Thermal characteristics of a medium- and high-temperature latent heat storage system under various operating conditions

• A medium-temperature thermal energy storage system with single-tube helical fins was developed. • Through visualization windows, the charging and discharging processes were concretized. • Discussions were held on the system's performance under different operating conditions. • The graded heating achieved a stabilized average power of 1.234 kW during charging. Thermal energy storage units effectively address the spatiotemporal mismatch between energy production and demand. However, research on medium- and high-temperature latent thermal energy storage systems remains relatively scarce. This paper presents a small-scale, single-tube setup employing a spiral finned tube as the heat exchanger, leveraging its large surface area and capacity for natural convection. In the experiment, heat transfer oil, heated to a specific temperature in the boiler, enters the latent heat storage unit, exchanging heat with the phase-change material through the tube and spiral fins. The effects of heat transfer fluid flow rate and direction on the system's heat transfer performance were investigated by varying the oil's volumetric flow rate and inlet position. The underlying principles governing heat exchange are also discussed. Furthermore, a graded heat storage experiment was conducted, comparing the instantaneous power of the heat transfer oil with that of isothermal heat storage, revealing a potential method for power smoothing. A novel feature of the rig is the installation of a visualization window on one side, allowing for observation of the salt's state during both charging and discharging processes. The results show that the setup can store 5.5 MJ of energy in 33.35 kg of phase change material within 1.5 h. Increasing the heat transfer oil volumetric flow rate from 19.8 L/min to 33 L/min reduced the charging time by 33.2% and improved charging efficiency from 66.2% to 80.7%. During discharging, the solidification of PCM near the finned tube increased thermal resistance, resulting in a 36.1% reduction in discharging time, but only a slight increase in discharging efficiency from 65.7% to 69.9%. Experiments with varying inlet positions indicated that, at a flow rate of 33 L/min, charging should be initiated from the top of the heat exchange unit, while discharging should begin from the bottom. The graded heating process achieved a stabilized average power of approximately 1.234 kW during the charging process, significantly reducing the power fluctuation.