Solar domestic hot water: numerical and experimental study of the thermal stratification in a storage tank

A numerical study of the thermal stratification in a solar storage tank, validated with experiments is presented. The experimental study has been carried out on three solar domestic hot water systems. Each system is characterized by two parameters: the aspect ratio of the tank and the system design factor. The experimental approach consists of recording the temperature profile within each storage tank during one working day under the same real operating conditions. The numerical approach consists of simulating an operating day of the whole system. The effects on the temperature profile in the storage tank of the aspect ratio, the collector flow rate and the draw off time of hot water have been investigated. Results show that the quality of the thermal stratification depends on the critical aspect ratio, which is equal to 0.5. The daily efficiency and the solar fraction of the system increase inversely proportional to the aspect ratio. The developed simulation model allows to determine the appropriate moment of the day to proceed with the first draw-off of hot water with a maximum exergy output. This moment confirmed by the experimental results is found to be located at about three hours after solar noon