Development of a Structured Thermocline Thermal Energy Storage System

University of Arkansas

R. Paneer Selvam (PI)

Matt Strasser (GRA)

Paper # 0074

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Presentation Outline • Methods of Thermal Energy Storage • Benefits of a Structured Thermocline TES System • Numeric Model of a Structured Thermocline • Modeling Results and Summary • Conclusions • Acknowledgments • Questions

Goal: Develop a Thermal Energy Storage (TES) System to Increase Economic

Viability of Concentrating Solar Power Plants (CSPs)

INTRODUCTION

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• Latent Heat (Material Phase Change) • (+) high energy storage density • (-) more complex heat transfer designs

• Chemical Storage (Material Chemical Change) • (+) very high energy storage density • (-) numerous health and safety concerns, including toxic and flammable

chemicals

• Sensible Heat Storage (Material Temperature Change) • (+) relatively simple heat storage/retrieval • (-) lower energy storage density

METHODS OF THERMAL ENERGY STORAGE

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BENEFITS OF A STRUCTURED THERMOCLINE TES SYSTEM

• Thermocline TES vs. Two-Tank System • (+) Only 1 Stainless Steel Tank is Necessary • (+) Dual Media System Decreases Necessary Volume of

Expensive HTF • Estimated Cost of Packed Bed Thermocline TES is 35% Less

than Two Tank System Cost

• Structured Thermocline vs. Packed Bed Thermocline • (+) Issue of Thermal Ratcheting Avoided • (+) Filler Material Geometry can be Optimized for Optimum

Heat Transfer to and from HTF

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NUMERIC MODEL OF A STRUCTURED THERMOCLINE

Cross Section of Populated Thermocline Tank: Axisymmetric (Left) and Parallel Plate (Right)

• Finite Difference-Based Numeric Model Developed to Optimize Structured Concrete Geometry

• Two Models Investigated: Axisymmetric and Parallel Plate

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NUMERIC MODEL OF A STRUCTURED THERMOCLINE

• Model Parameters • Temperature: 300 C - 585 C • Length: 16 m • Number of Concrete Cells: 1 Cell

• Model Variables

• Inner/Outer Radius/Thickness • Heat Transfer Fluid Flow Rate • Charging/Discharging Cycle Time

Axisymmetric (Top) and Parallel Plate (Bottom) Cells Considered in Design (Note Hatched Region is Cell Considered in Parallel Plate Model)

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NUMERIC MODEL OF A STRUCTURED THERMOCLINE

Boundary Conditions • Adiabatic Exterior Surfaces • Constant Inlet/Outlet HTF

Temperatures: • Inlet: T_hot • Outlet: T_cold

• No Heat Transfer in ‘Z’ Direction Outside of the limits of 0 < ‘Z’ < L

• Rate of Heat Convection to the Concrete Surface from the HTF Equals Rate of Heat Diffusion from the Concrete Surface into the Concrete

Illustration of Axisymmetric Model’s Boundary Conditions

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MODELLING RESULTS AND SUMMARY

• Models Evaluated by 2 Criteria: • Thermal Stratification • Charge-Discharge Efficiency

• Numerous Trials of Each Model:

• 32 for Axisymmetric • 20 for Parallel Plate

• Optimized Charge-Discharge Efficiencies:

• 62.58% for Axisymmetric • 65.59% for Parallel Plate

Example of Thermocline Stratification During 5-Hour Charge Cycle (Axisymmetric Model)

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MODELLING RESULTS AND SUMMARY

TES System Evaluation • Parameters in Evaluating TES System Performance

• TES System Volume Considered: 1 m x 1 m x 16 m (length x width x height) • Optimized 5-hr Charge and Discharge Cycles • TES Charged Until: THTF,out = 385 C • TES Discharged Until: THTF,out = 490 C

• Energy Retrieved from Unit Cross Section of Each Model

• Axisymmetric: 12.22 kWh • Parallel Plate: 16.41 kWh

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CONCLUSIONS

• A Structured Thermocline TES is a Viable Option to Decrease TES Cost Compared to the Cost of a Two Tank System

• The Structured Filler Material’s Geometry Should be Optimized to Maximize Heat Transfer Between the HTF and Filler Material • A Parallel Plate Filler Material Model Provided Higher Discharge

Efficiency and Energy Storage Capacity than a Axisymmetric Model • Axisymmetric Model: 62.68 % and 12.22 kWh (Per Unit Cross

Section) • Parallel Plate Model: 65.59 % and 16.41 kWh (Per Unit Cross

Section)

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FUTURE WORK

• Alternative Operating Temperature Limits Could be Considered

• Alternative Structured Filler Material Arrangements Could be Considered

• A Cost Evaluation Could be Conducted to Scale the Viability of a Structured Thermocline TES System Against Alternatives • Dual-Tank, Single Medium • Single-Tank, Packed Bed Thermocline

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ACKNOWLEDGMENTS

This research work was supported by a grant from the U.S. Department of Energy (Grant # DE-FG36-o8G018147) through the University of Arkansas. The opinions expressed do not reflect those of the research sponsor.

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QUESTIONS

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MODEL VARIABLE RANGES AND OPTIMIZED MODELL VARIABLES

Variable Range

Inner Radius 0.0127 m - 0.03175 m

Outer Radius 0.04445 m – 0.0762 m

Time 4 hr – 6 hr

Velocity 0.0015 m/s – 0.01 m/s

Number of Tubes 1 tube

Length of the Thermocline 16 m

Temperature Range 300° C - 585° C

Variable Range

Inner Thickness 0.0127 m - 0.01905 m

Outer Thickness 0.0508 m – 0.0762 m

Time 4 hr – 6 hr

Velocity 0.001 m/s – 0.003 m/s

Number of Tubes 1 tube

Length of the Thermocline 16 m

Temperature Range 300° C - 585° C

Axisymmetric (TOP) and Parallel Plate (BOTTOM) Model Variables and Ranges

Model Axisymmetric Parallel Plate

RI or TI (m) 0.025 0.01905

RO or TO (m) 0.05 0.05715

0.0015 0.0015

5 5

ES (kWh) 0.153 1.43

0.0015 0.0012

5 5

ER (kWh) 0.0959 0.938

Eff. (%) 62.68 65.59

Optimized Variables for Each Model

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PARALLEL PLATE CHARGE AND DISCHARGE

Parallel Plate Model Charge (LEFT) and Discharge (RIGHT) Cycles (Final Condition of Charge Cycle is Initial Condition of Discharge Cycle)

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CURRENT WORK AT UNIVERSITY OF ARKANSAS

• Suitable Concrete Mixtures have been Designed • Thermal Cycling Tests: 300-600 C • Thermal Conductivity: 2 W/m^2 K • Specific Heat Capacity: 900 J/kg K • Cost: $0.78-$3.18/kWhtherm

• Large Scale Thermocline Test System Constructed

• Axisymmetric Model Being Tested: 4 in x 4 in x 36 in Beams • Operating Temperatures: 300-585 C • Testing is in Progress

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THERMOCLINE TES SYTEM TESTING

Thermocline Test System (LEFT) and Populated Thermocline Tank (RIGHT)