4.1 Types and properties of heat transfer fluids

Heat transfer fluids are crucial in concentrated solar power systems. They absorb, transport, and store thermal energy. This section explores different types of fluids, from molten salts to liquid metals, and their unique properties.

Understanding the thermal, flow, and stability characteristics of these fluids is key. We'll look at heat capacity, conductivity, viscosity, and corrosiveness. These factors affect system design, efficiency, and long-term performance in solar power plants.

Types of Heat Transfer Fluids

Molten Salts and Synthetic Oils

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• Molten salts function as efficient heat transfer fluids in concentrated solar power systems
  • Consist of mixtures of nitrate salts (sodium nitrate and potassium nitrate)
  • Operate at high temperatures (290°C to 565°C) without decomposing
  • Provide excellent thermal storage capabilities
• Synthetic oils serve as heat transfer fluids in parabolic trough systems
  • Include silicone-based and hydrocarbon-based oils
  • Operate at temperatures up to 400°C
  • Offer lower freezing points compared to molten salts

Water/Steam and Liquid Metals

• Water/steam systems utilize direct steam generation in solar collectors
  • Operate at temperatures up to 550°C
  • Eliminate the need for intermediate heat transfer fluids
  • Require careful control to prevent thermal shock and corrosion
• Liquid metals show promise as advanced heat transfer fluids
  • Include sodium, potassium, and liquid sodium-potassium alloys
  • Offer high thermal conductivity and low vapor pressure
  • Operate at temperatures exceeding 800°C
  • Present challenges due to their reactivity with air and water

Thermal Properties

Heat Capacity and Conductivity

• Thermal conductivity measures a fluid's ability to conduct heat
  • Expressed in watts per meter-kelvin (W/m·K)
  • Higher values indicate better heat transfer efficiency
  • Liquid metals (sodium: 142 W/m·K) exhibit superior thermal conductivity compared to molten salts (0.5 W/m·K)
• Specific heat capacity quantifies the amount of heat required to raise the temperature of a unit mass of fluid
  • Measured in joules per kilogram-kelvin (J/kg·K)
  • Higher values allow for greater heat storage capacity
  • Water possesses an exceptionally high specific heat capacity (4,186 J/kg·K at 20°C)

Phase Change Characteristics

• Melting point determines the lowest operating temperature for a heat transfer fluid
  • Molten salt mixtures typically melt between 120°C and 220°C
  • Synthetic oils have lower melting points, often below 0°C
• Boiling point sets the upper temperature limit for fluid operation
  • Water boils at 100°C at atmospheric pressure
  • Synthetic oils have boiling points ranging from 300°C to 400°C
  • Molten salts and liquid metals have much higher boiling points, allowing for higher operating temperatures

Flow and Stability Characteristics

Viscosity and Thermal Stability

• Viscosity affects the fluid's resistance to flow and pumping requirements
  • Measured in pascal-seconds (Pa·s) or centipoise (cP)
  • Decreases with increasing temperature
  • Lower viscosity fluids (water: 0.001 Pa·s at 20°C) require less pumping power than higher viscosity fluids (some synthetic oils: 0.1 Pa·s at 20°C)
• Thermal stability describes a fluid's ability to maintain its chemical composition at high temperatures
  • Molten salts remain stable up to 600°C
  • Synthetic oils may degrade above 400°C, forming deposits and reducing heat transfer efficiency
  • Water remains thermally stable throughout its liquid range

Corrosiveness and Material Compatibility

• Corrosiveness impacts the selection of materials for pipes, tanks, and heat exchangers
  • Molten salts can be corrosive to certain metals, requiring careful material selection (stainless steels, nickel alloys)
  • Synthetic oils generally exhibit low corrosiveness but may degrade certain elastomers and plastics
  • Water can cause corrosion in the presence of oxygen, necessitating proper water treatment and material selection
• Material compatibility ensures long-term system integrity
  • Liquid metals require specialized containment materials due to their high reactivity
  • Synthetic oils may cause swelling or degradation of certain seals and gaskets
  • Proper material selection and regular maintenance mitigate corrosion and compatibility issues