Study of structure optimization of solar hot water storage tank with conical top

Abstract: Solar hot water storage tank is one of the key thermal storage equipment of the solar energy system and it has significant effect on the reliability of thermal energy supply. For the solar hot water storage tank, its optimal structure and reasonable fluid parameters are very important for improving the solar energy system efficiency. Published references indicate that the hot water storage tank with conical top has better performance than the hot water storage tank with plane top. But there is no study about the relationships among the cone vertex angles, fluid parameters and the thermal stratification effect for the hot water storage tank with conical top. This paper mainly aims to obtain the optimal structure of the solar hot water storage tank with conical top and the operating parameters. The standard k-ε model was adopted to analyze the influence of cone vertex angle and operating parameters on the thermal stratification of the hot water storage tank with conical top. The results show that the studied cone vertex angles have a little effect on thermal stratification and the hot water region slightly expands with the decreasing of cone vertex angle. In terms of the optimal water storage tank with the conical top height of 0.09 m and cone vertex angle of 159.6°, the hot water region is reduced with the increasing of cold water inlet velocity and the decreasing of hot water inlet temperature. However, higher hot water inlet temperature has less effect on the hot water region on condition of higher cold water inlet velocity than that of lower cold water inlet velocity. Thus, in practical engineering operation, to improve hot water inlet temperature under lower cold water inlet velocity can obtain better thermal stratification effect. For given fluid parameters, the difference between the hot water outlet temperature and cold water outlet temperature increases with the increasing of cold water inlet velocity. Increasing hot water inlet temperature under lower cold water inlet velocity makes a slightly larger contribution to improving the difference between the hot water outlet temperature and the cold water outlet temperature. To improve hot water inlet temperature from 333 to 343 K has no effect on the cold water outlet temperature in this study. One case with hot water inlet temperature of 333 K, cold water inlet temperature of 303 K, hot water inlet velocity of 0.05 m/s and cold water inlet velocity of 0.9 m/s and the other case with hot water inlet temperature of 343 K, cold water inlet temperature of 303 K, hot water inlet velocity of 0.05 m/s and cold water inlet velocity of 0.9 m/s are suitable to the running mode of "low flow flux and high temperature difference". One case with hot water inlet temperature of 333 K, cold water inlet temperature of 303 K, hot water inlet velocity of 0.05 m/s and cold water inlet velocity of 0.1 m/s and the other case with hot water inlet temperature of 343 K, cold water inlet temperature of 303 K, hot water inlet velocity of 0.05 m/s and cold water inlet velocity of 0.1 m/s are suitable for the users with large demand for hot water supply. In order to obtain high efficiency of solar energy system in practical engineering, many factors, such as types of solar collectors, local weather conditions, specific requirements of thermal users and operation modes must be considered simultaneously.