The melting process of a freezing molten salt pipe of concentracted solar power plant

Abstract

Concentrated solar power plants are potential to provide based load power of the future electrical power system. Commercial concentrated solar power plants using molten salt as the heat transfer fluid and the energy storage medium have been built and show great promising. However, due to high freezing point, the molten salt has a freezing possibility and may block the pipe. In practical, an electrical heat tracing is installed along the flow path of the molten salt to protect against freezing. Once the molten salt is freezing inside the pipe, the electrical heat tracing can be used to melt the molten salt. This paper studied the melting process of a molten salt (60% NaNO3 and 40% KNO3, referred to as Solar Salt) inside a horizontal pipe by an electrical heat tracing. The molten salt is a binary mixture, its freezing point (511 K) is higher than melting point (494 K). A solid-liquid mixture mushy zone exists between the melting point and the freezing point during the melting process. The enthalpy method was used to simulate the solid-liquid phase change phenomenon of the molten salt. The effect of the natural convection inside the pipe was also considerate. The effect of the installation position and the heating power of the electrical heat tracing on the melting process and the melting time were studied. During the heating process, the molten salt close to the electrical heat tracing melts first and forms a mushy zone. With the assistance of the natural convection, the molten salt in the mushy zone flows upward and gathers in the upper zone of the pipe. Thus, the upper zone of the pipe has a higher liquid fraction than the lower zone, which results in a liquid fraction gradient inside the pipe. The results show than the installation position of the electrical heat tracing effects the gradient of the liquid fraction during the melting process. The increase in the angle between the gravity and the installation position, γ , increases the liquid fraction gradient. It takes a same time to completely melt the molten salt when γ equals to 30° and 60°. When γ is 90°, due to a larger liquid fraction gradient, it needs more 5 min than γ is 60°. Therefore, large γ should be avoided for the practically installation of the electrical heat tracing. The results also show that the melting time is 165 min for a 35 W/m heating power, 85 min for 70 W/m, and 51 min for 140 W/m. Increasing the heating power from 35 W/m to 70 W/m can shorten 48.5% of the melting time, while increasing the heating power from 70 W/m to 140 W/m can only shorten 40% of the melting time. This is because a higher electrical heating power increases the liquid fraction gradient of the mushy zone during the melting process. Thus, the percentages of the shorten melting time decreases with the increases of the heating power. This study gives practical reference to the choose and the installation of the electrical heating tracing of the molten salt pipe.