At present, in-depth research is needed on the characteristics and optimization of solar energy storage systems in grid connected systems in the following areas:
(1) In terms of aging characteristics of solar energy storage systems: In previous studies on calendar aging testing of lithium-ion batteries, researchers chose a narrow range of temperature and state of charge. In terms of temperature, only a few researchers have studied the aging of lithium-ion batteries at low temperatures. In most studies, the storage temperature of lithium-ion batteries is higher than or equal to 25 ° C. In terms of state of charge, although many experiments cover a range of 0% to 100% change in state of charge, such experiments are usually set at a fixed temperature (such as 25 ° C or 40 ° C). In addition, researchers rarely consider the impact of calendar aging in the study of lithium-ion battery cycle aging. However, in reality, the time span of cycle aging experiments is large, and ignoring the impact of calendar aging will reduce the accuracy of the cycle aging model.
(2) In terms of battery pack thermal management: Current battery pack thermal management systems mostly adopt feedback strategies. Due to the time required to adjust the environment/working fluid temperature, the battery will inevitably continue to work at high temperatures for a period of time, which not only reduces the battery’s service life but also may cause safety issues. Therefore, it is necessary to develop feedforward active thermal management technology based on load forecasting. In addition, current battery pack thermal management systems are mostly developed based on the needs of thermal management for electric vehicle power battery packs. The capacity and power of fixed solar cell energy storage systems are different from those of electric vehicle power batteries, and the thermal dynamic response characteristics and requirements for thermal management systems are also different, So the key issues that need to be addressed in the thermal management of fixed solar cell energy storage systems are fundamentally different from those of general electric vehicle battery thermal management systems.
(3) In terms of renewable energy grid connection scheduling, the current renewable energy grid connection scheduling strategy is difficult to balance the two goals of efficient cooperation between renewable energy and the grid, as well as maximizing the economic efficiency of solar cell energy storage systems themselves. In addition, for renewable energy grid connected systems with large-scale solar cell energy storage systems, there is a lack of research and optimization on the joint operation of thermal management strategies and grid connection scheduling strategies for solar cell energy storage systems. The impact of introducing thermal management on the overall system economy is also yet to be evaluated.
Therefore, this study focuses on the renewable energy grid connected system composed of photovoltaic power generation system and lithium-ion battery energy storage system. Starting from the thermal and aging characteristics of lithium-ion batteries, a thermoelectric coupling model of lithium-ion batteries is constructed to study the thermal characteristics of lithium-ion battery energy storage system, Propose an active thermal management strategy based on load forecasting suitable for fixed solar cell energy storage systems and a renewable energy grid connection scheduling strategy containing solar cell energy storage systems. Research and optimize the joint operation of thermal management strategy and grid connection scheduling strategy in renewable energy grid connection systems. The specific research content is as follows:
1) Theoretical and experimental research on the thermal and aging characteristics of lithium-ion batteries. In the theoretical research section, starting from the basic structure and working principle of lithium-ion batteries, combined with relevant knowledge of electrochemistry and heat transfer, the heat generation mechanism and heat transfer characteristics of lithium-ion batteries are studied, and a heat transfer model of lithium-ion batteries is constructed. In addition, based on aging experiments, the aging mechanism of lithium-ion batteries is studied, and a high-precision lithium-ion battery life model is proposed. In the experimental research section, three types of cathode materials were studied for lithium-ion batteries, and the effects of temperature, current rate, and state of charge on the discharge capacity and internal resistance of solar cells were obtained, as well as the differences in characteristics of lithium-ion batteries with different cathode materials.
2) Study the thermal characteristics of individual lithium-ion batteries and battery packs separately. Firstly, based on theoretical analysis and experimental research, an equivalent circuit model of lithium-ion batteries is constructed. Afterwards, the thermal characteristics of the single cell battery were simulated in CFD software, and the temperature changes and temperature distribution of the single cell battery during charging and discharging at different current rates under different environmental temperatures were studied. The effectiveness of the equivalent circuit model was verified by constructing an experimental platform. Finally, study the temperature response and distribution of solar panels under different conditions, and analyze the effects of solar panel structure, inlet air temperature, and flow rate on the cooling effect of solar panels.
3) Based on the research results of the transient temperature field of solar panels and combined with load prediction, an active battery thermal management strategy is proposed. By controlling the inlet temperature and flow rate of cooling air, the thermal management of the battery pack is carried out, effectively controlling the maximum temperature of the battery pack and ensuring the temperature consistency of different cells within the battery pack. Subsequently, a comparative study was conducted on the effects of different thermal management strategies on the dynamic response of battery temperature, the highest temperature of solar cells, and battery consistency, to verify the effectiveness of the proposed active battery thermal management strategy.
4) Based on the lifespan model of lithium-ion batteries, an improved minimum maximum scheduling strategy is proposed for the operation control problem of photovoltaic lithium-ion battery renewable energy grid connection, ensuring the cooperation between the renewable energy grid connection system and the power grid, while achieving precise control of the battery’s state of charge, thereby extending the lifespan of lithium-ion batteries. Given the capacity of the photovoltaic system and the operating conditions of the power grid, the improved minimum to maximum scheduling strategy can be applied to determine the optimal battery capacity configuration and operating strategy of the system.
5) Study and optimize the joint operation of thermal management strategy and grid scheduling strategy in different seasons in the photovoltaic lithium-ion battery renewable energy grid connected system, and determine the optimal capacity configuration and operation strategy of the solar cell energy storage system. Based on the grid connection scheduling strategy, further study the thermal characteristics and operation of thermal management strategies of solar cell energy storage systems, combined with the lifespan model of lithium-ion batteries, evaluate the impact of introducing thermal management systems on the entire renewable energy grid connection system.