Thermal energy, commonly understood as heat, is the internal energy held by a system that is directly related to its temperature. This energy originates from the microscopic kinetic and potential energy of the atoms and molecules that compose the substance. Thermal energy storage (TES) is the process of capturing and retaining this heat or cold for use at a later time, effectively bridging the gap between when thermal energy is available and when it is needed. These systems enable a more consistent energy supply by allowing energy collected during times of surplus, like peak sun hours or off-peak electricity pricing, to be used later. Storing heat is a significant factor in balancing energy demand, enhancing the flexibility of renewable sources, and improving overall system efficiency.
The Molecular Mechanism of Thermal Storage
At the most fundamental level, thermal energy is stored within the substance’s atomic structure as internal energy. This energy is primarily contained in the constant, random motion of the constituent atoms and molecules. In a solid, this motion manifests as the vibration of atoms around their fixed positions in the crystal lattice.
As the temperature of a material increases, the amplitude and frequency of these atomic vibrations also increase, representing a greater amount of stored kinetic energy. Thermal energy is also stored as potential energy within the chemical bonds that hold the molecules together. Heating a material increases the average distance between atoms and stretches these bonds, which stores energy similarly to a compressed spring.
The amount of energy a substance can store for a given temperature change is quantified by its specific heat capacity. Materials with a high specific heat capacity require a large amount of energy to increase the motion and vibration of their molecules by just one degree. Water, for example, has an exceptionally high specific heat capacity, meaning it is an excellent medium for storing large quantities of thermal energy by simply raising its temperature.
Materials and Methods: Sensible vs. Latent Heat Storage
Thermal energy storage is practically categorized into two main methods based on how the material absorbs the heat: sensible and latent heat storage.
Sensible Heat Storage (SHS)
Sensible heat storage (SHS) involves the storage of thermal energy by changing the temperature of a material without changing its phase. The stored energy is directly proportional to the temperature difference between the charged and discharged states and the material’s specific heat capacity. Common materials for SHS include water, molten salts, concrete, and ceramics. Water is widely used for low-temperature applications due to its high specific heat capacity. For high-temperature applications, such as those found in concentrated solar power (CSP) plants, molten salts are the standard, capable of operating between approximately 290°C and 565°C.
Latent Heat Storage (LHS)
Latent heat storage (LHS) utilizes the energy absorbed or released when a material undergoes a phase change, typically from solid to liquid or vice versa. This phase transition occurs at a nearly constant temperature, meaning the energy is stored without a significant increase in the material’s temperature. The energy stored during this process is known as the latent heat of fusion. Materials used for LHS are called Phase Change Materials (PCMs), which are engineered to melt and solidify at specific temperatures relevant to the application. PCMs offer a significantly higher energy density than sensible heat materials, allowing for a more compact storage unit. This high-density storage is possible because the energy goes into breaking the molecular bonds during the phase change rather than just increasing molecular motion.
Deployment: Large-Scale Thermal Energy Storage Systems
The materials and methods of thermal storage are deployed within large physical systems to manage energy on an industrial scale.
Concentrated Solar Power (CSP)
One of the most prominent applications is in Concentrated Solar Power (CSP) plants, where thermal energy is stored in massive, insulated tanks. These systems use fields of mirrors to focus sunlight onto a receiver, heating a fluid, most commonly molten salt, to hundreds of degrees Celsius. The hot molten salt is then pumped into large storage tanks, typically a two-tank system. This stored heat can be used to generate steam and drive a turbine to produce electricity for many hours after the sun has set. More than half of the world’s commercial CSP facilities incorporate these systems, demonstrating the reliability of this technology for providing dispatchable power.
District Heating and Cooling
Thermal energy storage is also widely used in managing building energy demands through District Heating and cooling systems. Large water tanks or underground thermal energy storage (UTES) systems store hot or chilled water. This allows facilities to utilize lower-cost, off-peak electricity to create cold energy, such as ice, for cooling during the day, a practice known as peak shaving.
Industrial Waste Heat Recovery
Industrial waste heat recovery represents another significant deployment area. Systems are designed to capture and store excess heat generated from manufacturing processes. This heat, which would otherwise be released into the environment, is stored in solid materials like ceramics, rocks, or specialized packed beds. Storing this recovered heat allows industrial sites to reuse it efficiently, reducing both energy consumption and operational costs.