CFD Analysis of a Buffer Tank Redesigned with a Thermosyphon Concentrator Tube

Abstract

This study analyzes a buffer tank simulated in both continuous operation mode and heating mode using CFD techniques. The analysis is focused in the thermal behavior of the tank, especially in parameters such as heat exchanged, heating time, and temperature distributions into the tank, in order to propose a better design. The results of the different simulations show that the tank heats water extremely slowly and extremely evenly when producing domestic hot water (DHW), which negatively affects the thermal stratification that is critical for rapidly reaching the DHW temperature. Therefore, the main problem of the tank is an inefficient heat exchange and a poor distribution of temperature. In order to overcome these operational limitations, a new design is proposed by installing a tube inside the tank that encloses the heating coil and sends hot water directly to the tank top region such that high-temperature DHW is rapidly provided, and thermal stratification is improved. Several simulations are performed with different open and closed configurations for the outlets of the inner tube. The different results show that the heating times significantly improve, and the time needed to reach the 45 °C set point temperature is reduced from 44 to 15 min. In addition, the simulations in which the opening and closing of the water outlets are regulated, the outlet DHW temperature is kept within 45–60 °C, which prevents overheating to unsafe use temperatures. Furthermore, the results of the simulation in continuous operation mode show a clear improvement of thermal stratification and an increase in the heat transmitted to the inside of the tank.

Diagnostics of a Domestic Hot Water Storage Tank under Operating Conditions

“…Furthermore, improved stratification can be achieved by insulating the inner wall surface with an insulating material or other low thermal conductivity material compatible with the stored liquid [6][7][8]. At present, a frequent design solution for stratification of water in the storage tank is a tank with a built-in thermosiphon heat exchanger [9,10]. Due to the special design in the form of a thermosiphon inside the heat exchanger, hot water preparation time is reduced by up to 65% compared to a storage tank fitted with a conventional heat exchanger.…”