Thermal energy storage

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 260,000 m3 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 300,000 m3 rock caverns 50 m 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 1,100,000 cubic metres (39,000,000 cu ft) 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.

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