Current situation and technical conception of gravity energy storage power generation

On the basis of tracking and analyzing the development status of gravity energy storage power generation abroad, this paper puts forward a preliminary scheme of gravity energy storage power generation by analogy with pumped storage technology. By analyzing the reasonable net height difference and heavy operation scale of gravity energy storage power generation, the feasibility and theoretical calculation of core indicators such as single unit capacity and power generation efficiency are carried out. Through the introduction of the concepts of "gravity turbine", "upper bin" and "lower bin", the key scientific issues such as the conversion of heavy energy into rotating mechanical energy, the layout of upper and lower bin and the heavy transport system are discussed, and the relevant core technologies and exploration directions are emphasized, and the research concept of gravity energy storage power generation is initially proposed. It is hoped that through the discussion, attention will be paid to gravity energy storage power generation technology, cross-disciplinary cooperation will be promoted, systematic research will be carried out, and theoretical basis and technical support will be provided for the future development of gravity energy storage power generation in China.

1. Analysis of current status of gravity energy storage power generation

The basic principle of gravity energy storage power generation is similar to pumped storage technology. The basic process of energy storage and power generation is as follows: use rich power to lift heavy objects and store potential energy; When needed, by releasing the potential energy of heavy objects, the generator is converted to generate electricity. According to the current relevant foreign data reports, there are mainly four kinds of gravity energy storage power generation technologies: piston gravity energy storage, hanging gravity energy storage, concrete block energy storage tower and mountain gravity energy storage.

The piston Gravity energy storage proposed by California Gravity Power Company [31] is based on pumped-storage unit and uses heavy piston in shaft to replace water body for energy storage, as shown in Figure 1. When the power is rich, it is pumped and pressurized by the pump turbine to lift the heavy piston and store energy, that is, the water body does not store energy directly; When generating electricity, the heavy piston falls, and its potential energy is transferred to the water flow, which is converted by the pump turbine into mechanical energy to drive the generator. Because the density of heavy weight is higher than that of water, the power generation water head and energy density can be increased under the same height difference condition. In other words, compared with pumped storage power stations with the same potential energy, piston gravity energy storage power generation technology can reduce the construction height, reduce the dependence on geographical conditions and water resources, and facilitate the site selection and layout of power stations. The technical scheme retains the core equipment of pumped storage unit, and the pump turbine technology of pumping and generating power is mature, high efficiency, and has unique advantages. However, some technical problems need to be discussed, such as the technically and economically feasible size and scale, and the sealing scheme between them, which need to be paid attention to the follow-up research progress. At present, the capacity of piston gravity energy storage is limited, and it may be suitable for some small and short-term energy storage.

Gravitricity [37-40], a company that applies suspended gravity energy storage technology, plans to build a 250 kW gravity energy storage pilot power plant in Leith Harbour, Edinburgh, to store local excess power. The project mainly plans to use abandoned drilling platforms and mines, adopt 500-5,000 t heavy drilling RIGS as heavy objects, and realize the storage and release of electric energy by repeatedly lifting and lowering the drilling rig in a 150-1,500 m long drilling rig. When the electric energy is abundant, the rig is pulled above the abandoned mine by electric winch, and the electric energy is stored as heavy potential energy. When electricity is needed, it is generated by dropping the rig to drive the generator.

Gravitricity believes its advanced winch and control system will give it enough flexibility to respond quickly in less than one second to meet the grid's peak load balancing needs. Thomas et al. [41] introduced the technical scheme of using suspended weights for energy storage and power generation, and discussed the potential of applying this technology to the transformation of abandoned deep mines. Because a unit only uses one heavy weight to cycle, the corresponding total energy storage and continuous power generation time are very limited.

It is said that the project can operate for up to 30 to 40 years, and the cost will be half the cost of existing grid-scale battery energy storage solutions. When the power is sufficient, the crane lifts the concrete blocks from the ground and piles them up high like building blocks, converting the energy into potential energy of the concrete block tower, that is, the energy storage stage; When it is necessary to generate electricity, the concrete blocks are dropped in turn to release the potential energy of the heavy objects and convert them into electricity. Concrete block tower seems to be a relatively simple energy storage method, but there are mainly the following problems: due to the cyclic lifting of heavy blocks, the whole "tower" is not fixed, belongs to a kind of variable structure, its stability, reliability, safety and so on need to be carefully designed; Considering its own structural characteristics, the scale of a single concrete energy storage tower may be small and the energy storage capacity is limited. In addition, the potential energy stored by each concrete block from the bottom of the tower to the top of the tower is different, and there are no relevant technical reports on how to design lifting, stacking and energy conversion, and whether the expected effect can be achieved. Energy Vault said it has reached an agreement with India's Tata Power company to deploy a 35 MW·h storage system that can generate power within 2.9s with a peak power of 4 MW, which is worth further attention to the development progress.