As solar panels and wind turbines generate more electricity, one challenge is becoming increasingly difficult to ignore. What happens when renewable energy is available but nobody needs it right then? Instead of letting surplus power go unused, a growing class of thermal energy storage systems is exploring an unusually simple solution—heating sand.Known as sand batteries, these systems convert electricity into heat and store it inside large, heavily insulated containers filled with sand or similar solid materials. The heat can then be released hours, days, or potentially months later, providing energy for district heating networks and industrial processes. Unlike conventional batteries, however, they are primarily designed to store and deliver heat rather than electricity.Turning surplus electricity into stored heatThe principle behind a sand battery is relatively straightforward. When electricity is abundant or inexpensive, power from the grid or renewable sources such as wind and solar is used to operate electric heating elements. These elements heat air or another heat-transfer medium, which circulates through pipes or channels inside a sand-filled container.As the hot air moves through the storage material, it transfers thermal energy to the surrounding sand. Some systems can heat the material to around 600 degrees Celsius (1,112 degrees Fahrenheit), depending on their design.The container is surrounded by thick insulation that slows the escape of heat. Sand is particularly useful because it is inexpensive, widely available, chemically stable at high temperatures, and can hold substantial amounts of thermal energy. It also avoids the need for the lithium, cobalt, and other materials associated with certain electrochemical batteries.The amount of energy stored depends on the mass of the material and how much its temperature increases. In simple terms, a larger quantity of sand heated to a higher temperature can hold more energy.How does the system release energy?When heat is required, the process works in reverse. Cooler air is circulated through the hot storage material, absorbing its thermal energy before passing through a heat exchanger.The recovered heat can be transferred to water for district heating networks, used to produce steam for industrial equipment, or supplied as hot air for manufacturing and drying processes. The sand remains inside the insulated container, allowing the system to be recharged when electricity becomes available.This makes sand batteries particularly interesting for industries that require large, steady supplies of heat. Food processing, textiles, chemicals, paper production, and other manufacturing activities often rely on fossil fuels to generate process heat. A thermal storage system can help replace some of that demand with electricity generated from renewable sources.It also addresses a mismatch in renewable energy production. Wind and solar output can fluctuate throughout the day, while factories and heating networks need reliable energy supplies. A sand battery can absorb electricity when supply is high and release heat when demand rises.From Finland to larger-scale energy storageOne of the best-known examples comes from Finnish company Polar Night Energy, which developed a commercial sand battery for the energy utility Vatajankoski in Kankaanpää, Finland. The installation began operating in 2022 and uses about 100 tonnes (approximately 110 US tons) of sand inside a steel container about 7 meters tall (approximately 23 feet tall). It has a storage capacity of 8 megawatt-hours (MWh) and supplies heat to a local district heating network.The company has since developed a much larger installation in Pornainen, Finland. Rather than ordinary sand, the system uses approximately 2,000 tonnes (approximately 2,200 US tons) of crushed soapstone, a material that can also store thermal energy. With a capacity of 100 MWh, it is designed to supply heat to the town’s district heating network and reduce its reliance on fossil fuels and wood chips.These projects show how solid-material thermal storage can be scaled to municipal and industrial heating. Researchers are also exploring systems that can use stored heat to generate electricity. The US National Renewable Energy Laboratory’s ENDURING concept, for example, uses electrically heated silica particles stored in insulated silos. When electricity is needed, the hot particles transfer heat to a working gas that drives machinery connected to a generator.What are the limitations?Sand batteries are not a direct replacement for lithium-ion battery storage. Their main advantage is the ability to store large quantities of thermal energy using relatively inexpensive materials. But converting that stored heat back into electricity requires additional equipment, such as heat exchangers, turbines, or other power-generation systems, and introduces energy losses.There are also engineering challenges. Sand has relatively low thermal conductivity, meaning heat does not move through it particularly quickly. Systems must be designed to distribute heat effectively, maintain useful output temperatures, and limit losses through insulation. Their economics also depend on access to inexpensive electricity and a nearby demand for heat.The strongest case for sand batteries is not powering phones, cars, or homes directly with electricity. It is storing renewable energy in a form that heating networks and factories can use. As more wind and solar power enters electricity grids, turning occasional surpluses into dependable heat could become an important part of making renewable energy work around the clock.