9.2 Latent heat storage and phase change materials

Latent heat storage uses phase change materials (PCMs) to store and release thermal energy during phase transitions. PCMs come in organic, inorganic, and eutectic varieties, each with unique properties like melting point and latent heat of fusion.

Encapsulation techniques protect PCMs and improve their stability. Heat transfer enhancement strategies, like adding high-conductivity materials or optimizing geometry, boost energy storage and release rates. These innovations make PCMs a promising option for thermal energy storage.

Phase Change Materials (PCMs)

Characteristics and Properties

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• Store and release thermal energy during phase transitions at a constant temperature
• Utilize the latent heat of fusion, the energy absorbed or released when a material changes phase between solid and liquid
• Melting point is a critical property that determines the temperature at which the phase change occurs and energy is stored or released
• Classified into organic PCMs (paraffins, fatty acids), inorganic PCMs (salt hydrates), and eutectic mixtures (combination of two or more PCMs)

Types of PCMs

• Organic PCMs include paraffins (alkanes like octadecane) and fatty acids (palmitic acid, stearic acid)
  • Advantages: high latent heat, low supercooling, chemically stable, non-corrosive
  • Disadvantages: low thermal conductivity, flammable
• Inorganic PCMs primarily consist of salt hydrates (calcium chloride hexahydrate, sodium sulfate decahydrate)
  • Advantages: high latent heat, high thermal conductivity, non-flammable, low cost
  • Disadvantages: supercooling, phase segregation, corrosive
• Eutectic mixtures are combinations of two or more PCMs that melt and freeze congruently
  • Advantages: sharp melting point, high energy density
  • Disadvantages: limited availability, high cost

PCM Encapsulation and Stability

Encapsulation Techniques

• Encapsulation contains the PCM in a stable shell material to prevent leakage and interaction with the environment
• Macroencapsulation involves encapsulating PCM in large containers (tubes, spheres, panels)
  • Advantages: easy to handle, high PCM content
  • Disadvantages: low surface area to volume ratio, risk of leakage
• Microencapsulation involves encapsulating PCM in microscopic particles (1-1000 μm)
  • Advantages: high surface area to volume ratio, improved heat transfer, reduced leakage
  • Disadvantages: complex manufacturing process, high cost

Stability Challenges

• Supercooling occurs when a PCM remains liquid below its melting point, reducing energy storage capacity
  • Addressed by adding nucleating agents (borax, carbon nanotubes) to promote crystallization
• Thermal cycling refers to repeated melting and freezing cycles that can degrade PCM performance over time
  • Addressed by using compatible encapsulation materials, adding thickening agents, and ensuring proper PCM selection

Heat Transfer Enhancement

Strategies for Improving Heat Transfer

• Heat transfer enhancement improves the rate of energy storage and release in PCM systems
• Increasing the thermal conductivity of PCMs through the addition of high-conductivity materials (graphite, metal foams, carbon nanotubes)
  • Example: dispersing graphite nanoparticles in paraffin wax increases thermal conductivity by 10-fold
• Optimizing the geometry and configuration of PCM encapsulation to maximize surface area and heat transfer
  • Example: using finned tubes or heat pipes to increase the heat transfer area between the PCM and the heat transfer fluid
• Incorporating convection enhancement techniques (stirring, bubbling, vibration) to improve heat transfer within the PCM
  • Example: using ultrasonic vibration to enhance convective heat transfer in a salt hydrate PCM system