Five types of energy storage systems : Mechanical type（I）

In recent years, energy storage systems have become a new big hit in the field of energy development. To be more specific, lithium battery storage systems are among the leaders. But lithium battery storage is not the only one on the market. Depending on the storage medium, there are now five major categories of energy storage technologies commonly used. These five categories are mechanical energy storage, electromagnetic energy storage, thermal energy storage, chemical energy storage and electrochemical energy storage. So why are lithium battery storage suddenly becoming popular? How about let's first understand the difference between these five types of energy storage systems. This article will first introduce the mechanical type of energy storage system.

The application forms of mechanical energy storage are pumped storage, compressed air storage and flywheel energy storage.

I. Pumped storage system

1) Basic principle

The working principle of pumped storage is to use excess power as liquid energy media water from low elevation reservoirs to high elevation reservoirs at low valley; the water in high elevation reservoirs flows back to the lower reservoirs at peak load of the power grid to drive the turbine generator to generate electricity. In simple terms, it means that during peak load of the power system, the water from the upper pool flows back to the lower pool to push the turbine generator to generate electricity; during low load, the water from the lower pool is pumped back to the upper pool to store up energy. Observe the following diagram to understand this more clearly.

2) Characteristics

The characteristic of this technology is that it is a large-scale, centralized energy storage, and the technology is quite mature for energy management and peaking of the power grid. The efficiency is generally about 65%-75%, and up to 80%-85%. In addition, pumped storage has a fast load response (10% load change in 10 seconds), from full stop to full load pumping in about 1 minute, from full stop to full load pumping in about 5 minutes. And with daily regulation capability, it is suitable to be used in conjunction with nuclear power plants, large-scale wind power generation, and ultra-large-scale solar photovoltaic power generation. This is very reliable in large scale projects at national level.

3) Disadvantages

However, the technical disadvantages of pumped storage are also obvious. As we can see in Figure 1, this technology requires upper and lower pools for site selection, which is very dependent on geographical conditions and has certain difficulties and limitations. And the address may not always be close to the load center, and a suitable construction address may be tens or even hundreds of kilometers away, necessitating long-distance transmission. In this way, a large amount of power is wasted midway.

4) Application

According to incomplete statistics from CNESA, as of 2019, the cumulative installed capacity of pumped storage systems is the largest in the global energy storage market, accounting for 92.6% with 171GW of generation capacity. In second place is electrochemical energy storage, with an installed capacity share of 5.2%. Due to its low cost, stable and very sufficient power generation, pumped storage system has been occupying the absolute leading position in the global energy storage market.

II. Compressed air energy storage system

1）Basic principle

Large-scale compressed air storage uses excess electricity to compress and seal air stored in an underground structure (such as underground caves, end-of-life mines, sunken Haitian gas storage tanks, expired oil and gas wells or new gas storage wells, etc.), and then mixes compressed air with natural gas when needed, burning and expanding to drive a gas turbine to generate electricity. In a nutshell, this is an energy storage method that uses electrical energy for compressed air during the low load period of the grid, and releases compressed air to drive the turbine to generate electricity during the peak load period of the grid.

2) Advantages

This kind of energy storage system has the function of peak regulation and is suitable for large-scale wind farms. Because the mechanical work generated by wind energy can directly drive the compressor rotation, reducing the intermediate conversion into electricity, thus improving efficiency.

3) Disadvantages

Previously developed is a non-adiabatic compressed air energy storage technology. A major disadvantage of this technology is similar to pumped storage systems, which require large caves to store compressed air. The conditions for construction are closely related to geographic conditions and suitable locations are very limited. And a gas turbine is required to work within the system, meaning that a certain amount of gas is needed as fuel, suitable for use as energy management, load leveling and peak shaving. This makes the system seem not completely "clean". However, the non-combustion technology has been developed and will be in operation for the first time in China starting in 2020, and it is likely that in a few years this technology will drive a new energy revolution.

4) Applications

Until recently, only Germany and the United States had operational compressed air energy storage stations. Germany's Hundorf station was put into operation in 1978, with a compressed power of 60MW and a generating power of 290MW (later improved to 321MW), operating continuously for 2 hours and starting tens of thousands of times, with a 97% start-up reliability rate. Mcintosh, Alabama, USA, commissioned in 1991, 110MW, with continuous output of 100MW for 26 hours, had collapse accident due to geological instability. In addition, several large compressed air energy storage plants are under construction in the United States and have not yet been commissioned.

In June 2020, the UK Department for Business, Energy and Industrial Strategy (BEIS) announced that it would allocate £10 million to help build the world's largest liquid air energy storage project, which is scheduled to be operational again in 2022. This will be the world's first commercial liquid air energy storage system. China also invested in the world's largest 400MWh compressed air energy storage plant in 2020. This shows that the research and development of compressed air energy storage is also on the rise, and there should be more applications in the future.

III. Flywheel energy storage

1) Basic principle

In a flywheel energy storage system, electrical energy accelerates a rotor placed inside a vacuum enclosure, i.e. a large mass cylinder made of solid material (tens of thousands of revolutions/minute), thus storing electrical energy in the form of kinetic energy. This is done by using the inertial energy stored in the large rotor.

2) Advantages

Flywheel energy storage has very significant advantages. Not only does it have a lifetime of 15-30 years, it requires less maintenance and is more stable. It has a high power generation efficiency of 90% and a high power density, with a very fast response time of milliseconds.

3) Disadvantages

Due to the low energy density, flywheel energy storage can only last for a short period of time, ranging from a few seconds to a few minutes. And due to bearing wear and air resistance, it will have some self-discharge.

4）Application

Flywheel battery is a new concept battery proposed in the 90s, which breaks through the limitations of chemical batteries and realizes energy storage by physical methods, and will not cause pollution because of the mutual transformation of electrical energy and mechanical energy. Currently flywheel energy storage systems are mostly used in industry and UPS for power distribution system operation for frequency regulation and can be used as a UPS without battery. when the power supply fails (e.g. power supply interruption, voltage fluctuation, etc.), the power can be quickly transferred to maintain a small system with a stable frequency for some time to ensure power quality. The main structure and operation of this technology has basically been clarified, and small systems have been developed and can be applied in practice. However, there are very many difficulties for the research of large flywheel systems, such as rotor design, magnetic bearings, power electronic circuits and mechanical backup bearings.

The advantages of flywheel energy storage system are very outstanding and have a wide range of applications. If the large scale system can be developed successfully, it will be a new revolution in the field of energy storage.