Towards an energy efficiency optimization of solar horizontal storage tanks and circulation pipes integrating evacuated tube collectors through CFD parametric studies

This paper aims to simulate heat generation in two devices, namely the horizontal thermal storage tank and the circulation pipe, which are considered as the main devices constituting solar water heating systems. For this purpose, two dimensional simulations were performed, so as to numerically investigate the proposed configurations. The storage tank and the circulation pipe were considered as a partial discrete heating sources, while the other parts were maintained constantly adiabatic. The heat pipes are located at the tank's bottom wall where an isothermal temperature or a heat flux conditions are applied. Moreover, parametric studies regarding the geometry of the heat pipes were carried out. Three cases were studied: circular, rectangular and triangular configurations. A Prandtl number equal to 7 was used for the simulated case studies. The used equations were also described under detailed assumptions. Isotherms and average temperature evolutions were presented for various configurations in order to provide the achieved results. The storage tank and the circulation pipe were optimized and the energy transfer between the solar collector and the storage tank unit were maximized. Increasing the heat pipe's number and varying their shape could affect the mean time required to heat the stored water inside the horizontal storage tank. It is found that the rectangular shape of the heat pipe is the most effective due to its large heat exchange area, and would heat water to a temperature of 45 °C during 10 h, if the configuration of 6 heat pipes arrangement was used. While the required time to heat water to a temperature of 55 °C is 12.5 h using the configuration of 10 heat pipes arrangement. Besides, in order to achieve the maximal performance of the circulation pipe, the water inlet and outlet must be located respectively at the bottom and at the top to take advantage of the heated water's stratification.