RIO 5 - World Climate & Energy Event

Thermal energy storage is an essential component of solar heating systems. A numerical analysis of thermal stratification in hot water storage tanks suitable for solar water heating applications is presented in this paper. Transient, axi-symmetric, turbulent natural convection in fully and partially charged storage tanks for varying Rayleigh Number in the range of 10 to 10 is studied. Mix Number quantifies mixing inside the storage tank whereas Exergy efficiency gives performance based on the energy storage capacity. Numerical results showed good agreement with published experimental results of Abdoly and Rapp [5]. In fully charged storage tank, higher aspect ratio supports stratification. The transient temperature profiles in the bulk fluid reveal formation of stratified layers in fully charged tank and degradation of stratified layers in partially charged tanks due to mixing. Mix Number values, which decrease with increasing aspect ratio, show greater mixing for fully charged tank. The tank charged to 1⁄2 capacity showed less mixing compared to those charged to 1⁄4 and 3⁄4. Providing outer tank wall insulation reduces mixing considerably. The results indicate that the heat loss through the walls and insulation to the ambient is dominant compared to thermal diffusion across the thermocline. Exergy efficiency of the storage tanks shows higher values at larger aspect ratio which is attributed due to decrease in mixing in comparison to smaller aspect ratios. It is also seen that with larger aspect ratio, smaller wall thickness and better insulation, mixing can be reduced considerably. The influences of natural convection in the trends are discussed. Introduction Natural convection plays a major role in many thermal systems including thermal energy storage systems. Accurate designing of solar heating or cooling thermal energy storage tanks generally requires an account of stratification within the storage tanks, since the overall system performance is significantly affected by the temperature distribution inside the tank. The heat loss from the stored fluid to the ambient decreases the temperature of the fluid near to the tank wall, thereby increasing its density. Due to buoyancy effects, the dense fluid layer moves downward, resulting in development of temperature gradients inside the storage tanks. Natural convective heat transfer and thermal diffusion between the hot and cold fluid are responsible for mixing inside the fully and partially charged storage tank. The operation of hot water storage tanks for thermal energy storage is classified as static mode and dynamic mode. The static mode is further classified into fully charged and partially charged mode. Dynamic mode of operation includes charging and discharging cycle. In static fully charged mode, the storage tank is initially completely filled with hot water at a constant temperature (T1) and is subjected to convective heat loss from the tank walls to the ambient. In static partially charged mode, the storage tank is initially charged with different Natural Convection in Stratified Hot Water Storage Tanks 268 levels of hot and cold water separated by thermocline. In dynamic discharging mode the storage tank is initially filled with warm water, which is drawn from the top of the tank to the load and the returning cold water from the load is charged at the bottom of the storage tank. In dynamic charging mode the storage tank is initially filled with cold water and hot water is charged at same flow rate as the cold water is discharged at the bottom. Many numerical studies on thermal stratification have been performed showing the resultant temperature distribution inside the storage tank. Hsieh and Lien [1] made a numerical analysis of the turbulent natural convection in enclosures with differentially heated vertical walls showing the best overall performance in terms of mean velocities, temperature and turbulence quantities. Bouhdjar and Harhad [2] developed a two-dimensional model of mixed convection flow in thermal storage tank with varying aspect ratios using finite volume method to determine the transient thermal storage efficiency of thermal energy storage tank. Nelson et al. [3] presented one-dimensional transient heat conduction model to describe the decay of the thermocline in a stratified water tank. The parameters influencing the operation of stratified thermal energy storage for cool storages was examined. Shyu et al. [4] presented theoretical and experimental studies on the stratification decay in stratified tanks and the effects of tank wall thickness and insulation resistances. Abdoly and Rapp [5] neglected tank wall conduction but considered the effects of thermal insulation, and showed that the heat loss through the insulation to the ambient is more than the heat conduction across the thermocline. The study of literature reveals a lack of information on the mixing inside the tank and the resultant energy capacity of the stored fluid in the heat storage tanks. In the present study, an axi-symmetric transient conjugate heat transfer model that accounts for fully charged and partially charged hot water storage tank is considered. The model also considers heat transfer from the fluid to the ambient and effects of aspect ratio, initial charge in partially charged tank, and also energy loss due to the decay of thermocline. Mix Number and Exergy efficiency defines the performance of the hot water storage tanks. Physical Model and Governing Equations Numerical investigation is carried on cylindrical fibreglass hot water storage tank with aspect ratio (length/diameter) ranging from 1 to 4 is shown in Figure. 2.