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.

1. What Is Thermal Energy Storage?

Thermal energy storage refers to the capture of heat or cold for later use. The stored thermal energy can be extracted and applied when demand is high, or when supply from renewable sources is low. Unlike electrical storage, which requires conversion to electricity, TES preserves energy in a thermal form, reducing conversion losses and enabling direct use in heating, cooling, or power generation processes.

Key characteristics of TES:

2. Why Thermal Energy Storage Matters

The modern energy landscape is dominated by variable renewable resources—solar and wind—that fluctuate with weather and time of day. Balancing supply and demand requires flexibility. TES offers a low‑cost, high‑efficiency pathway to store surplus heat or cold, thereby:

1. Stabilizing the grid – by absorbing excess renewable heat during sunny or windy periods and releasing it when demand spikes.
2. Reducing energy waste – capturing waste heat from CHP plants or industrial processes that would otherwise be discarded.
3. Lowering operational costs – enabling off‑peak electricity use for heat pumps, a practice known as peak shaving.
4. Facilitating seasonal balancing – storing summer heat for winter heating or winter cold for summer cooling.

These benefits make TES a cornerstone technology for energy systems that aim to be almost or completely fed by renewable sources.

3. Core Technologies and Storage Media

TES systems are broadly categorized by the medium that stores the energy. The choice of medium depends on the required temperature range, storage duration, and application.

3.1 Water and Ice‑Slush Tanks

• Principle – Water’s high specific heat allows large amounts of energy to be stored with modest temperature changes.
• Ice‑slush – By partially freezing water, the latent heat of phase change can be harnessed, enabling storage at lower temperatures.
• Applications – Common in district heating, industrial cooling, and seasonal thermal storage.

3.2 Ground‑Source Systems

• Borehole heat exchangers – Heat is transferred to or from masses of native earth or bedrock through a network of boreholes.
• Deep aquifers – Water confined between impermeable strata can act as a natural heat reservoir.
• Shallow pits – Lined gravel or sand pits, insulated at the top, store thermal energy in the surrounding earth.
• Use cases – Ground‑source TES is widely used for building heating and cooling, and for large‑scale district networks.

3.3 Phase‑Change Materials (PCMs) and Eutectic Solutions

• PCMs – Materials that absorb or release latent heat during phase transitions (solid ↔ liquid).
• Eutectic solutions – Mixtures that melt at a precise temperature, providing a stable storage point.
• Benefits – High energy density and stable temperature storage make PCMs attractive for applications requiring precise thermal control.

4. Sources of Thermal Energy for Storage

TES systems are fed by various heat or cold sources, often derived from existing power or industrial processes.

4.1 Heat Pumps and Off‑Peak Electricity

• Heat pumps extract heat from ambient air, ground, or water, and can be powered by electricity.
• Off‑peak electricity – During periods when electricity is cheap or abundant, heat pumps can generate heat, which is then stored for later use.
• Peak shaving – By shifting heat production to low‑cost times, operators reduce peak demand on the grid.

4.2 Combined Heat and Power (CHP) Plants

• CHP plants simultaneously generate electricity and useful heat.
• The excess heat, often at high temperatures, can be directed into TES systems for later deployment.

4.3 Renewable Energy Surpluses

• Solar thermal – Excess solar heat during peak daylight hours can be stored.
• Wind power – When wind turbines produce more electricity than the grid can absorb, the excess can be converted to heat via heat pumps and stored.

4.4 Industrial Waste Heat

• Many industrial processes produce waste heat that would otherwise be lost.
• Capturing and storing this heat can improve overall plant efficiency and reduce environmental impact.

5. Applications Across Sectors

TES finds use in a wide array of contexts, each leveraging its unique advantages.

5.1 District Heating and Cooling

• Cities and towns can build large‑scale TES networks that balance seasonal demands.
• Heat from industrial plants or renewable sources is stored in underground or surface tanks and distributed through pipelines.

5.2 Residential and Commercial Buildings

• Small‑scale TES units (e.g., ice‑slush tanks) can store cooling for nighttime use.
• Ground‑source TES systems provide year‑round heating and cooling with minimal energy consumption.

5.3 Industrial Processes

• Processes that require steady temperature control can use PCMs or water‑based storage to maintain process conditions.
• Waste heat recovery and storage improve overall plant economics.

5.4 Power Generation

• TES can be integrated into power plants to smooth output, storing excess thermal energy during low demand and releasing it during peak periods.
• This capability supports the integration of variable renewable electricity into the grid.

6. Integration with Renewable Energy Systems

TES plays a pivotal role in achieving high penetration of renewables.

• Balancing variable supply – Surplus renewable heat can be stored and released when generation falls.
• Grid decoupling – By storing heat during off‑peak times, TES reduces the need for peaking power plants.
• Energy efficiency – Direct use of stored heat avoids conversion losses, improving overall system efficiency.

The ability to store heat for hours, days, or even months makes TES a versatile tool for managing the inherent intermittency of solar and wind resources.

7. Historical Development and Milestones

While the concept of storing thermal energy dates back centuries, modern TES technologies have evolved alongside advances in materials science, heat transfer, and renewable energy deployment.

• Early 20th century – Basic ice‑storage and water‑tank systems were used in industrial settings.
• Late 20th century – Ground‑source TES and phase‑change materials gained prominence with growing interest in energy efficiency.
• 21st century – Integration of TES with renewable electricity and CHP plants has become a key strategy for decarbonization.

The continuous development of new materials and system designs keeps TES at the forefront of sustainable energy solutions.

8. Economic and Environmental Benefits

8.1 Cost Savings

• Peak shaving reduces the need for expensive peaking plants.
• Utilization of waste heat turns a cost into a revenue source or cost offset.

8.2 Emission Reduction

• By enabling higher renewable penetration, TES cuts reliance on fossil‑fuel‑based heat generation.
• Lower overall energy consumption translates into reduced greenhouse gas emissions.

8.3 Energy Independence

• Communities can store locally generated renewable heat, decreasing dependence on external power supplies.

9. Challenges and Future Directions

Despite its promise, TES faces several challenges:

• Capital costs – Large‑scale storage systems can require significant upfront investment.
• System integration – Seamless coordination with existing heating and cooling infrastructure is necessary.
• Material durability – Long‑term performance of PCMs and other media must be assured.

Future research focuses on:

• Advanced PCMs with higher energy densities and tailored melting points.
• Hybrid systems that combine TES with electrical storage for broader flexibility.
• Smart controls that optimize charge–discharge cycles based on real‑time demand and renewable output.

10. Conclusion

Thermal energy storage is a versatile, scalable technology that captures surplus heat or cold for later use. By enabling the balancing of energy demand and supply, particularly in the context of variable renewable resources, TES reduces costs, cuts emissions, and enhances the resilience of energy systems. From small residential units to large district networks, TES continues to evolve, driven by advances in materials, design, and integration strategies.

FAQ

How long can thermal energy be stored? TES can keep thermal energy for hours, days, or even months, depending on the technology and storage medium.

What are the main types of storage media used in TES? Common media include water or ice‑slush tanks, masses of earth or bedrock accessed via boreholes, shallow gravel pits, deep aquifers, eutectic solutions, and phase‑change materials.

What does “peak shaving” mean in the context of TES? Peak shaving refers to using off‑peak, lower‑cost electricity to power heat pumps, generating heat that is stored for use during peak demand periods, thereby reducing grid load.

Why is TES important for renewable energy integration? TES allows surplus renewable heat to be stored and released when needed, smoothing supply fluctuations and enabling a higher share of variable renewables in the energy mix.

Can industrial waste heat be used in TES? Yes, waste heat from industrial processes can be captured and stored, improving overall plant efficiency and reducing environmental impact.