Brief introduction on what gravity energy storage is

Green, environmental protection and safety are the prerequisites for the sustainable development of energy storage technology, and flexible layout and strong scenario adaptability are conducive to responding to the needs of the power system and enhancing industrial value.

Compared to lithium batteries and pumped storage, gravity energy storage technology is easier to expand and modular, and it will not produce harmful substances, or rely on compressed air and flywheels to pose safety hazards or fire risks. Therefore, compared to compressed air energy storage and flywheel energy storage, gravity energy storage has great advantages.

1. The basic principle for gravity energy storage

If purely theoretical, gravity energy storage is the simplest way to store energy, and its principle is similar to pumped storage, which is to store energy with potential energy caused by gravity. When there is excess power, the driver lifts the heavy object to a high position, and when power is needed, the power of the heavy object is used to drive the generator to generate electricity.

2. The advantages and disadvantages of gravity energy storage

● The principle is simple and the technical threshold is low.

At the same time, due to the use of physical media to store energy, its energy storage efficiency is as high as 90%, it takes only 2.9 seconds to increase the output power from 0 to 100%, and the service life is more than 30 years. And there is no need for high site selection requirements such as those for pumped storage power stations, so its construction cost is only one-third that of pumped storage, and the cost of electricity is only two-thirds that of pumped storage.

● The energy density is low, and the construction scale is too large.

The towers required for gravity storage are more than 100 meters tall on average, and their output power is only equivalent to that of a wind turbine of the same height. In addition, this technology requires very high precision tower cranes.

Tens of meters of cable must be made of 5000 bricks, each with a position error of less than a few millimeters. Each tower requires thousands of cement blocks, and pouring cement blocks emits a lot of carbon dioxide, which can result in more carbon emissions than increasing the proportion of new energy generation.

3. Application of gravity energy storage

Although the principle of gravity energy storage is very simple, it is affected by conservative concepts, and the power grid is equipped with large-scale energy storage facilities, but progress is slow. Therefore, gravity energy storage has not really appeared in the public eye as a way of storing energy for a long time. The crane works on the same principle as a pumped storage system, loading the system.

The system can respond to fluctuations in electricity demand or other support required by grid operators in milliseconds and can provide a base load for renewable energy 24 hours a day. In addition, some companies have also developed energy storage systems such as railway energy storage and ridgeline energy storage with similar principles.

4. Analysis of the current situation of gravity energy storage power generation

The basic principle of gravity energy storage and power generation is similar to pumped storage technology, and the basic process of energy storage and power generation is: using abundant electricity to lift heavy objects and store potential energy. When needed, the potential energy of heavy objects is released and converted to drive the generator to generate electricity.

● Piston gravity energy storage

Based on the pumped storage unit, the heavy piston in the shaft is used to replace the water body for energy storage. When electricity is rich, the water is pumped and pressurized by the pump turbine to lift the piston of heavy objects 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 transmitted to the water flow, which is converted into mechanical energy by the pump turbine to drive the generator to work. Since the density of heavy objects is greater than that of water, the power generation head can be increased under the same height difference conditions, and the energy density can be increased. In other words, compared to pumped storage power stations with the same potential energy.

This technology can reduce the construction height, reduce dependence on geographical conditions and water resources, and facilitate the site selection and layout of power stations. This technical solution offers unique advantages. But there are some technical issues that need to be explored. Currently, piston gravity energy storage has a limited capacity and may be suitable for some small, short-term energy storage.

● Suspended gravity energy storage

The system can control the falling speed of heavy objects, change the power generation time. This energy storage technology operates in a closed mine, reducing the impact on the natural environment and having a high safety factor. How to improve the working stability of the electric winch, reduce the rotation and shaking of heavy objects and fix problems is the focus of research.

● Mountain gravity energy storage

It mainly uses the terrain of steep mountain areas to store energy through the potential energy of sand and gravel. When electricity is abundant, an electric system similar to a ski lift is applied to lift containers filled with sand and gravel to the top of the mountain for storage. At peak power consumption, gravity is used to transport sand and gravel from the top back to the ground, and generate electricity by releasing potential energy from the sand.

A mountain gravity energy storage system is a longer-lasting and larger scale energy storage method than a lithium battery energy storage system. Mountain gravity energy storage seems simple and easy, but the efficiency of the applied cable car system is not easy to improve, the comprehensive benefits of the energy storage power generation system may not be ideal, and the relevant technical solutions need to be developed.

This energy storage system uses mountain terrain and rail vehicles, which can achieve large-capacity energy storage in outdoor environments, but the civil cost of leveling the hillside is high, the chain transmission stability is poor and easy to wear, and further structural optimization is required.

This energy storage system uses natural hillsides and gravel as an energy storage medium to reduce construction costs, but the cable car's carrying capacity is low, the outdoor environment has a great impact on the operation of the cable car, and how to achieve stable and efficient energy recovery is a difficult research point for this system.

The wheel-rail support structure of the permanent magnet linear synchronous motor is adopted, and the electric power generation is completed by the linear motor. Or use multiple electric winches to pull vehicles and segment energy storage. The use of mountain drops for energy storage structure stability, with no risk of collapse, can achieve larger scale gravity energy storage.

5. Analogy to pumped storage technology

Replace fluid water with solid weights, and pump storage units into gravity energy storage units. When the power grid is low and there is excess power, the transmission system will lift the heavy objects in the lower warehouse to the upper warehouse in a higher position, and convert the surplus electric energy into potential energy of the heavy objects for storage and use.

When the peak of electricity consumption and the power supply is insufficient, the heavy objects fall in turn, converting the potential energy of heavy objects in the upper warehouse into mechanical energy and electrical energy. Gravity energy storage power generation concept The water-dielectric gravity energy storage system is inferior to the traditional pumped storage in terms of power and storage capacity, but the response time is shorter and the location is more flexible.

Since the solid heavy energy storage system does not require water pump and turbine structure, it can theoretically achieve higher energy storage efficiency than pumped storage, and the response time is shorter, and different energy storage structures can be flexibly selected according to different terrains and needs.

There are many structures of gravity potential energy storage schemes, each with its own advantages and disadvantages, and it is appropriate to design gravity energy storage systems according to different terrains and energy storage needs. Among them, gravity energy storage based on mountain drop and underground shaft is more promising.

The power and capacity of the gravitational potential energy system are related to the mass and lifting height of the lifted object, which is more suitable for the construction of medium power and capacity energy storage system, but by building multiple gravity energy storage system clusters, greater capacity and power can be obtained, so as to achieve its large-scale utilization.

6. Advantages of gravity energy storage power generation

Compared with other energy storage technologies, gravity energy storage power generation has the following significant advantages:

● Pure physical methods, high safety, environmental friendliness.

In the workflow of heavy material transportation, potential energy storage, mechanical energy power generation, etc., no chemical reaction is involved, and the operation is safe and reliable.

Gravity energy storage power generation is clean and low-carbon, and has little impact on the natural environment. Of course, it should be noted that the impact of heavy processing on the environment should be strictly controlled, and the processing technology and material problems should be solved, such as the use of recycled materials such as construction waste as much as possible, or local materials, in line with the concept of sustainable and green development.

Strong environmental adaptability, can be flexibly arranged according to needs, and is suitable for distributed energy storage. There are no special conditions or requirements for the storage, transportation and power generation processes of heavy objects, so the gravity energy storage power station basically has no external conditions such as site selection or weather, and the application is very flexible.

In addition to areas with concentrated electricity loads, gravity energy storage power stations can also be configured near power stations such as wind, solar and nuclear power, which can realize flexible layout on the commercial and home energy storage according to the needs of the power system. Pumped storage site planning needs to be based on appropriate geographical conditions and water resources, making full use of gravity.

● Long cycle life and low cost.

Using concrete or local materials as the main material, or other recycled materials, it can be recycled for decades with low losses during operation. If the material is used appropriately, the cost can be greatly reduced. It is foreseeable that, after the development of technology, the cost of gravity energy storage power generation will be at a relatively low level.

● Long energy storage time and no self-discharge problems.

The expansion of gravity energy storage power station is relatively easy, and there will be no loss during the storage of heavy potential energy, which has the convenience and innate advantages of long-term energy storage. In summary, it can be seen that gravity energy storage power generation has the potential to become an ideal new energy storage method for power systems, and has broad application prospects.

7. What are the economic benefits of gravity energy storage

Regarding the commercial prospects and economic benefits of gravity energy storage, there are not many clear or unified views in the market. Under the current more common energy storage technology routes, the levelized cost of chemical energy storage (lifepo4 battery, etc.) is 0.64 RMB to 1.26 RMB, while the cost of pumped storage is as low as 0.21 RMB to 0.25 RMB. This means that gravity storage technology costs somewhere in between, but the geographical requirements are low. The construction period of general gravity energy storage projects is about half a year.

8. Conclusion

 According to the current development trend of power system energy storage, pumped storage power stations will continue to maintain a high proportion. In the long run, large-scale energy storage technology will inevitably develop in a diversified direction to meet the needs of different application scenarios. Gravity energy storage has significant advantages such as environmental friendliness, flexible layout, high safety, long life, and no self-discharge.

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