Numerical analysis of thermal energy storage system using packed-bed

ABSTRACT

Solar energy offers an affordable solution to cooking and other household problems of day-to-day life. However, its intermittent nature limits its applications and hence storage is required for the same. In this paper, the numerical simulation of a thermal energy storage system (TES) has been presented. In this analysis, a model is built to estimate the charging and discharging time of packed bed thermal energy storage (PBTES) system. The model consists of a cylindrical container filled with a concrete pebble. The heat transfer fluid used is air passed through the packed-bed. Several parameters are modified to study the impact of pebble diameter and void fraction over the charging and discharging time of PBTES system. The system is built to store energy for18 h with a temperature range between 300 and 120°C, so that it delivers 2.5 kW after charging.

KEYWORDS:

• Sensible thermal energy storage
• concentrated solar power (CSP)
• packed bed
• heat transfer fluid (HTF)

Nomenclature

$Q$	=	Thermal energy stored, W
$C_p$	=	Specific heat, kJ/(kg.°C)
$C{p}_f$	=	Fluid specific heat (air), kJ/(kg.°C)
$C{p}_s$	=	Specific heat of storage solid material (concrete), kJ/(kg.°C)
$m$	=	Mass, kg
$\mathrm{\Delta }T$	=	raise in temperature during charging process, K
$\epsilon$	=	Void fraction into container
${\rho }_f$	=	Density of the fluid, kg/m3
${\rho }_s$	=	Density of the solid, kg/m3
$T_f$	=	Fluid temperature (interior air), K
$T_s$	=	Storage solid material temperature, K
$T_w$	=	Wall temperature, K
$T_{air}$	=	External air temperature, K
$T_{int}$	=	Inlet fluid temperature
K	=	Outlet fluid temperature K
$dt$	=	Time infinitesimal element, s
$v$	=	Velocity of fluid (air), m/s
$d_p$	=	Mean diameter of pebbles, m
$G=\epsilon \rho v$	=	Fluid mass flow rate per unit area in the empty heat storage system,
$\kappa =\frac{\lambda }{\rho Cp}$	=	Diffusivity, m2/s
$h_v$	=	Volumetric heat transfer coefficient, W/(m3.K)
$h_s$	=	Surface heat transfer coefficient between the wall and the solid storage material, W/(m2.K)
$h_f$	=	Surface heat transfer coefficient between the wall and the fluid, W/(m2.K)
$E_s$	=	Energy lost through the wall by the solid, W
$E_f$	=	Energy lost through the wall by the fluid, W
$N{u}_L=0.1{\left(GrPr\right)}^{\frac{1}{3}}$	=	Nusselt number (Warner and Bayley)
$\lambda$	=	Thermal conductivity coefficient
W/(m.K)	=	Thermal conductivity coefficient of fluid, W/(m.K)
${\lambda }_s$	=	Thermal conductivity coefficient of storage solid material, W/(m.K)
${\lambda }_w$	=	Thermal conductivity coefficient of wall, W/(m.K)
$v$	=	Kinematic viscosity of the air, m2/s
$e$	=	The mean space between the solid storage and the wall, m
$\beta$	=	Coefficient of thermal expansion of the fluid,
K	=	Prandtl number
$Re=\frac{\epsilon v{d}_p}{v}$	=	Reynolds number into the container
$Gr=\frac{g\beta \mathrm{\Delta }T{e}^3}{\nu }$	=	Grashof Number

Additional information

Notes on contributors

Paolo Jaunet

Paolo Jaunet is studying in University of Orlens, France. He has done his internship under student exchange program from University of Petroleum and Energy Studies, Dehradun India. Paolo Jaunet lives in France.

Ram Kunwer

Ram Kunwer is working as an Assistant Professor in University of petroleum and energy studies (UPES), Dehradun India. He expertise in solar thermal energy. He has done his post-graduation from Indian Institute of Technology, Delhi. Currently he is pursuing PhD from UPES. Ram Kunwer lives in Dehradun, India.

Geetanjali Raghav

Geetanjali Raghav is working as an Assistant Professor in University of petroleum and energy studies (UPES), Dehradun India. She expertise in solar thermal energy. She has done his post-graduation from Dehradun Institute of Technology (DIT) Dehradun India. Currently she is pursuing PhD from UPES. Geetanjali lives in Dehradun, India.