9.1 Sensible heat storage materials and systems

Sensible heat storage is a key method for storing thermal energy. It involves raising the temperature of materials like water or molten salts. This approach is used in various applications, from hot water tanks in homes to large-scale energy storage in power plants.

The effectiveness of sensible heat storage depends on material properties like specific heat capacity and thermal conductivity. Understanding these properties helps engineers design efficient storage systems. Proper insulation and smart system design are crucial for minimizing heat loss and maximizing energy storage potential.

Material Properties

Specific Heat Capacity and Thermal Mass

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• Specific heat capacity measures the amount of heat required to raise the temperature of a substance by one degree Celsius per unit mass
  • Higher specific heat capacity materials can store more thermal energy for a given mass and temperature change
  • Water has a high specific heat capacity (4.18 kJ/kg·K), making it an effective sensible heat storage medium
• Thermal mass refers to a material's ability to absorb, store, and release thermal energy
  • Materials with high thermal mass, such as concrete and brick, can help regulate temperature fluctuations in buildings
  • Incorporating thermal mass in construction can reduce heating and cooling loads, improving energy efficiency

Thermal Conductivity and Insulation

• Thermal conductivity measures a material's ability to conduct heat
  • Materials with high thermal conductivity, such as metals (copper, aluminum), allow heat to flow easily through them
  • Low thermal conductivity materials, like air and insulating materials (fiberglass, polyurethane foam), resist heat flow
• Insulation helps reduce heat transfer between a storage system and its surroundings
  • Effective insulation minimizes heat losses in sensible heat storage systems, maintaining the stored thermal energy for longer periods
  • Common insulation materials include mineral wool, cellulose, and aerogel

Storage Systems

Hot Water Tanks

• Hot water tanks are a common form of sensible heat storage for domestic and industrial applications
  • Water is heated using electricity, gas, or solar energy and stored in insulated tanks for later use
  • Stratification occurs within the tank, with hot water rising to the top and cooler water settling at the bottom
• Hot water tanks are used for space heating, domestic hot water supply, and industrial processes
  • In solar thermal systems, hot water tanks store excess solar energy collected during the day for use at night or during periods of low solar radiation
  • Industrial applications include process heat storage and combined heat and power (CHP) systems

Molten Salt and Underground Thermal Energy Storage

• Molten salt storage uses high-temperature molten salts as a sensible heat storage medium
  • Salts, such as sodium nitrate and potassium nitrate, have high specific heat capacities and can operate at temperatures up to 565°C
  • Molten salt storage is used in concentrated solar power (CSP) plants to store excess thermal energy and generate electricity during periods of low or no sunlight
• Underground thermal energy storage (UTES) utilizes the subsurface as a sensible heat storage medium
  • Types of UTES include aquifer thermal energy storage (ATES), borehole thermal energy storage (BTES), and cavern thermal energy storage (CTES)
  • UTES systems can store large amounts of thermal energy seasonally, with applications in district heating and cooling, industrial processes, and greenhouse heating

Heat Transfer

Heat Transfer Fluids and Thermal Stratification

• Heat transfer fluids (HTFs) are used to transport thermal energy between the heat source, storage system, and heat sink
  • Common HTFs include water, thermal oils, and molten salts
  • The choice of HTF depends on factors such as operating temperature range, thermal stability, and compatibility with system components
• Thermal stratification is the formation of layers with different temperatures within a storage system
  • Stratification occurs naturally in hot water tanks due to the density difference between hot and cold water
  • Maintaining thermal stratification improves the efficiency of sensible heat storage systems by reducing mixing between hot and cold regions
  • Inlet and outlet designs, such as diffusers and baffles, can help maintain stratification and minimize mixing in storage tanks