Storage tanks

Stratified Thermal Storage Tanks: A Comparative Study of One-dimensional Models

ABSTRACT
In this paper, a survey of the stratified thermal storage tank one-dimensional models available in the literature was carried out. Twelve one-dimensional models used in single stratified tanks were compared. Kadhem and Hamed’s model [18] takes into account the heat loss due to conduction and convection ofthe tank wall and considers the degradation of thermocline during relaxation periods. Alizadeh’s model predicts 1.4% of heat loss from the tank to the ambient.

INTRODUCTION
Energy stores play an important role in conserving energy. Thermal storage devices are widely used in many thermal systems and applications that are characterized by the delay between energy production and consumption,such as thermal solar systems. Water is the most used storage tank medium for its abundance, high specific heat and low cost [1,2]. Due to their simplicity and low cost, these kind of thermal storage tanks and water have been used for load management and energy conservation in an increasing number of installations, especially in many sectors in engineering, such as petroleum, chemical industry, foodindustry, solar energy systems, industrial processes, refrigeration and air conditioning, among others, require the use of thermal storages to optimize the performance of their systems [2, 3, 4]. .Especially many…

Several studies have been carried out to study the performance of thermal storage tanks. One-dimensional models have been reported since the 70s and they have been used to predictthe thermal behavior of storage tanks [3, 4, 5, 6, 9, 10, 11, 18]. More complex models have appeared in the literature. Two-dimensional models have been studied to predict the mixing effect of hot and cold water [7, 8, 12, 13, 14, 15, 16, 17] and three-dimensional models are used to evaluated the effect of storage tank geometry [2, 15, 19]. While two- and three-dimensional models are more capablein accounting for different factors affecting the thermal storage tank performance, they are not suitable for use in large energy systems load management programs due to computational cost. Simpler one-dimensional models may be advantageous since they are computationally more efficient and predict the effect for industrial processes.

In this paper a survey of several types of one-dimensionalmodels for thermal stratification tanks are studied. The objective of this study is to give a suggestion for using the model according to the necessity of its case of study, including advantages and disadvantages.

ONE-DIMENSIONAL MODELS
Oppel et al. [5] developed an explicit finite difference model. The model covers through-flow conditions for charging or discharging the thermal storage tankand conduction and turbulent mixing within the water. The authors developed the “buffer-tank” that allows variable rate flow and eliminates the pseudo-mixing in the algorithm. The energy equation is solving by splitting it in two equations, the conduction and the convection cases. The parameters AMIX and FLOW (Courant number) are used to ensure stability in the numerical simulation. The model wascompared with experimental data and there was good agreement in the comparison.

The model of Zurigat et al. [9] is known as the “effective diffusivity model”. This model is a finite difference model which accounts for turbulent mixing in the tank and heat loss to the ambient surroundings. The energy equation for turbulent flow is solving by splitting it in two equations representing theconduction and convection cases, and handling them with different computational time steps.

The model used by Zurigat et al. was compared with the experiment carried out by them. The results show that the model proposed may break down for Ri m:Ti,n= Vi-∆VTi,n-1+∆VTi-1,n-1 + α2iT0+α1i[ Ti,n-1-T(i-1,n-1)]/α(i) | α1i=KtACpρ∆t∆zi α2i=ULPCpρ∆t∆zi αi= Vi+α2i+α1iA /(2∆zi2) |
Displacement mixing...