Development and Comparison of Alternative Experimental Measurement Methods for Phase State Determination in Latent Heat Thermal Energy Storage Systems

Kurzfassung

Latent heat thermal energy storage is a type of energy storage, which utilizes the latent heat associated with the phase transition of the storage material to achieve high storage energy densities within small temperature ranges. For efficient use of such systems, charging and discharging must be planned, requiring knowledge of the storage system's state of charge. Given the isothermal nature of phase transitions, no universally applicable and reliable method for determination of the state of charge exists today. A novel approach for determining the state of charge in the solar salt and eutectic mixtures of sodium nitrate and potassium nitrate is presented, which observes the discontinuity in mechanical properties occurring during phase change of these materials. The measurement setup consists of an electric motor driving a rotating probe submerged in the phase change material. Shaft speed and electric current of the motor are observed and combined into a metric correlated with an estimate of the state of charge derived by spatial interpolation of temperature measurements conducted throughout the phase change material. The setup is tested in several experiments with both materials, across different motor settings and thermodynamic boundary conditions. The detectable range of state of charge is found to be narrow and strongly dependent on the material used, while measurement accuracy within this range is insufficient for reliable state of charge determination.