Conceptual design of eccentric micro annular channel electric heater for a thermal energy storage system

Molten salt electric thermal energy storage stands as one of the prominent large-scale thermal energy storage technologies. In this system, low-temperature molten salt from a cold tank is heated using electric heater and then stored in a hot tank. There is a pressing demand for specifically designed high-voltage and high heat flux electric heaters for molten salt electric thermal energy storage, as they offering significant cost savings. For the first time, the horizontal eccentric micro annular channel electric heater, operating at an input voltage of 6.6 kV, is proposed to efficiently heat a chloride salt-based thermal energy storage system. The performance of heating tube is validated through the dielectric voltage-withstand test and thermal endurance test. The simulated method for flow and heat transfer is validated using the test setup based on tap water. The effects of eccentricity and heat flux are discussed. The results reveal that the heating tube can withstand a test voltage of 18.0 kV for one minute without failure and withstand approximately 270 heating and cooling cycles corresponding to temperatures ranging from 673.15 K to 1093.15 K. The flow in the horizontal eccentric micro annular channel electric heater is turbulent under low Reynolds number conditions, and the maximum relative experimental error of the Nusselt number is 25.5 %. As the eccentricity increases, both the friction factor and mean Nusselt number decrease. Conversely, the higher heat flux results in a larger mean Nusselt number but a smaller friction factor. Furthermore, the overall performance of the horizontal eccentric micro annular channel electric heater is evaluated using the entropy production rate. The thermal entropy production rate is much greater than the flow entropy production rate. The horizontal concentric micro annular channel electric heater exhibits the lowest entropy production rate. Finally, an optimal electric heater is proposed based on the entropy production minimum principle, the heat flux of which is at least 5.1 times of traditional heater.