An experimental study of a non-eutectic mixture of KNO3 and NaNO3 with a melting range for thermal energy storage

• Enhanced capacities are possible by using anhydrous salt mixtures in the melting range.
• As a suitable binary salt mixture was sodium nitrate and potassium nitrate identified.
• A distribution of the enthalpy of fusion within the melting range was determined.
• Cyclic stabilities of the selected salt mixture were measured.
• The effect of dimension on the cyclic stability was examined in various test rigs.

Thermal energy storage is a key technology for reduced cost solar thermal power generation. This high-temperature application requires storage operation above 100 °C. Possible options are sensible, latent and thermochemical heat storages. A combination of sensible and latent heat storage seems a promising option for thermal energy storage with an increased specific heat capacity. Salt mixtures with a melting range as opposed to a melting point combine the effects of both latent and sensible heat storage. These provide the possibility of utilizing not only latent but in addition sensible heat during the melting and solidification process. The present paper focuses on a binary mixture of 30 wt.% potassium nitrate (KNO3) and 70 wt.% sodium nitrate (NaNO3). The measurement systems include a differential scanning calorimeter, a melting point apparatus, a custom-built adiabatic calorimeter and a lab-scale storage unit. The sample masses ranged from about 20 mg to 156 kg. Tests with the lab-scale storage unit indicate that salt mixtures with a melting range may be successfully utilized in large-scale applications.