Gravity energy storage technology mainly works by converting between three energy forms: gravitational potential energy, electrical energy, and kinetic energy. During periods of low electricity prices and when the power grid has a surplus, the gravity energy storage system drives a motor to move heavy objects to higher ground, converting electrical energy into gravitational potential energy for storage. During periods of high electricity prices and when the power grid urgently needs electricity, the system converts gravitational potential energy into kinetic energy to drive a generator to produce electricity.
In terms of types, the most famous gravity energy storage technology is pumped hydro storage. In addition, four new types of gravity energy storage technologies have emerged internationally in recent years: gravity piston-driven pumped hydro storage, mountain railway gravity storage, tower crane gravity storage, and mine-type gravity storage batteries.
Pumped storage
Among various gravity energy storage technologies, pumped hydro storage is currently the most widely used and technologically mature. It utilizes water, the most abundant natural resource, to store and release energy through the mutual conversion of water potential energy and electrical energy. It boasts advantages such as large storage capacity (100-3000MW), high efficiency (65-85%), long lifespan (40-60 years), and unrestricted energy storage cycle (minute-level response).
Since Switzerland built the world's first pumped storage power station in 1882, the installed capacity of pumped storage has continued to grow globally.
According to data from the National Energy Administration, China added 8.8 GW of pumped storage capacity in 2022, bringing the cumulative installed capacity to 45.19 GW by the end of 2022, a 24.18% increase compared to 2021. Guided by policy, the construction of pumped storage power stations will accelerate further, with the installed capacity expected to reach 62 GW by 2025 and approximately 120 GW by 2030.
However, site selection severely limits the development of pumped storage power stations. The combination of abundant water resources and elevation differences means that pumped storage power stations cannot be flexibly located. In addition, factors such as large construction scale, high cost, long cycle, and cumbersome project planning and approval processes further restrict the development of pumped storage technology.
Gravity piston-driven pumped storage technology
The gravity piston-driven pumped storage technology works as follows: when there is a surplus of electricity, the piston is lifted to pump water into the bottom of the piston for energy storage; when energy is needed, the gravity piston is released to compress the water and drive the motor to release the energy.
Compared to pumped hydro storage, this energy storage technology changes the energy storage medium from water to a heavy block piston. The water only pushes the heavy block piston up and down, without providing energy storage.
For this reason, this energy storage technology can significantly reduce the geographical limitations on pumped storage power stations, reducing the land area required for pumped storage power stations of the same scale to 1-2 hectares, with a scale of 40MW-16GW and a cost of approximately 1000 yuan/MWh.
However, this gravity energy storage technology requires the excavation of vertical shafts and waterways of specific dimensions, resulting in high initial investment for pilot projects. Furthermore, the sliding friction of the sealing sleeve within the vertical shaft has a significant impact on the system's energy conversion efficiency and the project's operational benefits.
Tower crane gravity energy storage technology
The tower crane gravity energy storage technology uses concrete blocks as the heavy object. When electricity prices are low, the tower crane is driven by electricity to lift the concrete blocks to store gravitational potential energy. When electricity is needed, the tower crane is used to lower the concrete blocks back to the ground, converting the gravitational potential energy into electrical energy.
The specific working principle is exemplified by the tower crane gravity energy storage test system located in Ticino, Switzerland. The system appears to be a concrete block tower about 110-120m high, composed of modules such as cranes and concrete blocks.
When there is sufficient power, the tower cranes sequentially lift individual concrete blocks, each weighing up to 35 tons, from the ground to the top of the block tower, making the central concrete block tower larger and converting electrical energy into gravitational potential energy.
When electricity is needed, the tower crane lowers the concrete blocks from the top of the block tower back to the ground. The gravitational potential energy of the blocks is first converted into kinetic energy during the descent, and finally drives the generator to generate electricity.
It is reported that the system can discharge in as little as 2.9 seconds, achieve an energy conversion efficiency of up to 90%, have a theoretical energy storage capacity of up to 35MWh, a lifespan of 30-40 years, and unlimited charge and discharge cycles. Meanwhile, the operating cost is about half that of similar electrochemical energy storage projects.
Mountain railway gravity energy storage technology
Mountain-type gravity energy storage technology is similar to pumped storage technology. Both rely on two heavy-duty storage tanks with significant differences in elevation to realize the conversion of gravitational potential energy. There are two differences: first, the heavy-duty material used in mountain-type gravity energy storage is a heavy block, not water; second, the transportation of heavy-duty materials in mountain-type gravity energy storage mainly relies on trains.
In practice, when there is a surplus of electricity from the power grid, the tram transports heavy objects to a high-level storage depot to store gravitational potential energy; when electricity is needed, the tram transports the heavy objects from a high-level storage depot to a low-level storage depot to convert the gravitational potential energy of the heavy objects into electrical energy for release.
Data from a relevant mountain railway-type gravity energy storage test system shows that the technology has no self-discharge storage loss, an energy conversion efficiency of 80%-85%, a lifespan of about 40-42 years, and an energy storage capacity of up to 12.5MWh.
Compared to pumped storage of the same scale, mountain railway gravity storage can reduce costs by half, and water resources do not need to be considered in site selection.
On the downside, if this energy storage technology cannot be used to rebuild decommissioned railways, a large investment will be required to construct railway tracks. In addition, the gradient of the railway track has a significant impact on its performance.
Mine-type gravity energy storage battery
Mine-type gravity energy storage battery systems are typically built on abandoned mine shafts, resembling a surface elevator shaft, equipped with a winch, motor, steel cables, and elevator frame. The winch, driven by electricity, changes the height of a suspended weight in the underground shaft, converting gravitational potential energy into electrical energy, thereby storing and releasing energy.
Regarding energy storage capacity, relevant research indicates that a 12m deep mine can achieve 11kWh of energy storage using a heavy object weighing approximately 50 tons, with a theoretical energy conversion rate of up to 90%, a lifespan of up to 50 years, and a power generation cost of approximately 1180 yuan/MWh.
The advantage of this technology is that it can make full use of abandoned mines in various places, reduce initial costs, and has significant advantages in energy storage capacity and lifespan.
The drawback is that if there are no abandoned deep wells in the area where the project is located, the initial investment will be relatively high.
