Gravity energy storage technology and applications

Principles of gravity energy storage

Gravity energy storage belongs to mechanical energy storage, and its energy storage medium is mainly solid matter and water. The basic principle of gravity energy storage is to lift and lower the energy storage medium based on the height difference, so as to complete the charging and discharging process of the energy storage system.

● When water is used as the gravity energy storage medium, the energy storage system can use pipes, shafts and other structures with better sealing performance. The biggest limitation of water medium is that its flexibility and energy storage capacity will be greatly limited by water source and terrain;

● When solid material is used as the medium, the solid heavy material needs to choose a material with higher density, such as metal, cement, stone sand, etc. to achieve a relatively high energy density. The new gravity energy storage will be realized through a variety of paths.

At the end of the 19th century, there were pumped storage power stations; the application of gravity energy storage in China was relatively late, and the first small hybrid pumped storage power station was built in 1968. At present, there are various gravity energy storage schemes according to the different energy storage medium and drop path: based on pumped storage, based on the height difference of structures, based on the mountain drop, and based on underground shafts.

Gravity energy storage technology classification

Gravity energy storage technology mainly includes piston gravity energy storage, suspension gravity energy storage, concrete block energy storage tower, mountain gravity energy storage and other gravity energy storage power generation technologies.

Piston gravity energy storage

The piston-type gravity energy storage proposed by the California Gravity Power Company is based on the pumped storage unit, which uses the heavy piston in the shaft to replace the water body for energy storage. When the power is abundant, the water is pumped and pressurized by the water pump and turbine to lift the piston of the heavy object to store energy, that is, the water body does not directly store energy;

When generating electricity, the weight piston falls, and its potential energy is transferred to the water flow, which is converted into mechanical energy by the water pump turbine to drive the generator to work. Compared with the pumped storage power station with the same potential energy, the piston-type 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 the power station. 

Suspended gravity energy storage

The Scottish company Gravitricity has proposed a mechanism that uses an abandoned drilling platform and a capstan rig for energy storage. They use abandoned drilling platforms and mines to repeatedly lift and lower 16-meter-long, 500-5,000-ton drilling rigs in 150-1500-meter-long wells.

Through the electric winch, the drilling rig is pulled up to the abandoned mine when the power consumption is low, and the drilling rig is dropped straight down when the power consumption peaks, thereby “releasing” the stored energy, the system can control the falling speed of heavy objects to change the power generation time and power generation. The company claims the system can react within a second, has a lifespan of up to 50 years, and is up to 90 percent efficient.

Concrete block energy storage tower

Gravitricity believes that the advanced winch and control system can make it flexible enough to respond quickly within 1s to meet the grid peaking demand. Energy Vault proposed a gravity energy storage power generation scheme based on concrete block energy storage towers. The project is said to run for up to 30 to 40 years, and the cost will be half the cost of existing grid-scale battery storage solutions.

When the power is abundant, the crane lifts the concrete blocks from the ground, stacks them up like building blocks, and converts the energy into the potential energy of the concrete block tower, that is, the energy storage stage; When power generation is required, the concrete blocks are dropped in sequence to release the potential energy of the heavy objects and convert them into electrical energy.

Mountain gravity energy storage

The Mountain Gravity Energy Storage (MGES) proposed by IIASA Research mainly utilizes the terrain of steep mountainous areas to store energy through the potential energy of sand and gravel. When the power is abundant, an electric system similar to a ski lift is used to lift the container full of sand and gravel to the top of the mountain for storage;

During peak electricity consumption, the sand and gravel are transported from the top to the ground by gravity, and the potential energy of the sand and gravel is released to generate electricity. The institute believes that the mountain gravity energy storage system is a longer-lasting and larger-scale energy storage method than the best rechargeable batteries lithium battery energy storage system.

Mountain gravity energy storage seems simple and easy to implement, but the efficiency of the cable car system used 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.

Advantages and disadvantages of gravity energy storage

Advantages

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%, and it only takes 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 higher requirements on site selection like pumped storage power stations, so its construction cost is only one-third of pumped storage, and the cost per kilowatt-hour is only two-thirds of pumped storage.

● Pure physical energy storage, high safety and environmental friendliness.

Gravity energy storage does not involve chemical reactions in work processes such as heavy object transportation, potential energy storage, and mechanical energy power generation, and its operation is safe and reliable. Gravity energy storage power generation is clean and low-carbon, with little impact on the natural environment.

● Strong environmental adaptability, suitable for “distributed” energy storage.

There are no special conditions and requirements for the storage, transportation and power generation of heavy objects. Therefore, there are basically no external conditions such as site selection and weather restrictions for gravity energy storage power stations.

The application is very flexible, and flexible layout can be realized on the grid side and the power supply side according to the needs of the power system.

● The power generation cycle life is long and the cost is low.

Heavy objects are mainly made of concrete or local materials, or use other recycled materials, which can be recycled for decades, and the loss of heavy objects is small during operation. If the materials are used properly, the cost of heavy objects can be greatly reduced.

● Long time and no self-discharge problem.

It is relatively easy to expand the upper and lower warehouses of the gravity energy storage power station, and there will be no loss during the storage of the potential energy of heavy objects. It has the convenience and inherent advantages of long term energy storage.

Disadvantages

The energy density is low and the construction scale is too large. The average high tower required for gravity energy storage is more than 100 meters, and its output power is only equivalent to a wind turbine of the same height; In addition, this technology requires very high precision for tower cranes. A cable with a length of tens of meters needs to make 5,000 bricks, and the position error of each brick is less than a few millimeters;

Each tower needs thousands of cement blocks, and pouring cement blocks needs to emit a lot of carbon dioxide.

Application scenarios of gravity energy storage technology

●Site selection of decommissioned thermal power plants

The vertical lifting of heavy objects in the gravity energy storage technology has a small footprint and flexible location selection, and decommissioned thermal power plants can be one of their preferred locations due to their power infrastructure.

●Compressed air energy storage + electrochemical energy storage synergy

As long-term energy storage, gravity energy storage has a good complementarity and synergy with short-term energy storage such as compressed air energy storage technology and electrochemical energy storage(such as ternary lithium battery and lithium iron phosphate battery).

●Comprehensive management of tailings

The use of tailings slag to manufacture gravity modules can solve the problem of tailings slag disposal and raw materials for gravity energy storage (unit modules), reduce construction costs, and at the same time obtain obvious economic and social benefits.

●Reuse of wind turbine blades

It is estimated that by 2025, the number of retired wind turbine blades in China will reach 8,112 tons. The wave of scrapping and the need for new processes to deal with degradation have made the recycling of decommissioned wind power plants a top priority.

●Zero-carbon urban unmanned agriculture concept

The gravity energy storage building is about 30 stories high, and the middle 9-22 floors are mostly idle space. Using these spaces, unmanned agriculture and smart vegetable factories, matsutake mushrooms, alfalfa and other high value-added crops can be built to maximize economic benefits.

●Data center concept

Similarly, building a data center on a vacant floor in a gravity energy storage building can solve the problem of a clean power source with high power consumption in the data center, and provide new ideas for achieving the “dual carbon” goal.

●Integration of wind and solar hydrogen storage and ammonia

With gravity energy storage as the core, wind power and photovoltaic power generation are transformed into stable power sources for the production of green hydrogen, and the problem of hydrogen storage and transportation is further solved by hydrogen-to-ammonia.

Bring sustainable, stable and growing economic and social benefits to the region, and maximize the value of resources.

●CO2 storage

Gravity energy storage devices can take advantage of their long lifespan and wide source of gravity module materials, providing favorable conditions for carbon dioxide sequestration. Using carbon sequestration technology may realize long-term or even permanent carbon dioxide sequestration.

The future of gravity energy storage

Although the gravity energy storage technology that has just been completed has many attractive commercial prospects, its practicality still needs to be proved by data.

The deficiencies in the existing technical indicators also need to be changed and improved in the future. With the gradual withdrawal of traditional energy from the stage of history and China’s determination to achieve the “dual carbon” goal, gravity energy storage may become the next outlet.

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