☀️Concentrated Solar Power Systems Unit 4 – Heat Transfer Fluids & Thermal Storage

Fundamentals of Heat Transfer

• Heat transfer involves the exchange of thermal energy between systems or within a system
• Three primary modes of heat transfer: conduction, convection, and radiation
  • Conduction occurs through direct contact between substances (solids, liquids, or gases)
  • Convection involves the transfer of heat via the movement of fluids or gases
  • Radiation is the transfer of energy through electromagnetic waves without requiring a medium
• Thermal conductivity is a material property that quantifies the ability to conduct heat
• Fourier's law describes heat conduction and relates heat flux to the temperature gradient and thermal conductivity
• Newton's law of cooling describes convective heat transfer and relates the rate of heat loss to the temperature difference and convective heat transfer coefficient
• The Stefan-Boltzmann law describes radiative heat transfer and relates the total radiant heat power to the fourth power of the absolute temperature and emissivity
• Thermal resistance is a measure of a material's ability to resist heat flow and is influenced by the material's properties and geometry

Types of Heat Transfer Fluids

• Heat transfer fluids (HTFs) are used to transport thermal energy in concentrated solar power (CSP) systems
• Commonly used HTFs include:
  • Molten salts (nitrate salts, chloride salts)
  • Synthetic oils (Therminol VP-1, Dowtherm A)
  • Water/steam
  • Air
  • Supercritical fluids (supercritical CO2)
• Molten salts have high thermal stability, low vapor pressure, and good heat transfer properties
• Synthetic oils have a wide operating temperature range and good thermal stability but lower thermal conductivity compared to molten salts
• Water/steam is readily available and has high specific heat capacity but requires high pressures for high-temperature applications
• Air is abundant and non-toxic but has low heat transfer coefficients and requires large heat exchangers
• Supercritical fluids have high heat transfer coefficients and low viscosity but require high operating pressures

Properties of Thermal Storage Materials

• Thermal storage materials are used to store excess thermal energy for later use in CSP systems
• Key properties of thermal storage materials include:
  • High specific heat capacity to store more energy per unit mass
  • High thermal conductivity for efficient heat transfer
  • High density to minimize storage volume
  • Chemical and thermal stability over the operating temperature range
  • Low cost and wide availability
• Sensible heat storage materials store energy by increasing their temperature (concrete, molten salts, rocks)
• Latent heat storage materials store energy through phase change at a constant temperature (phase change materials like paraffin wax, salt hydrates)
• Thermochemical storage materials store energy through reversible chemical reactions (metal hydrides, carbonates)
• Dual-media storage systems combine a solid storage medium (rocks, ceramics) with a heat transfer fluid (air, molten salt) for improved heat transfer

Heat Transfer Fluid Selection Criteria

• HTF selection depends on various factors related to the specific CSP application and operating conditions
• Operating temperature range is a crucial factor, as the HTF must remain stable and maintain its properties over the desired temperature range
• Thermal stability is essential to prevent HTF degradation and ensure long-term performance
• Compatibility with materials of construction (piping, heat exchangers, storage tanks) to minimize corrosion and material degradation
• Low vapor pressure to reduce the need for pressurization and minimize leakage risks
• High specific heat capacity and thermal conductivity for efficient heat transfer and storage
• Low viscosity to minimize pumping power requirements and improve heat transfer coefficients
• Environmental and safety considerations, such as toxicity, flammability, and ecological impact
• Cost and availability of the HTF, considering both initial investment and long-term operational costs

Thermal Storage System Design

• Thermal storage systems are designed to store excess thermal energy and dispatch it when needed
• Two-tank direct storage systems use separate hot and cold storage tanks, with the HTF serving as both the heat transfer and storage medium (molten salt systems)
• Two-tank indirect storage systems use a separate HTF and storage medium, requiring additional heat exchangers (oil-based systems with molten salt storage)
• Single-tank thermocline systems use a single tank with a thermal gradient, where hot and cold fluids are separated by buoyancy forces (molten salt or dual-media systems)
• Packed-bed thermal storage uses a solid storage medium (rocks, ceramics) with a heat transfer fluid flowing through the bed
• Thermal storage capacity is determined by the system's size, storage medium properties, and operating temperature range
• Insulation and tank design are critical to minimize heat losses and maintain thermal stratification
• Heat exchangers are designed to optimize heat transfer between the HTF and storage medium while minimizing pressure drops and thermal stresses

Efficiency and Performance Metrics

• Thermal storage efficiency is a key performance metric, defined as the ratio of energy recovered to energy stored
• Round-trip efficiency accounts for both charging and discharging processes and includes heat losses and parasitic energy consumption
• Exergy efficiency considers the quality of stored energy and its ability to perform useful work
• Capacity factor is the ratio of actual energy output to the maximum possible output over a given period
• Dispatchability is the ability to provide energy on demand and is enhanced by thermal storage
• Thermal cycling stability refers to the ability of the storage system to maintain performance over repeated charging and discharging cycles
• Heat transfer effectiveness quantifies the performance of heat exchangers in the storage system
• Pumping power and pressure drop should be minimized to reduce parasitic energy consumption and improve overall system efficiency

Safety and Environmental Considerations

• HTF leakage and spills can pose safety and environmental risks, requiring proper containment and mitigation measures
• Flammability and autoignition temperatures of HTFs must be considered in system design and operation
• Toxicity and ecological impact of HTFs should be evaluated, and appropriate handling and disposal procedures must be followed
• High-temperature molten salts can cause severe burns and require specialized personal protective equipment
• Pressure relief valves and emergency shutdown systems are essential to prevent overpressurization and minimize the consequences of equipment failures
• Regular maintenance and inspection of storage tanks, piping, and heat exchangers are necessary to ensure safe and reliable operation
• Environmental impact assessments should consider the entire life cycle of the thermal storage system, including material sourcing, manufacturing, operation, and decommissioning
• Proper training and safety protocols for personnel involved in the operation and maintenance of thermal storage systems are crucial

Emerging Technologies and Future Trends

• Advanced HTFs, such as nanofluids and ionic liquids, are being investigated to enhance heat transfer properties and thermal stability
• Phase change materials (PCMs) with high energy density and tunable melting points are being developed for latent heat storage applications
• Thermochemical storage systems using reversible chemical reactions are being explored for high-density, long-duration storage
• Supercritical CO2 power cycles are being investigated for their potential to improve CSP system efficiency and reduce storage costs
• Integrated thermal storage solutions, such as thermocline systems with dual-media storage, are being optimized for cost-effectiveness and performance
• Advanced insulation materials, such as aerogels and vacuum insulation panels, are being developed to minimize heat losses in storage tanks and piping
• Intelligent control strategies and predictive maintenance techniques are being implemented to optimize storage system operation and extend component lifetimes
• Hybridization of CSP with other renewable energy technologies, such as photovoltaics and wind power, is being explored to enhance dispatchability and grid integration