Molten salts storage

Energy is used to heat the storage medium, such as molten salt, without changing its phase, where it is stored as heat and used when needed. Electricity can be generated by using the heat stored in the molten salt to produce steam to drive a turbine. The period during which the system provides full rated output can range from eight to 24 hours or more. Molten salts are inorganic chemical compounds, typically a mixture of nitrate/nitrite salts (e.g. a mixture of potassium and sodium nitrate), which have high boiling points, low viscosity, low vapour pressure and high volumetric heat capacities, i.e. they require a relatively small storage tank. When selecting the chemical mixture, it is advantageous to have the lowest possible melting point and the highest possible boiling point to maximise the available temperature range for the molten salt. Salts that would normally be solid at ambient temperatures are maintained at temperatures above their melting points so that they are always in liquid, i.e. molten, form and can be heated to around 560°C. The efficiency of the system is maximised at higher temperatures, but the maximum operating temperature is limited by various factors such as corrosion, salt decomposition and salt vapour pressure. Molten salt storage is often associated with concentrated solar power (CSP) plants. The first large-scale implementation of this technology was the Andasol 1 CSP plant in Spain, which became operational in 2008. This was soon followed by Andasol 2 and Andasol 3 in 2009 and 2011 respectively. The storage system uses a mixture of 60% sodium nitrate and 40% potassium nitrate as the storage medium, and the three plants have a storage capacity of 1 010 MWh and 120 MW. Molten salts can also be used as stand-alone bulk thermal storage, known as Carnot batteries, where electricity is used to generate heat via a heat pump, which is stored in the molten salt and then connected to a turbine, which may be an existing one in a fossil fuel power station. Further research is aimed at reducing overall costs, in particular improving efficiency by increasing the operating temperature range of the molten salt.