A thermal battery is a device that stores energy as heat, then releases it later for heating, industrial processes, or electricity generation. Unlike lithium-ion batteries that store energy in chemical reactions between metals, thermal batteries work with something far simpler: they heat up a material (bricks, sand, molten salt, or other substances) and hold that heat until it’s needed. The concept is as old as heating a stone by a fire and using it to warm your bed, but modern versions operate at temperatures exceeding 3,000°F and can power factories.
Thermal batteries have gained attention as a way to bridge the gap between cheap, intermittent renewable electricity and the enormous heat demands of industry and buildings. When the sun is shining or the wind is blowing, excess electricity heats up the storage material. Hours or even days later, that stored heat can warm a building, generate steam for a factory, or be converted back into electricity.
How Thermal Batteries Work
At their simplest, thermal batteries use electricity to generate heat through resistance heating, the same principle behind a toaster or space heater. That heat transfers into a storage medium, materials chosen specifically for their ability to absorb and retain large amounts of thermal energy. When the stored heat is needed, air or another fluid passes over or through the hot material, carrying the heat to where it’s useful.
What happens next depends on the application. In industrial settings, the hot air can generate steam directly or heat equipment for manufacturing. Some systems use thermophotovoltaic panels, which convert radiated heat (essentially infrared light from the glowing hot material) back into electricity. Others run the heat through a traditional steam turbine. The company Malta, which spun out of Google X in 2018, stores energy in molten salt and regenerates grid-scale electricity from it. Antora Energy uses carbon-based blocks that both generate and store heat, then converts it back to electricity with thermophotovoltaic cells.
Three Types of Heat Storage
Not all thermal batteries store heat the same way. The differences come down to physics, and each approach suits different temperatures and uses.
Sensible Heat Storage
This is the most straightforward approach. You add heat to a material and its temperature rises. When you need the energy back, you extract heat and the material cools down. Water is the most common choice for low-temperature systems (below 100°C), which is why hot water tanks are technically a form of thermal battery. For higher temperatures, molten salts are widely used, particularly in concentrating solar power plants. Sand works well too, thanks to its abundance and thermal stability. Water has a specific heat capacity of 4.18 kJ per kilogram per degree Celsius, meaning it absorbs a lot of energy per unit of weight. Molten salt (around 1.56 kJ/kg·K) holds less per degree of temperature change but can operate at far higher temperatures without boiling off.
Latent Heat Storage
Instead of simply heating a material, latent heat systems store energy by melting it. During a phase change from solid to liquid, a material absorbs a large amount of energy without its temperature rising at all. Think of how ice absorbs heat as it melts but stays at 0°C until it’s fully liquid. The same principle works at much higher temperatures with engineered materials like paraffin waxes, salt hydrates, and metal alloys. Paraffin (n-heptadecane) stores about 244 kJ/kg during its phase change, while magnesium chloride hexahydrate stores around 169 kJ/kg. For solar power plants using parabolic troughs, molten salt phase-change materials are designed to melt and solidify in the 300°C to 500°C range.
Thermochemical Storage
The least mature but most energy-dense option uses reversible chemical reactions. Energy drives an endothermic (heat-absorbing) reaction in one direction, effectively locking the energy into the chemical bonds of the products. When you need the heat back, you allow the reaction to reverse, releasing energy as it proceeds in its exothermic (heat-releasing) direction. Iron hydroxide, for example, can store about 722 kJ/kg through thermochemical reactions, roughly three times the energy density of paraffin’s latent heat storage. The challenge is engineering these reactions to be reliably reversible over thousands of cycles.
Where Thermal Batteries Are Used
The biggest opportunity is in heavy industry. Manufacturing cement, steel, glass, chemicals, food and beverage products, and pulp and paper all require enormous amounts of heat, often at very high temperatures. Most of that heat currently comes from burning fossil fuels. Thermal batteries offer a path to replace that combustion with stored renewable electricity.
Electrified Thermal Solutions, an MIT spinoff, has developed conductive ceramic firebricks (a material used as insulation for centuries) that can discharge heat up to 3,272°F. That’s hot enough for virtually any industrial process, including steelmaking and cement production. The company is working with hundreds of industrial manufacturers across multiple sectors. The core idea is elegant: firebricks act as both the heating element and the storage medium, so running an electrical current through them generates and stores heat simultaneously, with no separate components needed.
On the residential side, thermal batteries are already available as compact alternatives to traditional hot water tanks. Sunamp, a UK-based company, makes heat batteries using phase-change materials that store 12 kWh of thermal energy, enough to supply a family’s hot water for an entire day, covering multiple showers and baths. Smaller units starting at 3 kWh are available for households with lower demand. These take up significantly less space than a conventional hot water cylinder because phase-change materials store more energy per liter than water alone.
Efficiency and Cost
Efficiency depends heavily on what you’re doing with the stored heat. If you store heat and use it directly as heat (for industrial processes or building heating), round-trip efficiency can exceed 90%, since you avoid the losses that come with converting heat back to electricity. When converting stored heat back into electricity, efficiencies drop. Current systems range from about 54% to 78% round-trip efficiency depending on the technology, with supercritical CO₂ systems reaching the higher end. Solar-to-electric conversion, which includes the initial step of capturing solar energy, sits around 32%.
The cost picture favors thermal batteries in many scenarios, particularly for long-duration storage. Analysis from Lawrence Berkeley National Laboratory found that the levelized cost of storage for thermal systems can be lower than lithium-ion batteries in building applications. The advantage grows with storage duration: lithium-ion batteries are cost-effective for a few hours of storage, but thermal systems scale more cheaply when you need to store energy for 8, 12, or even 24 hours. The storage medium itself (bricks, sand, salt) costs a fraction of what lithium and cobalt cost per unit of energy stored. One analysis of a solar-powered thermal storage system found a levelized cost of about $0.14 per kWh with a payback period of roughly 2.4 years.
Durability and Lifespan
Thermal batteries have a major structural advantage over chemical batteries: their storage materials don’t degrade the way electrode materials do. Bricks, sand, and molten salt don’t lose capacity over charge-discharge cycles in the same way lithium-ion cells do. There’s no equivalent of the slow chemical breakdown that limits a lithium-ion cell to a few thousand cycles before its capacity fades noticeably. Ceramic firebricks, the material used in some of the most advanced systems, have been used in furnaces and kilns for centuries precisely because they withstand repeated heating and cooling.
The components that do wear out tend to be the mechanical parts: pumps, valves, blowers, and heat exchangers. These are well-understood industrial components with established maintenance schedules and replacement procedures. The result is that thermal battery systems are generally designed for 20 to 30 years of operation, with the storage medium itself lasting the full life of the system.
Thermal Batteries vs. Lithium-Ion Batteries
- Best use case: Thermal batteries excel at storing large amounts of energy for long durations and delivering heat. Lithium-ion batteries are better for delivering electricity on demand, especially for short-duration needs like grid frequency regulation or powering electronics.
- Cost at scale: Thermal storage materials (bricks, salt, sand) are dramatically cheaper per kilowatt-hour than lithium-ion chemistry, making them more economical for large-scale, long-duration storage.
- Efficiency trade-off: If you need heat, thermal batteries are more efficient because there’s no conversion loss. If you need electricity, lithium-ion batteries win, since they avoid the heat-to-electricity conversion penalty.
- Physical size: Thermal batteries for industrial use are large installations. Residential thermal batteries using phase-change materials can be quite compact, but lithium-ion batteries generally offer higher energy density for electrical storage.
- Lifespan: Thermal storage media don’t degrade chemically, giving them a longevity advantage over lithium-ion cells, which typically lose noticeable capacity after several thousand charge cycles.
The two technologies aren’t really competitors so much as complements. Lithium-ion handles the electrical storage and rapid-response needs of a grid. Thermal batteries address the massive, slower-moving demand for heat in buildings and industry, a demand that accounts for roughly half of all global energy consumption.