Effect of initial temperature of water in a solar hot water storage tank on the thermal stratification under the discharging mode

Abstract

Thermal stratification in a storage tank is an important factor affecting hot water output. There are many factors affecting thermal stratification, and the initial temperature in the tank is a notable one. In this paper, a typical solar hot water storage (HWS) tank, which is widely used in practical applications, has been selected to study numerically the effect of the initial temperature on the thermal stratification under the discharging mode. The flow mechanism, temperature distribution, thermal stratification, and thermal mixing mechanism in the tank are analyzed with the numerically simulated results for five initial temperatures in the tank. In addition to the qualitative observation, a quantitative analysis of the effect of the initial temperature in the tank has also been conducted in terms of several key dimensionless parameters which quantify the extent of the thermal stratification in the tank. The results show that the dimensionless exergy decreases with the dimensionless time. When the dimensionless time increases from 0.1 to 0.8, the dimensionless exergy decreases by nearly 0.4. Additionally, a higher initial temperature leads to a smaller extent of the initial mixing and a larger output rate of hot water. When the initial temperature in the tank increases from 313 K to 353 K, the output rate of hot water increases by nearly 11%. Moreover, the extent of the thermal stratification increases almost linearly with the initial temperature in the tank. When the initial temperature in the tank increases from 313 K to 353 K, a 17% increase in the thermal stratification has been achieved indicating that the initial temperature in the tank has a significant effect on the thermal stratification, which will be insightful for the better design of solar hot water systems.