Measuring heat loss and temperature changes in molten salt storage tanks

An important component of a solar power plant is the thermal energy storage unit. Molten salts are widely used for solar energy storage, due to their large specific heat, low vapor pressure, low melting point and relative stability. Although many computational modeling studies have been carried out for molten salt systems, little experimental research has been reported.

Zhou et al. report experimental measurements of heat loss and temperature profile changes in the molten salt tank of a pilot solar plant.

The salt used in the study was a mixture of 60% sodium nitrate and 40% potassium nitrate, which is commonly used in commercial plants. The salt was heated to 560^oC with four 30 kW electric heaters near the bottom of the 2500 mm tall storage tank.

Thermocouples were placed inside the tank at various positions. These were used to probe the cool-down behavior of the tank by monitoring temperature and heat loss changes once the electric heaters were shut off.

The system heats up slowly; five or more days are required for the tank and its surroundings to reach a stable final temperature. Once the heaters are switched off, however, the temperature of both the salt and tank shell plummet rapidly. If the temperature falls below 220^oC in any part of the tank, the salt will solidify, damaging the power plant.

The investigators found that the cooling rate was essentially the same from the bottom to the top of the tank, including the vapor phase above the salt. The temperature of the molten salt did, however, have a large effect on heat loss. Ambient weather conditions, such as air temperature and wind, will be considered in future work.

Source: “An experimental investigation of temperature distribution and heat loss in molten salt tanks in concentrating solar power plants,” by Hao Zhou, Hua Shi, Yifan Zhu, and Wenfeng Fang, Journal of Renewable and Sustainable Energy (2020). The article can be accessed at https://doi.org/10.1063/1.5131071 .