The intermittency and volatility of renewable energy poses significant challenges to stable operation of power grids. Energy storage technology can address these issues effectively. Liquid air energy storage technology offers significant advantages of high energy storage density, being unconstrained by geographical conditions and atmospheric pressure storage. However, its round-trip efficiency is relatively low. To solve this problem, a liquid nitrogen and liquid air hybrid energy storage system (N-LAES) is proposed. By charging liquid nitrogen during energy release process, the gas flow in the expander increases, and gas pressure in front of the expander rises as well, thus the system’s round-trip efficiency increases. A thermodynamic model is developed, and the analysis results indicate that, for a typical scale N-LAES, the round-trip efficiency is increased to 66.47% compared with 56.90% for a standalone LAES. The net present value at the 30th year increases to 120 213 500 yuan, compared with 58 077 400 yuan for a standalone LAES, and the levelized cost of storage decreases to 0.809 4 yuan/(kW·h) from 0.897 2 yuan/(kW·h) for a standalone LAES. These findings demonstrate that both the thermodynamic and economic performance of the N-LAES is superior to that of the standalone LAES, offering a new approach for development of the liquid air energy storage technology.

In order to explore the type of underground cavern with compressed air energy storage from the perspective of thermodynamics, a numerical model of the first inflation and pressurization process of the cavern considering turbulence, heat transfer and real air characteristics is established, by using the computational fluid dynamics (CFD) method. The effects of different length-diameter ratios and inlet diameters of inflatable pipes on temperature rise of gas and lining materials in the cavern and the temperature distribution in the cavern are studied, and the control measures are put forward for the local high temperature phenomenon in the cavern. The main conclusions are as follows. When the length-diameter ratio is small (large tank gas storage), the temperature distribution in the cavern is relatively uniform. With the increase of the ratio of length to diameter (tunnel-type gas storage), the temperature distribution in the cavern appears stratification phenomenon, and the extremely high temperature zone appears at the end of the cavern (stuffy top effect). The temperature rise of the steel plate sealing layer is the largest in the process of inflation and pressurization of the cavern, the temperature change of the concrete lining is small, and the surrounding rock is almost not affected by temperature change in the cavern. Reducing the inlet diameter of the inflatable pipe can reduce the temperature in the cavern to a certain extent and promote the outward heat transfer. For the annular tunnel type cavern, the proposed improved inflation method can make the temperature distribution in the cavern uniform, avoid the stuffy roof effect, and provide a useful reference for engineering design.

A double-layer optimization site selection method for energy storage with grid-forming demand in novel power system is proposed, which considers the response of energy storage to peak shaving and frequency regulation in the power system and establishes a multi-objective double-layer optimization model. The operation layer counts the wind and solar power abandonment and network losses into the economic penalty, and takes the optimal annual operating cost of the system as the objective, considers the benefits of peak shaving and frequency modulation. The planning layer evaluates the security of the system and models the system by taking the optimal comprehensive annual operating cost of the system as the objective. Simulation and analysis of the algorithms are carried out using the improved empire competition algorithm. The peaking and frequency regulation economics and energy storage siting in the optimal scenario are illustrated through multi-scenario comparisons. Finally, the IEEE-33 node arithmetic system is simulated and analyzed to verify the validity of the proposed model. Furthermore, uncertainty factor indicators are selected to conduct sensitivity analysis on total costs, and the indicators that need more attention to affect economic costs are determined.

A combined reactive power control strategy for permanent magnet direct-drive wind turbine and distributed hybrid energy storage system is proposed. Firstly, the reactive power regulation capability of the permanent magnet direct-drive wind turbine and energy storage system is analyzed, and it is determined that both the wind turbine and energy storage system can participate in reactive power regulation through converter control. Secondly, the reactive power control strategy is put forward, which is presented in terms of signal reception, initial allocation, and internal allocation. In initial allocation, the equal margin allocation method is adopted. In internal allocation, the proportional allocation with the priority output of energy storage is considered. Finally, the effectiveness of the strategy is verified by simulation, it shows that the power grid voltage can be supported by fully utilizing the reactive capacity of the wind turbine and energy storage system.

As a large scale of physical energy storage technology, compressed air energy storage technology is widely used in consumption of renewable energy and peak shaving of power grids. A compressed air energy storage system coupled with molten salt thermal storage is designed, and the composite system is modeled using Ebsilon software. Based on the operating conditions of the energy storage system supplying hot water, steam, and electricity, the exergy efficiency, thermal efficiency, and economic performance under different operating modes are studied. The results indicate that, the composite system achieves the highest exergy efficiency (64.98%) at a storage pressure of 7 MPa and an exhaust temperature coefficient of 1.96. The highest thermal efficiency (91.55%) is attained at a storage pressure of 12 MPa. In the application scenario of combined heat, steam, and electricity cogeneration, the optimal energy storage duration is 6 hours. Additionally, at gas storage pressures of 7 MPa and 12 MPa, the optimal power generation durations are 6 hours and 8 hours. This research provides theoretical guidance for the study of cogeneration of power and heating using compressed air energy storage system coupled with molten salt thermal storage system.

Virtual Synchronous Generator (VSG), as one of the primary technologies in grid-forming controls, provides inertia support to the power grid. However, due to the limited capacity of individual converters, when larger inertia support is required, multiple VSGs must run in parallel, making the coordinated control of multiple VSGs a subject of significant interest. In this regard, a state-space model for multiple VSGs in parallel is established, and the system stability is analyzed through the eigenvalues of the state variable matrix. Concurrently, a coordinated control strategy for multiple VSGs based on model predictive control is proposed, which introduces the angular frequency deviation and power angle difference as performance indicators to design the objective function. The optimal active power increment required is solved, and the output angular frequency is dynamically adjusted through the power-frequency coefficient, enabling active support for the output frequency and effectively suppressing system frequency fluctuations caused by VSG paralleling, thus the grid stability is enhanced. The results indicate that, compared with the conventional VSG paralleling systems, the proposed MPC-VSG parallel control method can shorten the transient response time of the system and improve its robustness under transient conditions. The simulation result confirms the effectiveness of the proposed approach.

The impedance inside weak current network is large, and when unbalanced loads are connected, it is unable to maintain the stability of its own voltage, resulting in three-phase voltage imbalance and output power fluctuations. Virtual synchronous generator (VSG) technology, as a grid-forming control, can provide support for voltage of the weak current network. However, under the connection of unbalanced loads, the VSG technology cannot maintain output voltage balance. To solve this problem, an improved VSG sequence decoupling control strategy is proposed. Firstly, the principle of three-phase imbalance and power fluctuation in VSG output voltage caused by unbalanced load connection is investigated. Secondly, a dual synchronous coordinate system decoupling (DDSRF) is adopted to separate the positive and negative sequence voltages of the power grid, and a positive and negative sequence control strategy is employed to control the negative sequence voltage component in dq rotating coordinate system. Finally, a VSG simulation model is built and the simulation results indicate that the proposed improved VSG sequential decoupling control strategy can suppress the unbalanced voltage output of the VSGs.

Against the conflict between carbon emission and operation cost in integrated energy systems, a multi-objective optimal scheduling method for wind-solar-thermal-storage integrated energy system considering carbon capture is proposed. It explores how carbon capture equipment affects the renewable energy consumption, carbon emissions, and operating costs. Taking the electric load data of a typical day in a specific area as a reference and the improved IEEE 30-bus system as the example, the system economy is optimized. The results show that, compared with the wind-solar-thermal and wind-solar-thermal-storage scenarios without carbon capture, the operating costs of the integrated energy system considering carbon capture reduces by 5.19% and 2.86% respectively on typical days, and the carbon emissions decrease by 1 159 t and 1 013 t, respectively. The consumption rate of wind and solar power generation increases by 5.01% and 2.82%, respectively. Moreover, with the minimum system operation cost and carbon emissions as the optimization objectives, the non-dominated sorting genetic algorithm II is used for multi-objective optimization, and the system scheduling optimization scheme under different target weights is obtained by combining with the linear weighted sum method. The study finds that, increasing the weight of carbon emission target reduces the carbon emissions but raises the system operation costs and the cost per unit of carbon emission reduction. Specifically, when the target weight of carbon emissions rises from 0 to 0.5, the carbon emissions decrease by 5 159 t and the operating costs increase by 205 466 yuan. The carbon emission reduction cost per unit increases the least when the target weight shifts from 0.4 to 0.5. The most significant emission reductions occur when the target weight is within [0.2, 0.4]. The multi-objective optimal scheduling method considering carbon capture proposed above provides a reference for decision makers when weighing system carbon emissions and operating economy.

To address the frequency fluctuations and exceeding limits caused by load changes when wind hydrogen coupling system is connected to the weak current grid, a grid type virtual synchronous generator (VSG) moment of inertia self-adaptive control strategy based on dynamic feedback of hydrogen storage system pressure is proposed. Firstly, a physical simulation model of the grid type wind hydrogen coupling system is established, the closed-loop transfer function of active power is derived, and the influence of rotational inertia and damping coefficient on the power frequency oscillation characteristics of the system is analyzed. Then, considering the dynamic changes in pressure of the hydrogen storage system, the moment of inertia calculation is optimized in real time to ensure stable operation of the wind hydrogen coupling system under grid frequency fluctuations and load active power fluctuations. Finally, the strategy is validated using MATLAB/SIMULINK platform. The results show that, using the grid type self-adaptive method can accelerate the frequency recovery of the system, significantly improve the dynamic response ability of the system, and achieve stable operation of the wind hydrogen coupling system.

With the high proportion of new energy connected to power grid, multi-machine parallel coordinated control of energy storage inverter has become a key problem. Virtual synchronous generator (VSG) algorithm can provide damping, inertial support and stable voltage frequency for grid-forming energy storage system. However, the parallel synchronization, stability, state of charge (SOC) of each stack and impedance of each line should be considered in the coordinated control of parallel operation of multiple energy storage inverters. To solve this problem, a parallel mathematical model of energy storage inverter is established, the methods of reactive power allocation considering virtual impedance and active power allocation considering SOC are analyzed, and an improved VSG control strategy combining self-adaptive virtual impedance and SOC equalization is proposed. Finally, a model is built on the MATLAB/Simulink simulation platform, and the coordination control of each energy storage inverter is analyzed under the discharge condition with the all vanadium redox flow battery pack as the energy storage system. The validity of the improved VSG control strategy is verified, and the problem of over-discharge caused by voltage drop, reactive power and SOC difference caused by impedance difference is solved effectively, the utilization efficiency of the battery and the energy storage inverter is improved, and the life loss of the battery is reduced.