.This topic (“Urban Multi-energy System Networks with High Proportion of Renewable Energy”) aims to look into the key problems existing in the integrated multi-energy network system for the urban level and provide a detailed theoretical basis and experimental data for the design and operation of integrated micro-energy networks at the international academic level. The topic received several submissions within the field of renewable energy systems, including modeling and optimization, grid supply, the interaction between grid and EV/fuel cell cars, and advanced controlling strategy of off-grid/grid-connected microgrid systems, etc.Low-carbon development of energyFeng et al. [1] modeled the natural gas system, power system, and their coupling components, and proposed a low-carbon economic control framework based on MASAC. The objective was to minimize both operational costs and carbon emissions. Simulation results using a nodal system demonstrated that this method significantly reduces costs and emissions compared to traditional approaches. Hu et al. [2] developed a low-carbon scheduling model for electric vehicles that considers carbon quotas and dynamic electricity pricing. A two-level scheduling framework was applied to solve the model. Simulation analysis confirmed that the model reduced carbon emissions by 10.13 tons and user costs by 451.11 RMB. Yahya et al. [3] investigated the potential of converting waste tires into synthetic fuels in Saudi Arabia using vacuum pyrolysis technology. This method not only reduces landfill pollution but also decreases CO₂ emissions by 2.5 tons per ton of carbon recovered.Power facilities control methodsEswar et al. [4] proposed a renewable energy-based microgrid system for electric vehicle (EV) charging stations. They adopted a hybrid optimization approach (DOA-SBNN) to predict load demand and manage microgrid fluctuations. A four-phase inductively coupled interleaved boost converter (FP-ICIBC) and a fractional-order PID controller (FOPID) were designed to enhance power management efficiency. Experimental results demonstrated that this method outperformed existing technologies in terms of energy efficiency, total harmonic distortion (THD), and computation time. Wu et al. [5] proposed a Virtual Synchronous Generator (VSG) control strategy, integrated with Energy Storage Systems (ESS) and PV, to enhance system inertia. Results showed that the proposed PV-VSG method can increase PV penetration by 57.5% compared to scenarios without VSG. Kumar et al. [6] proposed a novel method for coordinated voltage and frequency regulation in isolated hybrid power systems (IHPS). A PI-TISMC controller was designed to coordinate inertial sources and suppress system oscillations.International energy cooperation and policyYan et al. [7] analyzed the necessity of cooperation between China and South Africa in the field of renewable energy, particularly solar and wind power. They identified key international challenges in energy development and proposed policy recommendations to support renewable energy collaboration between the two countries, contributing to South Africa’s low-carbon transition.This research project brings together pioneering studies focused on advancing the greening of energy systems. It addresses key challenges such as low-carbon equipment operation, stable device control, and international energy cooperation—factors that are particularly beneficial for the development of urban energy systems with a high share of renewable energy. Moreover, these studies contribute to a broader understanding of the role that urban decarbonization plays in achieving sustainable and efficient system operation, which could promote academical cooperation and communication in the future.