Thermal energy storage: Technologies, systems and applications

Overview

Thermal energy storage (TES) is the systematic storage of thermal energy for later reuse, employing a wide variety of technologies to bridge the temporal gap between energy production and consumption. This capability allows thermal energy to be retained for durations ranging from hours and days to entire months, depending on the specific application and media used. The scale of TES implementation varies significantly, extending from small-scale individual processes to large-scale district, town, or regional systems integrated into thermal energy networks. By decoupling the timing of heat or cold generation from its actual use, TES plays a critical role in balancing energy demand across different timeframes. Common usage examples include balancing energy demand between daytime and nighttime peaks, storing summer heat for winter heating, or capturing winter cold for summer cooling.

Storage Media and Technologies

The effectiveness of thermal energy storage depends heavily on the chosen storage media, which can be broadly categorized into sensible, latent, and thermochemical storage, though the specific implementation varies. Common storage media include water or ice-slush tanks, which are widely used for their high specific heat capacity and cost-effectiveness. Other methods involve using masses of native earth or bedrock, which are accessed with heat exchangers by means of boreholes to utilize the ground's thermal inertia. Deep aquifers contained between impermeable strata also serve as effective natural storage reservoirs. Additionally, shallow, lined pits filled with gravel and water, which are insulated at the top, provide another option for seasonal storage. More advanced approaches utilize eutectic solutions and phase-change materials (PCMs), which can store significant amounts of energy during the phase transition process. These diverse technologies allow for flexible integration into various energy systems, from small residential units to large industrial complexes, enhancing overall energy efficiency and grid stability.

What are the main types of thermal energy storage?

Thermal energy storage (TES) systems are generally classified into three primary categories based on the physical or chemical mechanism used to store energy: sensible heat storage, latent heat storage, and thermo-chemical heat storage. Each category employs distinct storage media and principles, offering different advantages regarding storage duration, density, and complexity.

Sensible Heat Storage

Sensible heat storage is the most common form of TES. It involves heating or cooling a storage medium, such as water, rock, or molten salt, without changing its phase. The energy stored is proportional to the mass of the medium, its specific heat capacity, and the temperature difference. Water tanks and bedrock masses accessed via boreholes are typical examples. This method is valued for its simplicity and low cost, making it suitable for district heating networks and large-scale seasonal storage.

Latent Heat Storage

Latent heat storage utilizes the energy absorbed or released during a phase change of a material, such as melting or solidification. Phase-change materials (PCMs), including ice-slush tanks and eutectic solutions, are used to store significant amounts of energy at nearly constant temperatures. This technology offers higher energy density compared to sensible storage, allowing for more compact systems. It is particularly effective for balancing energy demand between daytime and nighttime or storing winter cold for summer cooling.

Thermo-Chemical Heat Storage

Thermo-chemical storage involves reversible chemical reactions or physical absorption processes to store thermal energy. This method offers the highest energy density and minimal heat loss over long periods, making it ideal for seasonal storage. However, it typically requires more complex system designs and higher initial costs compared to sensible and latent heat storage.

Type	Principle	Media	Advantages
Sensible Heat	Temperature change	Water, rock, molten salt	Simplicity, low cost
Latent Heat	Phase change	Ice, PCMs, eutectic solutions	High energy density
Thermo-Chemical	Chemical reaction	Salts, hydrates	High density, low loss

Sensible heat storage technologies

Sensible heat storage relies on changing the temperature of a storage medium without undergoing a phase change. This method is widely used due to the relative simplicity of the media and systems involved. Water is a common medium, utilized in tanks for individual processes or larger district networks. The energy stored depends on the mass, specific heat capacity, and temperature difference, expressed as Q=mcpΔT. Water tanks are effective for balancing energy demand between daytime and nighttime usage patterns.

Molten salt technology is another prominent sensible heat storage method, particularly in concentrated solar power plants. Molten salts can store heat at high temperatures, allowing for efficient energy reuse. The Solana Generating Station utilizes molten salt storage to extend power generation beyond direct sunlight hours. Similarly, the Gemasolar Power Plant employs this technology to maintain operational status even during periods of variable solar input. These projects demonstrate the scalability of molten salt systems for large-scale energy storage.

Other materials such as silicon, aluminum, and rock or sand and concrete systems are also explored for sensible heat storage. Silicon and aluminum offer high thermal conductivity and stability at elevated temperatures. Rock, sand, and concrete systems provide cost-effective solutions for large-scale storage, often accessed via heat exchangers. The Cerro Dominador Solar Power Station integrates such technologies to enhance its thermal energy storage capabilities. These diverse media allow for flexibility in design and application, catering to different energy needs and environmental conditions.

Latent heat and phase-change materials

Latent heat storage utilizes phase-change materials (PCMs) to store thermal energy through the transition between solid and liquid states. Unlike sensible heat storage, which relies on temperature changes, latent heat storage captures significant energy at a nearly constant temperature. Common PCMs include salts, polymers, and ice-based systems. Salt hydrates and organic paraffins are frequently used for medium-temperature applications, while ice is prevalent in cooling systems. The energy density of a PCM is determined by its enthalpy of fusion, allowing for compact storage volumes compared to water tanks.

Ice-based and polymer technologies

Ice-based thermal energy storage is widely employed in air conditioning systems. By producing ice during off-peak nighttime hours, buildings can melt the ice to provide cooling during peak daytime demand. This shifts electrical load and reduces costs. Polymer PCMs offer advantages in flexibility and corrosion resistance. They are often encapsulated in microcapsules or macrocontainers to improve heat transfer rates. Polymers such as polyethylene glycol are used in textile and building applications for passive thermal regulation. These materials provide stable phase transitions over many cycles, ensuring long-term operational reliability in diverse environments.

Alloys and cryogenic storage

Miscibility gap alloys represent a specialized class of PCMs. These alloys undergo a phase separation upon heating, absorbing latent heat as they transition from a single-phase solid to a two-phase liquid mixture. This mechanism allows for high energy density at elevated temperatures, suitable for industrial process heat. Cryogenic energy storage involves liquefying gases like nitrogen or air at low temperatures. The stored cold energy is retrieved by evaporating the liquid, expanding the gas to drive turbines or provide direct cooling. This technology integrates well with renewable energy sources, storing excess electricity as cold liquid for later power generation or thermal use.

Thermo-chemical and molecular storage

Thermo-chemical energy storage represents a distinct class of thermal energy storage (TES) technologies that rely on reversible chemical reactions or physical adsorption processes to store energy. Unlike sensible or latent heat storage, thermo-chemical systems store energy in the chemical bonds of the storage medium, offering potentially higher energy density and lower heat loss over extended periods. This makes them particularly suitable for long-term seasonal storage, such as storing summer heat for winter heating or winter cold for summer cooling, as part of a broader thermal energy network.

Adsorption and Salt Hydrate Technology

Adsorption processes utilize solid materials, such as zeolites or silica gel, to absorb or desorb a fluid, typically water vapor or air. In a typical cycle, heat is applied to the adsorbent material, causing the fluid to desorb, thereby storing energy in the form of increased entropy and enthalpy. When the fluid is reintroduced, it adsorbs onto the solid surface, releasing the stored thermal energy. Zeolites, with their porous crystalline structure, are widely studied for their high adsorption capacity and stability. Similarly, salt hydrate technology involves the reversible hydration and dehydration of inorganic salts. These materials undergo phase changes between solid and liquid or different solid phases, absorbing or releasing latent heat. Salt hydrates are valued for their relatively low cost and high energy density, though challenges such as supercooling and phase segregation remain areas of active research.

Molecular Bond and MOST Systems

Emerging molecular storage systems focus on capturing energy in the molecular bonds of specific compounds. Molecular Solar Thermal (MOST) energy storage is a promising approach that converts solar energy into chemical energy through the isomerization of molecules. In this process, solar radiation triggers a structural change in the molecule, storing energy in the resulting metastable state. The energy can be released on demand, often through a catalyst or thermal trigger, reverting the molecule to its ground state. This technology offers the potential for high energy density and long-term storage with minimal losses, making it attractive for balancing energy demand between daytime and nighttime or across seasons. While still in developmental stages, MOST systems exemplify the innovation driving the diversification of thermal energy storage media beyond traditional water tanks and bedrock masses.

Thermal batteries and electric thermal storage

Thermal batteries represent a specific class of thermal energy storage systems designed to capture and release heat with high density and efficiency. These systems frequently utilize phase-change materials (PCMs) to store latent heat during the transition between solid and liquid states. In encapsulated thermal batteries, the PCM is contained within small capsules or panels, allowing for modular deployment and enhanced heat transfer surface areas. This configuration helps mitigate the low thermal conductivity often associated with pure PCM, enabling faster charging and discharging cycles for industrial or district heating applications.

Ground-based thermal storage systems

Ground heat exchange systems leverage the thermal mass of the earth or bedrock to store energy over extended periods. Vertical ground thermal batteries involve drilling boreholes into the subsurface, where heat exchangers circulate fluid to transfer thermal energy to or from the native earth. This method is particularly effective for seasonal storage, such as capturing summer heat for winter heating or storing winter cold for summer cooling. The impermeable strata surrounding deep aquifers can also serve as natural containment for these thermal reservoirs, minimizing heat loss to adjacent geological layers.

Electric thermal storage and storage heaters

Electric thermal storage (ETS) systems convert electrical energy into heat, which is then stored for later use. This approach is often employed to balance energy demand between daytime and nighttime, allowing consumers to utilize off-peak electricity rates. Storage heaters are a common implementation of ETS, typically using materials such as water, ice-slush, or insulated gravel beds to retain thermal energy. These heaters can be integrated into residential, commercial, or district heating networks, providing flexibility in energy consumption patterns. The stored heat is released gradually through convection or radiation, depending on the system design and the thermal properties of the storage medium.

Applications in renewable energy integration

Thermal energy storage (TES) serves as a critical mechanism for integrating variable renewable energy sources, such as wind and solar photovoltaics, by decoupling energy generation from consumption. As noted in the grounding data, TES technologies allow thermal energy to be stored for hours, days, or even months, providing flexibility for balancing energy demand between daytime and nighttime periods. This capability is particularly valuable in renewable energy systems where generation profiles often fluctuate significantly.

Seasonal Thermal Energy Storage

One of the most prominent applications of TES in renewable integration is seasonal thermal energy storage, which enables the storage of summer heat for winter heating, or winter cold for summer cooling. The Drake Landing project serves as a notable example of this approach, demonstrating how large-scale thermal storage can reduce reliance on conventional heating sources. Such systems typically utilize storage media including water or ice-slush tanks, masses of native earth or bedrock accessed with heat exchangers by means of boreholes, or deep aquifers contained between impermeable strata. These configurations allow communities to capture excess renewable energy during peak production periods and release it during times of higher demand.

Electricity-to-Heat Conversion Systems

Pumped-heat electricity storage and isentropic systems represent advanced approaches to converting variable electricity output into thermal form. These technologies enable the balancing of energy demand between daytime and nighttime, as described in the grounding data. By storing thermal energy in shallow, lined pits filled with gravel and water and insulated at the top, or in eutectic solutions and phase-change materials, these systems provide flexible storage options for renewable integration. The scale of such storage varies from individual processes to district, town, or regional applications as part of a thermal energy network.

Worked examples

Case Study 1: Helen Oy Water Cistern (Mustikkamaa, Finland)
Helen Oy operates a large-scale water cistern thermal energy storage facility in Mustikkamaa, Finland. This system utilizes water as the primary storage medium, leveraging its high specific heat capacity. The facility demonstrates the application of TES in balancing energy demand between daytime and nighttime periods. By storing thermal energy during off-peak hours and releasing it during peak demand, the system optimizes the efficiency of the local thermal energy network.

Case Study 2: Vantaa Underground Heat Storage Facility
The Vantaa underground heat storage facility employs boreholes to access masses of native earth or bedrock. Heat exchangers are used to transfer thermal energy into and out of the subsurface storage medium. This method allows for long-term storage, enabling the capture of summer heat for winter heating applications. The facility illustrates the scalability of TES technologies, extending from individual processes to district-level energy networks.

Case Study 3: Polar Night Energy Sand Battery (Finland)
Polar Night Energy has developed a sand battery technology in Finland. This system uses sand as a storage medium, heated to high temperatures to retain thermal energy for extended periods. The sand battery provides a solution for long-duration storage, capable of maintaining heat for months. This technology supports the integration of variable renewable energy sources by storing excess thermal energy for later reuse, enhancing grid stability and energy efficiency.