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Heat and Mass Transfer

, Volume 54, Issue 3, pp 831–839 | Cite as

Numerical model for the thermal behavior of thermocline storage tanks

  • Ismael A. S. Ehtiwesh
  • Antonio C. M. Sousa
Original

Abstract

Energy storage is a critical factor in the advancement of solar thermal power systems for the sustained delivery of electricity. In addition, the incorporation of thermal energy storage into the operation of concentrated solar power systems (CSPs) offers the potential of delivering electricity without fossil-fuel backup even during peak demand, independent of weather conditions and daylight. Despite this potential, some areas of the design and performance of thermocline systems still require further attention for future incorporation in commercial CSPs, particularly, their operation and control. Therefore, the present study aims to develop a simple but efficient numerical model to allow the comprehensive analysis of thermocline storage systems aiming better understanding of their dynamic temperature response. The validation results, despite the simplifying assumptions of the numerical model, agree well with the experiments for the time evolution of the thermocline region. Three different cases are considered to test the versatility of the numerical model; for the particular type of a storage tank with top round impingement inlet, a simple analytical model was developed to take into consideration the increased turbulence level in the mixing region. The numerical predictions for the three cases are in general good agreement against the experimental results.

Abbreviations

List of symbols

a

Fitting constant [m−1]

aw

Ratio between thermal losses area and tank volume [m−1]

C

Specific heat capacity [J/kg-K]

d

Exit diameter [m]

D

Diameter [m]

hw

Coefficient of thermal losses to the surrounding [W/m2-K]

hv

Volumetric interstitial heat transfer coefficient [W/m-K]

k

Thermal conductance [W/m-K]

lmix

Mixing length [m]

L

Height of the tank [m]

\( \dot{m} \)

Mass flow rate [m3/s]

Prt

Turbulent Prandtl number

r

Radius [m]

t

Time [s]

T

Temperature [K]

u

Velocity [m/s]

U

Average exit velocity [m/s]

V

Volume [m3]

x

Downstream distance [m]

Greek letters

α

Thermal diffusivity [m2/s]

ε

Porosity of the storage medium (void fraction)

νf

Kinematic viscosity [m2/s]

νt

Kinematic eddy viscosity [m2/s]

νtotal

Effective viscosity [m2/s]

Ө

Dimensionless temperature

ρ

Density [kg/m3]

σ

Standard deviation related to the spread of the profile across the centerline

δ

Distance from the centerline to the edge of the spreading [m]

Subscripts

*

Subscript indicating dimensionless quantities

eff

Subscript refering to the effective storage medium

f

Fluid

m

Momentum

s

Solid

t

Turbulent

w

Water

ZEF

Zone of flow establishment

Notes

Acknowledgements

The present work was sponsored by the State of Libya, under grant n° 469-2009.

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Copyright information

© Springer-Verlag GmbH Germany 2017

Authors and Affiliations

  • Ismael A. S. Ehtiwesh
    • 1
    • 2
  • Antonio C. M. Sousa
    • 1
  1. 1.Department of Mechanical Engineering, Centre for Mechanical Technology and AutomationUniversity of Aveiro, Campus Universitário de SantiagoAveiroPortugal
  2. 2.Department of Mechanical Engineering, Faculty of EngineeringSabratha UniversitySabrathaLibya

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