Experimental study on operating characteristics of nitrate salt-based latent heat thermal energy storage unit

•Thermophysical properties of the eutectic nitrate salt are measured and presented.•Influence of air inlet temperature on phase-change time is weakened with the increasing difference between air inlet temperature and the phase-change point.•Air mass flow rate has much more significant impact on melting time compared with that on solidification time.•Charging and discharging ratio are calculated according to a linear interpolation method of heat loss to the surroundings.•Effectiveness of storage is increased to 52.8% by reducing the air inlet temperature to 190 °C. Inorganic salts are potential phase-change materials for medium- and high-temperature thermal applications. It is essential to acquire knowledge of their behavior in latent heat thermal energy storage for the design of storage devices and the construction of an energy conversion and utilization system. In this study, a eutectic mixture of NaNO3 (mass ratio of 46%) and KNO3 was selected as phase change material, and a nitrate salt-based latent heat thermal energy storage unit was built to experimentally investigate its operating characteristics during charging and discharging. The thermophysical properties of the eutectic nitrate salt were measured and presented. The air outlet temperature remained almost unchanged during the melting of salt in the charging process, but it decreased gradually during the discharging process because the thermal resistance increased with the salt solidification. The melting time was shortened by 31.0% and 38.1% when the air inlet temperature was increased from 260 °C to 270 and 280 °C, respectively. The solidification time was shortened by 22.2% and 33.3% when the air inlet temperature was reduced from 210 °C to 200 and 190 °C, respectively. When the air mass flow rate was increased from 0.964 g/s to 1.446 and 1.962 g/s, the melting time was shortened by 32.4% and 57.4%, respectively, while the solidification was only shortened by 8% and 16%, respectively. The charging ratio or discharging ratio can be calculated through heat-loss evaluation to depict the thermal energy change in a charging or discharging process. The results indicated that charging ratio increased almost linearly with the melting of the salt. The air mass flow rate had a significant impact on this parameter. The influence of the air inlet temperature was gradually weakened with the increasing air inlet temperature. Owing to the coupling effect of heat loss and airflow, the influence of air parameter