Since solar panels generate electricity due to climatic conditions, solar panels can only generate electricity when there is sunlight during the day, and because the solar radiation intensity changes at any time, the solar panel power generation will change at any time, usually with the law of load power consumption does not match. Therefore, for the off-grid photovoltaic system, the energy storage device must be configured to store the excess energy generated by the square array during the sunshine for use in the evening or rainy days. In some areas, the power supply of the power grid is not very stable, and the load is important. The power supply cannot be interrupted, such as military, communications, medical, etc. Even the grid-connected system can be equipped with energy storage devices.
In the long run, the share of solar panel power generation in the energy consumption structure will gradually expand, and will dominate the energy supply by the end of the 21st century. However, the working characteristics of solar panel power generation are "sunrise, sunset and rest", and it is random. Solar panels and wind energy are interstitial energy sources. To become the world's main energy source, it is necessary to solve the problem of grid energy storage.
The main energy storage technology of solar panels, so far, people have explored and developed various forms of power storage methods, which can be mainly divided into mechanical energy storage chemical energy storage and electromagnetic energy storage.
Mechanical energy storage
Mechanical energy storage converts electrical energy into mechanical energy and converts mechanical energy into electrical energy when needed. At present, there are the following practical applications.
(1) Pumped storage energy
Pumping and storing energy needs to be equipped with two reservoirs upstream and downstream. During the period of low grid load, the pumped energy storage equipment works in the motor state, pumping the water from the downstream reservoir to the upstream reservoir, and converting the electric energy into gravitational potential energy for storage; when the grid load peaks, it works in the generator state and releases the upstream reservoir water to generate electricity.
The release time of pumped energy storage can range from a few hours to a few days. Some high dam hydropower stations have a large water storage capacity, which can be used as a pumped storage power station for power dispatching. This is a widely used form of large-scale, centralized power storage. According to statistics, as of March 2012, pumped storage accounts for more than 99% of the world's large-capacity storage power, with a total capacity of 127,000 MW.
The advantages of pumped water storage are that the technology is mature and reliable, the capacity can be made very large, and only limited by the reservoir storage capacity; the disadvantage is that the high and low reservoirs whose construction is limited by geographical conditions and need suitable drop are often far away from the load center, pumping and power generation. There are quite a lot of energy losses, the actual overall efficiency is 70% to 75%, the storage density is poor, the construction period is long, and the investment is large.
(2) Flywheel energy storage
Flywheel energy storage is the storage of energy in the form of kinetic energy in a high-speed rotating flywheel. The entire system consists of high-strength alloy and composite rotors, high-speed bearings, doubly-fed motors, power converters and vacuum safety shields. The electric power drives the flywheel to rotate at a high speed, and the electric energy is converted into a kinetic energy storage of the flywheel. When electric energy is required, the flywheel decelerates, the motor operates as a generator, and the acceleration and deceleration of the flywheel realize charging and discharging.
Developed countries such as theUnited States,Germany, andJapanhave carried out a lot of research and development work on flywheel energy storage technology.Japanhas produced the world's largest variable-capacity flywheel energy storage power generation system (capacity is 26.5MV.A, voltage is 1100V, speed is 510690r/min, and moment of inertia is 7.1 x 105kg.m2). The University of Maryland has also developed a 24kW.h electromagnetic suspension flywheel system for power peak shaving. The flywheel mass is 172.8kg, the working speed range is 11610~46345r/min, the breaking speed is 48784r/min, and the system output constant voltage is 110 to 240V, the overall efficiency is 81%, economic analysis shows that the full cost can be recovered after three years of operation. The French National Research Center in Europe, the Institute of Physics and High Technology inGermany, and the SISE inItalyare all conducting research on the flywheel energy storage system of high temperature superconducting magnetic suspension bearings. The largest flywheel energy storage system currently on the market can store up to 133 kWh of energy.
The flywheel system operates in a highly vacuum environment, and has the advantages of no friction loss, small wind resistance, long life, no impact on the environment, almost no maintenance, suitable for power grid frequency modulation and power quality assurance; To ensure the high cost of the system security, it can not reflect its advantages in small occasions, and is currently mainly used for the supplement of the battery system. The ultra-large capacity flywheel energy storage technology is still immature.
(3) compressed air energy storage
Compressed air energy storage is an energy storage method proposed in the 1950s. The system consists of two cycles: a gas compression cycle and an exhaust gas expansion cycle. When compressing, the doubly-fed motor acts as a motor, and drives the compressor with excess power when the grid load is low, and presses the high-pressure air into the underground gas storage hole; when the grid load peaks, the doubly-fed motor acts as a generator to store the compressed air first. After being preheated by the regenerator, the fuel is burned in the combustion chamber, and the expansion system is used to perform work (such as driving a gas turbine) to generate electricity. The construction of compressed air energy storage power stations is subject to terrain constraints and has special requirements for geological structures. Demonstration power stations have been built inGermany, theUnited States,JapanandIsrael.