As the world transitions towards a more sustainable energy future, the role of thermal energy storage (TES) has become increasingly critical. Thermal energy storage technologies enable the decoupling of energy generation and energy demand, allowing for the increased integration of variable renewable energy (RE) sources, like solar and wind, into power grids and end-use applications.
Materials for Thermal Energy Storage
At the heart of these advancements are innovations in thermal energy storage materials. Phase change materials (PCMs), for example, can store and release large amounts of energy during the phase transition between solid and liquid states. Researchers have developed PCMs with tailored melting points to match specific heating and cooling needs, from building insulation to industrial process heat recovery. Thermochemical energy storage systems, meanwhile, utilize reversible chemical reactions to store energy, offering even higher energy densities than PCMs.
Beyond these phase-change and thermochemical approaches, sensible heat storage in the form of molten salts or solid ceramic media has also progressed rapidly. Molten salt thermal energy storage integrated with concentrating solar power plants can cost-effectively store energy for dispatchable electricity generation, even after the sun has set.
Innovative Thermal Energy Storage Systems
Alongside advancements in storage materials, the industry has also witnessed the emergence of novel thermal energy storage system designs. Liquid air energy storage, for instance, uses electricity to liquefy air, which is then stored in insulated tanks. When power is needed, the liquid air is vaporized and expanded to drive a turbine. This system can provide long-duration energy storage to complement short-term battery storage.
Underground thermal energy storage systems, like borehole thermal energy storage, leverage the stable temperatures deep underground to store excess heat or cold for later use in buildings or industrial processes. These systems can dramatically reduce the energy consumption and carbon footprint of heating and cooling applications.
Applications of Thermal Energy Storage
The applications of thermal energy storage extend far beyond the power sector. In the built environment, TES is enabling the transition to net-zero energy buildings by storing heating and cooling energy for use when needed. Industrial facilities are also increasingly turning to TES to recover waste heat, improving overall energy efficiency and reducing greenhouse gas emissions.
Perhaps most significantly, thermal energy storage is playing a crucial role in the integration of renewable energy sources. By storing excess solar or wind energy in the form of heat, TES can help smooth the variability of these sources and provide dispatchable energy when the sun isn’t shining or the wind isn’t blowing. This is a critical capability as Europe accelerates its transition to a renewable-based energy system.
Thermal Energy Storage Technologies
Phase Change Materials
Phase change materials (PCMs) are a class of TES materials that store and release energy through the phase transition between solid and liquid states. PCMs can store up to 10 times more energy per unit volume than sensible heat storage materials, like water or masonry. By tailoring the PCM’s melting point, engineers can match the storage medium to the specific heating or cooling needs of a building, industrial process, or other application.
Leading PCM research has focused on developing new organic and inorganic compounds with enhanced energy storage density, thermal conductivity, and cycling stability. For example, eutectic salt mixtures and microencapsulated PCMs have demonstrated promising performance improvements over conventional paraffin-based PCMs.
Thermochemical Energy Storage
Thermochemical energy storage systems leverage reversible chemical reactions to store thermal energy. These systems can achieve even higher energy densities than PCMs, with the potential to store heat for long durations with minimal heat losses. Researchers have explored various thermochemical storage materials, including metal hydrides, salt hydrates, and chemical sorption systems.
One innovative thermochemical storage technology is the thermochemical battery, which uses a reversible chemical reaction to store and release energy on demand. These systems can decouple energy generation and consumption, making them well-suited for integrating variable renewable energy sources.
Sensible Heat Storage
While phase change and thermochemical approaches offer high energy densities, sensible heat storage systems remain an important and cost-effective option for many applications. Molten salt storage, for example, is widely used in concentrating solar power plants to store thermal energy for electricity generation during periods of peak demand.
Solid media, such as concrete or firebrick, also show promise for sensible heat storage, particularly in industrial waste heat recovery applications. These materials can be heated to high temperatures and then discharged to provide process heating or steam generation.
Thermal Energy Storage Integration
Built Environment Applications
Thermal energy storage is a key enabler for the transition to net-zero energy buildings. By storing heating and cooling energy, TES systems can decouple building energy demand from the power grid, reducing the need for costly grid reinforcements and enabling higher penetrations of renewable energy.
For example, borehole thermal energy storage systems, which use the stable underground temperatures to store heat or cold, have been deployed in numerous residential and commercial buildings across Europe. These systems can reduce a building’s heating and cooling energy consumption by 50% or more.
Industrial Waste Heat Recovery
Thermal energy storage also plays a crucial role in improving the energy efficiency of industrial processes. By recovering waste heat and storing it for later use, TES systems can dramatically reduce the overall energy consumption and greenhouse gas emissions of energy-intensive industries, such as steel, cement, and chemical manufacturing.
Molten salt storage and thermochemical systems are particularly well-suited for industrial waste heat recovery, as they can operate at the high temperatures required by many industrial processes. The stored thermal energy can then be used to generate steam, provide process heating, or even drive combined heat and power (CHP) systems.
Renewable Energy Integration
Perhaps the most significant application of thermal energy storage is in the integration of variable renewable energy sources, like solar and wind power. By storing excess renewable energy in the form of heat, TES systems can help smooth the inherent variability of these sources and provide dispatchable energy when needed.
This is a crucial capability as Europe accelerates its transition to a renewable-based energy system. Molten salt storage integrated with concentrating solar power plants, for example, can store thermal energy for several hours, allowing for the generation of electricity long after the sun has set.
Thermal Energy Storage Optimization
Computational Modeling and Simulation
Advancing the deployment of thermal energy storage technologies requires a deep understanding of their performance characteristics and optimization potential. Computational modeling and simulation tools have become essential for this purpose, enabling engineers to explore the complex thermodynamic and heat transfer processes involved in TES systems.
These models incorporate detailed descriptions of storage materials, system configurations, and operating conditions to predict the energy storage capacity, charge/discharge rates, and overall efficiency of TES technologies. By coupling these models with optimization algorithms, researchers can identify the most cost-effective and energy-efficient designs for specific applications.
Experimental Testing and Validation
While computational models provide valuable insights, experimental testing and validation remain critical for the development of thermal energy storage systems. Pilot-scale and demonstration projects allow researchers to assess the real-world performance of TES technologies under varying operating conditions, identify potential operational challenges, and validate the predictions of their computational models.
These experimental studies have been instrumental in advancing the thermal energy storage industry, leading to improvements in materials, system designs, and integration strategies. The lessons learned from these projects have helped to accelerate the commercialization and widespread adoption of TES technologies across Europe and beyond.
Performance Metrics and Benchmarking
To drive the ongoing innovation and optimization of thermal energy storage systems, industry stakeholders have developed a range of performance metrics and benchmarking frameworks. These metrics, which include energy storage density, round-trip efficiency, and cost-of-service, provide a common language for comparing the capabilities and competitiveness of different TES technologies.
By establishing these performance standards and benchmarks, the thermal energy storage industry can more effectively identify the most promising technologies, target areas for further research and development, and guide policymakers and investors in making informed decisions about TES deployment. As the industry continues to evolve, these metrics will play a crucial role in unlocking the full potential of thermal energy storage in Europe’s clean energy transition.
