Current situation and technical conception of gravity energy storage power generation

Measures such as vigorously developing non-fossil energy and promoting green energy use modes are the key to achieving China's "dual carbon" goal [1]. The medium - and long-term strategic goal of China's energy revolution clearly states that by 2030, non-fossil energy generation will account for 50% of total power generation, and by 2050, non-fossil energy consumption will exceed half [2]. At present, the renewable clean energy with large-scale development is mainly hydropower, wind energy and solar energy. Water energy is developed earlier, the most mature technology, is a high-quality green energy. At present, the scale of new construction of conventional hydropower in China has slowed down in general, and has entered a moderate and orderly stage from rapid and concentrated development [3-5]. The development space of wind energy and solar energy is huge, and from the perspective of global development trend, the growth rate is fast [6]. In December 2020, China announced at the Climate Ambition Summit that by 2030, the total installed capacity of wind and solar power in China will reach more than 1.2 billion kW. As we all know, wind energy and solar energy are greatly affected by natural factors, and the prominent problem is the randomness and intermittency of power generation. Large-scale access to the grid will bring many problems in power balance, power consumption, stability control and other aspects, especially after the increase of their proportion, high requirements and new challenges will be put forward for the rapid and flexible adjustment of the power system [7-10]. In order to ensure the safe and stable operation of the power system after the access of various types of fluctuating power supplies, energy storage is the most effective solution and can not be ignored. Therefore, energy storage technology has become one of the hottest topics in today's energy research [11-15].

Energy storage involves a wide range of fields, according to the way of energy storage, can be roughly divided into physical energy storage, chemical energy storage, electromagnetic energy storage and heat storage. Chemical energy storage mainly has a variety of battery class electrochemical energy storage, hydrogen energy storage and so on. Electrochemical energy storage can meet the needs of diverse scenarios, but it needs to face security risks, environmental pollution, operating costs and other unavoidable problems; Hydrogen energy storage is suitable for large-scale energy storage and long-period energy regulation, but it is still in the demonstration application stage. Electromagnetic energy storage, such as superconducting electromagnetic, supercapacitors, etc., has low energy density and high cost. Heat storage technology has good market value in specific application fields such as electric heating, electric heating, industrial heating, solar thermal energy storage system, etc. Physical energy storage mainly includes pumped storage, compressed air energy storage, flywheel energy storage, etc. Compared with other energy storage methods, it has significant advantages of low impact on the environment and high safety. At this stage, compressed air energy storage and flywheel energy storage do not have the conditions for large-scale commercial application, so pumped storage is the most widely used large-capacity energy storage technology [16-19]. On July 15, 2021, the National Development and Reform Commission and the National Energy Administration issued a document clarifying that "pumped storage and new energy storage are important technologies and basic equipment to support new power systems" [20]. Pumped storage power station has the functions of peak load and valley filling, frequency and phase regulation, emergency backup, etc. At present, most of the energy storage in the world is achieved by pumped storage. Objectively speaking, although the construction cycle of pumped storage is longer and the cost is higher, it has obvious technical advantages in many aspects such as storage capacity scale, technical maturity and service life. In recent years, China's pumped storage industry has developed rapidly, and has reached the world-class level in terms of applied head, single unit capacity and unit development technology. Pumped storage power stations have become the main body of energy storage in China's power grid [21-25]. Of course, the development of pumped storage power stations requires appropriate geographical conditions and water resources, which limits the application of pumped storage technology to a certain extent [26-27].

In recent years, based on the basic principle of pumped storage, there have been some foreign research reports on new gravity energy storage technologies [28-30]. Gravity energy storage is to lift heavy objects through transport, so as to convert rich electric energy and store it as the potential energy of heavy objects. In order to reduce the height of pumped storage power station and facilitate engineering layout, some scholars put forward an improvement scheme, namely piston gravity energy storage technology [31-36]. The technology retains the pumped storage unit, and the water body does not store energy directly, but uses a heavy piston to store energy. In addition, the gravity Energy storage power generation scheme reported by Scottish startup Gravitricity, Swiss energy storage company Energy Vault and Austrian research institute International Institute for Applied Systems Analysis (IIASA) does not involve water, but replaces the energy storage medium with a solid weight. Such as sand, rocks, concrete blocks and other similar weight materials [37-46]. At present, there are few relevant academic research materials in China, mainly for research reports relying on mountain energy storage for power generation [47-52], and some public patents on gravity energy storage for power generation [53-60]. The location of gravity energy storage power station is flexible, and it can use wasteland, mountains and other free Spaces to avoid agricultural and ecological land, which can get rid of the restrictions on geographical conditions to a certain extent, and has important value. However, as a potential green energy storage method, gravity energy storage power generation is still in the stage of exploration and development. Therefore, it is necessary to carry out extensive scientific proof and technical analysis.