Recently, the scale of new energy power generation has grown rapidly, and the proportion of output in the power system has been increasing. However, the output of new energy, mainly wind power and photovoltaic, has natural volatility and unpredictability, which increases the risk of grid operation and is prone to chain disconnection. With the continuous increase in the proportion of wind power and photovoltaics, the phenomenon of "abandoning wind" and "abandoning light" is serious.
There is a growing demand for reliable, long-term electrical energy in the power system. And that’s how compressed air energy storage appear. Energy storage technology can realize the orderly regulation of energy in time and space. Large-scale long-term energy storage technology can solve the volatility and intermittent problems of new energy power generation, and achieve smooth output on the power generation side. It can improve the quality of power generation, and can be used for peak shaving and valley filling.
Characteristics of compressed air energy storage
Compressed air energy storage has both large-scale and long-term energy storage characteristics. According to the difference of energy types, energy storage technologies can be divided into four categories: chemical energy storage, mechanical energy storage, thermal energy storage, and electromagnetic energy storage. It is generally believed that the large-scale energy storage technologies that can be used for 100 megawatts and above are only pumped hydro energy storage and compressed air energy storage.
At present, pumped hydroelectric storage (PHS) dominates the Chinese energy storage market. Due to its mature technology, low cost per kilowatt hour, and large capacity, it will still occupy the dominant position in the Chinese energy storage market in the short and medium term. However, pumped storage is highly resource-dependent, has high requirements on terrain and water sources, and has a long development cycle.
The application of this technology has high limitations, especially in some provinces with insufficient water resources, and it is difficult to develop pumped storage on a large scale. Therefore, compressed air energy storage is regarded as the physical energy storage technology with the most development potential, and it is one of the important ways to solve the problems of new energy consumption such as "abandoning wind" and "abandoning light".
The higher the proportion of wind power and photovoltaics in the power grid, the greater the demand for energy storage in the power system. When the proportion of wind power and photovoltaics reaches 80%, the demand for energy storage duration will reach the daily/weekly level. Therefore, the development of long-term energy storage is necessary and urgent. Pumped hydro storage can usually reach 6-10h energy storage time, and compressed air energy storage can reach 4-10h energy storage time.
Introduction of compressed air energy storage technology
Compressed air energy storage is divided into three processes: compression, storage, and energy release.
- Compression: When the power system is at a low power consumption, use excess electric energy to drive the compressor to compress air;
- Storage: Compress the air to a closed space (about 10MPa), and store the energy in the form of high-pressure air;
- Energy release: When the power system needs electricity, the compressed air is released from the storage space, heated and expanded to drive the turbine to do work, and drive the generator to generate electricity. The generated electrical energy is sent to the grid for use by end users. While in some certain condition, we should use off grid batteries system to make sure the perfomance of the devices.
The compressed air system device mainly consists of five parts:
- Air compressors, two or more stages, with coolers;
- Motor/generator, which provides kinetic energy for compressor or turbine unit;
- Turbines (turbines/expanders), including high-pressure and low-pressure turbines
- Cavities/containers for storing compressed air, which can be underground caverns, salt caverns produced by mining, abandoned natural gas wells, etc.;
- Fuel storage and heat exchanger equipment (traditional)/regenerative heat exchange device (regenerative type), etc.
New compressed air energy storage technology is emerging
The traditional compressed air energy storage technology is relatively mature, but there are problems such as dependence on large gas storage chambers and fossil fuels. Development and commercial applications are subject to various constraints. In response to the problems of traditional compressed air energy storage, experts and scholars have developed a variety of new compressed air technologies by optimizing the thermodynamic cycle, changing the phase state of the medium, and combining with other new technologies.
The mainstream technology of compressed air energy storage projects in China is thermal storage compressed air energy storage (TS-CAES). Compared with the traditional compressed air energy storage technology, the regenerative compressed air energy storage system cancels the combustion chamber, and the heat storage device can recover the heat generated during the air compression process.
When releasing energy, the heat is used to heat the air at the inlet of the expander to drive the generator to generate electricity. TS-compressed air energy storage technology realizes energy recovery and improves efficiency. According to the difference of heating temperature, TS-compressed air energy storage can be divided into three categories: high temperature, medium temperature and low temperature, and use different heat storage media.
The higher the temperature, the higher the conversion efficiency, but the high temperature conditions require high equipment, and the Chinese projects are mainly of medium and low temperature types.
Advantages and disadvantages of compressed air energy storage
Compressed air energy storage technology is a large-scale long-term energy storage technology with outstanding characteristics.
- Long time: 4-10h;
- Large scale: the size of a typical compressed air energy storage system is 3-400MW;
- Low capital cost: 4000~6000RMB/KW in China;
- Long lifespan: up to 30 years or more;
- Short construction period: about 2 years (about 18 months);
- The environmental impact is small, and the site selection is not restricted.
- Lower efficiency: 50-70% efficiency is lower than pumped storage (75-80%);
- The industrial chain is not mature enough;
- The cost of small-capacity projects is higher. Compared with pumped hydro energy storage, compressed air energy storage has a shorter manufacturing cycle and is less dependent on natural resources.
Compared with lithium battery energy storage, compressed air energy storage has the characteristics of long-term energy storage, large-scale capacity, and relative safety, which can be used as an effective supplement to electrochemical energy storage. In the future, large-scale and long-term compressed air energy storage technology will play an important role in the world energy storage market.
Industrial chain of compressed air energy storage
The upstream of compressed air energy storage mainly includes air compressors, expanders, heat storage heat exchangers, generators, gas storage devices and other equipment. The midstream includes the integration and construction of compressed air energy storage.
Downstream of the industrial chain, compressed air energy storage serves the power generation side, grid side, and user side terminals, and there are various business models, including renewable energy + energy storage, grid-side corresponding grid scheduling, and user-side peak-valley spread arbitrage, etc. In the compressed air system, the cost of core equipment is the highest, more than 40%, and the equipment accounts for nearly 60% of the cost.
The compressor and expander are the core devices, each accounting for 20%, the heat storage and heat exchange device accounting for 15-20%, the gas storage system accounting for 20-30%, the plant land accounting for 10%, and the others accounting for 10%. It should be noted that the compressed air project will have different requirements for project equipment due to differences in climatic conditions and geographical locations.
According to statistics, the capacity of compressed air energy storage projects launched in China has reached 7.29GW, of which 80MW has been put into operation, and 100MW is about to be put into operation; projects under construction/feasibility survey are 3365MW; contracted projects have reached 3750MW, The industrial scale of compressed air energy storage is rapidly expanding.
From the perspective of project participants, China Reserve National Energy, China Energy Construction, Tsinghua/China Salt Joint Venture, etc. are the main participants. China Reserve National Energy mainly focuses on commissioning, construction, feasibility study and survey projects, reaching 910MW.
Energy China currently focuses on contract signing, feasibility study and survey projects, reaching 3,550MW. The cost of compressed air energy storage core equipment accounts for the highest proportion and has the broadest market space. Compressed air energy storage is booming.