Table of Contents
Introduction
Thermal energy storage, often shortened to TES, is the practice of storing energy as heat or cold so that it can be used at a different time from when it was produced. Rather than storing electricity directly, TES focuses on the thermal form of energy. This is especially valuable in energy systems that use large amounts of heating and cooling, and in systems where renewable electricity can be converted to heat when it is abundant and inexpensive.
TES provides flexibility by shifting when heat or cold is produced compared with when it is needed. This helps match variable renewable generation with relatively predictable thermal demands in buildings, industry, and district heating or cooling networks.
Basic Principles Of Thermal Energy Storage
Thermal energy storage relies on materials that can absorb, hold, and release heat. When a material is heated, it stores energy by increasing its temperature, changing its phase, or undergoing a reversible chemical reaction. When the stored heat is needed, the process is reversed and the material cools or changes back, releasing energy.
The amount of thermal energy stored depends on three main factors. The first is how much material is used. The second is the temperature difference between the storage material and its initial state or reference temperature. The third is the thermal properties of the storage material, for example its specific heat capacity or its latent heat.
For sensible heat storage, where only temperature changes and no phase change occurs, the stored energy can be approximated by:
$$Q = m \, c \, \Delta T$$
where $Q$ is the amount of heat stored, $m$ is the mass of the storage material, $c$ is the specific heat capacity, and $\Delta T$ is the temperature change.
For latent heat storage, where the material changes phase, such as from solid to liquid, the stored energy can be approximated by:
$$Q = m \, L$$
where $L$ is the latent heat of the phase change.
Key thermal storage relations:
- Sensible heat storage:
$$Q = m \, c \, \Delta T$$ - Latent heat storage:
$$Q = m \, L$$
Higher mass, larger temperature differences, and materials with high $c$ or high $L$ allow more energy to be stored in a given system.
These simple relations show why materials like water, which has a relatively high specific heat capacity, and phase change materials, which have significant latent heat, are widely used in TES.
Main Types Of Thermal Energy Storage
There are three main categories of thermal energy storage: sensible heat storage, latent heat storage, and thermochemical storage. Each uses a different physical process and is suitable for different temperature levels and applications.
In sensible heat storage, energy is stored by raising or lowering the temperature of a material without changing its phase. Water tanks used for hot water or heating systems are a very common example. Other materials include rocks, concrete, or molten salts at higher temperatures. Sensible heat systems are generally simple, robust, and relatively inexpensive, but they require large volumes to store significant energy because they rely only on temperature changes.
Latent heat storage uses phase change materials. These are substances that absorb or release large amounts of heat as they change phase, usually from solid to liquid or liquid to solid, at a nearly constant temperature. Because the latent heat is much higher than the heat stored only by temperature variation, latent systems can achieve higher energy density than sensible systems. They are especially useful when a stable temperature is needed, for example for keeping a building within a narrow comfort range.
Thermochemical storage is based on reversible chemical reactions. Energy is stored when a reaction absorbs heat and drives the reaction in one direction. When heat is needed, the reaction is reversed and releases heat. Thermochemical systems can reach very high energy densities and can store energy for long periods with low losses, but they are more complex and are still less common in commercial use.
Typical Storage Media And Temperature Ranges
Different storage media are chosen according to the temperature level required and the specific application. Low temperature TES, which typically deals with temperatures close to room temperature up to roughly $100^\circ\text{C}$, often uses water as the storage medium. Domestic hot water tanks, buffer tanks for space heating, and chilled water for cooling are all examples. Water is attractive because it is cheap, available, and has a high specific heat capacity.
For medium temperature storage, up to a few hundred degrees Celsius, solids such as concrete, stones, or packed beds of rocks can be used. Air or oil may be used as heat transfer fluids to move heat into and out of these storage masses. Such systems are sometimes used in industrial processes or in solar thermal plants for process heat.
For high temperature storage, molten salts are common. In concentrated solar power plants, mixtures of nitrates are heated to several hundred degrees Celsius and stored in insulated tanks. The hot salt is later passed through a heat exchanger to generate steam for electricity production. Other high temperature solids, ceramics, and specialized salts or metals are also being explored.
Latent heat storage materials include paraffin waxes, salt hydrates, and specially designed phase change materials with melting points tailored for specific uses, such as 20 to 30 degrees Celsius for building cooling or around 50 to 70 degrees Celsius for domestic hot water. For thermochemical storage, materials such as certain metal oxides, salt hydrates, and sorbents are under development.
Charging And Discharging Processes
Charging a thermal storage system means putting heat in or taking heat out of a system in order to store cold. For hot storage, charging usually involves using a heat source to raise the temperature of the storage material or to drive a phase or chemical change. In a renewable system, this heat source may be solar thermal collectors, excess electricity converted to heat through resistive heaters or heat pumps, or waste heat from industrial processes.
Discharging is the opposite process. Heat flows from the storage medium to the demand side, such as heating water for a building, providing process heat in a factory, or running a steam turbine. For cold storage, discharging means absorbing heat from the building or industrial process to provide cooling.
Heat transfer into and out of storage is managed with heat exchangers, pipes, pumps, and in some designs, air channels or embedded tubes. The rate of charging and discharging depends on the temperature differences, the thermal conductivity of the materials, the flow rate of the heat transfer fluid, and the design of the storage system.
Heat losses are an important concern in TES. Insulation is used to reduce heat exchange with the environment so that stored energy can be kept over time. Short term storage such as daily storage for heating or cooling can tolerate some losses, while seasonal storage, which may hold energy over months, must be carefully designed to minimize gradual temperature changes.
Applications In Buildings And District Systems
In buildings, TES is widely used in simple forms such as hot water tanks. These tanks allow heat produced by boilers, solar collectors, or heat pumps to be stored and used when occupants need hot water, even if production and demand do not match in time.
Thermal storage is also used to balance space heating and cooling. In some systems, water tanks or phase change materials integrated into walls or ceilings store heat during off peak hours and release it when the building cools. For cooling, chilled water tanks and ice storage are common in large commercial buildings. A cooling system may produce cold at night when electricity is cheaper or when outside temperatures are lower. The cold is stored as chilled water or ice and then used during the day to provide air conditioning.
At the neighborhood or city level, TES can be integrated into district heating and cooling networks. Large hot water tanks, often placed near combined heat and power plants or large heat pumps, can store thermal energy for entire districts. Seasonal storage, such as borehole or aquifer thermal energy storage, can collect heat in summer, often from solar collectors or waste heat, and deliver it in winter for building heating. Similarly, cold can be stored from winter for use in summer cooling.
Role In Integrating Renewables And Managing Demand
Thermal energy storage is a powerful tool for increasing the share of renewable energy in heating, cooling, and electricity systems. When electricity from solar or wind is abundant and prices are low, electric heaters or heat pumps can convert that electricity into heat stored in water tanks, hot rocks, or molten salts. Later, when renewable electricity output is lower or demand increases, the stored heat can replace or reduce the need for fossil fuels.
In electricity systems, TES can help shift loads and flatten peaks by moving heating and cooling demands away from high price periods. For example, a building can preheat or precool its interior using cheap electricity and then coast for several hours with reduced active heating or cooling because the structure and any dedicated TES act as a buffer.
In industrial processes, TES can enable more flexible operation of heat intensive processes, even when using renewable electricity that fluctuates. Waste heat from industry can also be stored instead of being released to the environment, then used within the plant or supplied to nearby buildings.
In concentrated solar power plants, TES allows the plant to continue generating electricity after sunset by using stored high temperature heat to drive turbines. This helps provide more predictable and controllable renewable power.
Advantages And Limitations Of Thermal Storage
Thermal energy storage has several advantages. It often uses relatively simple and inexpensive materials such as water, rocks, or salts. Many TES technologies are mature and proven. The systems can be reliable, with long lifetimes and low maintenance requirements, especially for sensible heat storage. TES is particularly effective for decarbonizing heating and cooling, which represent a large share of global energy consumption.
TES also reduces the need to oversize electrical energy storage. Instead of storing all renewable electricity directly in batteries, part of the energy can be stored as heat or cold in thermal systems, which can be more cost effective in some applications.
However, TES also has limitations. Thermal energy is not as easily transported or converted as electricity. While heat can be used directly for heating and some industrial processes, converting heat back into electricity, for example through steam turbines, introduces efficiency losses. Long distance transport of heat requires district heating networks or insulated pipelines, which are not always available.
Another limitation lies in energy density, especially for sensible heat storage. Storing large amounts of energy often requires large volumes, which may not be convenient or feasible where space is limited. Heat losses over time are also a challenge, particularly for long term or seasonal storage. For very high temperature or thermochemical storage, technical complexity, material stability, and cost are ongoing issues.
Design Considerations And Safety Aspects
When designing a thermal energy storage system, several factors must be considered. The required storage capacity should match the typical demand patterns. Designers must choose appropriate storage temperatures and materials to achieve sufficient energy storage while avoiding safety or material issues. The size and insulation level of tanks or storage volumes must be balanced between cost and performance.
Integration with energy supply systems and loads is also critical. For example, a TES system in a building must work well with boilers, heat pumps, chillers, and distribution systems such as radiators or air handling units. Control strategies determine when to charge or discharge the storage, and these should reflect energy prices, renewable availability, and user comfort.
Safety aspects include structural stability of tanks, pressure management in closed systems, and thermal expansion of fluids. High temperature TES, especially with molten salts or oils, must prevent leaks, overheating, or chemical reactions that could be hazardous. For underground storage in aquifers or boreholes, hydrogeological impacts and risks of contamination must be evaluated.
Future Developments In Thermal Energy Storage
Research and innovation in TES aim to increase energy density, reduce costs, and improve integration with renewable systems. New phase change materials are being developed with tailored melting points, improved stability, and better thermal conductivity. Thermochemical storage technologies are being explored for long duration storage in combination with industrial processes and district heating.
There is growing interest in combining TES with electric heat pumps, smart controls, and digital optimization. For example, smart buildings can adjust their thermal storage strategies based on forecasts of renewable generation and electricity prices. In urban planning, larger seasonal storage projects are being considered to support climate neutral heating and cooling.
As the role of heat and cold in the energy transition becomes more prominent, thermal energy storage is likely to become an increasingly important element of flexible and sustainable energy systems.