Throughout history, human society has gone through two energy transformations: the first energy transformation occurred in the 18th century, and with the arrival of the Industrial Revolution, people replaced firewood with coal as the main energy source; The second energy transformation occurred in the 20th century, with the development of oil resources, the world’s energy structure shifted from coal to oil and natural gas. Since the 21st century, the continuous development of the global economy has led to an increase in people’s demand for energy, and climate change has raised concerns about the ecological environment, which has led to a third transformation of the world energy system. The goal of the third energy transformation is to shift the world’s energy structure from fossil fuels to zero carbon energy in the second half of this century. Fundamentally, the core goal of energy transformation is to reduce energy related carbon dioxide emissions to curb global climate degradation. Therefore, renewable energy is increasingly favored by people and has been continuously promoted around the world. According to IRENA’s statistics, the total installed capacity of renewable energy worldwide has increased from approximately 1223.53GW in 2010 to approximately 2799.99GW in 2020. Among them, wind and solar energy are the two most rapidly developing and typical renewable energy sources, with an installed capacity of approximately 6.51 times that of 10 years ago in 2020. In terms of actual power generation, affected by the COVID-19, the proportion of total renewable energy power generation in total global power production continued to increase by the end of 2020, reaching about 29%.
The proportion of wind and solar power generation has exceeded 20% in nine countries, including Denmark (about 63%), Uruguay (about 43%), Ireland (about 38%), Germany (about 33%), Greece (about 32%), Spain (about 28%), and the United Kingdom (about 28%) Portugal (approximately 27%) and Australia (approximately 20%). In China, the total power generation of wind power, solar power, and hydropower also exceeds 27% of the total power generation. However, due to changes in natural and climate conditions, renewable energy generation represented by wind and solar power has a high degree of uncertainty (caused by inaccurate predictions) and significant volatility (manifested as unstable output). As of the end of 2019, The estimated share of global renewable energy generation in terminal energy consumption is only 11.2%. These characteristics hinder the widespread promotion and utilization of renewable energy, and also pose significant challenges to the planning and operation of power systems in areas such as grid interconnection, power quality, and system reliability. Based on this, many countries have issued requirements for the integration of renewable energy into the grid. Taking photovoltaic power generation as an example, the Chinese national standard GB19964-2012 “Technical Regulations for Connecting Photovoltaic Power Stations to the Power System” stipulates that the fluctuation of photovoltaic grid connected power shall not exceed 10% of its installed capacity per minute. This has led to the abandonment of a large number of photovoltaic power generation that cannot meet the requirements in practical applications.
In order to address the volatility and uncertainty of renewable energy generation and its limitations in practical utilization, researchers have conducted extensive research on the characteristics and utilization methods of renewable energy. Technically speaking, there are mainly two methods:
One is to achieve smooth power transmission for renewable energy generation without battery storage system units, and the other is to integrate renewable energy generation systems with battery storage systems to form a renewable energy grid connected system, which is then utilized. Although methods that do not rely on battery energy storage system units (such as pitch angle control and kinetic inertia control in wind power generation) can reduce fluctuations in renewable energy generation, they also increase the complexity of system operation and cannot guarantee maximum renewable energy utilization. Therefore, combining battery energy storage systems with renewable energy systems to form a renewable energy grid connected system is considered the best way to achieve grid connected utilization of renewable energy at present.
At present, battery energy storage system technologies such as electrochemical, electromechanical, chemical, pumped storage, and thermal storage have been introduced into renewable energy grid connected systems. These technologies have significant differences in lifespan, volumetric energy density, volumetric power density, response time, efficiency, and economic cost. Solar cells are one of the most widely used battery energy storage system technologies, with mature technology and high flexibility in installation and use. They can be interconnected with various energy sources, easily stacked with modules, and increase the scale of battery energy storage systems. They are suitable for grid connection of renewable energy of various scales.
Figure 1 shows the structure of a typical solar cell renewable energy grid connected system. Among them, the photovoltaic power generation system and the solar cell energy storage system are respectively connected to the common coupling point through a power conversion system, and then connected to the power grid through a transformer device. Among them, solar cells supplement or absorb photovoltaic power generation through charging and discharging to deal with the volatility and uncertainty of photovoltaic power generation, so that the final energy sent to the grid by the entire system is stable and controllable.
At present, various types of solar cells are being researched and developed, some of which have been commercialized, while others are still in the laboratory stage. So far, the solar cells used in power system applications are generally deep cycle batteries (similar to the solar cells used in electric vehicles), with an energy capacity of about 17-40 MWh and an efficiency of about 70-80%. In fact, among various solar cell technologies, some seem more suitable (already) for application in power systems, including:
(1) Lead acid battery: A solar cell consists of a lead dioxide positive electrode and a sponge lead negative electrode, separated by a microporous material and immersed in a sulfuric acid electrolyte.
(2) Sodium sulfur battery: Solar cells are composed of molten sulfur at the positive electrode and molten sodium at the negative electrode, in solid form β Separation of alumina ceramic electrolytes. Electrolytes only allow sodium ions to pass through and combine with sulfur to form polysulfide sodium. During the discharge process, sodium ions flow through the electrolyte, electrons flow in the external circuit of the solar cell, and the working temperature of the solar cell is maintained
Around 300 ° C.
(3) Lithium ion battery: The cathode of a solar cell is a metal oxide of lithium, the anode is generally a graphite like carbon material, and the electrolyte is composed of lithium salts dissolved in organic carbonates. When the solar cell is charged, the lithium atoms in the cathode become ions and migrate to the anode through the electrolyte. They combine with external electrons on the anode and deposit as lithium atoms; When the solar cell is discharged, this process is reversed.
(4) Metal air cells: The anode of solar cells is usually a metal with high energy density, such as aluminum or zinc, which releases electrons during oxidation. The cathode or air electrode of a solar cell is usually made of a porous carbon structure or a metal mesh covered with appropriate catalysts. Electrolytes are usually good conductors of hydroxide ions, such as potassium hydroxide. Electrolytes can be in liquid form or solid polymer films saturated with hydroxides.
(5) Flow cell: A solar cell consists of two electrolyte storage layers, and the electrolyte circulates through an electrochemical solar cell that includes a cathode, anode, and separator (through a pump). When the two electrolytes flow through, chemical energy is converted into electrical energy in the electrochemical solar cell. These two electrolytes are stored separately in large storage tanks outside the electrochemical battery, and the size of the tanks and the number of electrolytes determine the energy density of these batteries. However, the power density of a mobile battery depends on the rate of electrode reaction between the anode and cathode. Based on the redox reaction between two electrolytes in the system, a flow battery is also known as an redox flow battery.
Among them, lithium-ion batteries have the advantages of high specific energy and high energy and power density compared to other solar cell technologies. In addition, lithium-ion batteries also have higher discharge rates, better high-power discharge capabilities, good round-trip efficiency, relatively long lifespan, and lower self discharge rates. Therefore, this project is based on lithium-ion battery energy storage system technology to study and optimize the characteristics of solar cell energy storage systems in renewable energy grid connected systems. From an economic cost perspective, although the cost of lithium-ion batteries has decreased compared to before in recent years with the development of technology and the maturity of industrialization, it is still relatively high. In addition, in the actual operation of renewable energy grid connected systems containing lithium-ion batteries, the thermal stability and safety issues of the batteries are crucial. Although lithium-ion batteries may have spontaneous failures (mainly internal short circuits), most accidents are caused by external abuse. External abuse includes mechanical abuse (destructive deformation and displacement of solar cells caused by applied force, such as collision, compression or puncture) Electrical abuse (such as external short circuits, overcharging, and over discharging of solar cells) and thermal abuse (generally caused by local overheating, including overheating caused by mechanical/electrical abuse and overheating caused by increased contact resistance due to loose external connectors). In addition, when lithium-ion batteries operate at higher temperatures, even if no accidents occur, excessively high battery temperatures can lead to accelerated aging of solar cells and shorten their lifespan.
Overall, in order to promote the development of renewable energy and increase the proportion of renewable energy in the energy structure, it is necessary to vigorously promote the combination of “renewable energy+energy storage system”. In the current situation, the main factors limiting the widespread promotion of renewable energy grid connected systems are their economy and safety. For renewable energy grid connected systems containing lithium-ion batteries, the economy of the system is mainly affected by the cost of battery energy storage systems, while safety is closely related to the stability of battery energy storage systems. Therefore, for the application of renewable energy grid connected systems containing lithium-ion batteries, we need to improve the economic efficiency of lithium-ion battery energy storage systems, as well as conduct thermal monitoring and management of lithium-ion batteries.