In order to improve the performance and efficiency of the battery in vanadium redox battery (VRB) system, a multi-stack equivalent loss circuit model is developed based on the composition and principles of VRBs, which includes electrochemical, hydrodynamic, temperature, bypass current, and vanadium batteries. Moreover, the effects of pumping loss and bypass current on vanadium batteries in the pipeline system of all-vanadium flow batteries are investigated. The relationship between pumping loss and pumping current, the bypass current model, and the equivalent circuit model of vanadium battery with multiple stacks connected in series are established, and the core mechanism of the vanadium battery taking into account the dynamic response is elaborated by transforming each model into a whole. The key factors involved in modelling of the VRB, including the pumping loss and bypass current, are discussed in detail. The influencing factors of battery performance and efficiency are also analyzed. The results show that, parameters such as the length and cross-sectional area of the pipeline affect the pipeline resistance, and the resistance of longer main and branch pipes will reduce the bypass current but increase the pumping loss current. Longer and thicker pipelines are conducive to the simultaneous reduction of both the bypass current and the pumping loss, which improves the energy efficiency of the battery. The research provides an idea for design of the manufacturer’s pipeline.

Against the cooling problem of engine heat exchangers, the flow and heat transfer characteristics of hydrogen in vertical and U-shaped tubes at supercritical pressures are studied. The influence of pressure and mass flow rate on heat transfer of the pipeline is studied by numerical method, and the heat transfer law is obtained. The heat transfer mechanism of the elbow section is discussed in depth, and the effect of dimensionless force on heat transfer is analyzed. The results show that, the closer the pressure is to the quasi-critical or when the mass flow rate increases, the convective heat transfer coefficient will increase, resulting in heat transfer enhancement. The bend section of the U-shaped tube can enhance heat transfer effectively, reaching a peak near θ=90°. There is a buoyancy effect in the straight pipe section at the inlet, but when the pressure is higher than 2.0 MPa, the buoyancy effect can be ignored after the hydrogen flows through the elbow section due to the influence of density difference. The Dean vortex caused by the secondary flow is the main factor to enhance the heat transfer performance of the elbow section, and the influence on the inlet section is significantly weaker than that on the outlet section.

The cost of underwater assets in offshore wind farms is relatively low compared with the total cost of offshore wind power projects, yet the quality of these assets is vital for safe and stable operation of the offshore wind farms. The quality issues that can arise with wind turbines, underwater steel structures and foundations of offshore substations, and submarine cables during the construction and operation and maintenance phases are analyzed. Based on cases from practical offshore wind power generation projects, the inspection requirements and methods stipulated by relevant standards are evaluated, and comprehensive and effective underwater inspection methods for various types of quality defects are put forward. These methods have been verified in real cases, forming industry standards and specifications, which provides significant technical guidance and reference value for quality control and operational maintenance of underwater assets in offshore wind farms.

In view of the complex variation law and strong autocorrelation of nitrogen oxides emission mass concentration of circulating fluidized bed (CFB) boiler, by using relevant variables and their historical information, ensemble learning online models of nitrogen oxides emission mass concentration are established. The ensemble learning online models include the autoregressive integrated moving average (ARIMA), random forest (RF), gradient boosting (GBDT), and eXtreme gradient boosting (XGBoost) model. The prediction results are compared and selected, among which the GBDT regressor is the best. In order to further improve the prediction effect of the model, a GBDT differential regression model is established by combining the first-order difference with the GBDT regression algorithm. The tests show that the established GBDT differential regression model has better prediction performance than the aforementioned models. The mean squared error of the predicted value is 20.2% lower than that of the simple GBDT regressor, and 46.5% lower than that of the online sequential extreme learning machine (OS-ELM) model used in the reference. The online model also fully considers avoiding the influence of the instrument purge process, and has strong practicability.

With the increasingly prominent problem of load fluctuations in high proportion renewable energy grids, improving the fast load response ability of large-scaled thermal power units under full operating conditions has become an urgent need to maintain the safe and stable operation of the power system. Therefore, a rapid load change strategy combined with the high-pressure bypass transformation technology under the flexibility demand of supercritical power units is proposed. Firstly, the high-pressure bypass is added to the high-pressure regenerative system to flexibly change the steam extracted amount from turbine, thus to accelerate the energy supply rate of once-through boiler. Secondly, to adapt to the high ramping rate, a load change scheme is designed with the limitations of main steam pressure, temperature and their change rate that boiler can withstand at sliding-pressure operation mode and the decoupling of load-main steam pressure for unit. Finally, test on a 600 MW coal-fired thermal power unit shows that, the unit can successfully achieve a high load ramping rate of 3%Pe/min under full operating conditions. Moreover, the main steam pressure, position of main steam valve and other parameters are maintained stable. In addition, the load regulation ability of the unit with load ramping rate of 5% Pe/min is verified, confirming the effectiveness of the proposed load rapid regulation strategy.

Direct discharging of saturated flue gas from coal-fired utility boiler can lead to significant low-grade waste heat loss. The saturated flue gas waste heat recovery and utilization for centralized heating system is constructed, the operating parameters of the heating system in coal-fired power plant are analyzed, and the feasibility that the saturated flue gas waste heat can be used to heat the return water of the heating system is verified. Finally, the economic efficiency of the centralized heating system is investigated for operation with different targets, and the influence characteristics of the operating parameters are revealed for the thermal performance of the centralized heating system. The research results show that, the temperature of flue gas waste heat can be increased by 30~40 ℃ by absorption heat pump. With the 350 MW coal-fired heating unit as an example, the absorption heat pump recovers the saturated flue gas waste heat with 50.23 MW for re-utilization. The economic benefits brought by enhancing the heating capacity are significantly better than those corresponding to the increase in power generation, and the heating capacity of the coal-fired power plant is increased by 13.4%, and the annual heating revenue is increased by 19 752 000~34 423 200 yuan. The research provides technical references for the saturated flue gas waste heat recovery and utilization in coal-fired power plants.

In response to significant increase in energy consumption caused by low-grade waste heat and energy waste in coal-fired power plants, a 350 MW unit is selected as the research object, the Ebsilon software is used to model and simulate different deep recovery schemes for low-grated waste heat and residue. The operating data of the unit under two schemes of “organic Rankine cycle (ORC)” and “Heater” are calculated. The energy consumption characteristics, revenue characteristics and differences are analyzed, and the mechanism and optimization plan for deep recovery of waste heat and energy are obtained. The results show that, the energy consumption characteristics of the unit improve significantly under both schemes, and the “heater” scheme has lower energy consumption. As the organic working fluid flow rate increases, the power generation of the ORC system gradually increases, but the thermoelectric efficiency of the ORC system gradually increases at first and then tends to stabilize and has a downward trend, with a range of 6.94%~7.75%. The organic working fluid flow rate has a relatively small effect on circulation efficiency of the ORC system. Both schemes are technically and economically feasible. The “ORC” scheme can bring direct electricity benefits to the power plant, while the “Heater” scheme is slightly more economical.

In order to provide effective repair and rust prevention treatment for hot surface pipe of utility boilers, and to ensure safe operation of the equipment, cold metal transfer (CMT) cladding process is adopted for water-cooled wall tubes. Four cladding process paths are designed, and the reliability of the numerical simulation data is verified through ANSYS numerical simulation and the CMT cladding experimental platform. Comparison based on the process developed in the simulation process shows that, design of the CMT heat source function has a higher degree of agreement in characterizing the temperature field than the conventional arc heat source. Along the material thickness direction, the change rule of the temperature gradient is consistent with the actual morphology of the specimen cross-section. In the case of the same heat input, the average temperature of the cross-melting path is 30 ℃ lower than that of the sequential melting path, the thermal effect of the cross-melting path on the substrate is smaller, and the stress is 22.0 MPa lower. Pipe deformation reduces by 0.18 mm in the cross-over reverse welding path compared with that in the sequential reverse welding path. Comprehensively considering the effects of the residual stresses and the deformations, the cross-reversed melting path is the optimal process route for CMT.

The calculation analysis and experimental study of blade fracture of a certain type of steam and static adjustable induced draft fan are carried out. Vibration signals and blade natural frequencies of fans at different speeds and openings are obtained through vibration tests and modal experiments. Performance curves of fans at different speeds are drawn according to the similarity theory, and the rotation-opening curves corresponding to the flow rate are obtained. The fan operation restricted zone is delimited by the stall line. The test results show that, when the rotation speed is constant, the amplitude of the blade passing frequency and its harmonic component increase with the stator blade opening. Combining with the modal experiment, it is found that the natural frequency of the blade and the blade passing frequency and its harmonic component coincide when the fan runs in the rotation speed range of 796~912 r/min, resulting in blade resonance. The fan should avoid running in the resonance speed range and avoid running in the speed restricted area.

With the large-scale development and grid integration of new energy sources, the fundamental control and operational mechanisms of power system have undergone significant changes to power balance and safety stability control. Low-frequency oscillation events in power systems have typically been simulated and analyzed using standard speed control system models. However, these standard models fail to reflect the regulation characteristics of the units and cannot accurately reproduce the low-frequency oscillation process. Based on the nonlinear characteristics of steam turbine valves and combined with the actual frequency control logic, a small frequency deviation compensation module and a valve flow module are introduced into the typical speed control system model to establish low-frequency oscillation model. The accuracy and effectiveness of the model are verified using actual operational data. Moreover, low-frequency oscillation evaluation indicators are established, by employing the analytic hierarchy process (AHP) and fuzzy evaluation methods, online identification and grading evaluation of low-frequency oscillations are achieved. On this basis, a phased suppression strategy for low-frequency oscillations on the prime mover side of thermal power units is proposed. The corresponding suppression measures are executed based on low-frequency oscillation evaluation results, and the low frequency oscillation model is used to carry out simulation verification. The results demonstrate the suppression strategy can effectively eliminate low-frequency oscillations on the prime mover side and improve operation safety of thermal power units.