A comparative study on the modeling of a latent heat energy storage system and evaluating its thermal performance in a greenhouse

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Abstract

Thermal Energy Storage (TES) systems can be compared with batteries. As batteries can be charged when electricity is available for using during the power failure, TES systems can do the same for the thermal energy, i.e., they can absorb the available heat in one cycle, called charge cycle, and release it in a consecutive cycle, called discharge cycle. Among different kinds of TES systems, Phase Change Materials (PCM) have drawn considerable attention, since by changing from one phase to another, they can exchange a significant amount of energy in a small temperature difference. In this quest, a one dimensional mathematical model is solved using two different techniques and the results are compared together; one method is based on the enthalpy and the other is based on the effective heat capacity as well. Secondly, through eight experiments designed by using factorial approach, effects of inlet air velocity and temperature on the outlet stream has been investigated. The results proved that having a determined temperature difference between the inlet air and the PCM in both hot and cold cycles can enhance the efficiency. Finally, the feasible applications of a LHTES system for reducing the temperature swing in a greenhouse is studied numerically and the results are compared with experimental values. As a result, by using this passive coolant system diurnal internal temperature can be reduced for 10 °C.

Nomenclature

a

Width of flat slabs [m]

A

Area of cross section of duct = ab [m2]

Ai

Area of each face of the greenhouse [m2]

AG

Total area of the greenhouse faces [m2]

b

Air gap between parallel slabs [m]

cp

Heat capacity of PCM [J/kg°C]

cpg

Heat capacity of air [J/kg°C]

De

Equivalent Diameter (m)

Δx

Spatial length [m]

Δt

Time step [sec]

E

Efficiency

h

Heat transfer coefficient between air and flat slabs [W/m2°C]

i

Spatial step counter

j

Time step counter

kp

Thermal conduction of PCM [W/m2°C]

L

Length of the Bed [m]

mACH

Mass flow rate of air change [kg/s]

mLHTES

Mass flow rate of air which passes through LHTES [kg/s]

mp

Mass of PCM [kg]

Nu

Nusselt

P

Perimeter [m]

Pe

Peclete number

Pr

Prantl number

Q

Transferred heat [J]

Re

Reynolds Number

Si

Solar radiation [w/m2]

t

Time [sec]

T

Temperature of the inside air of the LHTES [°C]

TE

Environmental temperature [°C]

TG

Temperature of the inside air of the greenhouse [°C]

Ti

Initial temperature [°C]

Tp

PCM temperature [°C]

U

Dimensionless parameter of regenerator

UG

Natural heat transfer coefficient between the greenhouse and outlet environment [W/m2°C]

Up

Overall heat transfer coefficient [W/m2°C]

V

Total volume of the greenhouse [m3]

x

Length variable [m]

τ

Process termination [s]

τG

Transmittance of the greenhouse cover to the direct solar radiation

ρ

Density of air [kg/m3]

β

Thermal Expansion [K−1]

α

Thermal diffusivity [m2/s]

ν

Air velocity [m/s]

δ

Thickness of slabs [m]

Λ

Dimensionless parameter of regenerator

Π

Dimensionless parameter of regenerator

γG

Constant of the proportion of solar radiation entering the greenhouse

Notes

Acknowledgements

The authors would like to thank “Iranian Fuel Conservation Organization” and research department of Tarbiat Modares University for their financial supports.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Process Engineering Department, Faculty of Chemical EngineeringTarbiat Modares UniversityTehranIran

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