Efficient storage of renewable energy is key to ensuring a reliable electricity supply and advancing the decarbonization of the economy and transportation. Emerging solutions such as salt caverns, compressed air energy storage (CAES), sand batteries, and gravity-based systems all aim to make the most of natural resources. Pioneering solutions that seek to combine these two qualities, storage and sustainability, are being deployed around the world.
From Switzerland to China, and from Finland to Chile, here are some examples.
Gravity-based energy storage
The company Energy Vault has devised a gravity-based energy storage system. From the outside, it looks like a set of enormous steel-frame towers or cranes holding large blocks made of composite materials such as concrete. The tower, already operating in both Switzerland and China, uses surplus renewable energy to lift these blocks to a higher position. When a nearby power grid needs to supply homes or businesses, the structure itself lowers the blocks in a controlled descent, using the force of gravity to generate kinetic energy that is then converted into electricity.
The first of these gravity storage facilities was built in 2019 in Arbedo-Castione, Switzerland, home to the company’s research and development center. In this case, six arms hang from a crane and, driven by electric motors powered by the surplus energy, pick up blocks and stack them around it. When the grid needs electricity, those same arms lower the blocks back to the ground, and the resulting kinetic energy generates electricity for the system. The concept is similar to the well-established technology used in pumped-storage hydroelectric plants, which use surplus electricity to pump water to upper reservoirs and then release it to generate power again when needed.
The towers can be modular, so they can be scaled up to store several gigawatt-hours, and are designed to be built with recycled materials. They can also be built alongside renewable energy installations, such as solar panels, to draw power directly from them, or operate as standalone systems that feed the grid when needed.
Salt caverns
Another notable initiative to improve energy storage takes its cue from the past.
A study by German researchers has focused on salt caverns, underground formations created by dissolving salt deposits with water. The result is a cavity with strong walls that provides a large storage space at a constant temperature. Their low permeability reduces the risk of the stored contents coming into contact with outside impurities. The contents can also be drawn off gradually as needed, while the rest remains safely underground. The experts conclude that these caverns are ideal for storing fuels thanks to their operational safety, low cost, and sealing capacity, among other factors.
These age-old caves are an option already being developed in the United States. The federal government has funded projects such as the Advanced Clean Energy Storage (ACES) facility in Utah, which already stores renewable hydrogen and other energy sources. The facility uses two salt caverns to store surplus renewable energy, converting it into hydrogen through electrolysis to supply a power plant that aims to gradually increase its share of this renewable fuel to 100% by 2045.
Compressed air energy storage
Meanwhile, China is working on compressed air energy storage (CAES). In this process, surplus energy is used to compress air and store it underground; when the grid needs electricity, the air is expanded through a turbine to generate it. The city of Changzhou, in the eastern part of the country, is home to one of these facilities and is also part of a project that combines CAES in salt caverns with rooftop solar installations and charging stations, among other elements.
Sand batteries
Among the materials and environments used to store renewable energy, one stands out for its abundance in nature: sand.
Sand batteries consist of containers or silos fitted with heating elements through which electricity flows, reaching high temperatures. The surrounding sand retains the heat, which it can hold for months thanks to its high heat capacity, releasing it when needed for space heating or other industrial processes.
These “batteries”, which can also use crushed soapstone (a rock known for its thermal properties), offer low cost and low environmental impact, since the materials are abundant. The resulting batteries have a service life of more than 30 years, are clean, and require little maintenance.
The shift toward steel silos
One example of this type of battery can be found in Finland, with a twist: the company Polar Night Energy has developed a steel silo, a giant “container” that holds the heat-storing material and works much like other sand batteries.
Through a closed circuit, the energy is stored as heat, and the interior can reach 600°C (1,112°F). When needed, the system releases water, steam, and air at output temperatures of up to 400°C (752°F), with an efficiency of 85% to 90% in harnessing the energy generated.
The company offers two modular models to match demand: 2 MW for smaller industrial facilities (space heating or district heating networks) and 10 MW for industrial processes. Last year, the company commissioned one of these silos to power the district heating system of the Finnish town of Pornainen.
Ultimately, the future of decarbonization depends not only on generating clean energy but also on the capacity and ingenuity to store it. By using natural elements such as salt, sand, and gravity as potential strategic reserves, science could close the sustainability loop, delivering an energy supply that is steady, efficient, and in harmony with the planet.
