Gravitational storage: old and new technologies for storing electricity
In the garden of Woolsthorpe Manor in Lincolnshire, England, you can still see the place where Isaac Newton is said to have had the inspiration for the theory of gravity. Although the tree from which the famous apple fell is no longer the same, the apples still fall the same way.
Although Newton was the first to describe gravity in mathematical terms, everyone has first-hand experience of it from a young age. It’s a fundamental force of nature, and its application is crucial in the field of energy.
What is gravitational potential energy?
Energy is neither created nor destroyed; it is transformed. When an object is in motion, it is endowed with kinetic energy, which is directly proportional to its mass (i.e., weight) and the square of its velocity at any given moment. But a stationary object is also endowed with energy – a "quiescent" form of energy, which is called potential energy, to express that it has the potential to transform into kinetic energy. When only the force of gravity is acting on an object, it’s called gravitational potential energy.
The gravitational potential energy indicates how much kinetic energy the object can develop if you drop it, and it also depends on the mass of the object, as well as the height from which it’s dropped: the greater the height difference, the greater the speed the object can reach, and therefore the greater its kinetic energy.
This principle of physics is called the conservation of energy, and it can be used to store energy so that it can be used at times when it’s needed: this is what’s called gravitational storage, or gravity storage.
How does gravitational energy storage work?
How gravitational energy storage works is simple. An object is lifted up to a certain height, then dropped at a given time: the electrical energy used to lift it is stored in the form of potential energy and later transformed into kinetic energy. The kinetic energy in turn can be transformed back into electrical energy by a generator driven by the motion of the falling object.
In order to be able to store electrical energy in large quantities, though, it’s important for objects to be very heavy and raised to a great height above the drop point. Various techniques have been developed to achieve this: while the basic concept is intuitive, in its practical implementation, the challenge is technological.
The advantages of different technologies
The first example, and the most widely used, is hydroelectric pumped storage: when electricity is available, it’s used to pump a large mass of water from a lower reservoir to an upper reservoir; later, the water is allowed to fall, thus driving hydroelectric turbines to generate electricity at times of higher demand. Technically, it’s an evolution of the classic hydroelectric power plant, but in which the plant can work in a closed circuit, without the need for the natural flow of a watercourse.
In order to take advantage of gravitational energy storage even where there is no immediate availability of large amounts of water, various types of systems using the weight of solid objects have been studied, and this kind of storage can also be called gravity batteries (or gravitational batteries).
One option is "elevation" gravitational energy storage: it consists of a series of lifting systems driven by electric motors, similar to freight elevators, that lift large bricks or concrete blocks and then drop them at a controlled rate, thus recovering the previously stored energy. It’s a solution that has the great advantage that it can be applied almost anywhere.
An alternative is to build plants underground, using rocky materials from disused mining sites as weights: this way, no surface space is taken up and there is little visual impact at the site. Alternative techniques consist of sliding the weights along an inclined plane, or involve the use of water pistons.
Gravity batteries have the great advantage of having a very high overall efficiency, theoretically as high as 80-85%, which is even higher than the already high efficiency of pumped hydro. This means that the percentage of energy that’s lost during the whole process is very low. In addition, there are limited environmental impacts in terms of raw materials: the construction of the plants requires very few rare materials, and the blocks to be dropped can be made from recycled materials, applying circular economy principles.
Why electricity storage is important
Gravitational energy storage is a promising solution to complement other storage technologies. Indeed, storing electricity is becoming increasingly important in light of the energy transition. Because "new renewables" are the fastest-growing sources of energy, storage is the best way to compensate for the intermittency of sources such as sun and wind: electricity can be stored at times of peak production and then released to the grid at times of highest demand, regardless of weather conditions.
On the other hand, having a certain amount of energy on hand to release at any time also fulfills another requirement: ensuring the necessary stability and proper flexibility of electricity grids – grids that are becoming increasingly complex and multidirectional due to the increasing flows of energy uploaded onto the grid from small power systems, especially by prosumers (a new word coined from the words producer + consumer).
The market is always looking for new solutions to diversify approaches to storage, maximize efficiency, and reduce costs: so every technological innovation, including gravity batteries, is worth looking into.
For Enel, a key player in renewables and the energy transition, storage systems play a key role and significant investments have been allocated to them in the new Strategic Plan 2024-2026, amounting to about 12% of the total. According to the plans, Enel Green Power's total storage capacity will increase 36-fold by 2026 compared to 2021.
Our strategy focuses on our role in supporting generation from renewable sources but also on grid services: our new battery system installed in Trino Vercellese, with more than 230 MW of capacity, is the largest in Europe of the stand-alone type, i.e., not directly integrated into a generation plant.
In terms of technologies, we adopt the most suitable solution according to need, from pumped hydro to batteries – we want to have the ability to choose the most suitable solutions for individual contexts from an energy and economic perspective each time.
The evolution of storage
The earliest form of electricity storage, in chronological order, is hydroelectric pumped storage: the first facility in Switzerland dates back as far as the early 1900s. Just as hydropower is the most important renewable source in terms of installed capacity, so pumped hydro is now the leading electricity storage technique: it makes up more than 94% of all storage in the world today. By its nature, however, it is only suitable as a support for generation.
This is why today there’s significant growth in the field of batteries, which due to their flexibility and speed in storing and releasing energy are particularly well suited to support grid balance. Other advantages are their small size and the possibility of modular use, that is, with successive expansions as needed. In addition, thanks to technological innovation and the possibility of recycling, they will use fewer and fewer rare materials.
Other storage systems, which are less common at the moment, include compressed air and flywheel batteries, but these require highly sophisticated machinery. Finally, storage using hydrogen as an energy carrier is gaining ground; when the electricity used to produce it comes from renewable sources, it’s called green hydrogen: the only truly sustainable one.
This is the landscape into which the rapidly maturing gravitational energy storage technology is finding its place: a Swiss startup, making advances in tower-type storage technology, built a 5-MW prototype in 2020 and now, near Shanghai, China, it’s building the world’s first commercial-scale model, which will produce 25 MW. The plant, which should be nearing completion in early 2024, stands next to a wind farm, to which it will provide the necessary storage capacity for the electricity generated. The crane is equipped with six booms and will be able to lift 30-ton blocks composed of local waste materials. We’ve come a long way from Newton's apple.