Detailed and latest report on gravity energy storage

1. Principle of gravity energy storage technology
   1. Gravity energy storage in solid medium
   2. Water medium gravity energy storage
2. Development status and representative projects
3. Technical route evaluation of gravity energy storage
   1. Site selection flexibility
   2. Scalability
   3. The simplicity of the system
   4. Energy conversion rate
4. Development trend outlook of gravity energy storage

Gravity energy storage in solid media is the main development direction for the commercialization of gravity energy storage in the near future. The new gravity energy storage technology in water media is still in the theoretical research stage, and the new gravity energy storage in water media other than traditional pumped storage has not yet been commercialized.

Principle of gravity energy storage technology

Gravity energy storage is to complete the storage and release of energy through the mutual conversion of gravitational potential energy and electric energy.

That is to say, use excess electric energy to lift heavy objects to a certain height for storage, and generate electricity through the process of falling heavy objects to generate electric energy when electricity is needed. Gravity energy storage can be divided into water medium and solid medium according to the energy storage medium.

Traditional pumped storage uses water as the medium to store the gravitational potential energy of water, which meets the standard of gravity energy storage in a broad sense. However, in order to distinguish traditional pumped storage from new gravity energy storage methods other than pumped storage that are charged and discharged through gravitational potential energy.

Common gravity energy storage includes piston gravity energy storage, suspended gravity energy storage, concrete block energy storage tower, mountain gravity energy storage and other types.

Gravity energy storage in solid medium

The physical energy storage of solid media is a method of charging and discharging energy by lifting and lowering solid heavy objects such as blocks. The main difference is that the structures with different heights are different.

In addition to directly stacking blocks to form energy storage towers, there are also ways to create height differences by means of mountain rail cars and abandoned mine shafts.

• EV1 gravity energy storage tower

The block gravity energy storage tower uses a crane to stack the blocks into a tower, and stores and discharges energy by lifting and lowering the blocks. The energy storage method was first proposed by Energy Vault, and a 35MWh energy storage commercial module was deployed in Switzerland in 2019 and named EV1. The target energy conversion rate of the energy storage device is 75%.

In the process of charging and discharging, assuming that the grasping and releasing do not consume electric energy, the electric energy consumed during lateral movement is equivalent to the battery loss, while the upward and downward movement process is for energy storage and release.

When the shape of the block and the external characteristics such as the characteristics of the mechanical device and the motor are determined, the total power of the energy storage device depends on the layout of the block.

• EVx gravity energy storage module

The characteristic of this method is that there is no difference in the energy storage capacity of each block. No matter where the block is, its rising and falling process, that is, the process of charging and discharging, the energy stored and consumed is the same, and a single block.

The block performs a charging and discharging cycle, and its displacement of horizontal movement is consistent, that is, the electric energy consumed in horizontal movement is the same. That is to say, the energy conversion rate of a single block should be consistent with the energy conversion rate of the whole system in theory.

• Mountain slope energy storage

Mountain slope energy storage is to use the drop of the mountain to lift and drop heavy objects to realize charging and discharging. Compared with artificial structures, this method has a more stable structure and stronger load-bearing capacity, but the disadvantage is that it occupies a larger area. Transmission methods used to lift heavy objects include rail cars, cable cars, winches, linear motors, and conveyor chains.

• Mine energy storage

Underground shaft gravity energy storage uses abandoned mine platforms and mines to lift heavy objects to achieve charging and discharging. The construction cost of this method is low, but the site selection is not flexible enough, and the size of the wellhead limits the scale of energy storage.

The technology was first proposed by the Scottish company Gravitricity, and a 250kW prototype was built in 2021, and the construction of the first 2MWh commercial energy storage project began in 2022.

Water medium gravity energy storage

Water energy storage refers to the use of water as a medium for gravity energy storage. Pumped storage is the most typical way of water energy storage, and the new type of pumped storage is a gravity energy storage derived from the original pumped storage principle. Including seawater pumped storage, subsea energy storage system and piston water pump system.

• Pumped hydro storage

Traditional pumped storage is the most widely used large-capacity energy storage technology. At present, most of the energy storage in the world is realized through pumped storage.

Pumped storage usually uses fresh water as the gravitational medium, and seawater pumped storage mainly builds reservoirs by the sea to establish a liquid level difference between the ocean and the reservoir, and then charges and discharges in combination with turbines or pumps.

Japan established the world’s first seawater pumped-storage power station in 1999, and my country has not yet had a seawater pumped-storage project. Compared with traditional freshwater pumped storage devices, the engineering of seawater pumped storage devices is more complicated, and problems such as seawater corrosion, marine organism adhesion, natural disasters, and groundwater pollution need to be solved.

• Undersea energy storage ball

The underwater energy storage system is charged and discharged through the static pressure difference of seawater combined with water pumps or turbines. Place multiple hollow spheres in the water, charge and discharge using internal and external water pressure, and the efficiency can reach 65%-70%.

This method is suitable for sea breeze absorption and utilization, but there are technical difficulties in the manufacture of hollow spheres, submarine reinforcement and submarine cables, and the feasibility of this technology is not yet clear.

• Piston water pump

The structure of the piston water pump uses the gravitational potential energy of the piston to form water pressure in a well-sealed channel for charging and discharging. The water pressure injected by the motor drives the piston to store energy. When electric energy is needed, the piston descends to generate electricity.

The energy storage capacity of this technology depends on the weight and lifting height of the piston, and can achieve energy storage for 6 to 14 hours, and the energy conversion efficiency is claimed to reach 75% to 80%.

The advantage of this technology is small footprint and high energy density, but the technical difficulty is that it is difficult to guarantee the sealing of the piston, and the requirements for site selection are high, and it needs to be built in areas with hard geology.

• Mine pumped storage

Mine pumped storage refers to energy storage in abandoned mines and/or mines and roadways in mines, using the original height difference of mines as the gravitational potential energy gradient of water to store and release energy.

Development status and representative projects

At present, the types of gravity energy storage with high market attention are mainly concentrated in the gravity energy storage system of solid medium, that is, block energy storage tower, mountain rail car and underground shaft energy storage system.

However, new gravity energy storage technologies using water as the medium, such as underwater energy storage balls and piston water pump technologies, have technical problems such as airtightness and sealing, and high research and development costs. Although the concept has been proposed for many years, it is still No commercial projects have emerged.

At present, the gravity energy storage technology development and demonstration projects in the world are all solid medium gravity energy storage, and the main implementing companies include Energy Vault, Gravitricity, and ARES.

Technical route evaluation of gravity energy storage

Advantages of gravity energy storage:

1. High energy density: The energy density of gravity energy storage depends on the gravity density of the energy storage medium and the height difference of the building, which can achieve a higher energy density.
2. N sustainability: Since gravity energy storage is stored through physical media, there is no energy attenuation problem of chemical batteries, and it can be used as medium and long-term energy storage.
3. N environmental protection: solid media can be used for waste utilization such as construction waste to reduce environmental pollution.

Technical difficulties and possible bottlenecks:

1. nBattery management system: Different from traditional lithium battery management system, gravity energy storage needs to design different battery management solutions according to different structure types.
2. N output power stability problem: Since the power generation of gravity energy storage is determined by the speed (solid medium) or pressure difference (water medium), it is necessary to control the power generation at a relatively stable level.
3. N mechanical structure service life problem: There is no loss in gravity energy storage, but the service life of its steel structure and other materials is affected by mechanical characteristics, and it needs to have strong deformation and corrosion resistance.

At present, new types of gravity energy storage in water media, such as piston water pumps and underwater energy storage balls, have high technical difficulties and high R&D costs, and it is difficult to realize commercialization in the short term.

The technology route of gravity energy storage in solid media is expected to be commercialized in the short term, that is block energy storage, mountain slopes and underground shafts, among which block energy storage is currently receiving more attention in the market.

Site selection flexibility

Among the three technical routes mentioned above, ARES’s rail train energy storage and Gravitricity’s underground shaft energy storage are both restricted by the site’s environmental conditions, requiring the use of natural mountains or abandoned mines to deploy energy storage systems.

The site and scale of block energy storage represented by Energy Vault are not limited, and can be built in corresponding locations according to market needs.

Scalability

Due to the constraints of natural conditions, mountain slopes and underground shafts will be restricted by natural conditions to limit their construction scale, while the block energy storage method has high scalability.

The simplicity of the system

The block energy storage system is the most complex. Whether it is an EV1 energy storage tower or an EVx energy storage module, its mechanical structure is relatively complex, and it involves software issues such as automatic control systems and path planning for each block, making technology development difficult.

With the help of rails or ropes on mountain slopes, the path planning of heavy objects is relatively simple. The energy storage capacity of underground shafts is limited by the wellhead, and the energy storage capacity is small, so the difficulty of planning the movement of heavy objects is between block energy storage and mountain slopes.

Energy conversion rate

According to the principle of gravity energy storage, the effective work is the movement of the weight in the vertical direction, while the work done in the horizontal direction is regarded as energy loss. Therefore, the theoretically achievable energy conversion rate of the technical route of mountain slopes should be lower than that of block energy storage and underground mines.

Development trend outlook of gravity energy storage

The current development of gravity energy storage technology mainly focuses on the direction of gravity energy storage in solid media, which stores and releases energy by lifting and dropping heavy objects. At present, the most mature pumped storage technology in gravity energy storage has the functions of both power generation and energy storage.

Although the energy conversion rate of pumped hydro storage is not high, it plays a pivotal role in power grid peak regulation and frequency regulation due to its large energy storage capacity, and the cost per unit of electricity is very low. It is difficult for other forms of gravity energy storage to surpass its economy in a short time.

Among the emerging gravity energy storage methods, the manufacturing process and technology research and development of piston water pumps and underwater energy storage balls are relatively difficult. In the short term, the technical directions with commercial prospects are block energy storage and mine energy storage.

According to calculations, the overall cost per unit of electricity of the new gravity energy storage has relatively technical advantages among different energy storage types, and its energy storage efficiency is high, which can theoretically be expanded on a large scale.

In the future, with the increasing proportion of new energy at the power generation end, gravity energy storage has a good application prospect in the direction of medium and long term energy storage.

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