4.2 Thermal energy storage technologies and materials
4 min read•august 9, 2024
Thermal energy storage is crucial for concentrated solar power systems, allowing them to operate even when the sun isn't shining. This section covers different storage types, from simple sensible heat to advanced thermochemical methods, and explores various system designs and materials.
Understanding storage properties like capacity, , and is key to optimizing CSP systems. We'll look at ways to enhance and storage efficiency, which are vital for making solar power more reliable and cost-effective.
Storage Types
Sensible and Latent Heat Storage
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- involves increasing the temperature of a storage medium without changing its phase
- Utilizes the specific heat capacity of the storage material
- Common materials include water, molten salts, and rocks
- Energy stored calculated using the formula , where Q is heat stored, m is mass, c is specific heat capacity, and ΔT is temperature change
- harnesses the energy absorbed or released during phase transitions
- Exploits the high energy density associated with phase changes (solid to liquid, liquid to gas)
- Offers higher compared to sensible heat storage for the same volume
- Typically operates at nearly constant temperature during phase change
- Common materials include paraffin waxes and salt hydrates
Thermochemical Storage and Phase Change Materials
- relies on reversible chemical reactions to store and release thermal energy
- Involves breaking and reforming chemical bonds
- Offers high energy density and potential for long-term storage
- Typical reactions include hydration-dehydration of metal oxides or carbonation-decarbonation processes
- (PCMs) transition between solid and liquid states to store and release latent heat
- Provide high energy storage density within a narrow
- Organic PCMs include paraffin waxes and fatty acids
- Inorganic PCMs include salt hydrates and metallic materials
- Selection criteria include melting temperature, latent heat of fusion, and thermal conductivity
Storage Systems
Two-Tank and Single-Tank Systems
- Two-tank storage system separates hot and cold storage media
- Consists of two separate tanks for storing hot and cold fluids
- Allows for clear separation of temperature levels
- Commonly used in parabolic trough and power tower CSP plants
- Molten salts or thermal oils serve as storage media
- Single-tank thermocline system utilizes temperature stratification within a single tank
- Hot fluid floats on top of cold fluid due to density differences
- Thermocline region acts as a thermal barrier between hot and cold zones
- Requires careful fluid management to maintain stratification
- Reduces storage system costs compared to two-tank systems
Solid Media and Encapsulated PCM Storage
- Concrete storage systems use solid concrete blocks as thermal energy storage medium
- Embedded heat exchanger pipes circulate heat transfer fluid through concrete
- Offers low-cost storage option with good thermal stability
- Challenges include relatively low heat capacity and potential for thermal stress cracking
- Encapsulated PCMs store latent heat in small containers or capsules
- PCM is enclosed in spheres, tubes, or other geometries
- Prevents leakage and improves heat transfer characteristics
- Can be integrated into various storage system designs (packed beds, shell-and-tube heat exchangers)
- Allows for easier handling and prevents phase separation in some PCM materials
Storage Properties
Storage Capacity and Energy Density
- Storage capacity refers to the total amount of thermal energy a system can store
- Measured in joules (J) or kilowatt-hours (kWh)
- Depends on the storage medium, system volume, and temperature range
- Sensible heat capacity calculated using
- Latent heat capacity includes both sensible and phase change energy: , where L is latent heat of fusion
- Energy density describes the amount of energy stored per unit volume or mass
- Higher energy density allows for more compact storage systems
- PCMs and thermochemical storage generally offer higher energy densities than sensible heat storage
- Typical values range from 50-100 kWh/m³ for sensible storage to 200-500 kWh/m³ for latent heat and thermochemical storage
Thermal Conductivity and Enhancement Techniques
- Thermal conductivity influences heat transfer rates within the storage medium
- Higher thermal conductivity allows for faster charging and discharging of the storage system
- Many high-capacity storage materials (PCMs, molten salts) have relatively low thermal conductivity
- Thermal conductivity enhancers improve heat transfer within storage materials
- Metal fins or foams increase effective heat transfer surface area
- Graphite or carbon additives boost thermal conductivity of PCMs
- Nanoparticle additives (aluminum oxide, copper) enhance thermal properties of fluids
- Heat pipes or thermosyphons facilitate rapid heat transfer in storage systems
- Microencapsulation of PCMs increases surface area-to-volume ratio, improving heat transfer rates
Key Terms to Review (18)
Aerogel insulation: Aerogel insulation is a lightweight, highly porous material known for its exceptional thermal insulating properties. Composed primarily of silica, aerogels are among the lowest density solids in existence, which enables them to trap air and reduce heat transfer, making them ideal for thermal energy storage applications.
Dispatchability: Dispatchability refers to the ability of a power generation system, particularly renewable energy sources, to produce electricity on demand and match supply with consumer demand effectively. This is crucial for maintaining grid stability and ensuring that electricity can be provided when it is needed, rather than solely relying on weather conditions or time of day.
Energy Density: Energy density refers to the amount of energy stored in a given volume or mass of a material or system. It is a crucial factor in determining the efficiency and effectiveness of various energy storage technologies, particularly in thermal energy storage applications. Higher energy density allows for more energy to be stored within a smaller space, making it vital for optimizing storage systems and enhancing overall performance.
Heat Transfer: Heat transfer is the process of thermal energy moving from one physical system to another due to temperature differences. This concept is crucial in understanding how energy is converted and utilized in various systems, particularly in the context of optimizing performance and efficiency in energy generation technologies, which involve the transformation and storage of thermal energy.
Latent heat storage: Latent heat storage is a method of thermal energy storage that utilizes the energy absorbed or released during a phase change of a material, such as melting or freezing. This type of storage is highly efficient as it allows for large amounts of energy to be stored or released at a constant temperature, making it an attractive option for managing thermal energy in various applications.
Load Shifting: Load shifting is a strategy used to manage energy demand by shifting energy consumption from peak periods to off-peak periods, helping to balance energy supply and demand. This approach is particularly important in renewable energy systems, where production can be intermittent, as it allows for the efficient use of generated energy by storing excess energy during low demand times and releasing it when demand is high.
Material characterization: Material characterization is the process of analyzing and understanding the properties and behaviors of materials, particularly in how they respond to various environmental conditions and operational demands. This process is crucial for selecting appropriate materials for specific applications, especially in thermal energy storage systems, where performance, efficiency, and longevity are paramount considerations.
Modeling and simulation: Modeling and simulation refer to the processes of creating abstract representations (models) of real-world systems and using those models to study and analyze the behavior of these systems over time. This approach allows for experimentation and understanding of complex interactions within a system without the need for physical prototypes. In the context of thermal energy storage technologies, modeling and simulation help in predicting performance, optimizing design parameters, and assessing the efficiency of various materials used for energy storage.
Molten salt storage: Molten salt storage is a thermal energy storage technology that uses molten salts to store and transfer heat, enabling Concentrated Solar Power (CSP) systems to generate electricity even when sunlight is not available. This technology plays a vital role in enhancing the efficiency of energy conversion cycles and facilitating the integration of renewable energy into the grid by allowing for energy dispatching when needed.
Paraffin wax: Paraffin wax is a solid hydrocarbon material derived from petroleum, commonly used in thermal energy storage systems due to its ability to store and release heat effectively. This phase change material (PCM) undergoes melting and solidifying at specific temperatures, which allows it to absorb excess thermal energy during the day and release it when needed, making it ideal for improving energy efficiency in various applications.
Phase Change Materials: Phase change materials (PCMs) are substances that absorb and release thermal energy during the process of melting and freezing. This ability to store heat allows them to effectively regulate temperature, making them crucial in various applications, including thermal energy storage in solar power systems, where they enhance efficiency by storing excess heat generated during the day for use at night or during cloudy periods.
Sensible Heat Storage: Sensible heat storage is a method of storing thermal energy by raising or lowering the temperature of a material without changing its phase. This process utilizes materials like water, concrete, or certain oils to store heat, which can later be released to provide energy for heating or electricity generation. The ability to store energy in this way is crucial for managing energy demand and supply, particularly in renewable energy systems.
Sodium Nitrate: Sodium nitrate is a chemical compound with the formula NaNO₃, commonly used in various thermal energy storage applications due to its favorable thermal properties. It acts as a heat transfer fluid and storage medium, allowing concentrated solar power systems to store and release thermal energy efficiently, thus enhancing their performance and reliability.
Storage capacity: Storage capacity refers to the amount of thermal energy that a thermal storage system can hold and later release when needed. In concentrated solar power systems, optimizing storage capacity is crucial for balancing energy supply and demand, particularly when sunlight is not available. This capability enables the efficient integration of renewable energy into the grid, improving overall system reliability and performance.
System Integration: System integration refers to the process of bringing together various subsystems and components to function as a cohesive unit within a larger framework. This involves ensuring that different technologies and processes can communicate and work together effectively, maximizing efficiency and performance. In the context of concentrated solar power, effective system integration is crucial for optimizing energy conversion, storage, and overall plant operation.
Temperature range: Temperature range refers to the span between the minimum and maximum temperatures that a system, material, or substance can effectively operate within. In the context of heat transfer fluids and thermal energy storage systems, understanding temperature range is crucial for selecting appropriate materials and optimizing performance, as different fluids and storage technologies have specific operating limits that influence their efficiency and safety.
Thermal Conductivity: Thermal conductivity is the property of a material that describes its ability to conduct heat. It is a crucial factor in determining how efficiently heat transfer fluids can operate and how effectively thermal energy storage materials can absorb and release heat. Materials with high thermal conductivity transfer heat quickly, which is essential for optimal performance in various energy systems.
Thermochemical storage: Thermochemical storage is a method of storing thermal energy by using reversible chemical reactions to absorb and release heat. This technique enables efficient energy storage by converting thermal energy into chemical potential energy and back, making it a promising option for applications in concentrated solar power systems. It stands out due to its ability to store energy at higher densities compared to traditional thermal storage methods, which enhances overall system efficiency.