Abstract
A thermal storage tank with internal heat exchange coils is commonly used in solar thermal systems with a collector area below $100m^2$. The coils are installed in the lower part of the tank because the temperature of the upper part of the tank can drop if the outlet temperature of the collector becomes lower than the upper temperature of the tank, which is a kind of temperature reversal. As an alternative to the well-mixed storage tank with lower coil only, we have proposed a tank with lower and upper coils and have achieved superior thermal stratification in the tank, which results in increased collector efficiency and solar fraction. But, the phenomenon of temperature reversal was often observed in the tank when the load or solar radiation changed rapidly. In the present work, revised control was successfully applied, i.e., to heat only the lower coil using a three way valve if temperature reversal occurs and to operate the collector at a low flow rate when the quality of solar radiation is not good.