Molten Salt

    Molten salts have high boiling points, low viscosity, low vapor pressure, and high volumetric heat capacities.A higher heat capacity corresponds to a smaller storage tank volume. In choosing the chemical mixture, it is advantageous to have the lowest possible melting point (and highest boiling point) to maximize the available temperature range for the molten salt. If the melting point is too high, additional heating may be required to prevent freezing.  Salts used for storage (such as sodium nitrate NaNO₃ and potassium nitrate KNO₃) have melting points between 300-500°C and volumetric heat capacities between 1670 - 3770 kJ/m³°C.  Sodium hydroxide NaOH has a melting point of 320°C and can be used up to 800°C, but is highly corrosive.  Commercially available "HITEC" salt used in solar plants consists of potassium nitrate (53% by weight), sodium nitrite NaNO₂ (40% by weight), and sodium nitrate (7% by weight) with a liquid temperature range of 149 - 538°C.