Thermal energy storage (TES) is a method to save heat or cold for future use. This technology can store energy for hours, days, or even months. The size of storage systems and how they are used can vary greatly, from small systems for one process to large systems that serve entire towns or regions. Examples of how TES is used include helping balance energy needs between day and night, storing heat collected in summer for winter heating, or storing cold from winter for summer cooling (called seasonal thermal energy storage). Materials used to store heat or cold include water or ice-slush tanks, large amounts of natural earth or bedrock that are heated or cooled through underground pipes, deep underground water sources trapped between layers of rock, shallow pits filled with gravel and water that are insulated, and special materials that change state when heated or cooled.
Other sources of thermal energy for storage include heat or cold created by heat pumps using electricity that is cheaper during off-peak times (a method called peak shaving), heat from power plants that produce both electricity and heat (combined heat and power plants), heat generated by renewable energy sources that create more electricity than needed, and heat wasted during industrial processes. Storing heat, both for short periods and over seasons, helps balance energy systems that rely heavily on renewable electricity and connects electricity and heating systems in energy networks that mostly or fully use renewable energy.
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Thermal energy storage can be divided into three main types: sensible heat, latent heat, and thermo-chemical heat storage. Each type has different strengths and weaknesses that affect how it is used.
Sensible heat storage (SHS) is the simplest method. It involves changing the temperature of a material, either increasing or decreasing it. This method is the most widely used because it is commercially available, while other methods are less developed.
Materials used for SHS are usually inexpensive and safe. One common example is a water tank. Other materials, like molten salts or metals, can store heat at higher temperatures and hold more energy. Energy can also be stored underground (UTES) in tanks or through a heat-transfer fluid (HTF) flowing through pipes placed vertically in U-shapes (boreholes) or horizontally in trenches. Another system is a packed-bed (or pebble-bed) storage unit, where a fluid, often air, flows through a bed of loose material, such as rock, pebbles, or ceramic bricks, to add or remove heat.
A drawback of SHS is that it depends on the properties of the material used. The amount of energy stored is limited by how much heat the material can hold. The system must be carefully designed to maintain a steady temperature when energy is released.
Sensible heat storage typically has low energy density, meaning it requires large spaces for storage tanks. It also loses heat slowly over time, even with proper installation.
A steam accumulator is a type of SHS that uses an insulated steel tank filled with hot water and steam under pressure. It helps balance heat production from a steady or changing source with the changing demand for heat. These accumulators may be important in solar thermal energy projects.
Heat storage tanks are used worldwide, especially in areas with district heating systems and in sunny regions for concentrated solar power. These tanks are used in homes, businesses, and industries for seasonal heating and to balance renewable energy grids. They are an example of SHS with both benefits and limitations.
Water has one of the highest thermal capacities at 4.2 kJ/(kg⋅K). Large hot water storage tanks are widely used in Nordic countries to store heat for several days, helping separate heat and power production and meet peak demand. Some towns use insulated ponds heated by solar power for district heating. Underground storage in caverns has been studied and may be cost-effective, as seen in Finland. For example, Helen Oy estimates a 11.6 GWh capacity and 120 MW thermal output for its 260,000 m³ water cistern under Mustikkamaa, operational from 2021. In 2018, Finland designated 300,000 m³ rock caverns 50 meters below sea level in Kruunuvuorenranta to store heat from seawater in summer and release it in winter. In 2024, Vantaa announced plans for an underground heat storage facility of over 1,100,000 cubic meters (39,000,000 cubic feet) with a 90 GWh capacity, expected to be operational by 2028.
Molten salt is used in SHS to store solar energy at high temperatures, known as molten-salt technology. Molten salts can retain heat collected by concentrated solar power systems, such as solar towers or troughs. The stored heat can later be converted into steam to generate electricity. This method was tested in the Solar Two project (1995–1999). In 2006, it was estimated that this system could retain 99% of stored energy before converting it to electricity. Mixtures of salts, such as sodium nitrate, potassium nitrate, and calcium nitrate, are commonly used. These systems are also used in non-solar applications, like in chemical and metal industries, as heat-transport fluids.
Molten salt melts at 131°C (268°F) and is kept liquid at 288°C (550°F) in a cold storage tank. When heated by solar collectors to 566°C (1,051°F), the salt is stored in a hot tank. With proper insulation, the heat can be stored for up to a week. When electricity is needed, the hot salt is used to produce steam for turbines. A 100-megawatt turbine would require a tank about 9.1 meters (30 feet) tall and 24 meters (79 feet) in diameter to operate for four hours.
A single tank with a divider to separate cold and hot molten salt is being developed. This design is more cost-effective than using two tanks, as molten-salt storage tanks are expensive to build. Phase-change materials (PCMs) are also used in molten-salt storage, and research continues on improving their stability using porous materials.
Most solar thermal power plants use molten-salt storage. The Solana Generating Station in the U.S. can store six hours of generating capacity. In 2013, the Gemasolar plant in Spain produced electricity continuously for 36 days. The Cerro Dominador plant, opened in 2021, stores heat for 17.5 hours.
Solid or molten silicon can store heat at much higher temperatures than salts, leading to greater energy capacity and efficiency. It is being studied as a potential improvement over current methods. Silicon can store over 1 MWh of energy per cubic meter at 1400°C. Silicon is also more abundant than the salts used for similar purposes.
Hot silicon thermal energy storage could store large amounts of heat at very high temperatures (around 1400–2000°C). This could be used to store excess electricity from renewable sources like solar and wind. This system may offer longer energy storage and lower costs compared to other SHS methods.
Another material used for thermal storage is molten aluminum. This technology, developed by the Swedish company Azelio, heats the material to 600°C. When needed, the heat is transferred to a Stirling engine using a heat-transfer fluid.
Molten aluminum is not widely used for energy storage due to challenges, such as its reactivity and the difficulty of handling solidification. However, research continues on how to use aluminum-based storage to integrate renewable energy sources like
Thermal battery
A thermal energy battery is a structure that stores and releases heat energy. This type of battery, also called a TBat, saves energy when it is available and releases it later when needed. The way thermal batteries work involves changes at the atomic level. When energy is added to or removed from a solid or liquid, the temperature of the material changes. Some thermal batteries use phase changes, such as melting or freezing, to store and release more energy because of the energy needed to change from solid to liquid or liquid to gas.
Thermal batteries are common in everyday life. Examples include hot water bottles and traditional cooking stoves made of stone or mud. Early thermal storage systems included rocks heated in fires and kilns, which are types of ovens that hold heat for long periods. In homes, thermal energy storage systems like heat batteries and thermal stores are widely used in the UK.
Thermal batteries are generally divided into four categories, each with different designs and uses. All of them store and retrieve heat energy, but they differ in how they store and release heat. Phase change materials are used in some thermal batteries because they can store and release large amounts of heat energy when they change from solid to liquid or liquid to gas. These materials are chosen based on the temperature needed for specific uses. Examples include salts, waxes, and water. Water, for instance, stores 334 joules of energy per gram when it changes from solid to liquid at 0°C (32°F).
Some systems use water or ice to store heat or cold. Ice can be melted to store heat and then frozen again to provide warmth. This method allows a material to absorb a lot of energy without changing temperature much, making the battery lighter or more efficient. However, phase change materials often have low thermal conductivity, which can slow how quickly heat is stored or released. Recent improvements focus on increasing energy storage capacity and stability by combining these materials with battery structures.
An encapsulated thermal battery is similar to a phase change battery but does not require a phase change. Instead, it stores heat by changing the temperature of a material like water, concrete, or sand. A key feature of these batteries is their volumetric heat capacity, which measures how much heat a material can store per unit of volume. For example, a residential water heater with a storage tank acts as a thermal battery. It is slowly charged over 30–60 minutes and quickly releases heat when needed. Some utilities use these systems to store extra energy from renewable sources, which can save money for homeowners.
In Finland, a district heating system uses sand or stone to store heat. This system, which holds 100 MWh of energy, uses surplus electricity to heat 2,000 tons of soapstone waste. It can provide heat for an entire area for a week. Similar systems in Canada store renewable energy as heat for use in buildings, though they are smaller and can only heat one building at a time. These systems can also collect waste heat from sources like computer servers or compost piles.
A ground heat exchanger (GHEX) is a type of thermal battery that uses the ground to store heat. Pipes are buried underground to transfer heat energy. In summer, heat from buildings is stored in the ground by running warm fluid through the pipes. In winter, cooler fluid is used to extract stored heat to warm buildings. This process repeats annually, making the system renewable. GHEX systems are tested using methods like Log-Time Curve Fit and Advanced Thermal Response Testing to measure their heat storage abilities.
An example of a GHEX system is shown in an ASHRAE study, which tracks how ground temperatures change seasonally. During winter, heat is taken from the ground to warm buildings, and during summer, heat is stored in the ground. This cycle creates a predictable pattern of heat storage and retrieval, similar to a battery charging and discharging over time.
Electric thermal storage
Storage heaters are commonly used in European homes where electricity is cheaper at night. These heaters use thick ceramic bricks or feolite blocks that are heated to high temperatures using electricity. They may or may not have good insulation and controls to slowly release heat over several hours. It is advised not to use them in homes with young children or places where there is a higher risk of fire because of poor housekeeping, due to the high temperatures involved.
As wind and solar power (and other renewable energy sources) provide a growing share of electricity, companies are exploring large-scale electric energy storage. Using extra renewable energy, heat is converted into high-temperature heat stored in well-insulated systems for later use. A new technology involves vacuum super insulated (VSI) heat storage systems. Using electricity to create heat, rather than direct heat from solar thermal collectors, allows very high temperatures to be achieved. This could enable storing heat in the summer from extra solar power generated during the day to use in the winter with very little heat loss.
Solar energy storage
Solar energy uses thermal energy storage. Many practical systems store energy for a few hours to a full day. Some places now use seasonal thermal energy storage (STES), which stores solar energy in summer to provide heat during winter. In 2017, Drake Landing Solar Community in Alberta, Canada, used solar energy to provide heating 97% of the time throughout the year, a world record made possible by using STES.
Both latent heat (heat stored during changes in material state, like melting) and sensible heat (heat stored as temperature changes) can be used with high-temperature solar thermal systems. Special metal mixtures, such as aluminum and silicon (AlSi12), have high melting points that work well for creating steam. Materials based on high alumina cement also store heat effectively.
Pumped-heat electricity storage
In pumped-heat electricity storage (PHES), a reversible heat-pump system stores energy as a temperature difference between two heat stores.
Isentropic systems use two insulated containers filled with materials like crushed rock or gravel. One container, called the hot vessel, stores thermal energy at high temperature and pressure. The other, called the cold vessel, stores thermal energy at low temperature and pressure. These containers are connected by pipes at the top and bottom, and the system is filled with an inert gas, such as argon.
During the charging process, off-peak electricity powers the system as a heat pump. For example, argon gas from the top of the cold vessel is compressed without heat loss to a pressure of about 12 bar, raising its temperature to approximately 500 °C (900 °F). The compressed gas moves into the hot vessel, where it flows through the gravel, transferring heat to the rock and cooling to ambient temperature. The cooled, pressurized gas exits the bottom of the hot vessel and is expanded without heat loss to 1 bar, lowering its temperature to −150 °C. This cold gas then flows upward through the cold vessel, cooling the rock while warming to its original condition.
To recover energy as electricity, the process is reversed. Hot gas from the hot vessel is expanded to power a generator and then sent to the cold vessel. Cooled gas from the bottom of the cold vessel is compressed, heating it to ambient temperature. This gas is then transferred to the bottom of the hot vessel to be reheated.
The compression and expansion steps are performed by a specially designed machine with sliding valves. Extra heat from inefficiencies is released into the environment through heat exchangers during the discharging cycle.
The developer reported that the system can achieve a round-trip efficiency of 72–80%. This is slightly lower than the efficiency of pumped hydro energy storage, which can reach over 80%.
Another proposed system uses turbomachinery and can operate at much higher power levels. Using phase change materials as heat storage could improve the system’s performance.