Thermal Energy Storage: The Future of Sustainable Power
As one of the most important energy sources utilized by humanity, thermal energy accounts for 40% to 50% of terminal energy consumption, with extremely wide-ranging applications. In the current energy development and utilization system, nearly all forms of energy conversion involve thermal energy. However, due to inevitable energy losses during conversion processes, thermal storage technologies are rarely directly applied to electrical energy storage (where both input and output are electrical energy). Instead, they appear more as a link in the energy conversion chain or are simply applied in thermal systems, often functioning as an efficient plant battery solution.
Thermal energy storage technologies - bridging the gap between energy production and demand
Classification of Thermal Energy Storage Technologies
Based on different working principles, thermal storage technologies are mainly divided into three categories: sensible heat storage, latent heat (phase change) storage, and thermochemical heat storage. Each technology offers unique advantages and applications in the evolving landscape of renewable energy systems, with the plant battery concept increasingly influencing their development and implementation.
Sensible Heat Storage
Utilizes temperature changes in materials to store and release heat without phase change
Latent Heat Storage
Stores energy through phase transitions, offering high energy density
Thermochemical Storage
Uses chemical reactions to store energy with extremely high density potential
1. Sensible Heat Storage
Sensible heat storage technology is a method that utilizes temperature changes in substances to achieve heat storage and release. Its working mechanism is relatively simple: heat is stored or released by heating or cooling a medium. During this process, the medium does not undergo chemical changes or phase transitions, making the entire system easy to control and operate stably. This reliability makes it an attractive component in plant battery systems designed for consistent performance.
Additionally, this type of heat storage material is diverse and low-cost, suitable for large-scale use. The technology not only has high maturity but also demonstrates broad application potential in multiple fields. However, sensible heat storage also faces several challenges, such as low energy storage density, large equipment volume, significant heat loss during long-term storage, and unstable output temperatures - factors that engineers are addressing to enhance its viability as a plant battery solution.
Sensible Heat Storage Media
Sensible heat storage media can be roughly divided into liquid and solid categories. Each category offers distinct advantages depending on the application requirements, with ongoing research improving their efficiency as components in modern plant battery systems.
Liquid Heat Storage Materials
-
Water: Possesses excellent safety and stability, with a storage temperature typically not exceeding 100°C. It is widely used in solar hot water supply systems and space heating. Its abundance and low cost make it a popular choice in many plant battery configurations.
-
Heat transfer oils: Offer excellent thermal conductivity and a wide operating temperature range. They were used in early medium and high-temperature thermal energy storage; however, their high cost, flammability, and potential high pressure risks in closed loops have led to them being gradually replaced by other better options in modern plant battery designs.
-
Molten salts: Characterized by low saturated vapor pressure, low viscosity, high thermal conductivity, non-flammability, and non-toxicity. They perform excellently at temperatures corresponding to mainstream steam parameters and are relatively low-cost, making them considered one of the ideal choices for solar thermal power conversion and effective plant battery implementations. However, at extremely high temperatures, molten salts may cause corrosion to pipelines and related equipment, requiring further research to improve their compatibility with stainless steel materials and heat resistance.
-
Liquid metals: Due to their extremely high thermal conductivity, they are regarded as potential future high-temperature (exceeding 600°C) heat storage solutions. However, given the extremely active chemical properties of these substances, additional safety protection measures are required to ensure normal system operation. Coupled with their high cost, they are still in the preliminary exploration stage for plant battery applications.
Solid Heat Storage Materials
Common solid heat storage materials include concrete, rocks, and refractory bricks. Compared to liquid heat storage materials, solid heat storage materials can operate at higher temperatures and can store more heat in the same space, meaning reduced material requirements and lower overall costs - significant advantages for scalable plant battery systems.
Advantages of Sensible Heat Storage
- Mature technology with proven reliability in various applications
- Low cost materials available in abundance
- Simple system design and easy maintenance
- Suitable for large-scale energy storage applications
- Compatible with many existing energy systems, facilitating plant battery integration
2. Latent Heat (Phase Change) Storage
Phase change heat storage technology mainly relies on latent heat as its primary form of energy storage. This technology utilizes the large amount of latent heat absorbed or released by substances during phase transitions to store heat. During this period, the temperature of the material remains almost constant. This heat storage technology offers significant advantages such as high energy storage density and small volume, making it an increasingly important component in advanced plant battery systems.
The process of a substance changing from one state to another is called a phase change. Typically, this change occurs at or near isothermal conditions and is accompanied by significant energy changes - the absorption or release of large amounts of heat, defined as latent heat of phase change. In particular, compared to sensible heat, the latent heat involved in the phase change process of most materials is much greater.
Taking water as an example, its specific heat capacity is approximately 4.2 kJ/(kg·°C), while during the transition from solid to liquid (ice melting into water), water can absorb 355 kJ/kg of energy as latent heat of phase change. This demonstrates that using phase change latent heat is significantly superior to simply relying on sensible heat in terms of energy density - a key advantage for compact plant battery designs.
Types of Phase Changes
Phase changes in substances mainly include four types: solid-solid, solid-liquid, solid-gas, and liquid-gas. Although solid-gas and liquid-gas transitions have high latent heat values, their application range is relatively limited due to the significant volume changes that occur, increasing operational complexity in plant battery systems.
Solid-Solid Phase Change
Occurs when a solid material transforms from one crystalline state to another. This phase change exhibits small volume changes and low supercooling but typically involves less heat release or absorption compared to other phase change processes. Research continues to improve these materials for specialized plant battery applications.
Solid-Liquid Phase Change
Involves substances transforming from solid to liquid. While this process requires specific containers to hold the liquid, the volume change is much smaller compared to solid-gas and liquid-gas phase changes, and the associated latent heat is significantly higher than that of solid-solid phase changes. Currently recognized as the most practical and widely applicable phase change heat storage method for plant battery systems.
Phase Change Materials
Currently, there are various types of materials used in phase change heat storage technology. Based on their chemical composition, they are mainly divided into organic and inorganic categories, each with distinct characteristics that make them suitable for different plant battery applications.
| Material Type | Examples | Temperature Range | Key Characteristics |
|---|---|---|---|
| Organic PCMs | Paraffins, alcohols, fatty acids | Low to medium temperatures | Chemically stable, low supercooling, non-corrosive |
| Inorganic PCMs | Hydrated salts, molten salts, metals/alloys | Medium to high temperatures | High energy density, good thermal conductivity |
Generally speaking, organic phase change materials are more suitable for thermal energy storage in the low to medium temperature range, while inorganic phase change materials exhibit better thermal energy storage performance under medium to high temperature conditions. This versatility allows phase change storage to be integrated into a wide range of plant battery systems, from residential heating to industrial processes.
3. Thermochemical Heat Storage
Thermochemical heat storage technology boasts extremely high energy density per unit volume, reaching the order of GJ/m³. In comparison, sensible heat storage materials have an energy density only about one-tenth of this, while latent heat storage materials are only half. Furthermore, by separating reactants, this technology can achieve zero-loss heat storage at room temperature, making it widely regarded as one of the most promising technical solutions for large-scale and long-term thermal energy storage in future plant battery systems.
Based on the differences in chemical bond changes involved in the energy storage process, thermochemical heat storage can be further divided into two categories: chemical adsorption storage and chemical reaction storage, each offering unique advantages for specialized plant battery applications.
Chemical Adsorption Heat Storage
Chemical adsorption heat storage is particularly suitable for applications in low-temperature environments. It relies on the formation and breaking of physical or chemical intermolecular forces (such as van der Waals forces, electrostatic forces, and hydrogen bonds) between solid adsorbents and gaseous adsorbates to achieve heat storage and release. This technology offers unique advantages for certain plant battery configurations requiring low-temperature operation.
The technology mainly includes two types of systems: one is a hydrated salt system using water vapor as the adsorbate, and the other is an ammonia complex system using ammonia molecules as the adsorbate.
| Material System | Storage Material | Storage/Release Temperature (°C) | Energy Density |
|---|---|---|---|
| Hydrated Salts | LiCl·H₂O | 85, 40 | 2622 kJ/kg |
| CaSO₄·2H₂O | 150, 60 | 277 kJ/kg | |
| Na₂S·5H₂O | 82, 66 | 27.89 GJ/m³ | |
| MgCl₂·6H₂O | 104, 61 | 17.82 GJ/m³ | |
| SrBr₂·6H₂O | 105, 52 | 4.14 GJ/m³ | |
| MgSO₄·7H₂O | 150, 25 | 21.99 GJ/m³ | |
| Ammonia Complexes | SrCl₂ | 96, 52 | 1724 kJ/kg |
| MnCl₂ | 162, 45 | 1296 kJ/kg |
Chemical Reaction Heat Storage
Chemical reaction heat storage is mainly applied under medium to high temperature conditions. Its systems are diverse and include methane reforming, ammonia synthesis and decomposition, metal hydrides, carbonates, metal oxides, and metal hydroxides. These methods achieve heat storage and release through the breaking and recombination of chemical bonds, offering tremendous potential for high-performance plant battery systems.
This type of energy storage offers large reaction enthalpies, high energy densities, and a wide operating temperature range. However, in practical applications, it still faces issues such as cost control, material corrosion, and gas storage, making it necessary to conduct in-depth research on relevant reaction mechanisms and optimize process flows to enhance overall performance in plant battery implementations.
Challenges in Thermochemical Storage
Thermochemical heat storage systems have complex structures and numerous auxiliary equipment, leading to high initial investment costs. Currently, their ultra-high energy density per unit volume has not been fully utilized in plant battery applications.
Additionally, due to the complexity of the chemical reaction mechanisms involved, precisely controlling reaction rates presents challenges. Moreover, some reaction processes have strict safety requirements, meaning the overall system efficiency still needs improvement. Therefore, further in-depth research on thermochemical heat storage technology is required to address these issues and enhance its viability as a plant battery solution.
The Future of Thermal Energy Storage
As renewable energy sources continue to dominate the global energy mix, the importance of efficient energy storage solutions becomes increasingly evident. Thermal energy storage technologies, including sensible, latent, and thermochemical systems, each offer unique advantages that can be harnessed in different applications. The development of the plant battery concept, integrating these storage technologies with energy generation systems, represents a significant step forward in creating sustainable, reliable energy infrastructure.
Continued research and innovation in materials science, system design, and integration will further enhance the performance and reduce the costs of thermal storage technologies. These advancements will be crucial in enabling the widespread adoption of renewable energy and in building a more sustainable energy future where the plant battery concept becomes a standard component of energy systems worldwide.
Learn more