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Annual Review of Heat Transfer, Vol. 15, p.131-177 https://doi.org/10.1615/AnnualRevHeatTransfer.2012004651 2.1.4. Molten salt systems For temperatures above 100°C, molten salts are attractive candidates for sensible heat storage in liquids. The major advantages of molten salts are low costs, high thermal stability and low vapor pressure. The low vapor pressure results in storage designs without pressurized vessels. In general there is experience with molten salts from a number of industrial applications related to heat treatment, electrochemical reactions and heat transfer. The application of salts requires the consideration of the lower temperature limit defined by the melting temperature. One major difficulty with molten salts is unwanted freezing during operation. Freezing must be prevented in the piping, the heat exchanger and in the storage tanks. Hence, often auxiliary heating systems are required. Other limitations of molten salt storage may arise due to aspects such as high costs of the storage media, the risk of corrosion and difficult handling of hygroscopic salts. The thermal stability of the salts defines the upper temperature limit. Salt mixtures, rather than single salts, have the advantage of a lower melting temperature. These mixtures can have similar thermal stability limits as the single salts of the mixtures. Hence, salt mixtures, such as eutectics, can have a wider temperature range compared to single salts. For solar thermal power plants, alkali nitrate salt mixtures, and to some extend alkali nitrite salts, are the preferred candidate fluids for TES. Typically a non-eutectic salt mixture of 60 wt% sodium nitrate and 40 wt% potassium nitrate is utilized. This mixture is commonly called solar salt. The eutectic mixture has a melting temperature of 222 °C and the thermal stability limit is at about 550 °C. A large scale example of a direct storage concept is the Solar Two central receiver power plant using molten salt as both, heat transfer fluid and heat storage medium (Figure 7). This demonstration power plant was erected in 1994 on basis of the Solar One facility and was operated until 1999. The maximum electrical power was 11 MW. The two tank storage system with a total volume of about 1700 m3 had an inventory of 1400 tons of a mixture of molten solar salt. The thermal capacity of the storage system operated between 565 °C and 290 °C was 107 MWh, allowing the operation of the turbine for three hours (Pacheco 2002a). 290°C Cold Salt Tank 565°C Steam generator Heliostat field Molten Salt loop Hot Salt Tank Turbine G Condensor Figure 7: Simplified scheme of Solar Two power plant with central receiver and direct storage of molten salt used as heat transfer fluid.PDF Image | Annual Review of Heat Transfer
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