Investigation of the effect of solar water heating of vacuum tube on heat loss coefficient

Abstract

In the solar thermal utilization system, it is necessary to study the factors that affect the heat loss of the hot water storage tank. Therefore, the heat loss coefficient of the hot water storage tank in the all-glass vacuum tubular solar water heater is calculated using experiments and measured data in the present study. Then the problem is analyzed in forced circulation and non-forced circulation conditions. According to the results of the investigation, the heat loss coefficient of the thermal storage tank in the forced circulation state is 12.6 times that of the non-forced circulation condition. The heat loss coefficient is impacted by the tank temperature, intake, and exit water temperatures, circulating water flow rate, wind speed, and ambient temperature, as determined by a linear regression analysis of the experimental data. It is found that in the non-forced circulation, wind speed significantly affects the heat loss coefficient, specifically, the heat loss coefficient increases by 0.49 W/(m2·℃) for every 1 m/s increase of wind speed when all other factors are constant. But wind speed can be neglected in the forced circulation condition. Moreover, it is observed that as the inlet water temperature increases, the corresponding heat loss coefficient increases.