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.

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 increase of wind power penetration in power systems, the inertia of the power system decreases and the frequency regulation resources are insufficient. The frequency response margin approaches the critical value, which results in great challenges to frequency security. To solve the problem, from the perspective of regulation mechanism, a wind-thermal cooperative frequency control model based on virtual inertia control of wind power is constructed. The kinetic energy of rotor is used for fast frequency regulation. Then, a load frequency controller of wind-thermal cooperative power system based on a novel active disturbance rejection control (ADRC) method is proposed and designed. This improves the anti-disturbance capability to uncertain wind power and load disturbances. The designed cascade extended state observer also solves the contradiction between high frequency noise suppression and fast response performance of conventional ADRC. The excessive regulation of wind power and thermal power units caused by frequency measurement noise can be avoided and the quality of wind-thermal coordination regulation can be improved. Finally, genetic algorithm based particle swarm optimization is used to optimize the parameters of the proposed load frequency controller. The simulation results show that, compared with the conventional ADRC and other conventional control methods, the proposed strategy can effectively improve the frequency response characteristics, and suppress the effect of measurement noise on the amplitude of system frequency response and control signal.

With the rapid development of renewable energy, the demand for grid-scale energy storage solutions is increasing to address the challenges posed by intermittent and variable power generation. As an integration of various mature electrothermal conversion and storage technologies, Carnot battery is gaining increasing attentions due to its scalability and independence from geographical constraints. The fundamental principles, key technologies, application prospects and current research status of Carnot battery are reviewed. The definition of high-temperature Carnot battery technology and the operational characteristics and technical challenges of related key equipment such as compressors and expanders are discussed. Additionally, practical application cases and technological prospects of Carnot battery systems based on electric heating and bidirectional cycles (such as Brayton and Rankine cycles) are analyzed, providing a reference for future research and technological development.

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.

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.

Against the shortcomings of intermittency and instability of photovoltaic power generation in microgrids, a hybrid energy storage system composed of vanadium redox batteries (VRB) and super capacitors (SC) is utilized to smooth out the power fluctuations in standalone microgrids, thus to improve the power supply reliability of standalone microgrids. Considering the capacity allocation problem of the hybrid energy storage system, a multi-objective hybrid energy storage system capacity optimization model that minimizes the average annual cost of the hybrid energy storage system and the load shortage rate is developed. Aiming at the poor local search ability of the conventional elite non-dominated solution sorting genetic algorithm (NSGA-II) algorithm for solving the multi-objective optimization problem, an NSGA-II algorithm based on the improved elite retention strategy is proposed. By introducing a new fitness function, the algorithm is sorted and reasonably retains the elite individuals, so it improves the optimization effect, thus to enhance the local search ability, continuously approach the Pareto true frontier, and obtain better capacity configuration solutions. Finally, the rationality of the proposed method is verified by arithmetic examples.

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.

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.

Against the dynamic operation characteristics of three-pressure reheat waste heat recovery steam generator (HRSG) of gas turbine combined cycle (GTCC) unit, the Modelica open-source programming language is used to build the simulation model for the HRSG, and the simulation results are compared with the actual operation data during peak load regulation process of the power plant. Under actual operating conditions, the established model can accurately predict the dynamic response laws of main parameters of the HRSG in the dynamic process of steady-state operation after the load drops from full load to low load, then increases from low load to full load, as well as unit shutdown and boundary parameter disturbance. When the unit load decreases from 320 MW to 280 MW, the power of the steam turbine decreases from 124.0 MW to 111.5 MW. The high pressure main steam flow rate decreases from 70.12 kg/s to 63.62 kg/s, and the high pressure main steam pressure decreases from 8 250 kPa to 7 612 kPa. The flue gas parameters at inlet of the HRSG change in about 300 s with the decrease of the unit load, while the steam parameters of the HRSG need about 600 s to complete dynamic response and reach steady state at low load, indicating there is a certain lag in steam parameters of the HRSG compared to the change of gas turbine exhaust gas parameters over time. In the dynamic process the unit shut down, the power of the steam turbine decreases from 130.4 MW to 5.4 MW, the high pressure main steam temperature of the HRSG decreases from 600.1 ℃ to 224.5 ℃, and the high pressure main steam flow rate reduces from 76.1 kg/s to 15.3 kg/s.