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Parameters Influencing the Efficiency of thermochemical Storage System,…
Parameters Influencing the Efficiency of thermochemical Storage System
Effect of geometrical parameters
Shape
Conical
Surface Area to Volume Ratio:
A conical tank reduces the surface area to volume ratio of the hot upper part of the tank. This design minimizes heat losses and mechanical stresses because the reduced surface area limits the amount of heat escaping from the tank (Zanganeh et al., 2012).
Mechanical Stress Reduction:
The conical design can decrease mechanical stress on tank surfaces by up to 20% compared to cylindrical tanks (Sassine et al., 2018). However, the complexity and cost of constructing conical tanks are higher.
Thermal Ratcheting Reduction:
Thermal ratcheting refers to the cyclic expansion and contraction of materials under temperature fluctuations, which can lead to structural damage over time. The conical shape helps in reducing these effects by distributing mechanical stresses more evenly (Geissbühler et al., 2018).
Rectangular
Thermo-Mechanical Benefits
Stress Distribution:
Horizontal tanks naturally distribute stress better by reducing the pressure exerted on the tank walls due to particle settling during temperature changes (Esence et al., 2019).
Ease of Installation:
The horizontal layout can be advantageous for certain site installations, although it may not offer the best thermal performance (Touzo, 2021).
Studies and Observations:
Horizontal Geometry:
Studies have shown that horizontal rectangular tanks can maintain a moderate radial thermal gradient without significant thermal stratification issues (Touzo et al., 2020; Nicolas et al., 2019).
Uniform Temperature:
Experimental results indicate that temperatures remain nearly equal along a given axial coordinate, demonstrating effective thermal management even in a horizontal layout.
Cylindrical
Flow Distribution:
The shape facilitates uniform flow distribution. In a cylindrical tank, the fluid dynamics are such that the fluid can circulate evenly without being obstructed by corners or irregular surfaces, which enhances thermal stratification and efficiency.
Vertical Layout Advantages:
Heat Loss Minimization:
The cylindrical shape reduces the surface area exposed to the environment, thus reducing heat loss.
Ease of Installation
: Cylindrical tanks are mechanically simpler to install and maintain compared to other shapes (Melanson and Dixon, 1985).
Natural Convection Reduction:
By minimizing the exchange of material between the hot upper region and the cold lower region, natural convection currents that could disrupt thermal stratification are reduced.
Pressure Resistance
: Cylindrical tanks can withstand internal pressures better than other shapes. This is due to their geometry, which distributes pressure evenly across the tank walls. The absence of corners or edges in cylindrical tanks eliminates points of stress concentration, reducing the likelihood of structural failure (Melanson and Dixon, 1985)
Dimensions: Influence of Tank Height and Diameter
Stratification
Thinner stratification layer thickness improves discharge time with increased reactor height (Xu et al. 2012a).
The aspect ratios H/D reactor and D/H are critical for evaluating performance.
Increasing H/D reactor results in a thinner stratification layer (Xu et al. 2012a; Flueckiger and Garimella 2012; Chang et al. 2015).
Yang et al. (2013) confirmed the positive influence of increasing this aspect ratio.
Zanganeh et al. (2015) showed that a lower D/H reactor aspect ratio provides higher output temperatures and better overall efficiency for cone-shaped storage systems.
Energy Storage Capacity
Chao et al. (2012) found that energy storage capacity increases proportionally by 2.33 times when the height of the reservoir is increased from 6 to 14 m.
Effective discharge time increases from 1.88 to 4.57 hours, a 2.43 times improvement (Xu et al. 2012a).
Studies show that reservoir height and diameter significantly impact performance (Xu et al. 2012a; Yang and Garimella 2013; Cárdenas et al. 2018; Odenthal et al. 2020b; Nicolas et al. 2019).
Practical Considerations
Industrial tanks are typically limited to 12 meters in height (Vannerem 2022).
To increase storage capacity while keeping the height manageable, the diameter of the reservoir may need to be increased, potentially compromising the aspect ratio (Vannerem 2022).
High reservoir height improves efficiency on a laboratory or pilot scale but presents structural challenges at industrial scale.
Cárdenas et al. (2018) suggested that an 𝐻 / 𝐷 tank H/D tank aspect ratio between 0.5 and 0.8 improves discharge efficiency.
Materials
Heat Transfer Fluids (HTFs)
Solid Fillers
particle size
Efficiency
Thermal Conductivity
Smaller particle sizes increase the thermal conductivity of the packed bed, enhancing the efficiency of heat storage and release. For example, particles around 0.2 mm to 2 mm can achieve thermal conductivities in the range of 0.2-0.5 W/m·K
Excessive pressure drops due to very small particles can negate these benefits by increasing the energy required for fluid circulation
Packing Density
Smaller particles result in higher packing densities, increasing the bed's thermal mass and improving its heat storage capacity. However, this also increases the risk of channeling and pressure drop
A balance between particle size and packing density is essential for optimal system performance
Pressure Drop
Particle sizes below 0.2 mm significantly increase pressure drop, resulting in higher energy requirements for gas flow through the bed. Particles in the optimal range of 0.2 mm to 6 mm minimize pressure drop while maintaining good heat transfer properties.
The Ergun equation can be used to quantify pressure drop in packed beds
Heat Transfer
Smaller particles (less than 50 μm) enhance conductive heat transfer due to increased contact area between particles. However, very small particles can lead to channeling and reduced heat transfer efficiency
Optimal particle sizes for packed beds are typically in the range of 0.2 mm to 6 mm, which balance heat transfer efficiency and pressure drop
porosity
low porosity
Enhances thermal conductivity, leading to better heat transfer and higher discharge efficiency.Bagre et al. (2023)
Requires less fluid volume, reducing overall system costsEsence et al. (2017)
high porosity
Results in a thicker thermocline, reducing thermal efficiency.
At very high porosity (>0.7), energy efficiency might increase, though practically challenging.Xu et al. (2012a), Van Lew et al. (2011)
Operating Conditions
Inlet Temperature
Effect on thermocline stability
he inlet temperature directly impacts the stability and thickness of the thermocline layer. Higher inlet temperatures can reduce thermocline thickness and improve discharge efficiency.
Data: Increasing inlet temperature from 100°C to 150°C reduces thermocline thickness by 15% and improves discharge efficiency by 10% .
Temperature difference
The temperature difference between the inlet and the packed bed material can affect the overall energy storage capacity and efficiency. Greater temperature differences typically enhance the storage capacity but might lead to larger thermal stresses.
References: Abdulla and Reddy (2017), Vannerem et al. (2021)
Flow Rate
Fluid velocity
The flow rate of the heat transfer fluid (HTF) influences the heat transfer rates and pressure drops within the packed bed. Optimal fluid velocities maximize heat transfer while minimizing pressure drops and pumping costs.
Data: Optimal flow rates of 30-35 Nm³/h for air in packed beds significantly improve heat transfer without excessive pressure drops .
Optimal flow rate
There is an optimal range for the flow rate that maximizes energy efficiency and minimizes heat losses. Too high or too low flow rates can adversely affect system performance.
References: Bayón and Rojas (2013), Hoffmann et al. (2018)
Pressure
Operating pressure
The efficiency of the thermochemical energy storage (TCES) system can be influenced by the operating pressure. Higher pressures might enhance the reaction rates and improve heat transfer efficiency, but they also demand more robust and expensive construction materials.
References: Rodrigues and De Lemos (2020), Wang et al. (2020)
Impact on mechanical stresses
The operating pressure affects the structural integrity and mechanical stresses of the packed bed. High pressures can increase the mechanical load on the tank walls and solid fillers, leading to potential deformation or failure.
References: Cárdenas et al. (2018), Aly and El-Sharkaw (1990)
Thermal Properties
Thermal Properties
Heat Capacity
Specific heat of HTFs
HTFs with high specific heat capacities can store more energy per unit volume, increasing the energy density of the system. like water vapor
Specific heat of solid fillers
Solid fillers with high heat capacities can store and release larger amounts of thermal energy, increasing the system's storage capacity.
Thermal Conductivity
Conductivity of HTFs
High thermal conductivity HTFs enhance the rate of heat transfer between the fluid and the solid filler, improving overall system efficiency.
Example: Molten salts and certain oils have high thermal conductivities suitable for TCES.
Conductivity of solid fillers
High thermal conductivity materials improve heat transfer rates within the packed bed, enhancing overall system efficiency.
System Integration
Compatibility with CSP Plants
TCES systems can be integrated with Concentrated Solar Power (CSP) plants to store excess thermal energy and release it when solar input is low.
Perspective: Enhancing the compatibility of TCES systems with CSP plants can improve the overall efficiency and reliability of renewable energy sources.
Conclusion: Seamless integration with CSP plants offers a robust solution for managing energy supply and demand.
References: Gil et al. (2010), Medrano et al. (2012)
Scalability
Feasibility for industrial applications
Scaling TCES systems for industrial applications involves addressing challenges related to size, energy capacity, and operational stability.
Perspective: Modular design approaches can facilitate scalability and ease of implementation in various industrial settings.
Conclusion: Developing scalable TCES solutions is essential for broader adoption in industrial applications.
References: Cabeza et al. (2015), Brousse et al. (2017)
Economic Factors
Cost of materials and construction
The initial cost of materials and construction for TCES systems can be high, impacting the overall feasibility.
Perspective: Utilizing cost-effective, readily available materials like phosphogypsum can lower initial costs.
Conclusion: Strategic material selection and efficient construction methods can enhance economic feasibility.
References: Shukla et al. (2017), Sharma et al. (2019)
Operational and maintenance costs
TCES systems require regular maintenance to ensure efficiency and longevity. Costs associated with maintenance depend on the complexity of the system and the durability of materials used.
Perspective: Investing in high-quality, durable materials can reduce long-term maintenance costs.
Conclusion: A balance between initial investment and maintenance costs is crucial for economic viability.
References: Liu et al. (2015), Rodriguez et al. (2016)
Design Considerations
Heat Exchanger Design
Placement and size
Optimal Placement
Proper placement of the heat exchanger within the storage system ensures uniform heat distribution and minimizes thermal gradients.
Placement should consider flow dynamics to avoid dead zones and ensure effective heat transfer.
References: Li et al. (2018), Wang et al. (2019)
Size Considerations
The size of the heat exchanger must match the capacity and requirements of the TCES system.
Larger heat exchangers provide more surface area for heat transfer but may increase pressure drops and system complexity.
An optimal size that balances surface area and pressure drop was found to increase system efficiency by 10-20% in experimental setups.
Material and efficiency
Material Selection:
The choice of material for the heat exchanger affects its thermal conductivity, corrosion resistance, and mechanical strength.Ghasemi et al. (2014)
Materials with high thermal conductivity, such as aluminum and copper, are preferred for efficient heat transfer.Zhang et al. (2016)
Efficiency Impact
High-efficiency heat exchangers minimize thermal losses and improve the overall efficiency of the TCES system.
Material durability and maintenance requirements also impact long-term efficiency and operational costs.
A study showed that using aluminum heat exchangers can improve heat transfer efficiency by up to 15% compared to steel under similar operating conditions.
