Thermal energy storage (TES) is the storage of thermal energy for later reuse. Employing widely different technologies, it allows thermal energy to be stored for hours, days, or months. Scale, both of storage and use, vary from small to large – from individual processes to district, town, or region as part of a thermal energy network. Usage examples are the balancing of energy demand between daytime and nighttime, storing summer heat for winter heating, or winter cold for summer cooling (seasonal thermal energy storage). Storage media include water or ice-slush tanks, masses of native earth or bedrock accessed with heat exchangers by means of boreholes, deep aquifers contained between impermeable strata; shallow, lined pits filled with gravel and water and insulated at the top, as well as eutectic solutions and phase-change materials.
Other sources of thermal energy for storage include heat or cold produced with heat pumps from off-peak, lower cost electric power, a practice called peak shaving; heat from combined heat and power (CHP) power plants; heat produced by renewable electrical energy that exceeds grid demand and waste heat from industrial processes. Heat storage, both seasonal and short term, is considered an important means for cheaply balancing high shares of variable renewable electricity production and integration of electricity and heating sectors in energy systems almost or completely fed by renewable energy.
Categories The kinds of thermal energy storage can be divided into three separate categories: sensible heat, latent heat, and thermo-chemical heat storage. Each of these has different advantages and disadvantages that determine their applications.
Sensible heat storage Sensible heat storage (SHS) is the most straightforward method. It simply means the temperature of some medium is either increased or decreased. This type of storage is the most commercially available out of the three; other techniques are less developed.
The materials are generally inexpensive and safe. One of the cheapest, most commonly used options is a water tank, but materials such as molten salts or metals can be heated to higher temperatures and therefore offer a higher storage capacity. Energy can also be stored underground (UTES), either in an underground tank or in some kind of heat-transfer fluid (HTF) flowing through a system of pipes, either placed vertically in U-shapes (boreholes) or horizontally in trenches. Yet another system is known as a packed-bed (or pebble-bed) storage unit, in which some fluid, usually air, flows through a bed of loosely packed material (usually rock, pebbles or ceramic brick) to add or extract heat.
A disadvantage of SHS is its dependence on the properties of the storage medium. Storage capacities are limited by the specific heat capacity of the storage material, and the system needs to be properly designed to ensure energy extraction at a constant temperature.
Sensible heat storages normally have a low energy density, which means that they require large volumes and space for storage tanks and a slow loss of thermal energy over time even with the installations alongside the sensible heat storage.
Heat storage in tanks, ponds or rock caverns
A steam accumulator consists of an insulated steel pressure tank containing hot water and steam under pressure. As a heat storage device, it is used to mediate heat production by a variable or steady source from a variable demand for heat. Steam accumulators may take on a significance for energy storage in solar thermal energy projects.
Heat storage tanks are being used globally, primarily in regions with established district heating networks and in sunny areas for a use of concentrated solar power. These tanks serve in residential, commercial, and industrial purposes, ranging from seasonal heating to balancing renewable energy grids. This is an example of a sensible heat storage device that has both its benefits and disadvantages.
Water has one of the highest thermal capacities at 4.2 kJ/(kg⋅K). Large stores, mostly hot water storage tanks, are widely used in Nordic countries to store heat for several days, to decouple heat and power production and to help meet peak demands. Some towns use insulated ponds heated by solar power as a heat source for district heating pumps. Intersessional storage in caverns has been investigated and appears to be economical and plays a significant role in heating in Finland. Energy producer Helen Oy estimates an 11.6 GWh capacity and 120 MW thermal output for its water cistern under Mustikkamaa (fully charged or discharged in 4 days at capacity), operating from 2021 to offset days of peak production/demand; while the rock caverns under sea level in Kruunuvuorenranta (near Laajasalo) were designated in 2018 to store heat in summer from warm seawater and release it in winter for district heating. In 2024, it was announced that the municipal energy supplier of Vantaa had commissioned an underground heat storage facility of over in size and 90 GWh in capacity to be built, expected to be operational in 2028.
Molten salt technology
The sensible heat of molten salt is also used for storing solar energy at a high temperature, termed molten-salt technology or molten salt energy storage (MSES). Molten salts can be employed as a thermal energy storage method to retain thermal energy. Presently, this is a commercially used technology to store the heat collected by concentrated solar power (e.g., from a solar power tower or solar trough). The heat can later be converted into superheated steam to power conventional steam turbines and generate electricity at a later time. It was demonstrated in the Solar Two project from 1995 to 1999. Estimates in 2006 predicted an annual efficiency of 99%, a reference to the energy retained by storing heat before turning it into electricity, versus converting heat directly into electricity. Various eutectic mixtures of different salts are used (e.g., sodium nitrate, potassium nitrate and calcium nitrate). Experience with such systems exists in non-solar applications in the chemical and metals industries as a heat-transport fluid.
The salt melts at . It is kept liquid at in an insulated "cold" storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats it to . It is then sent to a hot storage tank. With proper insulation of the tank the thermal energy can be usefully stored for up to a week. When electricity is needed, the hot molten salt is pumped to a conventional steam-generator to produce superheated steam for driving a conventional turbine/generator set as used in a coal, oil, or nuclear power plant. A 100-megawatt turbine would need a tank of about tall and in diameter to drive it for four hours by this design.A single tank with a divider plate to separate cold and hot molten salt is under development. It is more economical by achieving 100% more heat storage per unit volume over the dual tanks system as the molten-salt storage tank is costly due to its complicated construction. Phase-change materials (PCMs) are also used in molten-salt energy storage, while research on obtaining shape-stabilized PCMs using high porosity matrices is ongoing.
Most solar thermal power plants use this thermal energy storage concept. The Solana Generating Station in the U.S. can store 6 hours worth of generating capacity in molten salt. During the summer of 2013 the Gemasolar Thermosolar solar power-tower/molten-salt plant in Spain achieved a first by continuously producing electricity 24 hours per day for 36 days. The Cerro Dominador Solar Thermal Plant, inaugurated in June 2021, has 17.5 hours of heat storage.
Silicon
Solid or molten silicon offers much higher storage temperatures than salts with consequent greater capacity and efficiency. It is being researched as a possible more energy efficient storage technology. Silicon is able to store more than 1 MWh of energy per cubic meter at 1400 °C. An additional advantage is the relative abundance of silicon when compared to the salts used for the same purpose.
Hot silicon thermal energy storing technology would be able to store significant thermal energy at extremely high temperatures (around 1400-2000 °C). This would be utilized by using the white hot molten silicon to store excess electricity generated from surrounding renewable sources like solar energy and wind power. This system would enable efficient, lower costing, and a longer duration of energy storage compared to other sensible heat storage options.
Aluminum
Another medium that can store thermal energy is molten (recycled) aluminum. This technology was developed by the Swedish company Azelio. The material is heated to 600 °C. When needed, the energy is transported to a Stirling engine using a heat-transfer fluid.
Molten aluminum is not widely used for energy storage due to some disadvantages that have yet to be overcome. This includes molten aluminum's reactivity and the challenges that come along with handling the solidification of the aluminum. However, research is being continued on how aluminum thermal storage could be used due to its high energy density. These aluminum based storage technologies have the potential to grow and to integrate renewable energy sources like solar and wind into the grid. These energy storage technologies are promising candidates for long term storage with minimal loss.
Rock, sand, and concrete
Rock, sand and concrete has a heat capacity about one third of water's. On the other hand, concrete can be heated to much higher temperatures (1200 °C) by, for example, electrical heating and therefore has a much higher overall volumetric capacity. Thus in the example below, an insulated cube of about would appear to provide sufficient storage for a single house to meet 50% of heating demand. This could, in principle, be used to store surplus wind or solar heat due to the ability of electrical heating to reach high temperatures. At the neighborhood level, the Wiggenhausen-Süd solar development at Friedrichshafen in southern Germany has received international attention. This features a () reinforced concrete thermal store linked to () of solar collectors, which will supply the 570 houses with around 50% of their heating and hot water. Siemens-Gamesa built a 130 MWh thermal storage near Hamburg with 750 °C in basalt and 1.5 MW electric output. A similar system was scheduled for Sorø, Denmark, with 41–58% of the stored 18 MWh heat returned for the town's district heating, and 30–41% returned as electricity, but not retained.
Disadvantages to the use of hot rocks and concrete involve both of their heat capacity compared to water (about one third) meaning that much larger volumes of solid materials are required to store the same amount of energy, making it less suitable for storage areas with limited amount of space. Their challenges also relate to their implementations and maintenance. Over time with these materials heat loss is inevitable, the degradation of the materials, and their high initial costs all bring up contemplation of use. Relating to the integration difficulties, they require a challenging design to connect the storage and heat source to a distribution system.
Announced in 2022, "brick toaster" is an innovative heat reservoir operating at up to 1,500 °C (2,732 °F).
Polar Night Energy installed a thermal battery in Finland that stores heat in a mass of sand. It was expected to reduce carbon emissions from the local heating network by as much as 70%. It is about 42 ft (13 m) tall and 50 ft (15 m) wide. It can store 100 MWh, with a round trip efficiency of 90%. Temperatures reach 1,112 °F (600 °C). The heat transfer medium is air, which can reach temperatures of 752 °F (400 °C) – can produce steam for industrial processes, or it can supply district heating using a heat exchanger.